A new method for extracting DNA from the grape berry surface, beginning in the vineyard.

Isolating DNA from microbes on the surface of a grape berry is a challenge due to their adhesion to the thick berry skin and cuticle, making studies of the grape microbiome challenging. We developed a field-to-lab DNA extraction procedure that starts in the vineyard, disrupts the grape berry surface while en route to the lab through agitation, and efficiently extracts microbial DNA from the surface of the grape. It is cost effective and utilizes commonly available laboratory chemicals with low toxicity (Table 1). This protocol allows researchers to extract DNA from the grape berry surface in the field, therefore undergoing minimal manipulation of those microbiomes before DNA extraction.

The epiphytic microbiota of sour rot-affected grapes differs minimally from that of healthy grapes, indicating causal organisms are already present on healthy berries.

Sour rot is a disease complex produced by an interaction between grape berries and various species of yeast and acetic acid bacteria in the presence of Drosophila fruit flies. While yeast and bacteria are consistently found on healthy grape berries worldwide, we explored whether the composition of these epiphytic communities differed depending on the presence or absence of sour rot symptoms. Using high-throughput sequencing, we characterized the microbiome of sour rot-affected grapes from two geographical areas across two years. In 2015 and 2016, both healthy and sour rot-affected berries were collected from commercial and research vineyards in Geneva, NY and commercial vineyards in Tasmania, AUS. In this experiment, all associated organisms grouped together primarily by location, and not by presence/absence of symptoms or cultivar. The predominant difference between asymptomatic and symptomatic samples, regardless of location, was the abundance of Acetobacter species, which were significantly more plentiful in the symptomatic samples. Yeast genera such as Candida, Hanseniaspora, Pichia and Saccharomyces were abundant in both sets of samples, but varied by region. The consistent presence of yeast species and the increased abundance of acetic acid-generating bacteria is consistent with our understanding of their etiological role in sour rot development. In 2016, diseased grapes also were collected from vineyards in Fredonia, NY, and Modesto, CA. Consistent with our comparison study, all associated organisms again grouped together primarily by location. Yeast genera such as Candida, Hanseniaspora, Pichia and Saccharomyces were abundant in both sets of samples, but varied by region. The consistent presence of yeast species and the abundance of acetic acid-generating bacteria in both experiments is consistent with our understanding of their etiological role in sour rot development.

Grape Sour Rot: A Four-Way Interaction Involving the Host, Yeast, Acetic Acid Bacteria, and Insects.

Sour rot, a disease affecting berries of cultivated Vitis spp. worldwide, has not been clearly defined. Reported symptoms of the disease include browning of the berry skin, oozing of disintegrated berry pulp, and the smell of acetic acid, all in the presence of fruit flies (Drosophila spp.). We determined acetic acid concentrations in multiple collections of symptomatic berries, isolated and identified microbes from them, and inoculated commonly isolated organisms into healthy berries with and without concurrent exposure to wild-type or axenic Drosophila melanogaster. Coinoculations combining one of several yeasts (Metschnikowia spp., Pichia spp., and a Saccharomyces sp.) plus an acetic acid bacterium (an Acetobacter sp. and Gluconobacter spp.) reproduced sour rot symptoms, defined here as decaying berries with a loss of turgor and containing acetic acid at a minimum of 0.83 g/liter, based on observed field levels. Symptoms developed only in the presence of D. melanogaster, either wild type or axenic, indicating a nonmicrobial contribution of these insects in addition to a previously suggested microbial role. We conclude that sour rot is the culmination of coinfection by various yeasts, which convert grape sugars to ethanol, and bacteria that oxidize the ethanol to acetic acid, and that this process is mediated by Drosophila spp.

Two dominant loci determine resistance to Phomopsis cane lesions in F1 families of hybrid grapevines.

