Effect of Fall Application of Fungal Antagonists on Spring Ascospore Production of the Apple Scab Pathogen, Venturia inaequalis

Irrigation Targeted to Provoke Ejection of Ascospores of Venturia inaequalis Shortens the Season for Ascospore Release and Results in Less Apple Scab

Trials were carried out in apple orchards of Emilia-Romagna and Trentino-Alto Adige in northern Italy to investigate the effects of sprinkler irrigation on possible reduction in inoculum and subsequent disease pressure of Venturia inaequalis, the ascomycete causing apple scab. In spring, volumetric spore traps were placed above apple leaf litter containing pseudothecia with ascospores of the fungus. Pseudothecia matured more rapidly in irrigated plots, and 95% of the total number of spores trapped in a season was reached on average 164 degree days (base temperature 0°C) earlier in irrigated compared with nonirrigated plots. On average for seven location/year combinations, more than 50% of the ascospores were trapped following irrigations carried out for 2 h on sunny days before a forecasted rainfall. Subsequently, a much lower number of spores were trapped on rainy days following irrigation. Field trials with scab-susceptible apple cultivars were carried out in the two regions to evaluate the efficacy of sprinkler irrigation on disease. Irrigated and nonirrigated plots were either treated with different fungicide control strategies or not treated. Irrigation significantly reduced the incidence of apple scab at both sites, and the overall number of infected leaves and fruit was reduced by more than 50%. Midday sprinkler irrigation can significantly reduce the inoculum pressure of V. inaequalis in apple orchards. This may be a sustainable management strategy, especially in areas with extended dry periods.

The Use of Mycoendophyte-Based Bioformulations to Control Apple Diseases: Toward an Organic Apple Production System in the Aurès (Algeria)

The present study aims to investigate the effectiveness of bioformulations based on endophytic fungi to control apple scab and Valsa canker disease in two orchards in the Aurès region (Algeria). In both orchards, the results showed that the treatment of senescent apple leaves by invert emulsions containing Trichoderma longibrachiatum and Chaetomium globosum harmed the ascogenesis of winter forms of Venturia inaequalis by reducing the number of ascospore-ejecting asci, the number of morphologically mature asci, and a considerable increase in the immature asci number. This antifungal activity was more essential in soil-incorporated leaves, showing the importance of the combination of treatments with cultural practices to efficiently control the apple scab disease. Furthermore, the disease incidence decreased by 52.63% and 50.68% in R'haouat and Bouhmama orchards, respectively. Moreover, the treatment of Valsa ceratosperma cankers with a biogel containing the endophytic yeast Metschnikowia sp. led to wound healing varying from 43.52% and 87.97% after 120 days but remained more considerable than conventional treatment with Folicur (tebuconazol). The current results open real opportunities concerning the implementation of eco-friendly and potent apple protection systems.

Development of a Sensitive TaqMan qPCR Assay for Detection and Quantification of Venturia inaequalis in Apple Leaves and Fruit and in Air Samples

A TaqMan quantitative PCR (qPCR) assay based on the translation elongation factor 1-α gene was developed for the quantification of Venturia inaequalis in leaves and fruits of Malus × domestica and in spore trap samples. The designed primers and hydrolysis probe amplified a specific 86-bp fragment for V. inaequalis. The specificity of the assay was tested using 35 strains of V. inaequalis and 20 different fungal species, including common pathogens of apple and other species of Venturia. The limit of detection was 20 fg, which is lower than a single genome of V. inaequalis. The selectivity of the assay was tested using DNA from three cultivars of Malus × domestica, and no influence on pathogen amplification was found. The assay was also validated for repeatability and reproducibility. With this assay, it was possible to detect and quantify V. inaequalis in four cultivars (Ambrosia, Florina, Golden Delicious, and Mondial Gala) in both symptomatic and asymptomatic leaves and in symptomatic Golden Delicious apple fruit stored for 2 months. Furthermore, the assay was successfully tested on spore trap samples originating from apple orchards. The quantification of the molecular assay when compared with the estimated number of V. inaequalis cells, using an optical microscope, showed a correlation coefficient of 0.8186. The developed technique could be used to detect V. inaequalis in asymptomatic samples without any cross-reaction with other fungal species. Furthermore, to improve the efficacy of disease management with a timely application of fungicides, this assay could be used for the analysis of spore trap samples by using an implemented extraction method.

