Soil Disinfestation: From Soil Treatment to Soil and Plant Health

This feature article tracks 100 years of soil disinfestation, from the goal of eradicating soilborne pathogens and pests to much milder approaches, aimed at establishing a healthier soil, by favoring or enhancing the beneficial soil microflora and introducing biological control agents. Restrictions on the use of many chemical fumigants is favoring the adoption of nonchemical strategies, from soilless cultivation to the use of physical or biological control measures, with more focus on maintaining soil microbial diversity, thus enhancing soil and plant health. Such approaches are described and discussed, with special focus on their integrated use.

Soil solarization based on natural soil moisture: a practical approach for reducing the seed bank of invasive plants in wetlands

Soil solarization is a well-established method to disinfect soil for efficient weed control. However, the feasibility of applying this method in the restoration of invaded natural habitats is unclear. This is because soil moisture is necessary for the success of solarization, but pre-irrigation in natural ecosystems is often not applicable, or demands high labor investment, making it unsuitable for use in restoration. The present study was based on the idea that the relatively high soil moisture in wetlands might obviate the need for pre-irrigation, rendering this method much more applicable in natural habitats. We examined the efficacy of soil solarization using natural soil moisture to control the seed bank of the invasive plant, Acacia saligna, in a wetland, using large-scale experimental plots (0.38 ha each). An old, dense A. saligna grove was cut down and the roots were removed by a bulldozer. The plot was mulched with a transparent polyethylene sheet in early July and left on the soil for 14 weeks. Soil solarization significantly reduced the viability of seeds of A. saligna that had been experimentally buried. Additionally, viability of seeds in the natural seed bank was reduced, and seedling emergence was close to zero. Exposing seeds to soil temperature and soil moisture levels equivalent to those obtained during field soil solarization under controlled conditions significantly increased the release from dormancy of the seeds, suggesting that release from dormancy during the early stage of solarization is a critical stage leading to seed weakening or mortality in the soil. Soil solarization also decreased the cover and abundance of the natural vegetation; therefore, active revegetation is required to restore the natural vegetation and to conserve endangered and endemic species.

Rain-based soil solarization for reducing the persistent seed banks of invasive plants in natural ecosystems - Acacia saligna as a model

BACKGROUND

A large persistent seed bank of invasive plants is a significant obstacle to restoration programs. Soil solarization was demonstrated to be an effective method for reducing the seed bank of Australian acacias. However, use of this method in natural habitats might be limited due to the requirement to moisten the soil by irrigation. This study examined the possibility of replacing irrigation by trapping the soil moisture caused by the most recent rainfall, i.e. rain-based soil solarization (RBS).

RESULTS

Exposure of Acacia saligna seeds to 57 °C at 20% soil moisture for 68 h resulted in almost complete loss of seed viability. Similarly, RBS treatment significantly reduced the viability of A. saligna seeds buried at a soil depth of 1-19 cm as well as seed density in the natural seed bank, and almost completely eliminated seedling emergence from natural seed banks of A. saligna and other environmental weeds.

CONCLUSION

Our results indicate that RBS is an effective method for reducing the seed bank of invasive plants in natural habitats located in various climate regions characterized by different soil types. This is the first demonstration of a successful application of RBS for soil disinfestation. © 2018 Society of Chemical Industry.

Development and Deployment of Systems-Based Approaches for the Management of Soilborne Plant Pathogens

Biological suppression of soilborne diseases with minimal use of outside interventive actions has been difficult to achieve in high input conventional crop production systems due to the inherent risk of pest resurgence. This review examines previous approaches to the management of soilborne disease as precursors to the evolution of a systems-based approach, in which plant disease suppression through natural biological feedback mechanisms in soil is incorporated into the design and operation of cropping systems. Two case studies are provided as examples in which a systems-based approach is being developed and deployed in the production of high value crops: lettuce/strawberry production in the coastal valleys of central California (United States) and sweet basil and other herb crop production in Israel. Considerations for developing and deploying system-based approaches are discussed and operational frameworks and metrics to guide their development are presented with the goal of offering a credible alternative to conventional approaches to soilborne disease management.

Soil suppressiveness to fusarium disease: shifts in root microbiome associated with reduction of pathogen root colonization

Soil suppressiveness to Fusarium disease was induced by incubating sandy soil with debris of wild rocket (WR; Diplotaxis tenuifolia) under field conditions. We studied microbial dynamics in the roots of cucumber seedlings following transplantation into WR-amended or nonamended soil, as influenced by inoculation with Fusarium oxysporum f. sp. radicis-cucumerinum. Disease symptoms initiated in nonamended soil 6 days after inoculation, compared with 14 days in WR-amended soil. Root infection by F. oxysporum f. sp. radicis-cucumerinum was quantified using real-time polymerase chain reaction (PCR). Target numbers were similar 3 days after inoculation for both WR-amended and nonamended soils, and were significantly lower (66%) 6 days after inoculation and transplanting into the suppressive (WR-amended) soil. This decrease in root colonization was correlated with a reduction in disease (60%) 21 days after inoculation and transplanting into the suppressive soil. Fungal community composition on cucumber roots was assessed using mass sequencing of fungal internal transcribed spacer gene fragments. Sequences related to F. oxysporum, Fusarium sp. 14005, Chaetomium sp. 15003, and an unclassified Ascomycota composed 96% of the total fungal sequences in all samples. The relative abundances of these major groups were highly affected by root inoculation with F. oxysporum f. sp. radicis-cucumerinum, with a 10-fold increase in F. oxysporum sequences, but were not affected by the WR amendment. Quantitative analysis and mass-sequencing methods indicated a qualitative shift in the root's bacterial community composition in suppressive soil, rather than a change in bacterial numbers. A sharp reduction in the size and root dominance of the Massilia population in suppressive soil was accompanied by a significant increase in the relative abundance of specific populations; namely, Rhizobium, Bacillus, Paenibacillus, and Streptomyces spp. Composition of the Streptomyces community shifted significantly, as determined by PCR denaturing gradient gel electrophoresis, resulting in an increase in the dominance of a specific population in suppressive soils after only 3 days. This shift was related mainly to the increase in Streptomyces humidus, a group previously described as antagonistic to phytopathogenic fungi. Thus, suitable soil amendment resulted in a shift in the root's bacterial communities, and infection by a virulent pathogen was contained by the root microbiome, leading to a reduced disease rate.