KEY MESSAGE: Rapid characterization of novel NB-LRR-associated resistance to Phomopsis cane spot on grapevine using high-throughput sampling and low-coverage sequencing for genotyping, locus mapping and transcriptome analysis provides insights into genetic resistance to a hemibiotrophic fungus. Phomopsis cane and leaf spot, caused by the hemibiotrophic fungus Diaporthe ampelina (syn = Phomopsis viticola), reduces the productivity in grapevines. Host resistance was studied on three F1 families derived from crosses involving resistant genotypes 'Horizon', Illinois 547-1, Vitis cinerea B9 and V. vinifera 'Chardonnay'. All families had progeny with extremely susceptible phenotypes, developing lesions on both dormant canes and maturing fruit clusters. Segregation of symptoms was observed under natural levels of inoculum in the field, while phenotypes on green shoots were confirmed under controlled inoculations in greenhouse. High-density genetic maps were used to localize novel qualitative resistance loci named Rda1 and Rda2 from V. cinerea B9 and 'Horizon', respectively. Co-linearity between reference genetic and physical maps allowed localization of Rda2 locus between 1.5 and 2.4 Mbp on chromosome 7, and Rda1 locus between 19.3 and 19.6 Mbp of chromosome 15, which spans a cluster of five NB-LRR genes. Further dissection of this locus was obtained by QTL mapping of gene expression values 14 h after inoculation across a subset of the 'Chardonnay' × V. cinerea B9 progeny. This provided evidence for the association between transcript levels of two of these NB-LRR genes with Rda1, with increased NB-LRR expression among susceptible progeny. In resistant parent V. cinerea B9, inoculation with D. ampelina was characterized by up-regulation of SA-associated genes and down-regulation of ethylene pathways, suggesting an R-gene-mediated response. With dominant effects associated with disease-free berries and minimal symptoms on canes, Rda1 and Rda2 are promising loci for grapevine genetic improvement.

Weather During Critical Epidemiological Periods and Subsequent Severity of Powdery Mildew on Grape Berries.

Recorded severity of grape powdery mildew on berries of untreated, susceptible hybrid cultivars varied from 0.2 to 50.5% across a 30-year period in Geneva, NY; within 7 of those years, cluster disease severity ranged from 3.42 to 99.5% on Vitis vinifera 'Chardonnay'. Although existing temperature-driven risk models could not account for this annual variation, pan evaporation (Epan), an environmental variable influenced by the collective effects of temperature, vapor pressure deficit, solar radiation, and wind speed, did. Logistic regression analysis (LRA) was used to classify epidemics as either mild or severe. Recursive partition analysis (RPA) provided a simplified decision tree for calculation of powdery mildew risk and incorporated (i) an estimate of the relative primary inoculum levels based on temperatures in the previous late summer and (ii) the current season favorability for pathogen development during the grapevine phenological period critical for berry infection by Erysiphe necator. Although the LRA had fewer instances of misclassification, RPA provided a rapid means for seasonal risk classification. Both the RPA and LRA models are able to describe disease severity risk in real time or can be used to forecast risk, thereby allowing growers to adjust management programs in a responsive manner.

Cadophora species associated with wood-decay of grapevine in North America.

Cadophora species are reported from grapevine (Vitis vinifera L.) in California, South Africa, Spain, Uruguay, and Canada. Frequent isolation from vines co-infected with the Esca pathogens (Togninia minima and Phaeomoniella chlamydospora), and confirmation of its ability to cause wood lesions/discoloration in pathogenicity tests, suggest that C. luteo-olivacea is part of the trunk pathogen complex. In North America, little is known regarding the diversity, geographic distribution, and roles of Cadophora species as trunk pathogens. Accordingly, we characterized 37 Cadophora isolates from ten US states and two Canadian provinces, based on molecular and morphological comparisons, and pathogenicity. Phylogenetic analysis of three loci (ITS, translation elongation factor 1-alpha (TEF1-α) and beta-tubulin (BT)) distinguished two known species (C. luteo-olivacea and Cadophora melinii) and three newly-described species (Cadophora orientoamericana, Cadophora novi-eboraci, and Cadophora spadicis). C. orientoamericana, C. novi-eboraci, and C. spadicis were restricted to the northeastern US, whereas C. luteo-olivacea was only recovered from California. C. melinii was present in California and Ontario, Canada. Morphological characterization was less informative, due to significant overlap in dimensions of conidia, hyphae, conidiophores, and conidiogenous cells. Pathogenicity tests confirmed the presence of wood lesions after 24 m, suggesting that Cadophora species may have a role as grapevine trunk pathogens.

