The integrated use of genetic and chemical methods for control of lettuce downy mildew (Bremia lactucae Regel)

The options for integrating genetic and chemical methods for the control of Bremia lactucae (downy mildew) on lettuce are discussed. Used individually, all available control measures have their limitations, but there is considerable scope for combining procedures. Each option is considered in terms of its efficacy, potential durability and practicality. Maximum efficacy and durability in the future are likely to result from using mixed fungicide formulations on cultivars which have been bred to incorporate effective novel R genes into a ‘field-resistant’ background.     

Yield loss and management of downy mildew on field pea in Alberta,Canada

Downy mildew of field pea (Pisum sativum) caused by Peronospora viciae f. sp. pisi has become widespread in the main field pea production areas of central Alberta. Field experiments were conducted at naturally-infested field sites over several years to assess the effect of seeding depth, seeding date, seed treatment and foliar fungicides on downy mildew incidence and severity, and to estimate the relationship between severity and yield loss. Downy mildew was shown to cause substantial yield loss on field pea. Even a moderately severe infestation reduced pod numbers by 65% and seed yield by 75%. The loss (pod number or seed yield) was best explained by a linear model (y = −2.3114x + 10.086; R² = 0.9441 and y = −2.5165x + 10.378; R² = 0.9533, respectively). Depth of seeding (range 3–7 cm) did not affect downy mildew. Similarly, seeding date (early, mid and late May) did not have a consistent effect on disease levels. Late seeding occasionally reduced downy mildew, but always resulted in low seed yield. Several seed treatment fungicides reduced downy mildew, and metalaxyl-based products produced the highest yield. Several foliar-applied fungicides, including pyraclostrobin, azoxystrobin and metalaxyl, reduced downy mildew severity, but the results were not consistent across years. We conclude that cultural practices may not be sufficient for effective management of downy mildew, and that metalaxyl-based fungicides applied as seed treatments or foliar sprays could represent the best control option until downy mildew resistant pea cultivars become available.     

The effect of night temperature and glasshouse ventilation on the incidence of Botrytis cinerea in a late-planted tomato crop

The incidence of Botrytis cinerea Pers. was assessed on leaves, stems and fruits of tomato grown in night environments of 16°C with or without ventilation at night and 13°C with ventilation. Ventilation and temperatures by day were the same in all treatments. The experiment used nine independent glasshouse compartments in a row and column design. At 16°C, night ventilation gave significant reductions in grey mould on stems, leaves and fruits to c. 25% of that on unventilated plants. With night ventilation and a night temperature of 13°C, the incidence of B. cinerea on stems, leaves and fruit was doubled when compared with ventilated plants at 16°C. These differences could be explained partly in terms of the humidity which prevailed in the crops. No significant differences in yield were detected between treatments.     

Potassium phosphite for control of downy mildew of soybean

Downy mildew of soybean, caused by Peronospora manshurica, is widely spread throughout Brazil. The objective of this study was to evaluate the use of potassium phosphite to control this disease. Field experiments were conducted during the growing season of 2006/2007 and 2007/2008 in the state of Parana in southern Brazil. The experimental design consisted of completely randomized blocks in a factorial arrangement (4 × 2) with four replications. Four rates of potassium phosphite (0, 375, 750 and 1500 g P₂O₅ + K₂O ha⁻¹) were applied at two growth stages, V6 (fifth trifoliolate leaf) and R2 (full flowering), followed by one or two applications of pyraclostrobin and epoxiconazole (66.5 + 25 g a.i. ha⁻¹) at R3 (pod development) or R2 and R5.1 (10% of pod filling), mainly for the control of Asian soybean rust (Phakopsora pachyrhizi) and powdery mildew (Microsphaera diffusa). Field experiments were conducted to quantify the severity of downy mildew on leaves, nutrient content in leaf tissue (N, P and K), leaf area index (LAI), yield and seed weight. The maximum severity of downy mildew was observed at a growth stage of R5.3 (50% of pods were ripe), with 14% and 46% of the leaf area affected in 2006/2007 and 2007/2008, respectively. Also it was detected some effect of phosphite on Asian rust control but it was mostly in the trial of 2007/08 when the epidemic was very low (9.7-21.8% of severity). There was a linear reduction in the severity of downy mildew and a significant improvement in the LAI with an increase in the rate of phosphite applied. During the 2006/2007 growing season, a significant yield improvement was observed due to the application of the highest rate of phosphite. Two fungicide applications following phosphite application significantly improved the control of Asian soybean rust and powdery mildew, yield and seed weight when compared to a single fungicide application.     

