Gerald J. Holmes

Spatiotemporal Spread of Cucurbit Downy Mildew in the Eastern United States

The dynamics of cucurbit downy mildew, caused by Pseudoperonospora cubensis, in the eastern United States in 2008 and 2009 were investigated based on disease records collected in 24 states as part of the Cucurbit downy mildew ipmPIPE monitoring program. The mean season-long rate of temporal disease progress across the 2 years was 1.4 new cases per day. Although cucurbit downy mildew was detected in mid-February and early March in southern Florida, the disease progressed slowly during the spring and early summer and did not enter its exponential phase until mid-June. The median nearest-neighbor distance of spread of new disease cases was ≈110 km in both years, with ≈15% of the distances being >240 km. Considering disease epidemics on all cucurbits, the epidemic expanded at a rate of 9.2 and 10.5 km per day in 2008 and 2009, respectively. These rates of spatial spread are at the lower range of those reported for the annual spread of tobacco blue mold in the southeastern United States, a disease that is also aerially dispersed over long distances. These results suggest that regional spread of cucurbit downy mildew may be limited by opportunities for establishment in the first half of the year, when fewer cucurbit hosts are available for infection. The O-ring statistic was used to determine the spatial pattern of cucurbit downy mildew outbreaks using complete spatial randomness as the null model for hypothesis testing. Disease outbreaks in both years were spatially aggregated and the extent of spatial dependence was up to 1,000 km. Results from the spatial analysis suggests that disease outbreaks in the Great Lakes and mid-Atlantic regions may be due to the spread of P. cubensis sporangia from outbreaks of the disease near the Georgia/South Carolina/North Carolina border rather than from overwintering sites in southern Florida. Space-time point pattern analysis indicated strong (P < 0.001) evidence for a space-time interaction and a space-time risk window of ≈3 to 5 months after first disease outbreak and 300 to 600 km was detected in both years. Results of this study support the hypothesis that infection of cucurbits by P. cubensis appears to be an outcome of a contagion process, and the relative large space-time window suggests that factors occurring on a large spatial scale (≈1,000 km) facilitate the spread of cucurbit downy mildew in the eastern United States.

A Quantitative Review of Fungicide Efficacy for Managing Downy Mildew in Cucurbits

A meta-analysis of the effect of fungicides on cucurbit downy mildew was conducted using data previously published in Fungicide & Nematicide Tests and Plant Disease Management Reports from 2000 to 2008. Standardized mean effect size (the difference in disease intensity expressed in standard deviation units between the fungicide treatment and its corresponding untreated control) was calculated for each of the 105 field studies evaluating the effects of fungicides on cucurbit downy mildew. Fixed- and random-effects meta-analyses were performed on the log-transformed standardized mean effect sizes to estimate the overall mean effect size (L+), and to determine the variability in the effect size among studies. Fungicides led to a significant (P<0.001) reduction in disease with an L+ value of 1.198, indicating that, overall, fungicides were highly effective against cucurbit downy mildew. Fixed- and random-effects meta-analyses were then conducted to determine the effects of moderator variables on L+. The fixed-effects model resulted in narrower 95% confidence intervals and generally lower estimates of moderator subgroup mean effect size (L) than the random-effects models. Fungicide efficacy was significantly (P<0.001) greater in cucumber, with L being 26.5% higher in cucumber than in other cucurbits. Year of study, number of sprays, and disease pressure in the control significantly (P<0.001) affected L. Fungicide efficacy was significantly lower during the 2004-05 season than prior to or after the 2004-05 season. Studies in which disease pressure was moderate had a significantly higher L than studies with either low or high disease pressure. Fungicide efficacy was ≥22% in studies that received 5 to 6 sprays than in studies that received 1 to 4 or >6 sprays. Fungicide products led to a significant (P<0.001) reduction in disease, although there was substantial between-study variability. The pyridinylmethyl-benzamide group of fungicides (fluopicolide) was the most effective, followed by the carbamate (propamocarb) and quinone inside inhibitors (cyazofamid) group of fungicides, while the carboxylic acid amide group (mandipropamid and dimethomorph) was the least effective. A combination of protectant and systemic fungicides resulted in a proportional increase in L compared with sole application of either protectant or systemic fungicides.

Resurgence of Cucurbit Downy Mildew in the United States: A Watershed Event for Research and Extension

In 2004, an outbreak of cucurbit downy mildew (CDM) caused by the oomycete Pseudoperonospora cubensis (Berk. & M. A. Curtis) Rostovzev resulted in an epidemic that stunned the cucumber (Cucumis sativus L.) industry in the eastern United States. The disease affects all major cucurbit crops, including cucumber, muskmelon, squashes, and watermelon. Although the 2004 epidemic began in North Carolina, the cucumber crop from Florida to the northern growing regions in the United States was devastated, resulting in complete crop loss in several areas. Many cucumber fields were abandoned prior to harvest. The rapid spread of the disease coupled with the failure of fungicide control programs surprised growers, crop consultants, and extension specialists. The epidemic raised several fundamental questions about the potential causes for the resurgence of the disease. Some of these questions revolved around whether the epidemic would recur in subsequent years and the possible roles that changes in the host, pathogen, and environment may have played in the epidemic.