Soil Suppressiveness to Fusarium Disease Following Organic Amendments and Solarization

Soil suppressiveness to soilborne pathogens can evolve following the incorporation of plant residues in the soil and solarization. We studied its occurrence by assessing disease incidence and severity in sandy soil which was infested after the disinfestation treatment. Disease incidence and severity of crown and root rot in cucumber plants inoculated with Fusarium oxysporum f. sp. radicis-cucumerinum macroconidia were reduced by 20 to 80% when seedlings were planted in the tested soils 2 to 34 months after soil amendment. Residues of Diplotaxis tenuifolia (wild rocket [WR]), Artemisia dracunculus (tarragon), Salvia officinalis (sage), and Brassica oleracea var. italica (broccoli) were most effective for inducing soil suppressiveness. Effective soil suppressiveness continued to be evident after repeated inoculations and plantings in the same soil without additional treatment between inoculations. Moreover, residues of WR induced soil suppressiveness in two additional tested soils differing in their physical and chemical properties. Residues of Rosmarinus officinalis (rosemary), Coriandrum sativum (coriander), Mentha piperita (peppermint), and B. oleraceae var. botrytis (cauliflower) induced disease suppression at the first inoculated planting but not upon repeated inoculation and planting. The contribution of soil solarization to the evolution of soil suppressiveness, albeit evident, was inconsistent. Soil suppressiveness to Fusarium crown and root rot was also observed when cucumber seed were sown in soils which were initially amended with WR residues and later infested with F. oxysporum f. sp. radicis-cucumerinum chlamydospores. There is a potential for the use of plant residues for inducing soil suppressiveness and further contributing to the control of diseases caused by soilborne pathogens.

Microbial aspects of accelerated degradation of metam sodium in soil

Preplant soil fumigation with metam sodium is used worldwide to control soilborne diseases. The development of accelerated degradation of pesticides in soil, including metam sodium, results in reduced pesticide efficacy. Therefore, we studied microbial involvement in accelerated degradation of methyl isothiocyanate (MITC) following repeated soil applications of the parent compound, metam sodium. MITC degradation was reduced in soil with a history of metam sodium applications following sterilization, indicating the key role of microorganisms in accelerated degradation. Accelerated degradation of MITC was induced by inoculation of soil with no previous application of metam sodium with soil with a history of metam sodium applications. We developed a method to extract the active microbial fraction responsible for MITC degradation from soil with a history of metam sodium applications. This concentrated soil extract induced accelerated degradation of MITC when added to two different soils with no previous application of metam sodium. An extensive shift in total bacterial community composition in concentrated soil extracts occurred after a single metam sodium application. Two Oxalobacteraceae strains, MDB3 and MDB10, isolated from Rehovot soil following triple application of metam sodium rapidly degraded MITC in soil with no previous application of metam sodium. Polymerase chain reaction-denaturing gradient gel electrophoresis analysis of bacterial community composition showed relative enrichment of MDB3 following metam sodium application, suggesting its potential in situ involvement in accelerated degradation development in Rehovot soil. Responses of resident Oxalobacteraceae community members to metam sodium applications differed between Rehovot and En Tamar soils. Isolate MDB10 did not induce accelerated degradation of MITC in En Tamar soil and, with the slow dissipation of MITC, soil suppressiveness of accelerated degradation is suggested. The isolation and identification of MITC-degrading bacteria might be helpful in developing tools for managing accelerated degradation.

Accelerated degradation of metam-sodium in soil and consequences for root-disease management

We studied the development of accelerated degradation (AD) of methyl isothiocyanate (MITC) following repeated applications of its parent compound, metam-sodium (MS). Laboratory studies and four sets of field experiments were conducted during 2002-04 in three commercial fields in Israel. Repeated applications of MS to the three soils in the laboratory under controlled conditions demonstrated AD of MITC in some soils. In a peanut field, MS significantly reduced the incidence of Pythium pod rot and improved pod quality after a single application but its effectiveness was greatly reduced after two applications. In a second experiment, MS was significantly effective after a single application in controlling Verticillium wilt in potato but its efficacy diminished after three consecutive applications. In an additional experiment, fumigation with MS following single or double applications was more effective in reducing Verticillium wilt severity of potato compared with triple applications. Soils which did not develop AD of MITC were also recorded. Preplant MS fumigation of melon fields was effective at reducing sudden wilt following a single and two consecutive applications. Our study shows that development of AD of MITC might occur following repeated applications of MS in commercial fields. The data on MITC dissipation in soil following repeated MS applications under controlled conditions indicate the chemical's potential loss of activity under regular agricultural practices and the need for a management strategy to prevent such a development.

Controlled laboratory system to study soil solarization and organic amendment effects on plant pathogens

ABSTRACT A controlled laboratory system for simulating soil solarization, with and without organic amendment, was developed and validated using physical, chemical, and biological parameters. The system consists of soil containers that are exposed to controlled and constant aeration, and to temperature fluctuations that resemble those occurring during solarization at various depths. This system enables a separate analysis of volatiles and other components. We recorded a sharp decrease in oxygen concentration in the soil atmosphere followed by a gradual increase to the original concentration during solarization in the field and heating in the simulation system of soil amended with wild rocket (Diplotaxis tenuifolia) or thyme (Thymus vulgaris). The combined treatment of organic amendment and solarization (or heating in the controlled system) was highly effective at controlling populations of Fusarium oxysporum f. sp. radicis-lycopersici. Changes in soil pH, enzymatic activities, and microbial populations followed, in most cases, trends which were similar under both solarization and the heating system, when exposed to controlled aerobic conditions. The reliability and validity of the system in simulating physical, chemical, and biological processes taking place during solarization is demonstrated.