Mechanisms of Resistance to an Azole Fungicide in the Grapevine Powdery Mildew Fungus, Erysiphe necator.

We studied the mechanisms of azole resistance in Erysiphe necator by quantifying the sensitivity to myclobutanil (EC50) in 65 isolates from the eastern United States and 12 from Chile. From each isolate, we sequenced the gene for sterol 14α-demethylase (CYP51), and measured the expression of CYP51 and homologs of four putative efflux transporter genes, which we identified in the E. necator transcriptome. Sequence variation in CYP51 was relatively low, with sequences of 40 U.S. isolates identical to the reference sequence. Nine U.S. isolates and five from Chile carried a previously identified A to T nucleotide substitution in position 495 (A495T), which results in an amino acid substitution in codon 136 (Y136F) and correlates with high levels of azole resistance. We also found a nucleotide substitution in position 1119 (A1119C) in 15 U.S. isolates, whose mean EC50 value was equivalent to that for the Y136F isolates. Isolates carrying mutation A1119C had significantly greater CYP51 expression, even though A1119C does not affect the CYP51 amino acid sequence. Regression analysis showed no significant effects of the expression of efflux transporter genes on EC50. Both the Y136F mutation in CYP51 and increased CYP51 expression appear responsible for azole resistance in eastern U.S. populations of E. necator.

Identification of Eutypa spp. Causing Eutypa Dieback of Grapevine in Eastern North America.

Eutypa dieback of grapevine is caused by Eutypa lata in production areas with Mediterranean climates in California, Australasia, Europe, and South Africa. Eutypa dieback has also been described in the colder, eastern North American vineyards where cultivars adapted from native Vitis spp. (e.g., Vitis × labruscana 'Concord') are primarily grown. However, the causal agents associated with the diseases in this region have not been conclusively identified. Examination of 48 vineyards showing symptoms of dieback in the northeastern United States (Connecticut, Massachusetts, Michigan, New York, Ohio, and Rhode Island) and Ontario, Canada revealed that vineyards were mainly infected by Eutypa spp. other than E. lata. Multigene phylogenies (internal transcribed spacer ribosomal DNA, ß-tubulin, and RNA polymerase II) of isolates recovered from these vineyards indicated that Eutypa dieback is caused primarily by an undescribed Eutypa sp. and E. laevata. Eutypa sp. was recovered from 56% of the vineyards examined, whereas E. laevata and E. lata were less far common (17 and 6%, respectively). Fruiting body morphology and spore dimensions supported phylogenetic separation of the three taxa. Pathogenicity tests conducted on Vitis vinifera 'Chardonnay' in the greenhouse and in the field verified that all three species were able to cause wood canker and to infect pruning wounds, respectively.

Selection, Fitness, and Control of Grape Isolates of Botrytis cinerea Variably Sensitive to Fenhexamid.

Of 683 Botrytis cinerea isolates collected from a fungicide-trial vineyard, 31 were classified as putatively resistant to fenhexamid (50% effective concentration [EC50] ≥ 0.1 µg/ml). For the resistant isolates that survived and sporulated in culture, colony expansion and conidial germination frequency was significantly reduced relative to the mean of 30 representative baseline isolates (EC50 = 0.03 µg/ml). Grape berries were inoculated with four isolates representing a range of fenhexamid sensitivities and treated preventively or curatively with fenhexamid concentrations (150 to 600 mg/liter) representing 25 to 100% of the recommended rate. All treatments significantly delayed disease onset and progress caused by isolates with EC50 values of 0.03 and 0.15 µg/ml but provided little to no control of isolates with EC50 values of 0.32 and 62.5 µg/ml. The latter isolate exhibited a previously unreported F427V mutation of ERG27, an enzyme of ergosterol biosynthesis. In a duplex quantitative polymerase chain reaction test, the ratio of pathogen/host DNA increased significantly for 14 days after inoculation of untreated berries with a baseline isolate but declined slightly in berries treated with fenhexamid at 600 mg/liter 1 day post inoculation. In the vineyard, disease control was affected by the number and rate of fenhexamid applications but B. cinerea isolates with EC50 ≥ 0.1 µg/ml were not preferentially selected.

