Wayne F. Wilcox

Comparative Physical Modes of Action of Azoxystrobin, Mancozeb, and Metalaxyl Against Plasmopara viticola (Grapevine Downy Mildew)

The physical modes of action of azoxystrobin, mancozeb, and metalaxyl were evaluated on grapevine seedlings using Plasmopara viticola as a model pathogen. The protectant, postinfection, postsymptom, translaminar, and vapor activities of azoxystrobin were evaluated at a rate of 250 μg/ml. Azoxystrobin provided 100% disease control when applied 1 to 5 days before inoculation. Postinfection applications of azoxystrobin had little effect on the incidence of disease, but colony area and sporulation from the resultant lesions was reduced by 47 and 96%, respectively, relative to the check treatment when applied up to 5 days after inoculation. Postsymptom applications (6 days after inoculation) of azoxystrobin resulted in an 85% mean reduction of resporulation from diseased tissue relative to the check when seedlings were evaluated 1 to 14 days after treatment. Translaminar activity was greatest when the upper surface of the leaf was treated 7 days before inoculation of the lower leaf surface (94% disease control). In contrast, control was <50% when leaves were similarly inoculated 1 and 3 days after treatment. Vapor activity was not pronounced, providing maximum reductions of 5, 11, and 37%, with regard to incidence, colony area, and sporulation, relative to the check when seedlings were treated 1 to 7 days before inoculating adjacent, untreated leaves. Comparatively, mancozeb (1,790 μg/ml) provided complete control of the disease when applied 1 to 5 days before inoculation, but showed little postinfection activity in reducing disease incidence, although it exhibited moderate to high antisporulant activity when applied in postinfection and postsymptom modes (mean reductions of 38 and 89%, respectively, compared with the check treatments). Metalaxyl (260 μg/ml) also provided complete control of the disease when used in protectant mode, and also when applied 1 day after inoculation. Applications at 3 to 5 days after inoculation provided substantial reductions in disease severity and sporulation (mean reductions of 46 and 94%, respectively, compared with the check treatments), and postsymptom applications resulted in a mean 84% reduction in resporulation. Collectively, the results of this study illustrate the unique physical modes of action for azoxystrobin in comparison to that of two traditional protectant and systemic fungicides, and provide information on how azoxystrobin and other strobilurin fungicides with similar physical modes of action should be best used in disease management programs.

Distribution of Baseline Sensitivities to Azoxystrobin Among Isolates of Plasmopara viticola

An excised leaf disc assay was developed for determining the sensitivity of isolates of Plasmopara viticola (causal agent of grapevine downy mildew) to the strobilurin fungicide azoxystrobin. Five repeated assays with the same five single-sporangiophore isolates showed that the technique yielded reproducible results; that is, coefficients of variation ranged from 4.0 to 20% (mean 12%) for effective doses for 50% control (ED50 values) based on disease incidence and from 4.4 to 14% (mean 8.1%) for ED50 values based on disease severity. Then, the assay was applied to 81 single-sporangiophore isolates of P. viticola collected from 10 geographically distinct vineyards in western New York, providing a baseline distribution of sensitivities within this population. For disease incidence, individual ED50 values ranged from 0.05 to 0.94 μg/ml (mean 0.40 μg/ml), whereas for disease severity they ranged from 0.04 to 0.78 μg/ml (mean 0.24 μg/ml). When 61 of these isolates were similarly tested at a single discriminatory dose of 0.50 μg/ml, azoxystrobin provided 61% control of disease incidence versus 41 and 1.1% control for trifloxystrobin and kresoximmethyl, respectively; for disease severity (colony diameter), azoxystrobin provided 80% control versus 57 and 1.1% control for trifloxystrobin and kresoximmethyl, respectively. These results provide information that can be utilized in future monitoring of P. viticola resistance to azoxystrobin and indicate differences in the intrinsic activities of the three strobilurin fungicides against this pathogen.