Generation and Dissipation of Methyl Isothiocyanate in Soils Following Metam Sodium Fumigation: Impact on Verticillium Control and Potato Yield

The fate of methyl isothiocyanate (MITC) was studied in agricultural soils following metam sodium (MS) application in a controlled system and under field conditions as it was related to disease control. Soil samples were collected from 34 field sites in Israel with no history of MS application. The generation and dissipation curves of MITC in these soils, under controlled conditions, varied significantly among the soils, as reflected by the concentration by time (C × T) product. This value was significantly related with the mortality level of Fusarium oxysporum f. sp. radicis-lycopersici as a test organism and sand content of the soils. Seven field experiments were conducted in potato fields from 2001 to 2004. The MS treatments significantly reduced Verticillium wilt incidence and severity in five and four experiments, respectively, out of seven. Combining MS with formalin was more effective for controlling disease than MS alone in most cases. A significant relationship was found between mortality of F. oxysporum f. sp. radicis-lycopersici in soil samples to which MS was applied under controlled conditions and the incidence of Verticillium wilt disease in the field, and between C_MITC × T products and the incidence of Verticillium wilt disease in the field. These tests can be used for preplant assessment of potential MS efficacy.

Temperature Response of Chickpea Cultivars to Races of Fusarium oxysporum f. sp. ciceris, Causal Agent of Fusarium Wilt

Use of resistant cultivars and adjustment of sowing dates are important measures for management of Fusarium wilt in chickpeas (Cicer arietinum). In this study, we examined the effect of temperature on resistance of chickpea cultivars to Fusarium wilt caused by various races of Fusarium oxysporum f. sp. ciceris. Greenhouse experiments indicated that the chickpea cultivar Ayala was moderately resistant to F. oxysporum f. sp. ciceris when inoculated plants were maintained at a day/night temperature regime of 24/21°C but was highly susceptible to the pathogen at 27/25°C. Field experiments in Israel over three consecutive years indicated that the high level of resistance of Ayala to Fusarium wilt when sown in mid- to late January differed from a moderately susceptible reaction under warmer temperatures when sowing was delayed to late February or early March. Experiments in growth chambers showed that a temperature increase of 3°C from 24 to 27°C was sufficient for the resistance reaction of cultivars Ayala and PV-1 to race 1A of the pathogen to shift from moderately or highly resistant at constant 24°C to highly susceptible at 27°C. A similar but less pronounced effect was found when Ayala plants were inoculated with F. oxysporum f. sp. ciceris race 6. Conversely, the reaction of cultivar JG-62 to races 1A and 6 was not influenced by temperature, but less disease developed on JG-62 plants inoculated with a variant of race 5 of F. oxysporum f. sp. ciceris at 27°C compared with plants inoculated at 24°C. These results indicate the importance of appropriate adjustment of temperature in tests for characterizing the resistance reactions of chickpea cultivars to the pathogen, as well as when determining the races of isolates of F. oxysporum f. sp. ciceris. Results from this study may influence choice of sowing date and use of chickpea cultivars for management of Fusarium wilt of chickpea.

Development of crown and root rot disease of tomato under irrigation with saline water

ABSTRACT We studied the effect of water salinity on the incidence and severity of crown and root rot disease of tomato, as well as on the pathogen and on the plant's response to the pathogen. Irrigation with saline water significantly increased disease severity in tomato transplants inoculated with Fusarium oxysporum f. sp. radicis-lycopersici, and mineral fertilization further increased it. In one field experiment, disease incidence in plots irrigated with saline water (electrical conductivity [EC] = 3.2 +/- 0.1 dS m(-1)) and in those irrigated with fresh water (EC = 0.4 +/- 0.1 dS m(-1)) was 75 and 38%, respectively. Disease onset was earlier and yield was lower in plots irrigated with saline water. In a second field experiment, final disease incidence 250 days after planting, was 12% in plants which had been irrigated with saline water (EC = 4.6 +/- 0.1 dS m(-1)) and 4% in those irrigated with fresh water (EC = 1.2 +/- 0.1 dS m(-1)). Irrigation of tomato transplants with 20 mM NaCl did not inhibit plant development, but partial inhibition was observed at higher NaCl concentrations. Growth of the pathogen in culture or survival of conidia added to soil were not affected by saline water. Plants which were preirrigated with saline water were more severely diseased than those preirrigated with tap water. It was concluded that disease increases effected by saline water are associated with the latter's effect on plant response.

High-nitrogen compost as a medium for organic container-grown crops

Compost was tested as a medium for organic container-grown crops. Nitrogen (N) loss during composting of separated cow manure (SCM) was minimized using high C/N (wheat straw, WS; grape marc, GM) or a slightly acidic (orange peels, OP) additives. N conservation values in the resultant composts were 82%, 95% and 98% for GM-SCM, OP-SCM and WS-SCM, respectively. Physical characteristics of the composts were compatible with use as growing media. The nutritional contribution of the composts was assessed using cherry tomato (Lycopersicon esculantum Mill.) and by means of incubation experiments. Media were either unfertilized or fertilized with guano (sea-bird manure). Plant responses suggest that N availability is the main variable affecting growth. Unfertilized OP-SCM and WS-SCM supplied the N needed for at least 4 months of plant growth. Root-galling index (GI) of tomato roots and number of eggs of the nematode Meloidogyne javanica were reduced by the composts, with the highest reduction obtained by OP-SCM and WS-SCM, at 50% concentrations. These composts, but not peat, reduced the incidence of crown and root-rot disease in tomato as well as the population size of the causal pathogen, Fusarium oxysporum f. sp. radicis-lycopersici.

Effect of moisture on thermal inactivation of soilborne pathogens under structural solarization

ABSTRACT Structural solarization of greenhouses for sanitation by closing them involves dry heating to 60 degrees C and higher with a consequent low relative humidity (RH) ( approximately 15%), thus requiring an extended period for thermal inactivation of pathogens. In an attempt to enhance pathogen control by increasing moisture during the hot hours of the day, various regimes of inoculum moistening were studied. However, wetting inoculum of Fusarium oxysporum f. sp. melonis and F. oxysporum f. sp. radicis-lycopersici resulted in less effective pathogen control compared with that of dry heating. Fifty percent effective dose (ED(50)) values of thermal inactivation of wetted and dry inoculum for the former pathogen were 18 and 7 days, respectively, and for the latter, a respective 9 and 4 days. This was because wetting resulted in inoculum cooling due to evaporation, which eventually led to its drying. A model describing the drying of wet inoculum in a wetted greenhouse, based on the fact that there was an approximately 10 degrees C difference between greenhouse and ambient temperatures, was proposed. A double-tent system reduced this difference to 1 to 2 degrees C, reduced moisture loss, and led to improved inoculum inactivation of F. oxysporum f. sp. radicis-lycopersici. Thus, the ED(50) value of thermal inactivation was reduced from 15 days to 1 day, because this system provided both high temperature ( approximately 60 degrees C) and high RH ( approximately 100%), resulting in effective wet heating.