Characterization of Species of Diaporthe from Wood Cankers of Grape in Eastern North American Vineyards.

In eastern North America, Phomopsis cane and leaf spot, caused by Phomopsis viticola, is a foliar disease of grape but, in the Mediterranean climate of western North America, P. viticola is primarily associated with wood cankers, along with other Diaporthe spp. To determine the identity of wood-infecting Diaporthe spp. in eastern North America, 65 isolates were cultured from 190 wood-canker samples from 23 vineyards with a history of Phomopsis cane and leaf spot. Identification of 29 representative isolates was based initially on morphology, followed by phylogenetic analyses of DNA sequences of the ribosomal DNA internal transcribed spacer region, elongation factor subunit 1-α, and actin in comparison with those of type specimens. Three species were identified: P. viticola, P. fukushii, and Diaporthe eres. Inoculations onto woody stems of potted Vitis labruscana 'Concord' and V. vinifera 'Chardonnay' showed that D. eres and P. fukushii were pathogenic (mean lesion lengths of 7.4 and 7.1 mm, respectively, compared with 3.5 mm for noninoculated controls) but significantly less so than wood-canker and leaf-spot isolates of P. viticola (13.5 mm). All three species infected pruning wounds of Concord and Chardonnay in the field. Our finding of pathogenic, wood-infecting Diaporthe spp. in all 23 vineyards suggests a frequent co-occurrence of the foliar symptoms of Phomopsis cane and leaf spot and wood cankers, although the latter are not always due to P. viticola.

Effects of sunlight exposure on grapevine powdery mildew development.

Natural and artificially induced shade increased grapevine powdery mildew (Erysiphe necator) severity in the vineyard, with foliar disease severity 49 to 75% higher relative to leaves in full sun, depending on the level of natural shading experienced and the individual experiment. Cluster disease severities increased by 20 to 40% relative to those on check vines when ultraviolet (UV) radiation was filtered from sunlight reaching vines in artificial shading experiments. Surface temperatures of leaves in full sunlight averaged 5 to 8°C higher than those in natural shade, and in one experiment, filtering 80% of all wavelengths of solar radiation, including longer wavelengths responsible for heating irradiated tissues, increased disease more than filtering UV alone. In controlled environment experiments, UV-B radiation reduced germination of E. necator conidia and inhibited both colony establishment (hyphal formation and elongation) and maturity (latent period). Inhibitory effects of UV-B radiation were significantly greater at 30°C than at 20 or 25°C. Thus, sunlight appears to inhibit powdery mildew development through at least two mechanisms, i.e., (i) UV radiation's damaging effects on exposed conidia and thalli of the pathogen; and (ii) elevating temperatures of irradiated tissues to a level supraoptimal or inhibitory for pathogen development. Furthermore, these effects are synergistic at temperatures near the upper threshold for disease development.

Grapevine powdery mildew (Erysiphe necator): a fascinating system for the study of the biology, ecology and epidemiology of an obligate biotroph.