Seasonal Development of Ontogenic Resistance to Downy Mildew in Grape Berries and Rachises

ABSTRACT Clusters of Vitis vinifera and V. labrusca are reported to become resistant to Plasmopara viticola at stages of development ranging from 1 to 6 weeks postbloom. It has been suggested that resistance is associated with loss of the infection court as stomata are converted to lenticels, but the time of onset, cultivar variation, and seasonal variation in ontogenic resistance has remained uncertain, as has the comparative susceptibility of stem tissue within the fruit cluster. In New York, we inoculated clusters of V. vinifera cvs. Chardonnay and Riesling and V. labrusca cvs. Concord and Niagara at stages from prebloom until 5 to 6 weeks postbloom. Berries were infected and supported profuse sporulation until 2 weeks postbloom, and pedicel tissue remained susceptible until 4 weeks postbloom. Although berries on later-inoculated clusters failed to support sporulation, discoloration and necrosis of berry tissues was often noted, and necrosis of the pedicel within such clusters often led to further discoloration, shriveling, reduced size, or loss of berries. When the epidermis of discolored berries that initially failed to support sporulation was cut, the pathogen emerged and sporulated through incisions, indicating that lack of sporulation on older symptomatic berries was due to infection at an early stage of berry development followed by conversion of functional stomata to lenticels during latency. We repeated the study on Chardonnay and Riesling vines in South Australia and found that the period of berry and rachis susceptibility was greatly increased. The protracted susceptibility of the host was related to the increased duration and phenological heterogeneity of bloom and berry development in the warmer climate of South Australia. The time of onset and subsequent expression of ontogenic resistance to P. viticola may thus be modified by climate and should be weighed in transposing results from one climatic area to another. Our results can be used to refine forecast models for grapevine downy mildew to account for changes in berry and rachis susceptibility, and to focus fungicide application schedules upon the most critical periods for protection of fruit.

Identity of a Phytophthora species attacking raspberry in Europe and North America

Severe root rot of raspberry is caused in Europe and North America by a homothallic, non-papillate Phytophthora sp., which has been identified by different investigators as P. erythroseptica, P. fragariae or as a highly pathogenic variant of P. megasperma . Two collections of such highly pathogenic raspberry isolates from Europe and North America were compared with recognized specimens of the three above named species, which were originally isolated from potato, loganberry and strawberry, and a variety of perennial host plants, respectively. All such raspberry isolates, regardless of previous identification, formed an essentially homogeneous group with respect to colony and growth characteristics, the production and morphology of gametangia, the morphology and dimensions of sporangia, electrophoretic banding patterns of mycelial proteins and pathogenicity. They were readily distinguishable from recognized isolates of P. erythroseptica and P. megasperma with respect to cultural, morphological, and electrophoretic criteria. In contrast, they were very similar to isolates of P. fragariae from strawberry (and a single isolate from loganberry) although the two groups could be separated by differences in growth rate on some agar media, production of oospores in culture, and small differences in electrophoretic banding patterns, as well as in pathogenicity. It was concluded that the raspberry isolates should be assigned to P. fragariae , but that they should be given a subspecific epithet at the varietal level to distinguish them from strawberry isolates of the species i.e. P. fragariae var. rubi .

Phytophthora rosacearum and P. sansomeana, new species segregated from the Phytophthora megasperma “complex”

Phytophthora megasperma sensu lato was a conglomeration of morphologically similar but phylogenetically unrelated species. In this paper we continue the segregation of species from the old P. megasperma complex, formally naming two previously recognized isolate groups. Isolates recovered from rosaceous fruit trees (especially apple and cherry) are in ITS clade 6, related to but distinct from P. megasperma sensu strictu. They are named here Phytophthora rosacearum. They have been referred to previously as the “AC” or “high temperature small oospore” group of P. megasperma. A second group of isolates, earlier called “soybean race non-classifiable”, recovered from soybeans in Indiana and other Midwestern states, are morphologically similar to P. megasperma sensu strictu but unrelated to that species, falling in ITS clade 8. They are named here P. sansomeana. Isolates recovered from Douglas-fir seedlings in nurseries in the Pacific Northwest and various weedy hosts in New York State, referred to in earlier work as “P. megasperma DF1”, appear to be conspecific with the soybean isolates, although they include certain ITS DNA polymorphisms. Both new species are supported by a combination of new and previously published morphological, growth and molecular data.