Evaluation of a forecaster for downy mildew of onion (Allium cepa L.)

A forecaster of onion downy mildew, referred to as DOWNCAST, was evaluated under field conditions to determine its validity and effectiveness for timing fungicide sprays. DOWNCAST utilized criteria for predicting sporulation and infection by the downy mildew pathogen, Peronospora destructor. The criteria were based on quantitative and temporal relationships of temperature, rain, high humidity, rate of dew deposition and dew duration with the infection cycle of the pathogen. DOWNCAST correctly predicted sporulation incidence on 111 of 119 nights during two growing seasons. Refinement of a temperature criterion was needed to predict sporulation correctly on the remaining eight nights. Indirect evidence showed that DOWNCAST correctly predicted infection incidence. Sequences of weather conditions were used to predict ‘sporulation-infection periods’, when the pathogen sporulated, survived and infected the onions, and to identify infection cycles. Fungicide programmes of Ridomil-MZ, Sandofan M8, Bravosan, Vinicur M-SC, Manzate 200 or Bravo 500, begun on the sixth or eighth day of the first infection cycle, did not usually reduce sporulation incidence early in the second infection cycle. However, all of the programmes except Bravosan and Bravo effectively restored disease management a few days later. Programmes of Ridomil-MZ begun in the second infection cycle usually were less effective than those begun in the first cycle. Relative effectiveness of the various fungicide programmes was reflected in increased yield and size of the onions. It was concluded that fungicide programmes should start at about the time of the first sporulation-infection period.     

A bioeconomic model of downy mildew damage on grapevine for evaluation of control strategies

In order to reduce pesticide use in vineyards, we propose a bioeconomic model to evaluate different fungicide treatment strategies. This model estimates the development of the downy mildew Plasmopara viticola for a given year's weather on a grapevine plot, and predicts the damage done, the yield loss, and the resulting partial gross margin, depending on the chosen protection strategy.Grapevine growth and phenology are simulated with the STICS grapevine crop model according to the year's weather; fungal components quantify downy mildew development; damage onto leaves and fruits is characterized as a percent reduction of potential leaf area and yield; the effect of fungicide treatments is simulated as a partial protection against infection; the economic result is calculated at plot level, taking into account simulated yield, local economic conditions and costs of observations and sprayings.The model parameters were estimated using three sets of experimental data from vineyards in the French wine-growing region of Bordeaux. Using these parameter values, the model was used to evaluate the following five protection strategies: 3 systematic fungicide spraying strategies with 2-, 3- or 4-week intervals, the “Mildium” adaptive strategy, which includes field observations and decision rules, and a control untreated strategy. Yield losses and the resulting partial gross margins were calculated for 23 annual weather examples for each strategy and the statistics of these strategies were compared. The adaptive strategy was found to be slightly less protective on average against downy mildew than the 2-week systematic spray strategy. However its low variability ensures sustainability in terms of grower's income, while reducing by one third the number of sprayings. The model hypotheses and simulation results are discussed, in relation to the particular economic context of the “Bordeaux” protected designation of origin with the objective of reducing pesticide use in vineyards.     

Evaluation of systemic acquired resistance inducers for control of downy mildew on basil