UNLABELLED: Few plant pathogens have had a more profound effect on the evolution of disease management than Erysiphe necator, which causes grapevine powdery mildew. When the pathogen first spread from North America to England in 1845, and onwards to France in 1847, 'germ theory' was neither understood among the general populace nor even generally accepted within the scientific community. Louis Pasteur had only recently reported the microbial nature of fermentation, and it would be another 30 years before Robert Koch would publish his proofs of the microbial nature of certain animal diseases. However, within 6 years after the arrival of the pathogen, nearly 6 million grape growers in France were routinely applying sulphur to suppress powdery mildew on nearly 2.5 million hectares of vineyards (Campbell, 2006). The pathogen has remained a focus for disease management efforts ever since. Because of the worldwide importance of the crop and its susceptibility to the disease, and because conventional management with modern, organic fungicides has been compromised on several occasions since 1980 by the evolution of fungicide resistance, there has also been a renewed effort worldwide to explore the pathogen's biology and ecology, its genetics and molecular interactions with host plants, and to refine current and suggest new management strategies. These latter aspects are the subject of our review. TAXONOMY: The most widely accepted classification follows. Family Erysiphaceae, Erysiphe necator Schw. [syn. Uncinula necator (Schw.) Burr., E. tuckeri Berk., U. americana Howe and U. spiralis Berk. & Curt; anamorph Oidium tuckeri Berk.]. Erysiphe necator var. ampelopsidis was found on Parthenocissus spp. in North America according to Braun (1987), although later studies revealed isolates whose host range spanned genera, making the application of this taxon somewhat imprecise (Gadoury and Pearson, 1991). The classification of the genera before 1980 was based on features of the mature ascocarp: (i) numbers of asci; and (ii) morphology of the appendages, in particular the appendage tips. The foregoing has been supplanted by phylogeny inferred from the internal transcribed spacer (ITS) of ribosomal DNA sequences (Saenz and Taylor, 1999), which correlates with conidial ontogeny and morphology (Braun et al., 2002). HOST RANGE: The pathogen is obligately parasitic on genera within the Vitaceae, including Vitis, Cissus, Parthenocissus and Ampelopsis (Pearson and Gadoury, 1992). The most economically important host is grapevine (Vitis), particularly the European grape, V. vinifera, which is highly susceptible to powdery mildew. Disease symptoms and signs: In the strictest sense, macroscopically visible mildew colonies are signs of the pathogen rather than symptoms resulting from its infection, but, for convenience, we describe the symptoms and signs together as the collective appearance of colonized host tissues. All green tissues of the host may be infected. Ascospore colonies are most commonly found on the lower surface of the first-formed leaves near the bark of the vine, and may be accompanied by a similarly shaped chlorotic spot on the upper surface. Young colonies appear whitish and those that have not yet sporulated show a metallic sheen. They are roughly circular, ranging in size from a few millimetres to a centimetre or more in diameter, and can occur singly or in groups that coalesce to cover much of the leaf. Senescent colonies are greyish, and may bear cleistothecia in various stages of development. Dead epidermal cells often subtend the colonized area, as natural mortality in the mildew colony, the use of fungicides, mycoparasites or resistance responses in the leaf result in the deaths of segments of the mildew colony and infected epidermal cells. Severely affected leaves usually senesce, develop necrotic blotches and fall prematurely. Infection of stems initially produces symptoms similar to those on leaves, but colonies on shoots are eventually killed as periderm forms, producing a dark, web-like scar on the cane (Gadoury et al., 2011). Inflorescences and berries are most susceptible when young, and can become completely coated with whitish mildew. The growth of the berry epidermal tissue stops when severely infected, which may result in splitting as young fruit expand. Berries in a transitional stage between susceptible and resistant (generally between 3 and 4 weeks after anthesis) develop diffuse, nonsporulating mildew colonies only visible under magnification. Diffuse colonies die as berries continue to mature, leaving behind a network of necrotic epidermal cells (Gadoury et al., 2007). Survival over winter as mycelium in buds results in a distinctive foliar symptom. Shoots arising from these buds may be heavily coated with fungal growth, stark white in colour and stand out like white flags in the vine, resulting in the term 'flag shoots'. More commonly, colonization of a flag shoot is less extensive, and infection of a single leaf, or of leaves on one side of the shoot only, is observed (Gadoury et al., 2011).

Convenient, inexpensive quantification of elemental sulfur by simultaneous in situ reduction and colorimetric detection.