Modeling the survival of two soilborne pathogens under dry structural solarization

ABSTRACT Structural (space) solarization of a closed, empty greenhouse for sanitation involves dry heating to 60 degrees C and higher and low relative humidity (RH), under a fluctuating temperature and RH regime. Survival of inocula of Fusarium oxysporum f. sp. radicis-lycopersici and Sclerotium rolfsii during structural solarization was studied for 4 years (total of 12 experiments) in an attempt to develop a dynamic model for expressing the thermal inactivation of the pathogens. After 20 days of exposure, the populations of F. oxysporum f. sp. radicis-lycopersici and S. rolfsii were reduced by 69 to 95% and by 47.5 to 100%, respectively. The Weibull distribution model was applied to describe pathogen survival. The Weibull rate parameter, b, was found to follow an exponential (for F. oxysporum f. sp. radicis-lycopersici) and the Fermi (for S. rolfsii) functions at constant temperatures. To improve the applicability of the model, fluctuating conditions of both temperature and RH were utilized. The Weibull distribution derivative, expressed as a function of temperature and moisture, was numerically integrated to estimate survival of inocula exposed to structural solarization. Deviations between experimental and calculated values derived from the model were quite small and the coefficient of determination (R (2)) values ranged from 0.83 to 0.99 in 9 of 12 experiments, indicating that ambient RH data should be considered. Structural solarization for sanitation could be a viable component in integrated pest management programs.

Survival in Soil of Colletotrichum acutatum and C. gloeosporioides Pathogenic on Strawberry

The survival ability in soil of different inocula of strawberry isolates of Colletotrichum gloeosporioides and C. acutatum was studied under laboratory and field conditions. Two isolates of each species used in this study were identified according to morphological characteristics and by molecular techniques. Conidia of all four isolates survived for up to 1 year in autoclaved soil, whereas viability declined rapidly in untreated soils at 22% soil moisture (field capacity), with a 95% reduction in population recorded within 4.0 to 9.8 days. In methyl bromide (MB)-treated field soil at field capacity, a 95% decline in the viability of conidia of the two species was recorded within 8.9 to 12.9 days. At 11% soil moisture content, the time required for a 95% population reduction of the isolates of C. gloeosporioides and C. acutatum conidia was 124.5 and 114.4, and 72.8 and 74.2 days, respectively. C. acutatum was not recovered from naturally infected crowns after burial for 5 months in MB-fumigated and untreated soils at 10- and 20-cm depths under field conditions, but the decline was slower in the MB-fumigated soil. However, recovery of the pathogen from artificially inoculated mummified fruit after 5 months of burial ranged from 15 to 39%. Soil solarization for 4 weeks and MB fumigation treatments eradicated the pathogen from buried, artificially inoculated fruits. Based on this study, the potential contribution of conidia as well as mummified fruits to disease epidemics should be considered.

Weakening and Delayed Mortality of Fusarium oxysporum by Heat Treatment: Flow Cytometry and Growth Studies

ABSTRACT Survival of Fusarium oxysporum f. sp. niveum following heat treatments was studied using flow cytometric, physiological, and microscopic assays. We exposed germinating conidia to sublethal temperatures from 36 to 42 degrees C for 60 min, followed by rhodamine 123 staining and flow cytometry, and found increasing levels of fluorescence that reflect a change in mitochondrial membrane potential, indicating a weakening induced by stress. Viability of conidia or germinating conidia of the fungus exposed to heat decreased with increasing temperature, as assessed by fluorescent staining. However, viability was higher than that assessed with the 5-day-long plate count method and was further reduced 13 and 24 h after treatment, suggesting delayed mortality of the heat-treated germinating conidia. Delayed mortality was substantiated by observing these conidia with light and fluorescent scanning electron microscopy and by subculturing single germinating conidia that had been previously heated. Programmed cell death was not observed in heat-treated conidia or germinating conidia of F. oxysporum based on the detection of plasma membrane phosphatidylserine translocation, cell-cycle measurements, detection of DNA fragmentation, or microscopic observation of apoptotic bodies. We hypothesize that propagules, which survived the heating and apparently are alive, may undergo further irreversible detrimental processes, eventually leading to their death by yet unidentified mechanisms. These findings suggest that pathogen propagules also might be affected under lower temperatures, possibly facilitating pathogen control by heating.

Role of the Shrub Tamarix nilotica in Dissemination of Fusarium oxysporum f. sp. radicis-lycopersici

The saltcedar shrub Tamarix nilotica grows as a weed in the Arava region of Israel. This weed is commonly found in cultivated fields naturally infested with Fusarium oxysporum f. sp. radicis-lycopersici, the causal agent of tomato crown and root rot. Young bushes, 20 to 40 cm tall, were randomly uprooted from different fields. The roots were cut into segments which were placed on Fusarium-selective medium. Although the plants did not show any symptoms of disease, the roots of the shrub were colonized by the pathogen. The incidence of infected saltcedar plants and level of root colonization by F. oxysporum f. sp. radicis-lycopersici decreased with increasing distance of the sampling location from a tomato field infected with crown and root rot. F. oxysporum f. sp. radicis-lycopersici was also isolated from chaff of inflorescence samples taken from mature T. nilotica shrubs. Identity of the pathogen isolates obtained from T. nilotica roots and chaff samples was verified by pathogenicity and vegetative compatibility tests. Roots of T. nilotica plants sown under greenhouse conditions in soil naturally infested with F. oxysporum f. sp. radicis-lycopersici became colonized by the pathogen. Uprooting and removing saltcedar plants throughout the season from fields not cultivated with tomatoes lowered the inoculum density of F. oxysporum f. sp. radicis-lycopersici in the soil from 611 to 6 and from 176 to 10 CFU/g of soil in the 1998-99 and 1999-2000 growing seasons, respectively. These results demonstrate that T. nilotica may contribute to the buildup of the pathogen populations in the absence of a susceptible host. Colonization of saltcedar by F. oxysporum f. sp. radicis-lycopersici is an additional mechanism for survival of this pathogen in the fields and for dissemination through the spread of infested seed or chaff of T. nilotica.