Basil downy mildew, caused by Peronospora belbahrii Thines sp. nov., is a devastating foliar disease of fresh-cut basil first discovered in the U.S. in South Florida in 2007. Since then the pathogen has been found in over 20 U.S. states and has become a major threat to sweet basil production. In this study, acibenzolar-S-methyl (ASM, Actigard 50WG), DL-3-aminobutyric acid (BABA), isonicotinic acid (INA), salicylic acid (SA) and sodium salicylate (SS) were evaluated for their potential to control basil downy mildew in the greenhouse. Efficacy of these systemic acquired resistance (SAR) inducers varied in control of basil downy mildew depending on the rate, method and timing of application. Foliar sprays of ASM applied pre-, post- or pre- + post-inoculation at rates ranging from 25 to 400 mg l⁻¹ significantly (P = 0.05) reduced disease severity compared to the non-treated control in all experiments. ASM sprayed at 50 mg l⁻¹ three times on a weekly basis starting 3 and 7 days post- inoculation resulted in a 93.8 and 47.1% reduction in disease severity, respectively. Six weekly foliar sprays of BABA as pre- + post-inoculation at rates equal or higher than 125 mg l⁻¹ significantly suppressed downy mildew compared to the non-treated control. Foliar treatments of ASM or BABA followed by one or two post-inoculation sprays of a mixture of potassium phosphite (Prophyt) and azoxystrobin (Quadris) significantly improved efficacy for disease control. Sporangia counted on ASM treated leaves were significantly lower than leaves sampled from the non-treated control. ASM and BABA at concentrations lower than 1.0 mM did not inhibit sporangial germination in vitro. The effect of INA, SA and SS on disease reduction was generally inconsistent and not significant compared to the non-treated control.     

Non-chemical agents provide tenable,eco-friendly alternatives for the management of the major diseases devastating Indian mustard (Brassica juncea) in India

India is a leading producer of oilseed Brassicas, contributing approximately 23 percent of the country's total oilseed production. In India, the Indian mustard [Brassica juncea (L.) Czern. & Coss.] crop is ravaged by various diseases, including Alternaria blight, white rust, downy mildew, Sclerotinia rot and powdery mildew, which can contribute to fluctuations in crop yields. A field experiment examining an integrated disease management system for Indian mustard (B. juncea) was conducted under the All India Coordinated Research Project on Rapeseed-Mustard (Indian Council of Agricultural Research or ICAR) during three crop seasons (2006–09) at 11 locations to assess treatments suitable for the management of crop disease. The data from the different locations and years regarding disease severity and incidence were pooled and analyzed. Seed treatments with freshly prepared Allium sativum bulb aqueous extract (1 percent w/v) resulted in significantly higher initial plant stands, across locations and years. Seed treatment with A. sativum bulb extract, followed by its use as a foliar spray, resulted in significantly reduced Alternaria leaf and pod blight severity, reduced white rust severity, fewer stag heads per plot, reduced downy mildew and Sclerotinia rot incidence, and reduced powdery mildew severity, across locations and years. The combination also provided significantly higher seed yields compared with the control across locations and years and was at par with treatment by chemical fungicides. The combination used in the present study was as effective as the combination of seed treatment with Trichoderma harzianum and foliar spraying with Pseudomonas fluorescens and T. harzianum. Economic returns were higher when using biorational treatments (A. sativum bulb extract, T. harzianum, P. fluorescens) compared with chemical fungicides. The combination of seed treatments with T. harzianum followed by its use as a foliar spray (17.22), and the similar combination of seed treatments and foliar spraying with the A. sativum bulb extract (17.18), resulted in a higher benefit to cost ratio. This eco-friendly technology can help oilseed Brassica growers in India safeguard the crops from major diseases and increase the stability and productivity of the Indian mustard crop.     

Are there alternatives to copper for controlling grapevine downy mildew in organic viticulture?

Downy mildew caused by the obligate biotrophic oomycete Plasmopara viticola is one of the most important and devastating diseases of grapevine. Copper-based fungicides are used to control the disease in organic vineyards. However, since 2006, copper use has been limited by an European Commission regulation. In this study conducted from 2004 to 2007, we tested materials that might replace copper in organic viticulture, under controlled greenhouse and field conditions at two locations in Europe (northern Italy and Switzerland). We tested 112 different treatments, including biocontrol agents, materials of animal origin, homeopathic preparations, inorganic materials, microbial extracts, natural derivatives, plant extracts, physical methods and synthetic materials. Good control results were observed for two new copper formulations, but one of them induced phytotoxic effects. Two of the plant extracts provided good control on both leaves and clusters, and six of them controlled the infection on clusters only. One biocontrol agent reduced symptoms on leaves and bunches. Three of the other treatments provided good control on leaves, and four of them provided good control on bunches. Clay was as effective as copper on both leaves and bunches. Currently, even if there is no treatment that is as effective as copper for controlling grapevine downy mildew in organic vineyards, Yucca schidigera and Salvia officinalis extracts and Trichoderma harzianum, could be considered as promising candidates to be developed. This study may help the development of an integrated management program in which a less effective product is applied in combination with copper, to reduce the total quantities of copper applied.     

