Megan M. Kennelly

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.

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.

Evidence that prohexadione-calcium induces structural resistance to fire blight infection.

Mechanisms of fire blight control by the shoot-growth regulator prohexadione-calcium (ProCa) were investigated by comparing disease development in ProCa-treated potted apple trees (cv. Gala) to paclobutrazol (another shoot-growth regulator)-treated and nontreated trees and in ProCa-treated cv. McIntosh trees in the field. Twenty-eight days after inoculation with Erwinia amylovora Ea110, disease incidence on ProCa- and paclobutrazol-treated shoots was significantly reduced compared with that on nontreated shoots. Disease severity (percent shoot length infected) was also significantly lower on both ProCa- and paclobutrazol-treated shoots than on nontreated shoots. However, bacterial populations within inoculated shoots were high and bacterial growth occurred in all treatments. In addition, the mean cell wall width of the cortical parenchyma midvein tissue of the first and second youngest unfolded leaves of ProCa- and paclobutrazol-treated shoots was significantly wider both 0.5 and 2 cm from the leaf tips compared with the cell walls of the nontreated tissue. Taken together, these results suggest that reduction of fire blight symptoms by ProCa and paclobutrazol is not the result of reduced populations of E. amylovora in shoots. Moreover, because paclobutrazol also reduced disease severity and incidence, changes in flavonoid metabolism induced by ProCa but not paclobutrazol does not appear to be responsible for disease control as suggested in recent literature. Finally, although this study did not directly link disease control to the observed cell wall changes, the possibility that an increase in cell wall width impedes the spread of E. amylovora should be investigated in more depth.

Disciplinary, Institutional, Funding, and Demographic Trends in Plant Pathology: What Does the Future Hold for the Profession?

Summary assessment of challenges and recommendations for the future. x The committee was able to locate, col-lect, and analyze data that allowed all four areas of our charge (disciplinary balance, institutional erosion, funding, and age demographics) to be assessed quantitatively. However, the quantity and quality of available data were better for some areas (e.g., age demographics and funding) than for others (e.g., disci-plinary balance). With respect to disci-plinary balance, the most accessible and easily searched data reside within the APS membership database. Accord-ingly, the Committee recommends that APS develop the means by which indi-vidual member entries become suffi-ciently complete to allow more detailed analysis of disciplinary balance in the future. x The subdisciplines of nematology and forest pathology have experienced a re-duction in numbers over the last 20 years that would appear to be dispropor-tionate to the reduction in the general population of plant pathology faculty. The Committee recommends a focused review of how the profession could best preserve and enhance these subdisci-plines. x With respect to funding, the Committee was impressed by the ability of several departments to compete for funding across a broad spectrum of sources. Given that success in this area will be a defining characteristic of career success, the Committee recommends that gradu-ate and early-career training in grants-manship be approached as a skill set de-serving attention equal to that devoted to all other areas of academic training. x Although grant funds received increased substantially over 1996 levels, the com-mittee found that research funding among most departments surveyed had reached a plateau or declined slightly since 2002 when adjusted for inflation. Interviews with department chairs and heads indicated a progressive reduction in the number of institutionally, state, and federally funded graduate student assistantships since 1996, resulting in a trend toward the support of a reduced number of graduate students on com-petitive grant funds. The above trend was often mentioned in interviews as having a negative impact on the enthusi-asm of faculty to recruit graduate stu-dents due to uncertain funding. Given the coming need to expand graduate programs, the committee recognizes the identification and procurement of in-creased long-term funding specifically for graduate student training as a critical need for the continued health of the pro-fession. The level of preliminary data re-quired, short funding cycles, and rela-tively low success rates of many existing competitive grant programs conflict with the above need to have funds in place as a prerequisite to effectively re-cruit graduate students. x Institutional erosion was documented regionally and to a lesser degree nation-ally. At some institutions, this resulted in the complete loss of graduate programs in plant pathology and all undergraduate and graduate instruction in the disci-pline. At present, 14 large departments (20 to 40 plant pathology faculty) pro-duce 51% of the Ph.D. recipients in the discipline. Despite the loss of faculty within the larger departments, those sur-veyed had stable teaching programs, and most had obtained increased funding, which possibly offset the impact of de-creases in faculty numbers. Eighteen de-partments with 10 to 19 plant pathology faculty produced another 40% of the Ph.D. recipients. However, there appears to be a sharp break in plant pathology graduate student enrollment in depart-ments with fewer than 10 plant pathol-ogy faculty, as 26 such departments pro-duced only 9% of the Ph.D. recipients in the discipline. If this is indicative of a critical faculty size presently required to competitively operate a Ph.D. program in the discipline, it would appear that the midsized departments just above this level of staffing are at risk of losing graduate training programs in the disci-pline if further erosion occurs. Given the present substantial contribution of the midsized departments to the supply of new Ph.D. recipients, and the equally substantial number of faculty retire-ments in the immediate future, there ap-pear to be only two effective options: ei-ther the present capacity of midsized departments must be preserved, or the capacity the larger departments to pro-duce graduates must be nearly doubled. As the latter option seems unlikely, the Committee recommends that the na-tional impact of further institutional ero-sion among the midsized departments with respect to national needs for Ph.D. graduates in the discipline be made clear to relevant decision-makers at the insti-tutional, state, and national level. x While it was possible to deduce some trends from highly predictable age demographics, the Committee found it difficult to extrapolate most findings into the future due to the inherent unpre-dictability of scientific breakthroughs and the interdependence of the other is-sues under study. However, it is clear that the continued education of a suffi-cient number of both broadly trained and appropriately specialized plant pa-thologists represents the greatest imme-diate and future challenge to the profes-sion. The need to produce the replace-ment population occurs at a time when production of Ph.D. graduates in the discipline appears to have declined substantially below the minimal replace-ment rate, and when the instructional ca-pacity of some departments may be in jeopardy. x Perceptions of the coming retirement wave in the profession differed among different demographic groups. In par-ticular, graduate students, while mindful of the loss of some of their most talented mentors in the near future due to retire-ment, were nonetheless positive in their perception of future employment oppor-tunities. While the present economic re-cession may slightly moderate or delay the age demographic effects, they are ultimately inescapable: approximately one-half of the profession will retire within 10 years (Fig. 6). Even the most pessimistic projections allow for many, if not most of these positions to be even-tually refilled. The Committee recom-mends that the profession capitalize upon this positive aspect of demograph-ics and employment prospects in efforts to recruit graduates to the profession.

