Thermal Inactivation of Phytophthora nicotianae

Low Temperature Biodisinfection Effectiveness for Phytophthora capsici Control of Protected Sweet Pepper Crops in the Southeast of Spain

Biodisinfection using fresh sheep manure in August is effective in controlling Phytophthora root rot in greenhouses in southeast Spain, but this is not the case after the beginning of September. The effectiveness of biodisinfection of new amendments has been evaluated beginning in October in sweet pepper greenhouses to control Phytophthora capsici. The amendments used were: T1: wheat husk + fresh sheep manure (FSM), 3.5 kg m−2; T2: sunflower pellets 3.5 kg m−2; T3: FSM: 3.5 kg m−2; T4: Control. Temperatures above 40°C were obtained in some amendments; anoxic conditions were found in all amendments, and also a reduction of the viability of P. capsici oospores with respect to the control, as well as a higher yield. The contribution of fresh sheep manure to the amendments did not lead to an improvement in their effectiveness. Autumn biodisinfection under low temperature conditions using fresh organic amendments that enhance soil anaerobic conditions may be a promising strategy for the control of P. capsici in pepper greenhouses in southeastern Spain.

Thermal Inactivation of Inoculum of Two Phytophthora Species by Intermittent Versus Constant Heat

Research on solarization efficacy has examined the critical temperature and minimum exposure time to inactivate soilborne pathogens. Most mathematical models focus on survival of inoculum subjected to a constant heat regime rather than an intermittent heat regime that better simulates field conditions. To develop a more accurate predictive model, we conducted controlled lab experiments with rhododendron leaf disks infested with Phytophthora ramorum and P. pini. Focused in vitro experiments with P. ramorum showed significantly longer survival of inoculum exposed to intermittent versus constant heat, indicating that intermittent heat is less damaging. A similar trend was observed in soil. Damage was evaluated by comparing the reduction in subsequent survival time of inoculum subjected to different intensities of sublethal heat treatments. Inoculum exposure to continuous heat reflected an increasing rate of damage accumulation. Multiple sublethal heat events resulted in a constant rate of damage accumulation which allowed us to calculate total damage as the sum of damage from each heat event. A model including a correction for an intermittent heat regime significantly improved the prediction of thermal inactivation under a temperature regime that simulated field conditions.

Effect of biofumigation with brassica pellets combined with Brassicaceae cover crops and plastic cover on the survival and infectivity of inoculum of Phytophthora nicotianae Breda de Haan

BACKGROUND

Biofumigation with defatted seed meal of Brassicaceae in the form of pellets has several advantages over the incorporation of fresh Brassicaceae crops to control soil-borne diseases. Two field experiments were established to evaluate the effect of biofumigation with brassica pellets on the survival and infectivity of Phytophthora nicotianae Breda de Haan inoculum introduced before treatments. In the spring experiment the incorporation of additional Brassicaceae cover crop (Brassica nigra L. and Sinapis alba L.) was tested, and in the summer experiment two brassica pellet doses were applied.

RESULTS

Biofumigation with brassica pellets in spring (3000 kg ha(-1) with and without plastic) or in summer (3000 kg ha(-1) with or without plastic; 6000 kg ha(-1) without plastic) had no significant effect on the survival of P. nicotianae, regardless of the incorporation of additional Brassicaceae cover crop in spring. Reduction in infectivity in spring was related to the application of plastic, especially when combined with brassica pellets and Brassicaceae crop. In summer, soil temperature was the main factor in the inactivation of the inoculum, especially when plastic was applied, and no additional inactivation was achieved with brassica pellets.

CONCLUSION

In spring and summer, biofumigation with brassica pellets had no effect on the survival of P. nicotianae. Application of plastic in spring may reduce infectivity. Soil temperature is the main factor in the inactivation of inoculum in summer, especially when plastic is applied. © 2015 Society of Chemical Industry.