Bacteriophage Usage for Bacterial Disease Management and Diagnosis in Plants

In nature, plants are always under the threat of pests and diseases. Pathogenic bacteria are one of the major pathogen types to cause diseases in diverse plants, resulting in negative effects on plant growth and crop yield. Chemical bactericides and antibiotics have been used as major approaches for controlling bacterial plant diseases in the field or greenhouse. However, the appearance of resistant bacteria to common antibiotics and bactericides as well as their potential negative effects on environment and human health demands bacteriologists to develop alternative control agents. Bacteriophages, the viruses that can infect and kill only target bacteria very specifically, have been demonstrated as potential agents, which may have no negative effects on environment and human health. Many bacteriophages have been isolated against diverse plant-pathogenic bacteria, and many studies have shown to efficiently manage the disease development in both controlled and open conditions such as greenhouse and field. Moreover, the specificity of bacteriophages to certain bacterial species has been applied to develop detection tools for the diagnosis of plant-pathogenic bacteria. In this paper, we summarize the promising results from greenhouse or field experiments with bacteriophages to manage diseases caused by plant-pathogenic bacteria. In addition, we summarize the usage of bacteriophages for the specific detection of plant-pathogenic bacteria.

Mode of Action of Microbial Biological Control Agents Against Plant Diseases: Relevance Beyond Efficacy

Microbial biological control agents (MBCAs) are applied to crops for biological control of plant pathogens where they act via a range of modes of action. Some MBCAs interact with plants by inducing resistance or priming plants without any direct interaction with the targeted pathogen. Other MBCAs act via nutrient competition or other mechanisms modulating the growth conditions for the pathogen. Antagonists acting through hyperparasitism and antibiosis are directly interfering with the pathogen. Such interactions are highly regulated cascades of metabolic events, often combining different modes of action. Compounds involved such as signaling compounds, enzymes and other interfering metabolites are produced in situ at low concentrations during interaction. The potential of microorganisms to produce such a compound in vitro does not necessarily correlate with their in situ antagonism. Understanding the mode of action of MBCAs is essential to achieve optimum disease control. Also understanding the mode of action is important to be able to characterize possible risks for humans or the environment and risks for resistance development against the MBCA. Preferences for certain modes of action for an envisaged application of a MBCA also have impact on the screening methods used to select new microbials. Screening of MBCAs in bioassays on plants or plant tissues has the advantage that MBCAs with multiple modes of action and their combinations potentially can be detected whereas simplified assays on nutrient media strongly bias the selection toward in vitro production of antimicrobial metabolites which may not be responsible for in situ antagonism. Risks assessments for MBCAs are relevant if they contain antimicrobial metabolites at effective concentration in the product. However, in most cases antimicrobial metabolites are produced by antagonists directly on the spot where the targeted organism is harmful. Such ubiquitous metabolites involved in natural, complex, highly regulated interactions between microbial cells and/or plants are not relevant for risk assessments. Currently, risks of microbial metabolites involved in antagonistic modes of action are often assessed similar to assessments of single molecule fungicides. The nature of the mode of action of antagonists requires a rethinking of data requirements for the registration of MBCAs.

The Effect of Leaf Shredding on Apple Scab in South African Orchards

The South African apple industry currently relies entirely on chemical fungicides to control apple scab (Venturia inaequalis). In this study, the effectiveness of sanitation strategies in reducing scab incidence and severity in South African orchards was evaluated. Over three seasons, leaf shredding with no fungicide sprays was tested against a nonsprayed, nonshredded negative control, a positive control that followed a commercial fungicide program, and a combined treatment of a commercial fungicide program with leaf shredding. Two treatment replicates were applied in a randomized block design in each of two orchards. Scab incidence and severity on fruit and leaves were assessed weekly from green-tip until fruit-set in the following spring. Pooled data from the 3 years revealed that fruit scab incidence and severity and leaf scab severity (51, 55, and 39%, respectively, P < 0.05) and leaf scab incidence (33%, P < 0.1) were significantly lower in the leaf-shredding treatment than in the negative control. This is the first study to evaluate the effect of leaf shredding in reducing scab in South African orchards. Results indicate that this treatment is highly effective and should be integrated into scab management strategies in future, but should be customized to suit South African orchard conditions.