Disease Development Following Infection of Tomato and Basil Foliage by Airborne Conidia of the Soilborne Pathogens Fusarium oxysporum f. sp. radicis-lycopersici and F. oxysporum f. sp. basilici

ABSTRACT Fusarium oxysporum f. sp. radicis-lycopersici, the causal agent of Fusarium crown and root rot of tomato, and F. oxysporum f. sp. basilici, the causal agent of Fusarium wilt in basil, are soilborne pathogens capable of producing conspicuous masses of macroconidia along the stem. The role of the airborne propagules in the epidemics of the disease in tomato plants was studied. In the field, airborne propagules of F. oxysporum f. sp. radicis-lycopersici were trapped with a selective medium and their prevalence was determined. Plants grown in both covered and uncovered pots, detached from the field soil, and exposed to natural aerial inoculum developed typical symptoms (82 to 87% diseased plants). The distribution of inoculum in the growth medium in the pots also indicated the occurrence of foliage infection. In greenhouse, foliage and root inoculations were carried out with both tomato and basil and their respective pathogens. Temperature and duration of high relative humidity affected rate of colonization of tomato, but not of basil, by the respective pathogens. Disease incidence in foliage-inoculated plants reached 75 to 100%. In these plants, downward movement of the pathogens from the foliage to the crown and roots was observed. Wounding enhanced pathogen invasion and establishment in the foliage-inoculated plants. The sporulation of the two pathogens on stems, aerial dissemination, and foliage infection raise the need for foliage protection in addition to soil disinfestation, in the framework of an integrated disease management program.

Combined soil treatments and sequence of application in improving the control of soilborne pathogens

ABSTRACT The effects of reduced doses of methyl bromide (MB) or metham sodium, heating, short solarization, and soil microbial activity, alone or in combination, on survival of soilborne fungal pathogens were tested in a controlled-environment system and field plots. Sublethal doses of heating or MB delayed germination of Sclerotium rolfsii sclerotia. Combining MB and heating treatments was more effective than either treatment alone in controlling S. rolfsii and Fusarium oxysporum f. sp. basilici. The application heating followed by fumigation with MB, was significantly more effective in delaying and reducing germination of S. rolfsii sclerotia and in controlling F. oxysporum f. sp. basilici than the opposite sequence. Further, incubation in soil and exposure to microbial activity of previously heated or MB-treated sclerotia increased the mortality rate, indicating a weakening effect. Similarly, incubation of chlamydospores of F. oxysporum f. sp. melonis and F. oxysporum f. sp. radicis-lycopersici in soil in the field after fumigation further reduced their survival, confirming the laboratory results. In field tests, combining MB or metham sodium at reduced doses with short solarization was more effective in controlling fungal pathogens than either treatment alone. Treatment sequence significantly affected pathogen control in the field, similar to its effect under controlled conditions. This study demonstrates a frequent synergistic effect of combining soil treatments and its potential for improving pathogen control and reducing pesticide dose, especially when an appropriate sequence was followed.

Vegetative Compatibility Groups of Verticillium dahliae in Israel: Their Distribution and Association with Pathogenicity

A collection of 565 isolates of Verticillium dahliae, recovered between 1992 and 1997 from 13 host plant species and soil at 47 sites in Israel, was tested for vegetative compatibility using nitrate-nonutilizing (nit) mutants. Three vegetative compatibility groups (VCGs) were found and identified as VCG2A (28 isolates), VCG2B (158 isolates), and VCG4B (378 isolates) by using international reference strains. One isolate was heterokaryon self-incompatible. Of the VCG2B isolates, 92% were recovered from the northern part of Israel and 90% of VCG4B isolates were recovered from the south, with some overlap in the central region. Isolates of the minor group VCG2A were geographically scattered among the two major VCGs. Isolates of the same VCG resembled one another more than isolates from different VCGs based on colony and microsclerotial morphology, temperature responses, and, partially, pathogenicity. Different pathotypes were defined among 60 isolates tested, using cotton (cv. Acala SJ-2) and eggplant (cv. Black Beauty) as differentials. All isolates in VCG2A and 86% of the isolates in VCG4B, irrespective of their origin, induced weak to moderate symptoms on cotton and moderate to severe symptoms on eggplant and were similar to the previously described cotton nondefoliating patho-type. In contrast, all cotton isolates in VCG2B caused severe foliar symptoms, stunting, and often death, but little or no defoliation of inoculated cotton plants. These were defined as a cotton defoliating-like pathotype and induced only weak to moderate symptoms on eggplant. We concluded that vegetative compatibility grouping of V. dahliae in Israel is closely associated with specific pathogenicity and other phenotypic traits.

Efficacy of Fluazinam in Suppression of Monosporascus cannonballus, the Causal Agent of Sudden Wilt of Melons

Sudden wilt (vine decline) of melon, caused by Monosporascus cannonballus, is a worldwide problem in arid and semi-arid regions. Soil disinfestation by fumigation with methyl bromide before planting is a common treatment for disease management but, because methyl bromide is expected to be banned from use within the next 10 years, alternative measures for disease suppression are needed. The efficacy of 29 fungicides against M. cannonballus was evaluated in vitro. Among the fungicides tested, fluazinam and kresoxim methyl were the most effective and both totally inhibited the growth of M. cannonballus in culture at concentrations of 10 μg a.i /ml. Because fluazinam also was effective in inhibition of Pythium aphnidermatum, which also may be involved in sudden wilt syndrome, and kresoxim methyl was not, fluazinam was chosen for further tests. The effective dose of fluazinam for M. cannonballus that reduced mycelial growth by 50% was 0.09 μg a.i./ml. Fluazinam efficacy was evaluated in three field experiments conducted in the spring and in the late summer cropping seasons. In two of the experiments, applications of fluazinam resulted in approximately 87% wilt reduction, whereas in the third experiment it was only 32%. The mobility of fluazinam in soil was determined in samples taken from the field. Fungicide mobility in soil was relatively limited; most of the compound was adsorbed to soil particles, resulting in a zone of high concentration that decreased with depth and distance from the application site. Nevertheless, rates measured even at a depth of 25 cm were sufficient to control M. cannonballus. This study shows that fluazinam may be used as one component in an integrated approach for suppression of sudden wilt of melons.