Epidemiology and Population Biology of Pseudoperonospora cubensis: A Model System for Management of Downy Mildews

The resurgence of cucurbit downy mildew has dramatically influenced production of cucurbits and disease management systems at multiple scales. Long-distance dispersal is a fundamental aspect of epidemic development that influences the timing and extent of outbreaks of cucurbit downy mildew. The dispersal potential of Pseudoperonospora cubensis appears to be limited primarily by sporangia production in source fields and availability of susceptible hosts and less by sporangia survival during transport. Uncertainty remains regarding the role of locally produced inoculum in disease outbreaks, but evidence suggests multiple sources of primary inoculum could be important. Understanding pathogen diversity and population differentiation is a critical aspect of disease management and an active research area. Underpinning advances in our understanding of pathogen biology and disease management has been the research capacity and coordination of stakeholders, scientists, and extension personnel. Concepts and approaches developed in this pathosystem can guide future efforts when responding to incursions of new or reemerging downy mildew pathogens.

Influence of Wound Type and Storage Duration on Susceptibility of Sweetpotatoes to Rhizopus Soft Rot

Susceptibility of stored sweetpotato roots (cvs. Beauregard and Hernandez) to Rhizopus soft rot caused by Rhizopus stolonifer was tested at 4- to 6-week intervals over a storage period of 335 days in 1998-1999 (year 1) and 1999-2000 (year 2). In each experiment, roots were wounded by four methods (puncture, bruise, broken, and scrape), inoculated with freshly harvested spores from 4- to 10-day-old cultures, and compared with a nonwounded but inoculated control for their susceptibility to decay. Roots were totally resistant to infection after harvest for 60 days in year 1 and 30 days in year 2. The bruise wound type was most commonly associated with infection, with disease incidence peaking 100 and 175 days after harvest in years 1 and 2, respectively. Inoculation by the puncture method followed a similar pattern in Hernandez but was ineffective in Beauregard. Following the peak in disease incidence at 100 or 175 days, susceptibility of roots declined to levels comparable to that of freshly harvested roots. This period of heightened susceptibility was longer in Hernandez than in Beauregard. The effects of injury types broken and scrape were more variable and did not show the same trend in both years.

What's A Picture Worth? The Use of Modern Telecommunications in Diagnosing Plant Diseases

Plant disease diagnosis is an art as well as a science (8). The diagnostic process (i.e., the recognition of symptoms and signs) is inherently visual and requires intuitive judgment as well as the use of scientific methods. Photographic images of plant disease symptoms and signs are used extensively to enhance descriptions of plant disease and are invaluable in research, teaching, extension, and diagnostics. Modern telecommunications systems permit individuals to share high-resolution digital images among multiple locations within seconds. Plant pathologists can thus incorporate these digital imaging and digital image transfer tools into the process of plant disease diagnosis. As with the introduction of most new tools, techniques, or changes in protocol, there is controversy about how this tool should be used (5,12,13,16–18,20,21). The objective of this paper is to discuss the benefits, risks, and limitations associated with digitally assisted diagnosis (DAD) of plant diseases as well as the current status of this practice in the United States. Case scenarios included in this paper provide evidence that the transfer of high-quality digital images has been a successful tool in assisting long-distance consultation and the accurate diagnosis of plant problems. Examples of inappropriate uses of image transfer are also discussed.

Influence of Preceding Crop on Wireworm (Coleoptera: Elateridae) Abundance in the Coastal Plain of North Carolina

ABSTRACT Three studies were conducted to determine the effect of preceding crop on wireworm (Coleoptera: Elateridae) abundance in the coastal plain of North Carolina. In all three studies, samples of wireworm populations were taken from the soil by using oat, Avena sativa L., baits. Treatments were defined by the previous years crop and were chosen to reflect common crop rotations in the region. Across all three studies, eight wireworm species were recovered from the baits: Conoderus amplicollis (Gyllenhal), Conoderus bellus (Say), Conoderus falli (Lane), Conoderus lividus (Degeer), Conoderus scissus (Schaeffer), Conoderus vespertinus (F.), Glyphonyx bimarginatus (Schaeffer), and Melanotus communis (Gyllenhal). The effect of corn, Zea mays L.; cotton, Gossypium hirsutum L.; fallow; soybean, Glycine max (L.) Merr.; sweet potato, Ipomoea batatas (L.) Lam.; and tobacco (Nicotiana spp.) was evaluated in a small-plot replicated study. M. communis was the most frequently collected species in the small-plot study and was found in significantly higher numbers following soybean and corn. The mean total number of wireworms per bait (all species) was highest following soybean. A second study conducted in late fall and early spring assessed the abundance of overwintering wireworm populations in commercial fields planted to corn, cotton, peanut (Arachis hypogaea L.), soybean, sweet potato, and tobacco in the most recent previous growing season. C. lividus was the most abundant species, and the mean total number of wireworms was highest following corn and soybean. A survey was conducted in commercial sweet potato in late spring and early summer in fields that had been planted to corn, cotton, cucurbit (Cucurbita pepo L.), peanut, soybean, sweet potato, or tobacco in the most recent previous growing season. C. vespertinus was the most abundant species, and the mean total number of wireworms per bait was highest following corn.

