A. J. Gevens

Characterization of Phytophthora capsici from Michigan Surface Irrigation Water.

ABSTRACT Phytophthora capsici infects cucurbitaceous and solanaceous crops worldwide. In free water, P. capsici sporangia release zoospores that may be disseminated by moving surface water. Surface irrigation sources (river system, ponds, and ditches) in three Michigan counties with a history of P. capsici-susceptible crop production were monitored for the pathogen during four growing seasons (2002 to 2005). Pear and cucumber baits were suspended in water at monitoring sites for 3- to 7-day intervals and water temperature was recorded. Baits were washed and lesions were excised and cultured on water agar amended with rifampicin and ampicillin. P. capsici was detected at monitoring sites in multiple years, even when non-host crops were planted nearby. Recovered isolates (N = 270) were screened for sensitivity to the fungicide mefenoxam and characterized for mating type (MT). P. capsici isolates resistant to mefenoxam were common in water sources from southwest and southeast Michigan. Most monitoring sites yielded isolates of a 1:1 ratio of A1:A2 MTs. Amplified fragment length polymorphism analysis of select isolates from 2002 to 2004 indicated a lack of similarity groups persisting over time and in specific geographical locations. Data suggest that P. capsici did not overwinter in any of the surface water sources monitored. Water temperatures were correlated to positive P. capsici detection from all monitoring sites. The frequent detection of P. capsici in surface water used for irrigation in the primary vegetable growing regions in Michigan suggests that this is an important means of pathogen dissemination.

The 2009 late blight pandemic in the eastern United States - causes and results

The tomato late blight pandemic of 2009 made late blight into a household term in much of the eastern United States. Many home gardeners and many organic producers lost most if not all of their tomato crop, and their experiences were reported in the mainstream press. Some CSAs (Community Supported Agriculture) could not provide tomatoes to their members. In response, many questions emerged: How did it happen? What was unusual about this event compared to previous late blight epidemics? What is the current situation in 2012 and what can be done? Its easiest to answer these questions, and to understand the recent epidemics of late blight, if one knows a bit of the history of the disease and the biology of the causal agent, Phytophthora infestans.

Characterization of Phytophthora capsici Causing Foliar and Pod Blight of Snap Bean in Michigan

Green and yellow snap bean plants with water-soaked foliar lesions, stem necrosis, pod blight, and overall plant decline were observed in four commercial fields in three Michigan counties during 2003 to 2005. All fields were cropped to cucurbits that exhibited symptoms of Phytophthora capsici infection in recent years. In all, 680 isolates of P. capsici were obtained from bean stems, petioles, leaves, and pods; the pathogen was not recovered from roots. Kochs postulates were completed with representative isolates, confirming P. capsici as the causal organism. Select isolates also were pathogenic on cucumber fruit, causing symptoms consistent with P. capsici infection. The majority of the P. capsici isolates collected were sensitive to the fungicide mefenoxam and were of the A1 mating type. Under laboratory conditions, six P. capsici isolates from snap bean (2003) were pathogenic on 12 different commercial bean cultivars, including soybean. Infected bean plants exhibited water-soaked lesions, foliar necrosis, and wilting. We subjected 131 isolates collected from 2003 and 2004 to amplified fragment length polymorphism analysis to investigate diversity among isolates and geographical populations and to determine whether bean P. capsici isolates were similar to isolates from a cucurbit host. This is the first in-depth study of P. capsici on snap bean in Michigan. Although bean cultivars previously were considered a suitable rotation for crops susceptible to P. capsici, this is no longer a recommended practice.

A Detached Cucumber Fruit Method to Screen for Resistance to Phytophthora capsici and Effect of Fruit Age on Susceptibility to Infection