Assessment of phenotypic diversity of Plasmopara viticola on Vitis genotypes with different resistance

The lack of characterized isolates of Plasmopara viticola is a very limiting issue in the management of downy mildew of grapevine. Although molecular studies have previously confirmed a high diversity of this pathogen, there are still no phenotypically characterized pathotype strains or races available which could be used to study the mechanisms of interaction with host genotypes of different resistance. A leaf disk inoculation technique was used to assess the variability of reactions of six different Vitis genotypes infected with single sporangial clones of five field isolates of P. viticola from different geographical origins. The virulence of thirty P. viticola clones was characterized on grapevine cultivars (Müller-Thurgau, Regent and Cabernet Cortis) and wild species (Vitis vinifera ssp. sylvestris, Vitis rupestris and Vitis riparia) with different susceptibility to downy mildew. Five categories ranging from full susceptibility with profuse and not clearly restricted sporulation (type A) to complete resistance with no sporulation or necrotic reaction (type E) were defined to evaluate the pathogen phenotype. A high variation of pathotypes was found amongst the samples from different field accessions and an unexpected variability was observed even between the cloned strains from the same field. This also accounted for the fungicide tolerance against the two most common fungicides, metalaxyl-M (Phenyl Amide group) and dimethomorph (Carboxylic Acid Amide group), where clones showing high sensitivity and high tolerance to the one or both fungicides were found within the same field. The virulence patterns found within clones of single field isolates supports previous reports that numerous genotypes may be involved in the infection of a single plant or even a single leaf. Moreover, the study provides a methodology to identify and select specific host–pathogen combination suitable for future studies in mechanisms of grapevine downy mildew interaction.     

Sensitivity to fluopicolide of wild type isolates and biological characteristics of fluopicolide-resistant mutants in Pseudoperonospora cubensis

Cucurbit downy mildew caused by the oomycete pathogen Pseudoperonospora cubensis is a devastating disease that is distributed worldwide and affects cucumber in open fields and greenhouses. Fluopicolide, which was a novel systemic fungicide and was released in 2008, it is very effective in controlling downy mildew on cucumber and grape, potato late blight and pepper Phythophthora blight and reduces the loss caused by the diseases, but so far the potential for P. cubensis to develop resistance to fluopicolide has not been investigated. Hence, a laboratory study was undertaken to assess the risk of P. cubensis developing resistance to fluopicolide. Baseline sensitivity to fluopicolide was determined by using 75 P. cubensis isolates collected from cucumber-growing greenhouses in Hebei province, where no fluopicolide had been used for control of cucumber downy mildew before. Values of effective concentrations for 50% inhibition (EC₅₀) of sporulation ranged from 0.02 to 0.40 μg ml⁻¹ and were distributed as a unimodal curve, indicating that all 75 isolates were sensitive to fluopicolide. Sporangia of nine sensitive isolates were ultraviolet (UV)-irradiated, and four fluopicolide-resistant mutants were acquired at a mutation frequency of 7.4 × 10⁻⁷. Seven mutants resistant to fluopicolide were obtained from seven isolates by sporangia adaptation on fluopicolide-treated leaves of cucumber. The EC₅₀ values for all eleven fluopicolide-resistant mutants ranged from 3.37 to 13.06 μg ml⁻¹ with mean resistance factors of 7.9–118.0. After 10 sporangia transfers on fungicide-free leaves of cucumber, all eleven resistant mutants remained resistant to fluopicolide with mean resistance factors of 8.2–81.3. Seven resistant mutants from the selection for resistance and one resistant mutant from UV mutagenesis exhibited stable resistance; however, the other three resistant mutants from UV irradiation became significantly less resistant. Compared to their respective sensitive parents, the eleven resistant mutants exhibited diversity in latent period, infection frequency, lesion extension and sporulation ability. Five out of the eleven resistant mutants exhibited prolonged latent period and three out of the eleven resistant mutants provided decreased infection frequency (IF) compared to their respective parents, indicating that in some cases, resistance mutation might affect the latent period and IF of P. cubensis. There were significant differences in pathogenicity and ability to produce sporangia, but this seemed not to be caused by resistance mutation. No cross-resistance was detected between fluopicolide and azoxystrobin, metalaxyl, dimethomorph, or cymoxanil. In all, there could be a moderate to high risk of field populations of P. cubensis developing resistance to fluopicolide, and populations of P. cubensis should be monitored regularly for their shift of sensitivity over years of application.     