Physiochemical Characterization and Field Assessment of Lettuce Chlorosis Virus

Lettuce chlorosis virus (LCV) was purified and partially characterized, and polyclonal antisera were produced and used to assess disease in the field. The antisera reliably detected LCV by indirect enzyme-linked immunosorbent assay (ELISA) in Nicotiana benthamiana. In Western blots, the LCV antisera distinguished between LCV and lettuce infectious yellows virus (LIYV)-infected plants. LCV particle lengths in partially purified preparations, as observed by transmission electron microscopy, were variable, with the majority between 750 and 950 nm long. A single, high molecular weight dsRNA and several lower molecular weight dsRNAs were isolated from LCV-infected N. benthamiana. A single RNA isolated from purified virion preparations was estimated to be 8,625 nucleotides long and was suspected to be the genomic RNA of LCV. LCV was present in experimental field plots in Holtville, California, during the lettuce growing seasons of 1995 to 1997. The percentage of symptomatic plants and yield of lettuce heads treated with insecticide, as well as dsRNA and ELISA reactions for the plots, are reported. A dsRNA consistent in size with LCV was isolated from four weed species in the Imperial Valley of California.

Tillage Practices and Nitrogen Rates on Pickling Cucumber Production

Cucumbers require adequate nitrogen (N) for growth and development. This study was undertaken to determine the most effective tillage system and N rate for hand-picked, processing, cucumber (Cucumis sativus L.) production. Three tillage systems and five N fertilizer rates were compared for effects on yield, fertilizer N use efficiency (NUE), and effects on belly rot disease caused by Rhizoctonia solani. In both years conventional tillage had greater yields than no till and was greater than strip tillage one year. Cucumber NUE was not different between tillage systems or N rates; NUE averaged 35%. In one of two years, incidence of belly rot disease was greater under no till and increased with increasing N rate; in the other year, there was no affect of tillage on belly rot. In most years, conventional tillage for cucumbers will outperform strip tillage and no till.

Relationships of Preharvest Weather Conditions and Soil Factors to Susceptibility of Sweetpotato to Postharvest Decay Caused by Rhizopus stolonifer and Dickeya dadantii

Postharvest soft rots of sweetpotato caused by Rhizopus stolonifer (Rhizopus soft rot) and Dickeya dadantii (bacterial root rot) occur sporadically and can result in significant losses. A 3-year field study related preharvest conditions, including soil texture, chemistry, and fertility; air temperature; soil temperature and moisture; and various cultural practices (153 total variables), to postharvest susceptibility to both diseases in 75 sweetpotato fields in North Carolina and 63 sweetpotato fields in Louisiana. Storage roots were sampled from each field, cured, stored, and inoculated with each pathogen after 100 to 120 days in storage. Disease susceptibility was measured as incidence of diseased storage roots 10 days following inoculation. There was wide variation from field to field in incidence of both diseases (0 to 100% for Rhizopus soft rot and 5 to 95% for bacterial root rot) in both states in each year. Correlations between disease incidence and each of the preharvest variables revealed numerous significant correlations but the variables that correlated with disease incidence were different between North Carolina and Louisiana. Models for both diseases were built by first using forward stepwise regression to identify variables of interest, followed by a mixed-model analysis to produce a final reduced model. For North Carolina fields, postharvest Rhizopus soft rot susceptibility was described by the percentage of the soil cation exchange capacity occupied by calcium, amount of plant-available soil phosphorus, percent soil humic matter, mean air temperature, mean volumetric soil moisture at 40 cm in depth, and mean soil temperature at 2 cm in depth. Postharvest bacterial soft rot susceptibility was described by soil pH and the number of days of high soil temperature late in the season. For Louisiana fields, Rhizopus soft rot susceptibility was described by a complex of variables, including late-season air and soil temperature and late-season days of extreme soil moisture. For bacterial root rot, days of low air temperature and days of high soil temperature late in the season as well as days of low soil moisture best described variation. Although the influence of preharvest variables on postharvest susceptibility was profound for each disease, the complexity of factors involved and differences between the data for the two states makes development of a predictive system extremely difficult.