Screening of Cucumber Plant Introduction Accessions for Resistance to Phytophthora capsici

Fruit rot in cucumber (Cucumis sativus L.) caused by Phytophthora capsici is a major concern in cucumber growing areas in the country. To screen for resistance to P. capsici, the cucumber plant introduction (PI) collection from North Central Regional Plant Introduction Station, Ames, IA (1,025 PIs), was grown in a non-replicated trial at the Horticulture Teaching and Research Center, MSU. Approximately half of the accessions were tested in 2011 and the remainder in 2012. The detached fruit method by Gevens et al. (2006) was used to screen the PI accessions but with some modifications. To increase the ability to screen larger number of fruits and to avoid manifestation of resistance due to age-related resistance (ARR), young fruits (approximately 4dpp) were evaluated and zoospore suspension, instead of agar plugs, was used as inoculum. Five to ten fruits of each PI accession were surface sterilized and inoculated with 30ul zoospore suspension with a concentration of 1x10⁵ zoospores/mL. Symptom development on each fruit was monitored daily for five days after inoculation. The disease rating used was in a scale of 1–9 with 1 as no symptoms and 9 for tissue collapse. Disease development in response to P. capsici infection showed variation in symptoms across the PIs, and examples include: no symptom, water soaking, water soaking with necrosis, extensive water soaking with necrosis, mycelial growth or both with necrosis and mycelial growth, tissue collapse with or without mycelial growth. Three trends of disease development were observed among the PIs. Fruits were either highly susceptible, exhibited delayed symptoms, or showed potential resistance to P. capsici after 5 days post inoculation (dpi). The majority of the PIs tested exhibited high susceptibility to the pathogen. The mean rating for the population was 7.1. The susceptible commercial variety Vlaspik had a mean rating of 8.0. In our initial screen through the collection, a small number of accessions including several from India and Turkey showed delayed symptom development compared to the rest of the PIs tested. These PIs are now being retested. One accession from Turkey (PI175693) has consistently showed reduced susceptibility to the pathogen after two seasons of screening of young fruits collected from the field and one in the greenhouse with a mean of 3.58 at 5dpi, showing localized necrosis at the point of inoculation.  Based on this initial screening, there are possible sources of resistance to P. capsici for future cucumber breeding programs.