Integrated Control of Apple Scab and Powdery Mildew in an Organic Apple Orchard by Combining Potassium Carbonates with Wettable Sulfur, Pruning, and Cultivar Susceptibility

In a 4-year study in a whole-field sanitized organic apple orchard, the effectiveness of nine fungicide treatments, including potassium mono- and bicarbonate and their combinations with wettable sulfur, were evaluated for scab and powdery mildew control on two cultivars with different susceptibility to scab and powdery mildew, under two pruning treatments. The whole-field sanitation practice was performed by removal of infected fallen leaves. Treatment effects on phytotoxicity and yield were also determined. Pruning significantly reduced leaf scab incidence but only on the more scab-susceptible Idared. Pruning significantly reduced mildew incidence in most years and on both cultivars but the more mildew-susceptible Jonathan showed significantly higher mildew incidence than Idared. Among products approved for organic production, the best scab control was achieved with a potassium mono- or bicarbonate treatment combined with wettable sulfur, except for Jonathan in 2011 on leaf and in 2014 on both leaf and fruit, and for Idared in 2013 on fruit. The best mildew control was also achieved with potassium mono- or bicarbonate treatments combined with wettable sulfur, with exceptions on shoots of Idared in 2011 and 2013 and on fruit of Idared in 2012. Leaf phytotoxicity was significantly higher in all potassium carbonate treatments compared with untreated plots, except for Idared in 2012, while fruit russet in these treatments did not differ significantly from the untreated plots. However, phytotoxicity values of all carbonate treatments were significantly lower than the lime sulfur treatment in most years on both cultivars. Yield of the potassium mono- or bicarbonate treatments combined with wettable sulfur was significantly higher than the untreated plots in the pruned treatments for both cultivars in all years. The integrated control approach designed for organic disease management against the two pathogens is discussed.

Fungal endophytes: modifiers of plant disease

Many recent studies have demonstrated that non-pathogenic fungi within plant microbiomes, i.e., endophytes ("endo" = within, "phyte" = plant), can significantly modify the expression of host plant disease. The rapid pace of advancement in endophyte ecology warrants a pause to synthesize our understanding of endophyte disease modification and to discuss future research directions. We reviewed recent literature on fungal endophyte disease modification, and here report on several emergent themes: (1) Fungal endophyte effects on plant disease span the full spectrum from pathogen antagonism to pathogen facilitation, with pathogen antagonism most commonly reported. (2) Agricultural plant pathosystems are the focus of research on endophyte disease modification. (3) A taxonomically diverse group of fungal endophytes can influence plant disease severity. And (4) Fungal endophyte effects on plant disease severity are context-dependent. Our review highlights the importance of fungal endophytes for plant disease across a broad range of plant pathosystems, yet simultaneously reveals that complexity within plant microbiomes presents a significant challenge to disentangling the biotic environmental factors affecting plant disease severity. Manipulative studies integrating eco-evolutionary approaches with emerging molecular tools will be poised to elucidate the functional importance of endophytes in natural plant pathosystems that are fundamental to biodiversity and conservation.

Toward an Integrated Use of Biological Control by Cladosporium cladosporioides H39 in Apple Scab (Venturia inaequalis) Management

Apple scab, caused by Venturia inaequalis, is the most important disease in apple production, reducing yield and quality of fruit. Control of apple scab in commercial orchards currently depends on multiple applications of fungicides. The potential of the antagonistic isolate Cladosporium cladosporioides H39, originating from a sporulating colony of V. inaequalis, to control apple scab development was tested in eight trials during 2 years in orchards in Eperjeske (Hungary), Dabrowice (Poland), and Bavendorf (Germany) planted with different cultivars. Treatments were conducted as calendar sprays or after infection periods. Additional trials in an orchard in Randwijk (The Netherlands) focused on the effect of timing of antagonist application before or after infection periods. The overall results of the field trials consistently showed-for the first time-that stand-alone applications of the antagonist C. cladosporioides H39 can reduce apple scab in leaves and fruit. This was demonstrated in an organic growing system as well as in conventional orchards by spray schedules applied during the primary or the summer season. In both systems, the same control levels could be reached as with common fungicide schedules. Efficacies reached 42 to 98% on leaf scab incidence and 41 to 94% on fruit scab. The antagonist was also effective if applied one or even several days (equivalent to approximately 300 to 2,000 degree h) after infection events in several field trials and a trial conducted in Randwijk with single-spray applications at different intervals before or after infection events. Better understanding of the biology of the antagonist will help to further exploit its use in apple scab control.