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.

Effect of Nozzle Type and Water Volume on Dollar Spot Control in Greens-Height Creeping Bentgrass

A field study was conducted in 2007 and 2008 to determine the influence of nozzle type and water volume on the control of dollar spot of greens-height creeping bentgrass, caused by Sclerotinia homoeocarpa F. T. Bennett. The nozzles utilized were XR, TurfJet, Air Induction, Turbo TwinJet, and Turbo Drop Twin Fan, all producing flat spray patterns. For each nozzle type, three flow rates were used to produce water carrier volumes equivalent to 0.5, 1.0, or 2.0 gal/1000 ft². Chlorothalonil (1.8 oz/1000 ft²) was applied on a 14-day interval. Disease was assessed by counting the number of dollar spot infection centers in plot centers where spray coverage was complete. Overall, all nozzles except for TurfJet provided similar dollar spot suppression, and water volume was rarely a factor. On one rating date in 2007, all nozzle-volume combinations reduced dollar spot compared to untreated turf except for the TurfJet nozzles at the lower water volumes. On one date in 2008, all nozzle and volume combinations reduced dollar spot compared to untreated turf, with the exception of plots receiving fungicide through XR and TurfJet nozzles at 0.5 gal/1000 ft² or Air Induction nozzles at 1.0 gal/1000 ft². Therefore, the use of a reduced water volume and an air induction drift reducing nozzle may be a practical option for managing dollar spot with a contact fungicide in creeping bentgrass.

A Dimorphic and Virulence-Enhancing Endosymbiont Bacterium Discovered in Rhizoctonia solani

The ubiquitous soilborne and plant-pathogenic basidiomycete Rhizoctonia solani, although classified as a single species, is a complex of anastomosis groups (AGs) that cause disease in a broad range of higher plants. Here, we investigated the persistent co-isolation of bacteria with R. solani from brown patch-infected, cool-season turfgrasses, and report the presence of endo-hyphal bacteria, related to members in the genus Enterobacter, in an isolate of R. solani AG 2-2IIIB. The intracellular localization of the bacteria was corroborated by fluorescence, confocal and electron microscopy, and DNA analysis. Furthermore, the Enterobacter sp., which is rod-shaped in the free-living form, exists as an L-form (spheroid) within the fungus, a phenomenon not previously reported in endosymbionts. Our findings also indicate that the bacterium is required for full virulence of R. solani on creeping bentgrass and production of wild type levels of the toxin phenylacetic acid in fungal cultures. The possible presence of ...