The control of isariopsis leaf spot and downy mildew in grapevine cv. Isabel with the essential oil of lemon grass and the activity of defensive enzymes in response to the essential oil

The aim of this study was to evaluate the potential of the essential oil of lemon grass (Cymbopogon citratus) for the control of isariopsis leaf spot (Pseudocercospora vitis) and downy mildew (Plasmopara viticola) in grapevine cv. Isabel, the effect of the essential oil on the productivity of the grapevines and the effect of the essential oil on the activity of the enzymes chitinase and catalase. The experiment was conducted in a commercial vineyard over two consecutive crop cycles. Each plant in the experiment was subjected to one of the following nine treatments: 0, 0.5, 1.0, 2.0 or 4.0 mL L⁻¹ essential oil, Tween^® 80%, Bordeaux mixture, Acibenzolar-S-methyl and mancozeb. An analysis of the area under the disease progress curve showed a quadratic response by both diseases to the doses of essential oil during the first and second crop cycles. The essential oil treatments also increased the number and mass of the clusters of fruit as well as the productivity and desirable chemical characteristics of the grape. The activity of chitinase increased as a result of the essential oil treatments, whereas the activity of catalase decreased. The essential oil at doses of 1.0 and 2.0 mL L⁻¹ can serve as an alternative means of controlling isariopsis leaf spot and downy mildew and can also serve to improve the fruit quality of grapes cv. Isabel in tropical regions.     

In vitro compatibility of microbial agents for simultaneous application to control strawberry powdery mildew (Podosphaera aphanis)

Biological control of plant pathogens on strawberries may be improved by the simultaneous application of different biological control agents (BCAs). Therefore, the compatibility of various BCAs which had previously shown to be effective against powdery mildew (Podosphaera aphanis (Wallr.) U. Braun & S. Takam) under laboratory conditions was examined in vitro. Inhibitory effects between fungal and bacterial BCAs were demonstrated in dual culture tests on two solid nutrient media. Leaf disc assays with single and multiple strain treatments demonstrated either unaffected or significantly improved control of P. aphanis for many multiple strain treatments, even if antagonistic interactions previously occurred in dual culture tests. Highest inhibition of powdery mildew conidiation (80.7% reduction) was achieved with multiple strain treatments with Bacillus subtilis FZB24 and Metarhizium anisopliae (p < 0.001). In this combination, conidiation was 3.7 times lower than in single treatments with B. subtilis indicating synergistic interactions between these BCAs. Combinations of Trichoderma harzianum T58 and B. subtilis FZB24 showed antagonistic interactions in dual culture tests as well as in leaf disc assays. In this combination, powdery mildew conidiation on leaf discs was four times higher compared to single treatments with T. harzianum T58.     

Susceptibility to downy mildew (Plasmopara viticola) of different Vitis varieties

The susceptibility of different varieties of Vitis vinifera and other Vitis species to downy mildew (caused by Plasmopara viticola) in field, greenhouse and laboratory tests was compared over a period of three years. Different degrees of susceptibility were detected. The most susceptible V. vinifera varieties were ‘Treixadura’ and ‘Albariño; ‘Cabernet Sauvignon’, ‘Mencía’ and ‘Chasselas Doré’ were the least susceptible. The non-vinifera varieties showed no symptoms of downy mildew in the field. Surprisingly, in laboratory and greenhouse tests, the rootstock 110-R was much more susceptible to infection than S04. No relationship was seen between susceptibility and berry colour, or the time of sprouting or fruit ripening. Neither was any correlation seen between susceptibility and the condition of being a traditional or introduced variety. Better knowledge of the susceptibility to downy mildew of different varieties would allow for growers and breeders to select those that are more resistant.     