Soil Solarization as a Component of an Integrated Program for Control of Raspberry Root Rot

Field and growth chamber studies were conducted during 2000 to 2005 to determine the efficacy of soil solarization for the control of Phytophthora root rot of raspberry (PRR). The exposure time that was lethal to Phytophthora rubi cultures on V8 juice agar plates was evaluated at intervals up to 240 h and at a range of temperatures from 20 to 35°C. Colonies incubated at 20 and 25°C nearly covered the plates in 240 h, while radial growth slowed at 27°C and ceased at temperatures ≥29°C. The exposure times required to kill P. rubi were estimated to be 222 h at 29°C, 168 h at 31°C, 108 h at 33°C, and 52 h at 35°C. Previous exposure to shorter durations at temperature ≥29°C slowed growth of colonies when they later were incubated at 20°C. Field trials were established in 2000 and 2003 at three locations in Washington State to evaluate soil solarization for the management of PRR. Cumulative hours with soil temperatures >29°C at 30 cm soil depth in solarized plots exceeded 200 h in each trial. In the 2000 trial, combinations of solarization, bed shape, and amendments of gypsum were evaluated. Over the 3 years after planting PRR-susceptible raspberry 'Malahat' and 'Willamette', primocane growth and survival were greater (P < 0.05) in raised bed plots that were solarized than in solarized and nonsolarized flat bed plots or hilled bed plots with gypsum. In 2003, trials were initiated to evaluate solarization in combination with applications of mefenoxam and fosetyl-Al. In 2004, solarization increased (P < 0.05) primocane growth of 'Malahat' and 'Qualicum' raspberries at both locations compared to application of fungicides alone and nontreated control plots. At both locations in 2005, density and growth of 'Qualicum' primocanes were greatest in solarized plots, while canes in solarized and fungicide only plots of 'Malahat' were similar. Incidence of diseased canes was lowest in plots that received fungicides. Primocane survival and fruit yields were very low at both locations in the third season because of favorable conditions for PRR and plant stress in late spring. These results indicate that soil solarization can be an effective component of integrated management of PRR in the Pacific Northwest, especially when combined with raised beds and gypsum amendments.

Impact of Different Preplant Cultural Treatments on Survival of Phytophthora nicotianae in Soil

During the life of a citrus planting, the population of Phytophthora pathogens can build to significant levels in orchard soil. A study was initiated to examine the impact of some nonchemical cultural practices on survival of P. nicotianae, the most prevalent Phytophthora sp. in Arizona citrus groves, in soil formerly planted to citrus. In three trials over a 3-year period, P. nicotianae could not be detected at a depth of 10 cm after soil naturally infested with the pathogen was subjected to a dry summer fallow period of at least 31 days in the desert southwest region of Arizona. The mean temperature of soil at this depth during these trials ranged from 37 to 39°C. Furthermore, in two of these trials, after summer dry fallow periods of 38 and 45 days, the pathogen could not be detected at a depth of 15 to 20 cm and was detected in only one of 19 soil samples at a depth of 25 to 30 cm. In comparison, the pathogen was recovered from a high proportion of soil samples subjected to a dry winter fallow period or maintained in the greenhouse and planted with a seedling of citrus, alfalfa, or irrigated without the presence of any plant, where mean temperature of soil ranged from 15 to 30°C. In regions with a hot and dry summer climate, a dry summer fallow treatment of soil after removal of an existing citrus planting and before establishment of a new grove could provide a rapid and relatively inexpensive means of lowering the population of P. nicotianae to virtually nondetectable levels to at least a depth of 30 cm.

Recovery of Phytophthora ramorum Following Exposure to Temperature Extremes

We examined the impact of exposure to high and low temperature extremes on recovery of Phytophthora ramorum both as free chlamydospores and within infected rhododendron tissue over a 7-day period. Chlamydospores held in moistened sand were incubated at 30, 35, 40, 0, -10, and -20°C for up to 7 days. Infected Rhododendron 'Cunningham's White' leaf disks held in sandy loam, loam, or sand at two different soil moisture levels also were subjected to these temperatures for up to 7 days, and to a variable temperature regimen for 12 weeks. Recovery was characterized by growth of P. ramorum on selective agar medium following exposures to temperature treatments. Chlamydospores held in moistened sand showed a high rate of recovery at 30°C, steadily declining recovery at 35°C, and no recovery at 40°C over the 7-day period. Chlamydospores were recovered from 0°C after 7 days, with little or no recovery observed at -10 or -20°C. In infected rhododendron tissue, P. ramorum was recovered at 20 and 30°C after 7 days but, at 35°C, the pathogen showed a decline within 2 days and no recovery by 4 days. A 40°C treatment allowed no recovery of P. ramorum from infected tissue after 2 days. For cold treatments, P. ramorum was recovered from infected leaf disks at 0 and -10°C after 7 days. At -20°C, recovery declined rapidly after 1 to 3 days and no recovery was obtained after 4 days. P. ramorum showed nearly 100% recovery from leaf disks subjected to a 12-week variable temperature treatment based on ambient summer temperatures in Lewisburg, TN. The results suggest that P. ramorum is capable of surviving some highly adverse temperature conditions for at least 7 days both as free chlamydospores in sand and within infected host tissue. Thus, P. ramorum present as free chlamydospores or within tissue of infected plants shipped to the eastern United States has the potential to survive some of the adverse conditions encountered in summer and winter in many eastern states.