Rapid, inexpensive, and convenient methods for quantifying elemental sulfur (S(0)) with low or sub-µgg(-1) limits of detection would be useful for a range of applications where S(0) can act as a precursor for noxious off-aromas, e.g., S(0) in pesticide residues on winegrapes or as a contaminant in drywall. However, existing quantification methods rely on toxic reagents, expensive and cumbersome equipment, or demonstrate poor selectivity. We have developed and optimized an inexpensive, rapid method (∼15 min per sample) for quantifying S(0) in complex matrices. Following dispersion of the sample in PEG-400 and buffering, S(0) is quantitatively reduced to H(2)S in situ by dithiothreitol and simultaneously quantified by commercially available colorimetric H(2)S detection tubes. By employing multiple tubes, the method demonstrated linearity from 0.03 to 100 µg S(0) g(-1) for a 5 g sample (R(2)=0.994, mean CV=6.4%), and the methodological detection limit was 0.01 µg S(0) g(-1). Interferences from sulfite or sulfate were not observed. Mean recovery of an S(0) containing sulfur fungicide in grape macerate was 84.7% with a mean CV of 10.4%. Mean recovery of S(0) in a colloidal sulfur preparation from a drywall matrix was 106.6% with a mean CV of 6.9%. Comparable methodological detection limits, sensitivity, and recoveries were achieved in grape juice, grape macerate and with 1g drywall samples, indicating that the methodology should be robust across a range of complex matrices.

Effects of acute low-temperature events on development of Erysiphe necator and susceptibility of Vitis vinifera.

Growth and development of Erysiphe necator (syn. Uncinula necator) has been extensively studied under controlled conditions, primarily with a focus on development of grapevine powdery mildew within the optimal temperature range and the lethal effects of high temperatures. However, little is known of the effect of cold temperatures (above freezing but <8 degrees C) on pathogen development or host resistance. Pretreatment of susceptible Vitis vinifera leaf tissue by exposure to cold temperatures (2 to 8 degrees C for 2 to 8 h) reduced infection efficiency and colony expansion when tissues were subsequently inoculated. Furthermore, nascent colonies exposed to similar cold events exhibited hyphal mortality, reduced expansion, and increased latent periods. Historical weather data and an analysis of the radiational cooling of leaf tissues in the field indicated that early-season cold events capable of inducing the foregoing responses occur commonly and frequently across many if not most viticultural regions worldwide. These phenomena may partially explain (i) the unexpectedly slow development of powdery mildew during the first month after budbreak in some regions and (ii) the sudden increase in epidemic development once seasonal temperatures increase above the threshold for acute cold events.

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.

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.

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.

Vapor Activity and Systemic Movement of Mefenoxam Control Grapevine Downy Mildew.

Metalaxyl is translocated from roots to leaves to control a number of oomycete pathogens, but systemic movement from vegetative organs into fruit and vapor activity against Plasmopara viticola, the causal agent of grapevine downy mildew, has not been examined experimentally. We inoculated fruit clusters of grapevines with P. viticola at prebloom, bloom, or 1 week postbloom. We then selectively applied mefenoxam (288 mg/liter), the active enantiomer of metalaxyl, to the leaves or stem tissue 12 to 48 h after inoculation. Little to no downy mildew developed on fruit when mefenoxam was applied to leaf tissue, stem tissue, or both. In contrast, downy mildew symptoms were severe on inoculated clusters on untreated shoots. When potential vapor activity was blocked, we observed fungicidal activity on seedling foliage in response to apparent systemic movement from treated stems and soil, but not from leaves. However, when vapor activity was permitted, mefenoxam residues on treated leaves controlled disease on other, untreated leaves. In subsequent vineyard experiments, vapor and systemic activity provided equivalent and near-complete suppression of downy mildew on clusters 48 h post inoculation. Furthermore, inoculated grape seedlings that were placed near mefenoxam-treated seedlings in open and closed systems developed nil to trace levels of downy mildew compared with controls, further indicating that the material has strong vapor activity.

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.

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 Youden's 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.