Identification and utilization of resistance to Phytophthora capsici could provide the basis for a viable management strategy against cucumber fruit rot, a persistent threat in cucumber (Cucumis sativus) production. Our objectives were to develop a method for testing detached, nonwounded, cucumber fruit for resistance to P. capsici, and to screen cucumber cultivars and plant introductions (collectively referred to as cultigens) for resistance. Four P. capsici isolates (differing in their sensitivity to the fungicide mefenoxam and compatibility type) were compared for their fruit infection capability in 1999 and 2000. No significant differences were found among isolates, and a single isolate was used for all subsequent screens. From 1999 to 2004, 480 cucumber cultigens were grown according to standard practices at Michigan State University research farms in four fields with no history of P. capsici. Commercially mature fruit were harvested, inoculated with P. capsici, and rated for lesion diameter, pathogen sporulation diameter, and density of pathogen sporulation. Although no fruit exhibited complete resistance to P. capsici, some cultigens exhibited limited pathogen sporulation. In the process of screening, it was observed that younger, smaller fruit were comparatively more susceptible than older, larger fruit. Replicated trials with hand-pollinated fruit showed that the transition from susceptible to more resistant appeared to coincide with the transition from the period of rapid fruit elongation to the period of increased fruit diameter. This is the first report using a nonwounded fruit screen to analyze cucumber resistance to P. capsici.

Characterization and Distribution of Three New Clonal Lineages of Phytophthora infestans Causing Late Blight in Wisconsin from 2009 to 2012

Phytophthora infestans causes late blight of potato and tomato, a disease that has been estimated to cost U.S. potato growers

Characterization of kudzu (Pueraria spp.) resistance to Phakopsora pachyrhizi, the causal agent of soybean rust.

287.8 million annually. We collected isolates of P. infestans from Wisconsin from 2009 to 2012 and determined distribution of clonal lineages and mating types and sensitivity to the systemic fungicide mefenoxam. We also sought to evaluate the current utility of an analysis of the Glucose-6-phosphate isomerase (Gpi) allozyme locus for predicting mefenoxam sensitivity with the aim of delivering timely information to growers. Overall, 143 isolates were collected from 52 locations in 20 Wisconsin counties from 2009 to 2012. Three clonal lineages, US-22, US-23, and US-24, were identified and were novel to Wisconsin and the U.S. US-22 is of the A2 mating type and sensitive to mefenoxam, with Gpi 100/122. US-23 and US-24 are of the A1 mating type and primarily intermediately sensitive to mefenoxam, with Gpi 100/100 and 100/100/111, respectively. Because of this close correlation and the unique Gpi patterns for each lineage present, we were able to predict mefenoxam sensitivity directly from samples using the allozyme assay and quickly deliver management information to growers. Both mating types were present in Wisconsin in 2009 and 2010 but were spatially separated and no evidence of sexual recombination or soil persistence was detected. The presence of new clonal lineages of P. infestans in Wisconsin indicates a need for continued close monitoring of late blight to facilitate generation of timely information for enhanced short-term and long-term late blight management.ResumenPhytophthora infestans causa el tizón tardío de la papa y el tomate, una enfermedad a la que se le ha estimado un costo para los productores de papa de los EUA de

First Report of Late Blight Caused by Phytophthora infestans Clonal Lineage US-24 on Potato (Solanum tuberosum) in Wisconsin

287.8 millones de dólares anualmente. Colectamos aislamientos de P. infestans de Wisconsin de 2009 a 2012 y se determinó la distribución de linajes clonales y grupos de compatibilidad y sensibilidad al fungicida sistémico mefenoxam. También buscamos evaluar la utilidad actual de un análisis del locus de la aloenzima Glucosa-6-fosfato isomerasa (Gpi) para la predicción de la sensibilidad al mefenoxam, con el propósito de entregar información oportuna a los productores. En total, se colectaron 143 aislamientos de 52 localidades en 20 condados de Wisconsin del 2009 al 2012. Se identificaron tres linajes clonales, US-22, US 23 y US 24, que fueron novedosos para Wisconsin y para los EUA. El US-22 es del grupo de compatibilidad A2 y sensible a mefenoxam, con Gpi 100/122. El US-23 y el US-24 son del tipo A1, y al principio de sensibilidad intermedia al mefenoxam, con Gpi de 100/100 y 100/100/111, respectivamente. Debido a esta correlación tan estrecha y a los patrones tan singulares de Gpi para cada linaje presente, estuvimos en condiciones de predecir la sensibilidad al mefenoxam directamente de muestras usando el ensayo de las aloenzimas y proporcionar rápidamente información de manejo a los productores. Los dos grupos de compatibilidad estuvieron presentes en Wisconsin en 2009 y 2010 pero estuvieron separados espacialmente y no se detectó evidencia de recombinación sexual o persistencia en el suelo. La presencia de nuevos linajes clonales de P. infestans en Wisconsin indica la necesidad de monitoreo continuo y cercano del tizón tardío para facilitar la generación de información oportuna para un manejo aumentado a corto y largo plazo del tizón tardío.