Multilocation evaluation and selection of stable genotypes for reaction to downy mildew in pearl millet

Twenty-five pearl millet (Pennisetum typhoides (Burm. f.) Stapf and Hub. = P. americanum (L.) K. Schum.) genotypes were evaluated for resistance to downy mildew (Sclerospora graminicola (Sacc.) Schroet.) at 10 different locations in India and Africa during 1979 and 1980. The objective of this work was to identify genotypes with stable and generalized resistance. Genotypic differences in linear regression coefficients were found and regression lines for downy mildew incidence in the various genotypes tended to converge. It was concluded from this that selection on the basis of disease severity would enable genotypes that are superior at all disease levels to be selected. Seven genotypes, namely 700516, 700651, J-1486 × 700787-2-10, P-7, EB-83-2, NC-7174 and SSC-7218, showed low disease scores, average regressions and non-significant deviations, and therefore expressed stable, generalized resistance. Genotype 700251 was particularly suitable for high-disease environments. BJ-104, an important Indian hybrid cultivar, was very unstable and susceptible to downy mildew.     

Influence of energy-saving night temperature regimes on Botrytis cinerea in an early-season glasshouse tomato crop

Night minimum temperatures of 9°C and a regime with periods of one month at 16°C,and 5°C, followed by 13°C, were compared with a night regime of 13°C for their effects on Botrytis cinerea Pers. in an early-season glasshouse tomato crop. Day temperatures were the same in all treatments. Nine independent compartments were used in a randomized block design in which sub-plots were treated with or without a programme of fungicidal sprays of iprodione started at one of three stages in relation to the first infection by B. cinerea. The low-temperature regime of 9°C significantly increased numbers of lesions caused by B. cinerea on stems and leaves during April and May, but not in subsequent months. Increases during May and June of fallen fruits from 9°C plants were not statistically significant. Stem and leaf infections were not increased by the 16/5/13°C energy-saving regime, although losses due to fallen fruit were greater during May. Treatment with iprodione sprays caused significant decreases in disease in all night temperature regimes and was similar in effect to an increase in night temperature from 9°C to 13°C. Responses to the timing of the first application of iprodione were never significant and differed during the first month of disease assessment only. Yields were significantly lower in the first month of cropping at 9°C but there were no differences in yield between the 13°C and the 16/5/13°C regimes.     

Efficacy of azoxystrobin for the control of cucumber downy mildew (Pseudoperonospora cubensis) and fungicide residue analysis

During a five-year trial (2007–2011), the efficacy of azoxystrobin (Quadris, 250 g a.i. L⁻¹, Syngenta) in two doses (187.5 g a.i. ha⁻¹ and 250 g a.i. ha⁻¹) and chlorothalonil (Bravo 720-SC, 720 g a.i. L⁻¹, Syngenta) at a rate of 1.44 kg a.i. ha⁻¹ was tested for the control of cucumber downy mildew (CDM). Cultivars that were susceptible or resistant to CDM (Regal and Haros, respectively) were tested for their response to fungicide applications. Differences in both disease severity and yield of the cultivars among resistance levels and fungicide treatments were observed. A highly significant and negative correlation was obtained between AUDPC and yield. Higher yields can be achieved by planting more resistant cultivars in combination with lower doses of fungicides. This is an indication that CDM contributes significantly to yield losses in cucumber production in Serbia. While monitoring the degradation of azoxystrobin residues, a decrease in residue levels to 1.0 mg kg⁻¹ below the maximum residue level (MRL) was observed at the end of the pre-harvest interval (PHI).     