Effect of Sanitation Treatments on Leaf Litter Density and Leaf Spot Incidence in Integrated and Organic Sour Cherry Orchards

A 3-year study was conducted to determine the effect of five sanitation treatments on leaf litter density (LLD), leaf spot incidence, and percent defoliation on two cultivars ('Újfehértói fürtös' and 'Érdi bőtermő') in two sour cherry orchards: one managed by integrated pest management principles with conventional fungicides and the other managed organically. The following sanitation treatments were compared: sprays of urea or lime sulfur in autumn, removing fallen leaves after leaf fall, straw mulch cover in late winter, sprays of urea or lime sulfur followed by mulch cover, removing fallen leaves followed by mulch cover, and a nonsanitized control. In both orchards and all years, LLD decreased by 2 to 28% in all treatment plots from early December to mid-May. LLD reduction was two to four times higher in the organic orchard compared with the integrated orchard. All treatments, except the lime sulfur or urea treatment alone, resulted in significant (P < 0.05) reduction of LLD in both the integrated and organic orchards compared with nonsanitized plots. Only leaf removal alone or in combination with mulch significantly (P < 0.05) reduced cherry leaf spot incidence and percent defoliation (by 11 to 70% and 15 to 72%, respectively) compared with nonsanitized plots. The application of these sanitation treatments in orchard management practices is discussed.

Culturable fungi of stored 'golden delicious' apple fruits: a one-season comparison study of organic and integrated production systems in Switzerland

The effects of organic and integrated production systems on the culturable fungal microflora of stored apple fruits from five matched pairs of certified organic and integrated 'Golden Delicious' farms were studied at five representative production sites in Switzerland. Isolated fungi were identified morphologically. Colonization frequency (percentage of apples colonized), abundance (colony numbers), and diversity (taxon richness) were assessed for each orchard. The standard quality of the stored fruits was comparable for both organic and integrated apples and complied with national food hygiene standards. Yeasts (six taxa) and the yeast-like fungus Aureobasidium pullulans were the dominant epiphytes, filamentous fungi (21 taxa) the dominant endophytes. The most common fungi occurred at all sites and belonged to the "white" and "pink" yeasts, yeast-like A. pullulans, filamentous fungi Cladosporium spp., Alternaria spp., and sterile filamentous fungi. Canonical correspondence analysis of the total fungal community revealed a clear differentiation among production systems and sites. Compared to integrated apples, organic apples had significantly higher frequencies of filamentous fungi, abundance of total fungi, and taxon diversity. The effects of the production system on the fungal microflora are most likely due to the different plant protection strategies. The incidence of potential mycotoxin producers such as Penicillium and Alternaria species was not different between production systems. We suggest that higher fungal diversity may generally be associated with organic production and may increase the level of beneficial and antagonistically acting species known for their potential to suppress apple pathogens, which may be an advantage to organic apples, e.g., in respect to natural disease control.

Infection Potential of Pleospora allii and Evaluation of Methods for Reduction of the Overwintering Inoculum of Brown Spot of Pear

The capacity for germination and pathogenicity to pear leaves of ascospores of Pleospora allii, the teleomorph of Stemphylium vesicarium, causal agent of brown spot of pear, were studied in vitro. Most ascospores germinated within 1 h at temperatures between 15 and 20°C, and the optimum temperature for germination was 18.9°C. Infections developed on wounded and non-wounded detached pear leaves, but were more frequent on wounded leaves. The minimum infective dose was one ascospore per wound. Biological, chemical, and mechanical methods for decreasing overwintering inoculum of P. allii were evaluated. Ascospores were discharged from March to May, depending on the orchard and year. Leaf shredding or removal were the most effective methods of reducing overwintering inoculum. Biological control methods based on application of Thichodermasp. formulations were partially effective. Chemical methods based on copper and urea treatments were ineffective.

Effect of Timing of Application of the Biological Control Agent Microsphaeropsis ochracea on the Production and Ejection Pattern of Ascospores by Venturia inaequalis