First Report of Late Blight Caused by Phytophthora infestans Clonal Lineage US-23 on Tomato and Potato in Wisconsin, United States

Kudzu (Pueraria spp.) is an accessory host for soybean rust (SBR) (caused by Phakopsora pachyrhizi) that is widespread throughout the southeastern United States. An expanded survey of kudzu sites was conducted in 2008 to determine the proportion of natural resistance in the north-Florida kudzu population. Of the 139 sites evaluated, approximately 18% were found to be free of SBR infection, while 23% had reduced sporulation. Ten accessions of kudzu from north-central Florida were characterized for their response to challenge by a single isolate of P. pachyrhizi under laboratory conditions. Three outcomes were observed: tan lesions with profuse sporulation (susceptible); reddish-brown lesions with delayed, reduced sporulation (resistant); and an immune response in which no lesions developed (immune). Of the 10 accessions, 6 were susceptible, 3 were immune, and 1 was resistant. Cytological examination revealed that resistant interactions were typified by early onset of a multicell hypersensitive response (HR) while typical immune interactions were the result of cell wall depositions that blocked penetration in combination with early onset of the HR. Quantitative real-time polymerase chain reaction was performed to determine the extent of colonization. After 15 days, there was 10-fold less P. pachyrhizi DNA present in resistant compared with susceptible kudzu, while the amount of P. pachyrhizi DNA present in the immune kudzu was below the detection level. Susceptible kudzu had approximately half the amount of P. pachyrhizi DNA present when compared with a susceptible soybean cultivar.

Potato Common Scab: a Review of the Causal Pathogens, Management Practices, Varietal Resistance Screening Methods, and Host Resistance

Potato (Solanum tuberosum) crops are grown on over 25,090 ha in Wisconsin annually. Late blight, caused by Phytophthora infestans (Mont.) deBary, is a potentially devastating disease that affects tomato and potato crops in Wisconsin every few years when inoculum is introduced and weather conditions favor disease. Incidence and severity of late blight are highly variable in these few years due to differences in pathogen clonal lineages, their timing and means of introduction, and weather conditions. Prevention of this disease through prophylactic fungicide application can cost producers millions of dollars annually in additional chemical, fuel, and labor expenses. Populations of P. infestans in the U.S. have recently undergone significant genetic change, resulting in isolates with unique clonal lineages and epidemiological characteristics (1). In 2010, late blight epidemics were of low severity in discrete portions of a few fields and were seen exclusively on potato in two counties of central Wisconsin. Symptoms included water-soaked to dark brown circular lesions with pale green haloes accompanied by white fuzzy pathogen sporulation typically on leaf undersides in high humidity conditions. Infected plants were collected by professional crop consultants and submitted to the authors at the University of Wisconsin Vegetable Pathology Laboratory in Madison, Wisconsin. Eight isolates of P. infestans were generated from individual leaf samples, representing separate fields, by removing sporangia from sporulating lesions and placing onto Rye A agar amended with rifampicin and ampicillin. Axenic, single zoospore-derived cultures of isolates were generated from parent cultures and maintained on Rye A agar for further characterization. Mycelium was coenocytic with hyphal diameter of 5 to 8 μm (n = 50). Sporangia were limoniform to ovoid, semi- to fully papillate, caducous, had short pedicels, and were 36.22 × 19.11 μm (height × width; n = 50). The average length-width ratio was 1.91. Allozyme banding patterns at the glucose-6-phosphate isomerase (Gpi) locus indicated a 100/100/111 profile, consistent with the US-24 clonal lineage (3,4). Mating type assays confirmed the isolates to be A1 and intermediate insensitivity to mefenoxam was observed in vitro (4). Genomic DNA was extracted with a phenol:chloroform:isoamyl alcohol solution and restriction fragment length polymorphism (RFLP) analysis was performed using the RG-57 probe on a representative isolate and resulted in banding patterns consistent with US-24 (2,3). Clonal lineages of P. infestans documented in Wisconsin in previous epidemics included US-8 in the mid-1990s and US-1 in the 1970s. The US-24 (A1) clonal lineage was very widespread in the U.S. in 2010 and its presence in Wisconsin in the same year as identification of US-22 (A2) posed great concern for potential sexual recombination, oospore production, and soil persistence. Fortunately, the opposite mating types were separated spatiotemporally. To the best of our knowledge, this is the first report of the P. infestans clonal lineage US-24 causing late blight on potato in Wisconsin. References: (1) K. Deahl. (Abstr.) Phytopathology 100:S161, 2010. (2) S. B. Goodwin et al. Curr. Genet. 22:107, 1992. (3) Hu et al. Plant Dis. 96:1323, 2012. (4) A. C. Seidl and A. J. Gevens. (Abstr.) Phytopathology 101:S162, 2011.