Local and systemic control of powdery mildew (Leveillula taurica) on pepper plants by foliar spray of mono-potassium phosphate

A foliar spray of 1% (w/v) solution of the fertilizer mono-potassium phosphate (MKP) (KH₂PO₄) on the upper surfaces of lower leaves of greenhouse-grown peppers induced local and systemic control against Leveillula taurica, as compared with control plants. This protection was expressed by a reduction in the leaf area covered with sporulating colonies and in conidial production on leaf tissue, 24 or 48 h post-treatment, when MKP was applied on lower leaves of plants that had been exposed to the source of inoculum. Foliar application of MKP, initiated before or after exposure to heavily diseased plants as the source of inoculum, was effective in controlling powdery mildew. Application of MKP efficiently suppressed powdery mildew as expressed by inhibition of the development of new sporulating colonies, as well as the conidial production of the fungus on infected tissue. Microscopic examination indicated destruction of both hyphae and conidial structures on MKP-treated leaves. The efficacy of MKP in controlling powdery mildew on greenhouse-grown plants was compared with a sterol-inhibiting systemic fungicide. Both treatments significantly inhibited powdery mildew as compared with non-treated control plants, although the fungicide-based treatment seemed to be slightly more effective (not significant) in controlling the disease. Phosphate solutions were not phytotoxic to plant tissue and did not affect the yield, as compared with the fungicide treatment. However, a lower yield was recorded for the non-treated control plots because of mildew infection on leaves. These data indicate that MKP spray may be applied as an alternative practice for the control of powdery mildew in peppers.     

A comparative evaluation of post-infection efficacy of mefenoxam and potassium phosphite with protectant efficacy of azoxystrobin and potassium phosphite for controlling leather rot of strawberry caused by Phytophthora cactorum

Leather rot, caused by Phytophthora cactorum, is one of the most important fruit-rotting diseases of strawberry worldwide. Efficacy of mefenoxam and potassium phosphite against leather rot, when applied in a post-infection fungicide program, made in response to rain events was evaluated over 3 years of testing. Post-infection treatments of potassium phosphite and mefenoxam were compared with calendar-based treatments of azoxystrobin or potassium phosphite sprayed weekly, starting at late bloom (fruit set). In order to obtain high-risk conditions for infection (splash dispersal of the pathogen and subsequent infection periods), plots were flooded until standing water was observed between the rows. Post-infection applications were made within 36 h after the initiation of a flooding event. Leather rot incidence in the untreated controls ranged from 15 to 66% over the 3 years. All fungicide treatments had significantly (P < 0.001) less leather rot incidence than in the untreated control. There were no significant differences in leather rot incidence between the different fungicide treatments. Percent control (the percentage reduction in incidence relative to the check) was as high as 100% with all fungicide treatments. Mefenoxam and potassium phosphite post-infection (after flooding) provided control equal to that obtained with a calendar-based spray program, but with from 1 to 3 fewer fungicide applications.     

Response of progeny bred from Bolivian and North American cultivars in integrated management systems for leaf spot of peanut (Arachis hypogaea)

Early and late leaf spot (caused by Cercospora arachidicola and Cercosporidium personatum, respectively) are major yield-reducing diseases of peanut (Arachis hypogaea L.) in the southeastern U.S. Effective suppression of both early and late leaf spot, collectively called ‘leaf spot’, can be reached with integrated disease management (IDM) systems utilizing peanut cultivars with moderate levels of resistance to these pathogens. A Bolivian landrace cultivar, Bayo Grande, and a series of progeny breeding lines were evaluated under two IDM systems consisting of various reduced application fungicide regimes and/or tillage treatments in field experiments over two years. In the absence of fungicides or when averaged across fungicide regimes, Bayo Grande and breeding lines had better leaf spot resistance when compared to cultivars Georgia Green, MDR-98 and C-99R in all experiments across years. The addition of three or four fungicide applications to Bayo Grande and several of the breeding lines suppressed leaf spot to levels similar to those genotypes and Georgia Green under the full regime. Conservation tillage reduced the need for fungicides in most genotypes in both years. Yields were higher for Bayo Grande and the breeding lines compared to Georgia Green in three of the four experiments across years. When compared across genotype, yields were greatest for the full season fungicide regime, except in 2002 when yields were similar for all treatments. Leaf spot severity was reduced in conservation tillage compared to conventional tillage in most varieties in the absence of fungicides. However, yields tended to be lower in conservation tillage treatments than in conventional tillage. Under conservation tillage, yields of Bayo Grande and two of the breeding lines were negatively impacted in one year and were overall lower in the other year. Several of these new Bolivian-derived genotypes show promise for use in a reduced fungicide and/or conservation tillage IDM system with the potential to lessen fungicide use compared to standard production practices while maintaining low leaf spot levels and high yields.