Spatial distribution and temporal development of fusarium crown and root rot of tomato and pathogen dissemination in field soil

ABSTRACT The spatial distribution and temporal development of tomato crown and root rot, caused by Fusarium oxysporum f. sp. radicis-lycopersici, were studied in naturally infested fields in 1996 and 1997. Disease progression fit a logistic model better than a monomolecular one. Geostatistical analyses and semivariogram calculations revealed that the disease spreads from infected plants to a distance of 1.1 to 4.4 m during the growing season. By using a chlorate-resistant nitrate nonutilizing (nit) mutant of F. oxysporum f. sp. radicis-lycopersici as a "tagged" inoculum, the pathogen was found to spread from one plant to the next via infection of the roots. The pathogen spread to up to four plants (2.0 m) on either side of the inoculated focus plant. Root colonization by the nit mutant showed a decreasing gradient from the site of inoculation to both sides of the inoculated plant. Simulation experiments in the greenhouse further established that this soilborne pathogen can spread from root to root during the growing season. These findings suggest a polycyclic nature of F. oxysporum f. sp. radicis-lycopersici, a deviation from the monocyclic nature of many nonzoosporic soilborne pathogens.

Sudden Wilt of Melons in Southern Israel: Fungal Agents and Relationship with Plant Development

Fungi belonging to five genera, Monosporascus sp., Pythium aphanidermatum, Rhizoctonia solani, Olpidium sp., Fusarium solani, and F. proliferatum, were the species most frequently isolated from the root systems of wilted melon. Diseased plants were collected from 24 fields in the northern and central Arava region of southern Israel during the fall seasons of 1994 and 1995. In pathogenicity tests conducted under field conditions, in artificially inoculated microplots, the first wilt symptoms were observed at various stages of fruit maturation. High mortality levels (73 to 97%) were recorded for inoculation combinations in which Monosporascus sp. was involved. Inoculations with the other fungi listed resulted in lower incidences of wilt. The combination of F. solani and P. aphanidermatum resulted in higher mortality than that caused by each pathogen alone. Monosporascus sp. seems to be the primary pathogen, although other fungi could also induce wilt. The dry weight of plants grown in naturally infested soil ceased to accumulate 33 days after transplanting, in contrast to plants grown in methyl bromide-treated soil. At this stage, the first wilt symptoms were observed. Fruit load affected wilt incidence. At the end of the growing season, 98% mortality was recorded for plants having the normal fruit load (2.5 fruits per plant) compared with 75 and 12% for plants that had their fruits thinned to one or zero per plant, respectively.

Biological Control of Sclerotium rolfsii and Verticillium dahliae by Talaromyces flavus Is Mediated by Different Mechanisms

ABSTRACT Ten wild-type strains and two benomyl-resistant mutants of Talaromyces flavus were examined for their ability to secrete the cell wall-degrading enzymes chitinase, beta-1,3-glucanase, and cellulase, to parasitize sclerotia of Sclerotium rolfsii, to reduce bean stem rot caused by S. rolfsii, and to secrete antifungal substance(s) active against Verticillium dahliae. The benomyl-resistant mutant Ben(R)TF1-R6 overproduced extracellular enzymes and exhibited enhanced antagonistic activity against S. rolfsii and V. dahliae compared to the wild-type strains and other mu tants. Correlation analyses between the extracellular enzymatic activities of different isolates of T. flavus and their ability to antagonize S. rolfsii indicated that mycoparasitism by T. flavus and biological control of S rolfsii were related to the chitinase activity of T. flavus. On the other hand, production of antifungal compounds and glucose-oxidase activity may play a role in antagonism of V. dahliae by retardation of germination and hyphal growth and melanization of newly formed microsclerotia.

Sporulation of Fusarium oxysporum f. sp. lycopersici on Stem Surfaces of Tomato Plants and Aerial Dissemination of Inoculum

ABSTRACT Plants exhibiting symptoms of wilt and xylem discoloration typical of Fusarium wilt caused by Fusarium oxysporum f. sp. lycopersici were observed in greenhouses of cherry tomatoes at various sites in Israel. However, the lower stems of some of these plants were covered with a pink layer of macroconidia of F. oxysporum. This sign resembles the sporulating layer on stems of tomato plants infected with F. oxysporum f. sp. radicis-lycopersici, which causes the crown and root rot disease. Monoconidial isolates of F. oxysporum from diseased plants were assigned to vegetative compatibility group 0030 of F. oxysporum f. sp. lycopersici and identified as belonging to race 1 of F. oxysporum f. sp. lycopersici. The possibility of coinfection with F. oxysporum f. sp. lycopersici and F. oxysporum f. sp. radicis-lycopersici was excluded by testing several macroconidia from each plant. Airborne propagules of F. oxysporum f. sp. lycopersici were trapped on selective medium in greenhouses in which plants with a sporulating layer had been growing. Sporulation on stems was reproduced by inoculating tomato plants with races 1 and 2 of F. oxysporum f. sp. lycopersici. This phenomenon has not been reported previously with F. oxysporum f. sp. lycopersici and might be connected to specific environmental conditions, e.g., high humidity. The sporulation of F. oxysporum f. sp. lycopersici on plant stems and the resultant aerial dissemination of macroconidia may have serious epidemiological consequences. Sanitation of the greenhouse structure, as part of a holistic disease management approach, is necessary to ensure effective disease control.

Reduced Dosage of Methyl Bromide for Controlling Verticillium Wilt of Potato in Experimental and Commercial Plots

The use of gas-impermeable films to reduce the dosage of methyl bromide (MB) required to control Verticillium wilt in potatoes was examined in field experiments, conducted in soils naturally infested with Verticillium dahliae. The incidence and severity of Verticillium wilt were significantly reduced (by 74 to 94%) by fumigation with MB at 50 g/m² under standard low density polyethylene (LDPE) or at 25 g/m² under gas-impermeable films. Fumigation at 25 g/m² under LDPE was less effective. Disease severity was inversely correlated (r² = 0.89 to 0.91) with chlorophyll content in the leaves. Fumigation also reduced (by 89 to 100%) stem colonization by the pathogen. Potato yield in the fumigated plots was significantly higher (26 to 69%), than in their nonfumigated counterparts, and was inversely correlated with disease index (r² = 0.69 to 0.9). The percentage of high-value tubers (above 45 g) was 52 to 56% of total yield in the fumigated plots as compared with 32 to 40% in the nonfumigated controls. Thus, fumigation also improved the commercial quality of tuber yield. Effective control of V. dahliae and yield increases following MB fumigation at the recommended dosage or at a reduced dosage with gas-impermeable films was also observed in a consecutive crop. These results were verified in a large-scale field experiment using commercial applications, further demonstrating the feasibility of reducing MB dosages under farm conditions, without reducing its effectiveness in terms of disease control and yield improvement.