Evaluation of tactics for managing resistance of Venturia inaequalis to sterol demethylation inhibitors

The impact on the selection and control of subpopulations of V. inaequalis resistant to the sterol demethylation inhibitor (DMI) fenarimol or to dodine were evaluated with respect to several tactics of apple scab control. Experiments were conducted in an experimental orchard with elevated levels of DMI and dodine resistance over a period of three consecutive seasons. The DMI-resistant subpopulation was poorly (14%) controlled at a fenarimol rate of 15 mg/liter (sprayed to run-off), whereas control was significantly improved (54%) at twice that rate. Mancozeb mixed with the low rate of fenarimol also improved the control of DMI-resistant isolates, but the improvement was due to the indiscriminate control of both the DMI-sensitive and -resistant populations provided by mancozeb. The selection of fenarimol-resistant isolates resulting from poor control of the resistant subpopulation by the low rate of fenarimol was equivalent whether fenarimol was applied singly or in mixture with mancozeb. Consequently, the use of high DMI rates in mixture with a protective fungicide is expected to delay the build-up of resistant subpopulations by limiting their increase through two separate principles of control. For dodine in mixture with fenarimol, it was found that each mixing partner applied alone selected both fe-narimol- and dodine-resistant isolates. This selection pattern was partly explained by the possibility that one of the multiple genes underlying fenarimol and dodine resistance confers resistance to both fungicides, in addition to the selection of double-resistant isolates. Regardless, a mixture of fenarimol with dodine each employed at a low rate controlled both the fenarimol-and the dodine-resistant subpopulation at least as effectively as the individual components at twice their mixture rate, and an accelerated selection of double-resistant isolates was not detected. In commercial orchard trials, mixtures of DMIs with either a protective fungicide or with dodine provided equivalent control even when levels of DMI resistance, dodine resistance, or both were moderately elevated. With the exception of orchards with high levels of DMI or dodine resistance, dodine might be an alternative to protective fungicides as a mixing partner with DMIs.

Effects of Diffuse Colonization of Grape Berries by Uncinula necator on Bunch Rots, Berry Microflora, and Juice and Wine Quality

ABSTRACT Production of grape (principally cultivars of Vitis vinifera) for high-quality wines requires a high level of suppression of powdery mildew (Uncinula necator syn. Erysiphe necator). Severe infection of either fruit or foliage has well-documented and deleterious effects upon crop and wine quality. We found that berries nearly immune to infection by U. necator due to the development of ontogenic resistance may still support diffuse and inconspicuous mildew colonies when inoculated approximately 3 weeks post-bloom. Fruit with diffuse mildew colonies appear to be healthy and free of powdery mildew in late-season vineyard assessments with the naked eye. Nonetheless, presence of these colonies on berries was associated with (i) elevated populations of spoilage microorganisms; (ii) increased evolution of volatile ethyl acetate, acetic acid, and ethanol; (iii) increased infestation by insects known to be attracted to the aforementioned volatiles; (iv) increased rotting by Botrytis cinerea; and (v) increased frequency of perceived defects in wines prepared from fruit supporting diffuse powdery mildew colonies. Prevention of diffuse infection requires extending fungicidal protection until fruit are fully resistant to infection. Despite a perceived lack of improvement in disease control due to the insidious nature of diffuse powdery mildew, potential deleterious effects upon crop and wine quality thereby would be avoided.