Demonstration of an Integrated Pest Management Program for Wheat in Tajikistan

Wheat is an important food security crop in central Asia but frequently suffers severe damage and yield losses from insect pests, pathogens, and weeds. With funding from the United States Agency for International Development, a team of scientists from three U.S. land-grant universities in collaboration with the International Center for Agricultural Research in Dry Areas and local institutions implemented an integrated pest management (IPM) demonstration program in three regions of Tajikistan from 2011 to 2014. An IPM package was developed and demonstrated in farmer fields using a combination of crop and pest management techniques including cultural practices, host plant resistance, biological control, and chemical approaches. The results from four years of demonstration/research indicated that the IPM package plots almost universally had lower pest abundance and damage and higher yields and were more profitable than the farmer practice plots. Wheat stripe rust infestation ranged from 30% to over 80% in farmer practice plots, while generally remaining below 10% in the IPM package plots. Overall yield varied among sites and years but was always at least 30% to as much as 69% greater in IPM package plots. More than 1,500 local farmers—40% women—were trained through farmer field schools and field days held at the IPM demonstration sites. In addition, students from local agricultural universities participated in on-site data collection. The IPM information generated by the project was widely disseminated to stakeholders through peer-reviewed scientific publications, bulletins and pamphlets in local languages, and via Tajik national television.

Resistance of Kansas Sclerotinia homoeocarpa Isolates to Thiophanate-Methyl and Determination of Associated β-Tubulin Mutation

Ostrander, J. C., Todd, R. B., and Kennelly, M. M. 2014. Resistance of Kansas Sclerotinia homoeocarpa isolates to thiophanate-methyl and determination of associated β-tubulin mutation. Plant Health Progress doi:10.1094/PHP-RS-13-0120. Eighty-two isolates of Sclerotinia homoeocarpa from 12 sites in Kansas were evaluated for in vitro sensitivity to the methyl benzimidazole carbamate (MBC) fungicide thiophanate-methyl at the discriminatory dose of 10 µg/ml. Seventeen isolates were sensitive to thiophanate-methyl and the remaining isolates were resistant. Of the 65 isolates from golf course putting greens, two isolates were sensitive and the remaining 63 isolates were resistant. Six resistant and five sensitive isolates were also evaluated in greenhouse assays on fungicide-treated plants. The isolates that were sensitive to thiophanate-methyl in vitro did not cause any disease on thiophanate-methyl-treated plants, and those that were resistant in vitro caused blighting on treated plants equivalent to the nontreated controls. The entire β-tubulin gene was sequenced for four resistant and four sensitive isolates. The resistant isolates all harbored a substitution of alanine for glutamic acid at codon 198 (E198A). These results provide a starting point for further surveys and monitoring of fungicide sensitivity.

Rootstocks shape the rhizobiome: Rhizosphere and endosphere bacterial communities in the grafted tomato system

Root-associated microbes are critical to plant health and performance, although understanding of the factors that structure these microbial communities and theory to predict microbial assemblages are still limited. Here we use a grafted tomato system to study the effects of rootstock genotypes and grafting in endosphere and rhizosphere microbiomes that were evaluated by sequencing 16S rRNA. We compared the microbiomes of nongrafted tomato cultivar BHN589, selfgrafted BHN589, and BHN589 grafted to Maxifort or RST-04-106 hybrid rootstocks. OTU-based bacterial diversity was greater in Maxifort compared to nongraft controls, whereas bacterial diversity in the controls (selfgraft and nongraft) and the other rootstock (RST-04-106) was similar. Grafting itself did not affect bacterial diversity; diversity in the selfgraft was similar to the nongraft. Bacterial diversity was higher in the rhizosphere than in the endosphere for all treatments. However, despite the lower overall diversity, there was a greater number of differentially abundant OTUs (DAOTUs) in the endosphere, with the greatest number of DAOTUs associated with Maxifort. In a PERMANOVA analysis, there was evidence for an effect of rootstock genotype on bacterial communities. The endosphere-rhizosphere compartment and study site explained a high percentage of the differences among bacterial communities. Further analyses identified OTUs responsive to rootstock genotypes in both the endosphere and the rhizosphere. Our findings highlight the effects of rootstocks on bacterial diversity and composition. The influence of rootstock and plant compartment on microbial communities indicates opportunities for the development of designer communities and microbiome-based breeding to improve future crop production. Importance Understanding factors that control microbial communities is essential for designing and supporting microbiome-based agriculture. In this study, we used a grafted tomato system to study the effect of rootstock genotypes and grafting on bacterial communities colonizing the endosphere and the rhizosphere. Comparing the bacterial communities in control treatments (nongraft and selfgraft plants) with the hybrid rootstocks used by farmers, we evaluated the effect of rootstocks on overall bacterial diversity and composition. These findings indicate the potential for using plant genotype to indirectly select bacterial taxa. In addition, we identify taxa responsive to each rootstock treatments, which may represent candidate taxa useful for biocontrol and in biofertilizers.