Delayed and enhanced biodegradation of soil-applied diphenamid, carbendazim, and aldicarb

Recent studies have demonstrated that interaction between various agrochemicals and soil microorganisms may either slow down or enhance processes of degradation. Soil disinfestation is employed for the control of soil-borne pathogens and weeds. Soil application of such broad-spectrum biocides, as well as some more selective chemicals, has a strong effect on microbial activity, which may result in drastic reduction in the rate of degradation of pesticides applied to such treated soils. Application of pesticides to previously disinfested soils may extend their biological activity, which in the case of herbicides could cause phytotoxic damage to the next crop. In contrast, repeated application of the same or structurally related pesticides may result in a selective buildup of microbial populations capable of degrading the pesticide at much faster rates. Cases of accelerated degradation were reported for pesticides belonging to various chemical groups. Studies were conducted to evaluate the mechanisms of accelerated degradation. For several pesticides it has been shown that soil fungi are involved in their normal degradation, but not in their accelerated degradation. The shift in the rate of degradation of pesticides such as diphenamid, benomyl, and S-ethyl dipropylthiocarbamate, in soils that have acquired accelerated degradation, seems to be associated with the buildup of populations of bacterial degraders. Moreover, it has been shown that for the herbicide diphenamide, accelerated degradation is apparently linked to the induction of an oxidative demethylation process in soil bacteria, which might be analogous to the development of resistance in pests. Contrary to studies demonstrating accelerated degradation of the systemic insecticide aldicarb and accumulation of aldicarb sulfoxide in non-history soils, our work has shown that repeated application of this carbamate at several locations in Israel over a 10-year period did not induce accelerated degradation. It should be pointed out that in the Israeli soils there was only minimal formation of aldicarb sulfoxide. These studies were performed in soils with a pH ranging from 7.8 to 8.3, which is higher than the pH reported for soils where accelerated degradation was detected.

RFLP mapping of I1, a new locus in tomato conferring resistance against Fusarium oxysporum f. sp. lycopersici race 1

The inheritance and linkage relationships of a gene for resistance to Fusarium oxysporum f. sp. lycopersici race 1 were analyzed. An interspecific hybrid between a resistant Lycopersicon pennellii and a susceptible L. esculentum was backcrossed to L. esculentum. The genotype of each backcross-1 (BC1) plant with respect to its Fusarium response was determined by means of backcross-2 progeny tests. Resistance was controlled by a single dominant gene, I1, which was not allelic to I, the traditional gene for resistance against the same fungal pathogen that was derived from L. pimpinellifolium. Linkage analysis of 154 molecular markers that segregated in the BC1 population placed I1 between the RFLP markers TG20 and TG128 on chromosome 7. The flanking markers were used to verify the assignment of the I1 genotype in the segregating population. The results are discussed with reference to the possibility of cloning Fusarium resistance genes in tomato.

Involvement of fungi and bacteria in enhanced and nonenhanced biodegradation of carbendazim and other benzimidazole compounds in soil

The relationship between chemical structure and the enhancement of microbial degradation of three benzimidazole compounds in soil was determined. Preapplication of methyl benzimidazole-2-ylcarbamate (carbendazim or MBC), 2-aminobenzimidazole (2AB), and benzimidazole enhanced their degradation upon repeated application (self-enhanced degradation). MBC and 2AB cross-enhanced the degradation of each of these two compounds, whereas benzimidazole did not enhance the degradation of MBC. Thiabendazole (TBZ) did not enhance its own degradation or cross-enhance the degradation of MBC. No increase in the number of MBC-degrading fungi or in the capacity of soilborne fungi to degrade MBC was detected in soil exhibiting enhanced MBC degradation (MBC-history). A sharp increase in esterolytic activity in the microsomal fraction of Alternaria alternata capable of degrading MBC in culture was induced by the presence of MBC in the growth medium. 2AB was the main metabolite of MBC that accumulated in A. alternata cultures and in cell-free preparations. MBC was degraded much faster by mixed bacterial cultures that originated from MBC-history soil than in cultures from MBC-nonhistory soil. Fluctuations in the MBC degrading capacity of mixed bacterial cultures occurred during repeated subculturing of the mixed culture. Inoculation of nonhistory soil with mixed bacterial cultures resulted in enhanced MBC degradation, whereas inoculation with A. alternata did not enhance MBC degradation. It is suggested that while fungi contribute to MBC dissipation in soil, bacteria have a greater role in enhanced biodegradation of MBC in soil.

An RFLP marker in tomato linked to the Fusarium oxysporum resistance gene I2

The locus, I2, which in tomato confers resistance against Fusarium oxysporum f. sp. lycopersici race 2, was introgressed into Lycopersicon esculentum from the wild species L. pimpinellifolium (P.I. 126915). We searched for restriction fragment length polymorphisms (RFLPs) between nearly isogenic lines (NILs) in clones that map to the region introgressed from the wild species. Since I2 maps to chromosome 11, we used DNA clones from this chromosome as hybridization probes to Southern blots containing bound DNA of the NILs digested with 23 restriction enzymes. Of the 14 chromosome 11 clones, 9 exhibited polymorphism. These clones were further hybridized to "verification" filters that contained DNA from resistant and susceptible L. esculentum varieties digested with the enzymes that gave the polymorphism. One clone, TG105, was found to be associated with I2; 19 susceptible lines showed a different RFLP with this probe than 16 resistant lines, including the original L. pimpinellifolium accession used as a source for the resistance gene. These results together with our mapping analysis indicate that TG105 is closely linked to the resistance gene.

