The oomycete plant pathogen Phytophthora capsici Leonian affects the cucurbit industry annually, in some cases causing 90% to 100% crop loss (Babadoost, 2000; Meyer and Hausbeck, 2012). Michigan is a leading producer of processing squash, pumpkins, and cucumbers in the United States with more than 68,500 acres of vegetable crops susceptible to P. capsici grown annually (Anonymous, 2013). In the midwest and eastern United States, P. capsici commonly causes a fruit rot on cucurbits and is a limiting factor in production (Babadoost, 2004; Hausbeck and Lamour, 2004; McGrath, 2000; Meyer and Hausbeck, 2012). Entire truck loads of processing squash, pumpkins, and cucumbers can be rejected at the processing facility as a result of the fruit becoming infected just before or during harvest and rotting during transit (Hausbeck and Lamour, 2004). The susceptibility of all commonly cultivated cucurbits (Cafe et al., 1995; McGrath, 2000) and the rapidity with which epidemics on squash and pumpkin fruit can develop make growers vulnerable. Specifically, in Michigan, fruit rot of processing cucurbits can be a major issue (Hausbeck and Lamour, 2004; Meyer and Hausbeck, 2012) as well as crown and root rots of squash and other vegetables (Hausbeck and Lamour, 2004; Meyer and Hausbeck, 2012; Quesada-Ocampo and Hausbeck, 2010). Young fruit are especially susceptible during the first week after anthesis. Gevens et al. (2006) observed in a detached fruit assay a high susceptibility to P. capsici in cucumber fruit within 7 dpp. Other researchers (Ando et al., 2009; Hausbeck and Lamour, 2004; Meyer and Hausbeck, 2012) noted that diverse cucurbit fruit including squash, pumpkin, melon, and cucumber were most susceptible to P. capsici at 3 dpp. Protecting the fruit throughout development is crucial but is difficult to achieve as a result of fungicide cost, a dense foliar canopy, and the long maturation time needed by some cucurbit cultivars. Depending on the nature of plant types (i.e., bush or vining), the fruit may lay directly on bare soil (Ando and Grumet, 2006), increasing the chances of P. capsici infection. Even when cucurbits are grown on black polyethylene plastic, vining cucurbit types will likely develop fruit along the vines that have trailed off of the plastic mulch. Although root and crown rot can be serious issues in certain cucurbit growing systems (Café-Filho and Duniway, 1995; Ristaino, 1991), fruit infection poses the most serious management challenge (Babadoost and Zitter, 2009).
Phytophthora capsici can survive in soil for 5 years or more through oospores in the absence of a susceptible host (Hausbeck and Lamour, 2004). This feature, along with the ability to produce large numbers of sporangia and zoospores in wet field conditions, is responsible for the pathogen’s high infection potential. Control of P. capsici is not always sufficient with the use of traditional fungicide and cultural management practices. Growers of processing cucurbits (i.e., hard squash and pie pumpkins) are limited in cultural management options because the fruit are harvested mechanically and there is a relatively low profit margin. Therefore, raised plant beds, plastic mulch, and drip irrigation are not routinely used by growers of cucurbits for processing, although they are used with limited success by growers of cucurbits for the fresh market (Jackson et al., 2010). Fungicide use can be a limiting factor economically during seasons with high disease pressure as a result of the long maturity period of hard squash and pumpkins. In addition, the vining nature of the plants quickly cover the fields’ surface making any sprays applied with a ground rig after the vines have filled in the rows difficult to achieve without damaging the crop. As a result of the difficulties encountered in managing P. capsici on processing cucurbits, alternative control methods have been sought.
Ontogenetic resistance (age-related resistance) is the ability of plants or plant organs to more aptly defend themselves against biotic and abiotic factors as they mature (Ficke et al., 2002). Cucurbit fruit develop resistance as they mature, and that resistance coincides with the completion of the fruit elongation phase, ≈1 to 3 weeks after anthesis, depending on species (Ando et al., 2009; Gevens et al., 2006). This form of resistance to pathogen infection may provide an opportunity to improve fungicide application timing (Ando et al., 2009; Gadoury et al., 2003; Kim et al., 1989; Roberts et al., 2000). For example, age-related resistance in commercial pickling cucumbers has facilitated optimal timing of fungicide application during the crop’s susceptible early growth stages. Fungicides are recommended to be applied when fruit are 1 inch, 3 inches, and 5 inches in length (Hausbeck and Lamour, 2004).
Identifying cucurbit accessions with Phytophthora fruit rot resistance to P. capsici would be a useful addition to cucurbit breeding programs attempting to integrate resistance into commercial varieties. Sources of root rot resistance have been identified and studied in wild cucurbit relatives (Chavez et al., 2011; Padley et al., 2008), and observations in the field indicate that commercial cucumber cultivars have tolerance to P. capsici root infection (Hausbeck and Lamour, 2004). However, fruit rot resistance has yet to be identified. Although fungicide and cultural management options have expanded, host resistance is especially desirable for low-input systems. The objectives of this study were 1) to identify resistance to fruit rot in wild germplasm previously identified as having crown/root rot resistance; 2) to test ontogenetic fruit resistance to two P.capsici isolates 7 to 10 dpp and 21 to 24 dpp in 10 C. moschata and C. pepo accessions; and 3) to determine if resistance correlates with changes in fruit size and pericarp thickness.
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