Evaluating Predictors of Apple Scab with Receiver Operating Characteristic Curve Analysis

ABSTRACT Apple scab (Venturia inaequalis) is a perennial threat to apple production in temperate climates throughout the world. In the eastern United States, apple scab is managed almost exclusively through the regular application of fungicides. Management of the primary phase of disease is focused on preventing infection by ascospores. Management of secondary cycles of infection is largely dependent on how well primary infections were controlled. In this study, we used receiver operating characteristic curve analysis to evaluate how well mid-season assessments of the incidence of apple scab on cluster leaves, clusters (i.e., the whorl of cluster leaves), or immature fruit can serve as predictors of apple scab on harvested fruit (harvest scab) and whether these mid-season assessments of scab could be used reliably to manage scab under various damage thresholds. Results showed that assessment of scab on immature fruit was superior at predicting harvest scab than were assessments made on clusters or cluster leaves at all damage thresholds evaluated. A management action threshold of 7% scab incidence on immature fruit was identified by Youdens index as the optimal action threshold to prevent harvest scab incidence from exceeding 5%. Action thresholds could be higher or lower than 7% when economic assumptions were factored in to the decision process. The utility of such a predictor is discussed.

Powdery Mildew Severity as a Function of Canopy Density: Associated Impacts on Sunlight Penetration and Spray Coverage

Variable canopy densities and associated differences in sunlight distribution within the fruit zones on clusters were quantified and correlated with variable severities of powdery mildew within vineyards in the states of New York, Washington, and South Australia. Canopy density was measured with enhanced point quadrat analysis (EPQA), and the number of shading layers and the photon flux within the fruit zone of individual vines indicated that less disease developed on clusters with more exposure to sunlight. When clusters were categorized as heavily shaded (≤10% photosynthetic photon flux), moderately exposed, or well exposed (≥51% photosynthetic photon flux), vines with the least disease were also shown to have a significantly greater proportion of clusters in the well-exposed category relative to vines with the highest powdery mildew ratings. Consequently, these latter vines had significantly more heavily shaded clusters. The correlation remained strong and the relationship linear even with biweekly applications of either 2 kg/ha or 9 kg/ha of wettable sulfur during the growing season. Additionally, through the use of a fluorescent tracer and EPQA assessments, the deposit of spray materials on clusters was shown to be linearly related to their degree of exposure. Thus, canopy management practices designed to optimize sunlight exposure of grape clusters for fruit quality purposes should also significantly assist in the management of powdery mildew.

Primary Infection, Lesion Productivity, and Survival of Sporangia in the Grapevine Downy Mildew Pathogen Plasmopara viticola

ABSTRACT Several aspects of grapevine downy mildew epidemiology that are fundamental to model predictions were investigated. Simple rainfall-, temperature-, and phenology-based thresholds (rain > 2.5 mm; temperature > 11 degrees C; and phenology > Eichorn and Lorenz [E&L] growth stage 12) were evaluated to forecast primary (oosporic) infection by Plasmopara viticola. The threshold was consistent across 15 years of historical data on the highly susceptible cv. Chancellor at one site, and successfully predicted the initial outbreak of downy mildew for 2 of 3 years at three additional sites. Field inoculations demonstrated that shoot tissue was susceptible to infection as early as E&L stage 5, suggesting that initial germination of oospores, rather than acquisition of host susceptibility, was probably the limiting factor in the initiation of disease outbreaks. We also found that oospores may continue to germinate and cause infections throughout the growing season, in contrast to the widely-held assumption that the supply of oospores is depleted shortly after bloom. Lesion productivity (sporangia/lesion) did not decline with age of a lesion in the absence of suitable weather to induce sporulation. However, the productivity of all lesions declined rapidly through repeated cycles of sporulation. Extremely high temperatures (i.e., one day reaching 42.8 degrees C) had an eradicative effect under vineyard conditions, and permanently reduced sporulation from existing (but not incubating) lesions to trace levels, despite a later return to weather conducive to sporulation. In fair weather, most sporangia died sometime during the daylight period immediately following their production. However, over 50% of sporangia still released zoospores after 12 to 24 h of exposure to overcast conditions.