Role formation and division of work in multiprofessional human service organizations

Many Human Service Organizations (HSOs) employ professionals and non-professional workers drawn from different occupational groups. The author presents seven factors that might affect the processes of role formation and division of work: the profession, the organization, power relationships among workers, the dominant profession, consensual decision making in workers' groups, client effects, and individual characteristics. The discussion highlights the dearth of knowledge on these issues, and the necessity for systematic and comprehensive study directed toward identifying the factors affecting the emergence of a variety of role formation and division of work patterns in HSOs.

The effect of sublethal heating on sclerotia of Selerotium rolfsii

Sublethal heating of sclerotia of Sclerotium rolfsii at 50 °C for 30 min increased leakage of 14C-labelled water-soluble organic compounds from the sclerotia, and increased their colonization by bacteria and streptomycetes 574- and 1420-fold, respectively. Disease incidence in bean plants inoculated with heat-treated sclerotia was 43% less than disease incidence produced by unhealed ones. Scanning electron microscope observations demonstrated that heating increased the frequency of surface cracks on the sclerotia and the concentrations of bacteria on or around these cracks about 10 times.

Binding of (14C) parathion in soil: a reassessment of pesticide persistence

A steady decrease of extractable [14C] parathion residues in soils over a 1-month incubation period was accompanied by an increase of unextractable, bound 14C-labeled residues, resulting finally in total recoveries of extracted plus bound residues of 80 to 87 percent of the applied radiocarbon. Soils containing bound residues were nontoxic to fruit flies. Binding of 14C-labeled residues was related to the activity of soil microorganisms; soil sterilization resulted in a reduction of binding by 58 to 84 percent. Under flooded (anaerobic) conditions, the binding of compounds labeled with 14C doubled, and parathion was reduced to aminoparathion. Reinoculation of sterilized flooded soil fully reinstated the binding capacity. [14C] Aminoparathion was preferentially bound to soil, since its binding within 2 hours was 30 times greater than that of [14C] parathion. Because of the existence of formerly "unseen," unextractable residues, the concept of "persistent" and "nonpersistent" pesticide residues might have to be reconsidered.

Scanning electron microscopy of the septal pore apparatus of Rhizoctonia solani

The septal pore apparatus of Rhizoctonia solani was studied in hyphae of different ages by scanning electron microscopy. The formation of the septal pore apparatus begins with an annular swelling followed by the development of a dome-shaped perforated pore cap, which usually has three apertures. In older hyphae, the septal pore apparatus is absent, and the septal pore may be plugged. In some cases, four apertures are observed in the pore cap.

Sequential production of polygalacturonase, cellulase, and pectin lyase by Rhizoctonia solani

The sequence of appearance of cell wall degrading enzymes of Rhizoctonia solani propagules was followed. Polygalacturonase (PG; EC 3.2.1.15) was induced earlier by sodium polypectate (NaPP) as compared with the induction of cellulase (Cx; EC 3.2.1.4) by carboxymethyl cellulose (CMC), cellobiose, or fibrous cellulose powder. Increasing CMC concentration to 0.5% shortened the time of Cx appearance. In Czapek medium containing citrus pectin, pectin lyase (PL; EC 4.2.2.10) was produced faster and at higher amounts than in a medium containing NaPP as the sole carbon source. PG appearance also preceded that of PL in media simultaneously supplemented with their respective inducers. NaPP, which induced production of PG, repressed Cx production. Among the Cx inducers, only CMC and cellobiose repressed PG production to any extent. At pH 6.0, either in a synthetic medium or on autoclaved bean hypocotyl segments, a delay in PG production as compared with Cx and Pl production was observed. Optimal pH levels for enzyme production and activity were 4.0 and 5.0 for PG, and 5.5 for Cx, and 8.0 and 7.5 for PL. PG was less repressed than Cx by glucose, cellobiose, and monogalacturonic acid, while PL was not affected.

Release of cell-bound polygalacturonase and cellulase from mycelium of Rhizoctonia solani

Propagules of Rhizoctonia solani grown in modified Czapek's medium containing sodium polypectate or carboxymethyl cellulose as a sole carbon source produced both extracellular and cell-bound polygalacturonase (PG), and cellulase (Cx), respectively. The cell-bound enzymes can be released to various extents by shaking the germinating propagules in solutions of NaCl, KCl, phosphate buffer, Na2EDTA (ethylenediaminetetraacetate), detergents such as Triton X-100 (octyl phenoxypolyethoxyethanol), Tween 80 (polyoxyethylene sorbitan monooleate), Celmusol, and distilled water. Sodium dodecyl sulfate (SDS) inactivated both PG and Cx but did no affect Cx activity in phosphate buffer solution. PG was more easily released by salts from the mycelium of R. solani than Cx. The release of both enzymes was a passive process and was not due to an osmotic effect. The amount of the cell-bound fraction was correlated with the total amount of the extracellular fraction rather than with the mycelial growth. At least one-third of the cell-bound fractions of both enzymes was found to be associated with the cell wall fraction of the mycelium.

Effect of propagule size on the in vitro production of polygalacturonase and cellulase by Rhizoctonia solani

The physiological behavior of large (250–500 microns (μ)) infective and small (50–150 μ) non-infective mycelial fragments (propagules) of Rhizoctonia solani was compared. Liquid Czapek's medium containing sodium polypectate (NaPP) as the sole carbon source was inoculated with equal amounts (on a dry weight basis) of each kind of propagule. At NaPP concentrations of 0.005%–0.25% large propagules produced more polygalacturonase (PG) after a shorter period of incubation than small ones, whereas no differences in PG activity were observed at 1% NaPP. Differences in PG activity between large and small propagules were also observed in a medium consisting of 1:50 dilution of potato extract, but not in richer media such as potato dextrose broth and bean hypocotyl medium. These differences did not depend on pH of the medium or dry weight of the mycelium. Germination percentage of the large propagules and length of their emerging hyphae were greater than those of the small ones in media containing 0.005%–0.25% NaPP. Extracellular cellulase activity in Czapek's medium containing 0.25% carboxymethyl cellulose was also higher and appeared earlier in large propagules as compared with small ones. Viscometric and colorimetric methods for detecting PG and cellulase activities produced similar results. It is suggested that the capacity of the large propagules to produce hydrolytic enzymes at low nutrient levels may be important for their parasitic activity.