ABSTRACT Field and in vitro trials were conducted to establish the influence of the biological control agent Microsphaeropsis ochracea on the ejection pattern of ascospores by Venturia inaequalis and on apple scab development, and to establish the best timing of application. The ejection pattern of ascospores was similar on leaves sprayed with M. ochracea and on untreated leaves. Fall application of M. ochracea combined with a delayed-fungicide program was evaluated in orchards with intermediate and high scab risk. For both orchards, it was possible to delay the first three and two infection periods in 1998 and 1999, respectively, without causing significant increase or unacceptable leaf and fruit scab incidence. To evaluate the best timing of application, sterile leaf disks were inoculated with V. inaequalis and then with M. ochracea 0, 2, 4, 6, 8, 10, 12, 14, and 16 weeks later. After incubation under optimal conditions for pseudothecia development, the number of ascospores was counted. Similarly, M. ochracea was sprayed on scabbed leaves on seven occasions from August to November 1999 and 2000. Leaves were overwintered on the orchard floor and ascospore production was evaluated the following spring. Ascospore production was reduced by 97 to 100% on leaf disks inoculated with M. ochracea less than 6 weeks after inoculation with V. inaequalis, but ascospore production increased with increasing period of time when M. ochracea was applied 8 to 16 weeks after the inoculation with V. inaequalis. In the orchard, the greatest reduction in production of ascospores (94 to 96% in 2000 and 99% in 2001) occurred on leaves sprayed with M. ochracea in August. The production of ascospores was reduced by 61 to 84% in 2000 and 93% in 2001 on leaves sprayed with M. ochracea in September, reduced by 64 to 86% in 2000 and 74 to 89% in 2001 on leaves sprayed in October, and reduced by 54 and 67% in 2000 and 2001, respectively, on leaves sprayed in November. It was concluded that M. ochracea should be applied in August or September and that ascospore maturation models and delayed-fungicide program could be used in orchards treated with this biological control agent.

Effect of Urea, CaCO3, and Dolomite on Pseudothecial Development and Ascospore Production of Mycosphaerella citri

Citrus greasy spot, caused by Mycosphaerella citri, produces lesions on leaves, followed by premature defoliation, and rind blotch on fruit. Ascospores produced in leaf litter represent the major source of inoculum. The effect of treatment of leaf litter with urea, CaCO₃, or dolomite on the development of pseudothecia and ascospore production was evaluated. In laboratory experiments, one urea application reduced production of pseudothecia and ascospores by up to 90%, but did not affect time of production of pseudothecia or ascospores or rate of leaf decomposition. Two applications of urea delayed leaf decomposition. As the rates of CaCO₃ or dolomite were increased, pseudothecial incidence, density, time to ascospore production, and total numbers of ascospores decreased and the rate of leaf compostion increased. Immature pseudothecia on leaves treated with urea or CaCO₃ degenerated and produced fewer ascospores per pseudothecium. The results observed in microplot studies in the field were similar to those observed in laboratory experiments. The number of days to pseudothecia and ascospore production and the pseudothecial incidence and density were negatively related to the rate of CaCO₃ or dolomite applied. Application of CaCO₃ dolomite, or urea to leaf litter can reduce inoculum and be useful in an integrated program of greasy spot management.

Reduction of Defoliation in Citrus Caused by Mycosphaerella citri with a Novel Biocompatible Fungicide

A novel, biocompatible fungicide, CD-2346, was evaluated for management of defoliation associated with greasy spot (Mycosphaerella citri Whiteside) in orange (Citrus sinensis (L.) Osbeck) and grapefruit (Citrus × paradisi Macfad.). This product contains potassium bicarbonate, potassium carbonate, urea, and a surfactant. CD-2346 (7.0 kg/ha) and a half-rate combination of CD-2346 + benomyl (0.55 kg/ha) were compared with a higher rate of benomyl (1.1 kg/ha) and commercial standards of oil (petroleum distillate FC-435-66 [70 to 93.5 liters/ha]) or oil (46.8 liters/ha) + copper sulfate (2.2 to 2.8 kg/ha). Fungicide efficacy was assessed using the area under the defoliation progress curve (AUDPC) and final defoliation in entire trees or final greasy spot severity in individual leaves. All fungicide treatments reduced the AUDPC, and all but benomyl reduced either final defoliation percentage or disease severity in individual leaves. CD-2346 and the reduced-rate combination of CD-2346 + benomyl were generally equal or superior to the full rate of benomyl in decreasing the AUDPC; CD-2346 + benomyl was equivalent to oil in reducing the AUDPC. Oil and CD-2346 treatments similarly reduced final defoliation, but oil + copper sulfate was superior to CD-2346 in reducing the AUDPC and final greasy spot defoliation or severity.

