491 J. AMER. SOC. HORT. SCI. 131(4):491–498. 2006. J. AMER. SOC. HORT. SCI. 131(4):491–498. 2006. Evaluation of Altered Cucumber Plant Architecture as a Means to Reduce Phytophthora capsici Disease Incidence on Cucumber Fruit Kaori Ando and Rebecca Grumet 1 Department of Horticulture and Graduate Program in Plant Breeding and Genetics, Michigan State University, East Lansing, MI 48824 ABSTRACT. Fruit rot induced by Phytophthora capsici Leonian is an increasingly serious disease affecting pickling cucumber (Cucumis sativus L.) production in many parts of the United States. The absence of genetically resistant cultivars and rapid development of fungicide resistance makes it imperative to develop integrated disease management strategies. Cucumber fruit which come in direct contact with the soil-borne pathogen are usually located under the canopy where moist and warm conditions favor disease development. We sought to examine whether variations in plant architecture traits that influence canopy structure or fruit contact with the soil could make conditions less favorable for disease development. As an extreme test for whether an altered canopy could facilitate P. capsici control, we tested the effect of increased row spacing and trellis culture on disease occurrence in the pickling cucumber ‘Vlaspik’. Tempera- ture under the canopy was lowest in trellis plots, intermediate in increased spacing plots, and highest in control plots. Disease occurrence in the trellis plots was significantly lower than in other treatments, indicating that preventing fruit contact with the soil reduced disease occurrence. The effect of currently available variation in plant architecture was tested using nearly-isogenic genotypes varying for indeterminate (De), determinate (de), standard leaf (LL), and little leaf (ll) traits. Plants with standard architecture had higher peak mid-day temperatures under the canopy and greater levels of P. capsici infection; however, levels of disease occurrence were high for all genotypes. Screening a collection of ≈150 diverse cucumber accessions identified to serve as a representative sample of the germplasm, revealed variation for an array of architectural traits including main stem length, internode length, leaf length and width, and number of branches; values for ‘Vlaspik’ were in the middle of the distribution. Plant architectures that may allow for more open canopies, including reduced branching habit and compact growth, were tested for disease incidence. One of the compact lines (PI 308916), which had a tendency to hold young fruit off the ground, exhibited lower disease occurrence. The reduced disease occurrence was not due to genetic resistance, suggesting that architecture which allows less contact of fruit with the soil could be useful for P. capsici control for pickling cucumber. Fruit rot caused by the oomycete pathogen, Phytophthora capsici, is currently one of the most serious diseases affecting cucumber production in many parts of the United States (Haus- beck and Lamour, 2004). In Michigan, the top pickling cucumber producing state, pickling cucumber acreage has increased over the past several years (Michigan Dept. of Agriculture, 2001), however, maintaining and increasing yield has become difficult due to losses caused by P. capsici. Phytophthora capsici was first reported by Leonian (1922) as a host specific pathogen of chili pepper (Capsicum annum L.) oc- curring in New Mexico. Later, it was found to infect a wide variety of woody plant and vegetable hosts (Erwin and Ribeiro, 1996). Although incidence of cucumber infection was first reported in Colorado in 1936 (Kreutzer, 1937), it has only recently become a common problem in cucumber production areas (Babadoost, 2004; Hausbeck and Lamour, 2004). Once cucumber fields are infected by P. capsici, severe yield loss due to infection of the fruit is frequently observed (Babadoost, 2004; Hausbeck and Lamour, 2004). Cucumber fruit infected by P. capsici typically develop a Received for publication 17 Nov. 2005. Accepted for publication 29 Apr. 2006. We thank the Pickle Seed Research Fund and MSU-GREEEN for support of this project. We thank Dr. Jack Staub, the Plant Introduction Station (Ames, Iowa), and Seminis Vegetable Seed Inc. for providing seed. We thank Steve Suzura and Dr. Mary Hausbeck and her lab members, R. Bounds, B. Cortright, A. Gevens, and B. Harlan, and for their help in field and fruit screening experiments and Bill Chase, Gary Winchell, and Ron Gnagney for farm assistance. We also thank Drs. Jim Kelly and Mary Hausbeck for their helpful reviews of the manuscript. 1 Corresponding author: phone: 517-355-5191 xt 1431; fax: 517-353-0890; email: grumet@msu.edu depressed fruit surface with a water soaked appearance followed by white powdery mycelium covering the affected region. Optimal growth conditions for P. capsici include a warm and wet environment, particularly around 25 to 30 ºC (Hausbeck and Lamour, 2004). Phytophthora capsici sporangia have a recogniz- able lemon shape and produce asexual motile zoospores which are the primary cause of infection during the growing season, since they can be spread by irrigation, rain, and free surface water on plants (Erwin and Ribeiro, 1996; Hausbeck and Lamour, 2004). In addition, P. capsici produces a unique thick walled structure called an oospore, which enables it to overwinter. Since oospores are able to survive in the soil for more than 5 years, crop rotation becomes impractical, contributing to the difficulty in controlling this disease (Lamour and Hausbeck, 2001a, 2002, 2003). Heavy dependence on chemical control has resulted in the development of resistance, as has been identified with respect to mefanoxam (Ridomil Gold; Novartis, Greensboro, N.C.), a fungicide commonly used in Michigan (Lamour and Hausbeck 2000, 2001a, 2001b). In addition, since many farms do not practice rotation with non-susceptible crops, spore populations can increase, and sexual recombination among mating types can allow for transfer of resistance genes (Lamour and Hausbeck, 2000, 2001a, 2001b, 2002). Gevens et al. (2006) screened 482 C. sativus accessions for fruit resistance, but no significant source of resistance has been identified to date. Consequently these condi- tions make it necessary to seek alternative control strategies. Field observations show that cucumber fruit are susceptible to P. capsici, while roots or crowns are much less susceptible. Fields will frequently appear healthy at the time of harvest as