Survival Dynamics of Aphanomyces cochlioides Oospores Exposed to Heat Stress

ABSTRACT To determine how exposure to heat effects their survival, oospores of Aphanomyces cochlioides isolate C22 were exposed in water to 35, 40, 45, or 50 degrees C for prescribed times and then examined for viability. The Weibull model was modified to represent the effects of temperature on survival of oospores. The final fitted model gave lethal doses for 50% of the oospores of 251, 49.8, 9.8, and 1.9 h at 35, 40, 45, and 50 degrees C, respectively. To determine if alternating high and low temperatures resulted in (i) recovery from heat damage during low temperature periods, (ii) increased susceptibility to heat damage, or (iii) if effects of heat damage were cumulative, oospores were examined after each of four 24-h cycles at 45 degrees C for 4 h and 21 degrees C for 20 h. Survival of oospores exposed to alternating high and low temperatures fit the cumulative effects model. Significant variability in heat tolerance among five isolates was observed (P< 0.001) but model parameters successfully accommodated this variability (R(2) = 0.96, P < 0.001). This research shows that under wet conditions, there are predictable patterns to mortality for A. cochlioides oospores exposed to continuous or fluctuating high temperatures.

Tolerance of Mycelium of Different Genotypes of Phytophthora infestans to Freezing Temperatures for Extended Periods

ABSTRACT Mycelium of Phytophthora infestans, the causal agent of potato late blight, can initiate crop infections over successive years by overwintering in infected potato tubers that survive as seed in fields or within cull piles. This study used four different genotypes of P. infestans to evaluate the influence of freezing temperatures on survival of mycelium in vitro. Sporangium-free mycelium of P. infestans US1, US8, US11, and US14 growing on rye agar plates was exposed to temperatures ranging from -20 and 0 degrees C (experiment A) for different periods up to 24 h and from -5 and 0 degrees C (experiment B) for periods up to 5 days. Cultures were incubated at 12 degrees C after exposure, and survival of the cultures was estimated after 28 days by a digital image analysis technique that measured the average reflectance intensity (ARI) of images of the mycelium of temperature-treated cultures. The ARI values of treated cultures were compared with the growth of mycelium in negative controls (mycelium not present) and positive controls (mycelium exposed to 12 degrees C for an equivalent period), and determination of recovery was based on statistical differences from the controls. There were significant differences in ARI values among genotypes, temperature treatment, and exposure periods in all experiments. An index of recovery was calculated for each genotype at all treatment temperatures and exposure periods for both experiments. In experiment A, exposure of mycelium of P. infestans (all genotypes) to -20 and -10 degrees C proved lethal for exposure periods of more than 1 h. All genotypes showed some degree of recovery up to 24-h exposure at -5 and -3 degrees C. In both experiments, exposure of mycelium of P. infestans to 0 degrees C was not lethal to any genotype tested for any exposure period. In experiment B, all of the genotypes survived exposure up to 3 days at -3 degrees C to some degree, but at -5 degrees C, exposure of 1 day was lethal to all genotypes. Tolerance of freezing temperatures by mycelium of P. infestans may be an ecologically important survival mechanism and the increased tolerance of US8 and US14 may explain their predominance in cooler climates such as north-central United States.

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.

United States Department of Agriculture-Agricultural Research Service research on alternatives to methyl bromide: pre-plant and post-harvest

Methyl bromide is a widely used fumigant for both pre-plant and post-harvest pest and pathogen control. The Montreal Protocol and the US Clean Air Act mandate a phase-out of the import and manufacture of methyl bromide, beginning in 2001 and culminating with a complete ban, except for quarantine and certain pre-shipment uses and exempted critical uses, in January 2005. In 1995, ARS built on its existing programs in soil-borne plant pathology and post-harvest entomology and plant pathology to initiate a national research program to develop alternatives to methyl bromide. The focus has been on strawberry, pepper, tomato, perennial and nursery cropping systems for pre-plant methyl bromide use and fresh and durable commodities for post-harvest use. Recently the program has been expanded to include research on alternatives for the ornamental and cut flower cropping systems. An overview of the national research program is presented. Results from four specific research trials are presented, ranging from organic to conventional systems. Good progress on short-term alternatives is being made. These will be used as the foundation of integrated management systems which begin with pre-plant management decisions and continue through post-harvest processing.