Spatial Distribution of Venturia inaequalis Airborne Ascospores in Orchards

ABSTRACT Apple scab (Venturia inaequalis) causes important economic losses in many apple production areas of the world. The disease is controlled by numerous fungicide applications regardless of the presence of ascospores in the orchard. Airborne ascospore concentration (AAC) can be measured in real time to time fungicide applications. However, the level of heterogeneity of the AAC in commercial orchards was unknown. Consequently, the spatial distribution of V. inaequalis ascospores was studied in a commercial apple orchard of 0.43 ha. The potential ascospore dose (PAD) and AAC were measured in 40 quadrats each of 108 m(2). In each quadrat, the AAC was monitored during the major rain events in spring 1999 and 2000 using spore samplers. The variance-to-mean ratio for the PAD and for most of the AAC sampling dates was >1, indicating an aggregated pattern of distribution. None of the frequency distributions of the most important ascospore ejection events followed the Poisson probability distribution, indicating that the pattern of distribution was not random. For all events, AAC had an aggregated pattern of distribution as suggested by the negative binomial distribution. The PAD followed neither the Poisson nor the negative binomial distribution. Geostatistical analyses confirmed the aggregated pattern of distribution. The cultivars had an effect on the PAD and AAC distribution pattern, but both PAD and AAC were not uniformly distributed within a block of the same cultivar. Therefore, the number, location, and height of samplers required to estimate AAC in orchards need to be investigated before using information on AAC for decision making.

Effect of Microsphaeropsis sp. on the Production of Perithecia and Ascospores of Gibberella zeae

The potential of Microsphaeropsis sp. (isolate P130A) as an antagonist of Gibberella zeae was tested under in vitro and field conditions. Firstly, an in vitro method of ascospore production was developed on wheat and corn residues. The plant type (corn or wheat), residue type (straw/stalk or grain), and incubation conditions (closed or open) had a significant effect on ascospore production. Perithecia were more abundant on wheat and corn grain incubated under open conditions. On these two substrates, the application of Microsphaeropsis sp. significantly reduced ascospore production. On wheat, the antagonist had a significant effect when applied 2 weeks before (-2), at the same time (0), and 4 weeks after (+4) inoculation with G. zeae, with 1.73, 0.31, 1.11, and 1.36 log ascospores per cm² for the control, -2, 0, and +4 weeks treatments, respectively. On corn, Microsphaeropsis sp. had a significant effect when applied 2 weeks before, at the same time, 4 weeks after, and 6 weeks after inoculation with G. zeae with 3.02, 0.23, 1.29, 2.35, and 2.22 log ascospores per cm² for the control, -2, 0, +4, and +6 weeks treatments, respectively. When applied to crop residues in the field as postharvest or preplanting applications, Microsphaeropsis sp. had no effect on the pattern of perithecia maturation, but significantly reduced the number of perithecia produced on two sampling dates, May 1998 and July 1999. There is a potential to biologically reduce the initial inoculum of G. zeae; however, more work is needed to optimize the efficacy of the biocontrol agent.

Effect of Microsphaeropsis sp. Strain P130A on Germination and Production of Sclerotia of Rhizoctonia solani and Interaction Between the Antagonist and the Pathogen

Microsphaeropsis sp. strain P130A was evaluated for the control of tuber-borne inoculum of Rhizoctonia solani based on the viability of sclerotia produced in vitro and on both the viability and production of tuber-borne sclerotia. The interactions between the antagonist and the pathogen, as well as the effect of the toxins produced by the antagonist on mycelial growth of R. solani were studied using transmission electron microscopy. On sclerotia produced in vitro, for all incubation periods (1 to 42 days), Microsphaeropsis sp. significantly reduced germination. Percent germination of sclerotia treated with Microsphaeropsis sp. decreased with increasing incubation period from an average of 82.0% after 1 day to stabilize at an average of 5.8% after 35 days. Similarly, percent germination of tuber-borne sclerotia was significantly lower when tubers were treated with Microsphaeropsis sp. Both 2% formaldehyde and Microsphaeropsis sp. treatments significantly reduced sclerotia germination to approximately 10% after 42 days of incubation at 4 degrees C. Furthermore, on tubers treated with the antagonist, the number of sclerotia per square centimeter decreased from 1.6 to 0.5 during the 8 months of storage at 4 degrees C, whereas an increase from 1.2 to 7.8 sclerotia per square centimeter was observed on untreated tubers. Microsphaeropsis sp. (strain P130A) colonized hyphae of R. solani within 4 days after contact on culture media. Transmission electron microscopic observations showed that the antagonist induced a rupture of the pathogen plasma membrane and that a chitin-enriched matrix was deposited at sites of potential antagonist penetration. Host penetration was not associated with pathogen cell wall alterations, which occurred at the time of progress of the antagonist in the pathogen cytoplasm. In the presence of a crude extract of Microsphaeropsis sp., cells of R. solani showed cytoplasm disorganization and breakdown of plasma membranes. Antibiosis and mycoparasitism were involved in the antagonism of R. solani by Microsphaeropsis sp., but the sequence by which these events occur, as well as the significance of wall appositions produced by R. solani, is yet to be established.