First Report of Late Blight Caused by Phytophthora infestans Clonal Lineage US-22 on Tomato and Potato in Wisconsin

Tomato (Solanum lycopersicum) and potato (S. tuberosum) crops are grown on over 67,000 acres (27,114 ha) in Wisconsin annually. Late blight, caused by Phytophthora infestans (Mont.) deBary, is a potentially devastating disease that affects tomato and potato crops in Wisconsin every few years when inoculum is introduced and weather conditions favor disease. Incidence and severity of late blight are highly variable in these few years due to differences in pathogen clonal lineages, their timing and means of introduction, and weather conditions. Prevention of this disease through preventative application of fungicides can cost producers millions of dollars per year in additional chemical, fuel, and labor expenses. In 2009, late blight caused by P. infestans clonal lineage US-23 was observed on potato very late in the season in Vernon County, southwestern Wisconsin, in very low incidence and severity. In 2010, US-23 again appeared but on tomato in two southeastern Wisconsin counties, Waukesha and Ozaukee, again in low incidence and severity. Clonal lineages of P. infestans documented in Wisconsin in previous epidemics included US-8 in the mid-1990s and US-1 in the 1970s. Populations of P. infestans in the United States have recently undergone significant genetic change, resulting in isolates with unique clonal lineages and epidemiological characteristics (1). Foliar symptoms included water-soaked to dark brown circular lesions with pale green haloes accompanied by white pathogen sporulation. On tomato fruit, lesions were firm, sunken, and brown. Isolates of P. infestans were generated from field-infected tomato and potato foliar and fruit tissues collected by the authors and professional crop consultants. In initial pathogen confirmation analysis in 2009, three isolates of P. infestans were generated from one potato plant exhibiting multiple lesions from one of eight fields tested by placing infected leaf excisions onto Rye A agar amended with rifampicin and ampicillin. Axenic, single zoospore-derived cultures of isolates were generated from parent cultures and maintained on Rye A agar for further characterization. In 2010, three US-23 isolates were recovered from three locations (two counties), out of 20 fields tested. Mycelium was coenocytic with hyphal diameter of 5 to 8 μm (n = 50). Sporangia were limoniform or ovoid, semi to fully papillate, caducous, had short pedicels, and were 26.16 μm high × 18.17 μm wide (n = 50). The average length/width ratio was 1.42. Allozyme banding patterns at the glucose-6-phosphate isomerase (Gpi) locus indicated a 100/100 profile, consistent with the US-23 clonal lineage (3) Mating type assays confirmed the isolates to be A1 and in vitro intermediate mefenoxam sensitivity was observed (4). Genomic DNA was extracted with a phenol/chloroform/isoamyl alcohol solution and RFLP analysis was performed using the RG-57 probe on a representative isolate and resulted in banding patterns consistent with US-23 (2,3). The P. infestans clonal lineage US-23 was present in epidemics in 2009 and 2010 in the United States. Disease symptoms associated with US-23 were observed exclusively on potato in 2009 and on tomato in 2010 in Wisconsin. To our knowledge, this is the first report of P. infestans clonal lineage US-23 causing late blight on tomato and potato in Wisconsin and represents a change in the composition of the pathogen population from previous epidemic years. References: (1) K. Deahl. (Abstr.) Phytopathology 100:S161, 2010. (2) S. B. Goodwin et al. Curr. Genet. 22:107, 1992. (3) Hu et al. Plant Dis. 96:1323, 2012. (4) A. C. Seidl and A. J. Gevens. (Abstr.) Phytopathology 101(suppl.):S162, 2011.