Squash and pumpkin (Cucurbita pepo) are important crops for vegetable growers in the United States. In 2018, ≈100,000 acres of squash and pumpkin were grown with a value of $350 million [U.S. Department of Agriculture (USDA), 2019]. These crops are cultivated for food as immature fruit (summer squash) and mature fruit (winter squash and processing pumpkins), and they are also grown for fall decorations, such as jack-o’-lanterns and gourds (Paris, 2016).
Phytophthora crown and root rot, caused by the oomycete pathogen Phytophthora capsici, is a severe disease affecting squash and pumpkin. Originally described in 1922 after being identified on pepper [Capsicum annum (Leonian, 1922)], it was first reported on cucurbits (Cucurbitaceae) in 1937 (Kreutzer, 1937). Now known to infect a wide range of species, other hosts of P. capsici include all cucurbits, eggplant (Solanum melongena), tomato (Solanum lycopersicum), and snap bean (Phaseolus vulgaris) (Lamour et al., 2012; Tian and Babadoost, 2004). Phytophthora capsici is widespread in cucurbit production regions of the United States, and the incidence of disease has increased in recent years (Hausbeck and Lamour, 2004; Meyer and Hausbeck, 2012, 2013b). Squash and pumpkin plants can be affected by P. capsici during various growth stages, resulting in seedling damping off, vine and foliar blight, and fruit rot (Babadoost, 2004; Babadoost and Islam, 2003; Hausbeck and Lamour, 2004; Krasnow et al., 2014; Meyer and Hausbeck, 2013a).
The presence of both mating types of P. capsici in a field results in sexual reproduction and the generation of thick-walled oospores capable of surviving in the soil for many years (Babadoost, 2004; Carlson et al., 2017). These oospores often serve as the primary source of inoculum, germinating after a period of dormancy and initiating the asexual cycle (Granke et al., 2012; Hausbeck and Lamour, 2004). During favorable conditions, asexual reproduction by P. capsici results in rapid proliferation of the pathogen, often leading to complete yield loss during severe outbreaks (Babadoost, 2000, 2004; McGrath, 2017). Runoff from infected fields may contaminate surface water sources, such as creeks, rivers, and ponds, which then transport the pathogen long distances and lead to new epidemics (Hausbeck and Lamour, 2004; Jones et al., 2014).
Control of phytophthora crown and root rot remains a challenge for squash and pumpkin producers. Total eradication of the pathogen from an infected field is typically impossible, which has forced growers to abandon entire fields in some cases (Babadoost, 2004). Fungicides can be effective in limiting disease in squash and pumpkin, although severe crop loss can still occur despite their use when environmental conditions are favorable (Granke et al., 2012). Furthermore, insensitivity to some of the most commonly used active ingredients, such as mefenoxam and cyazofamid, has been reported in P. capsici populations (Dunn et al., 2010; Jackson et al., 2012), increasing the difficulty of controlling this disease chemically. Biocontrol efforts with Trichoderma and Bacillus species have not been shown to be effective in controlling disease (Gilardi et al., 2015). Current strategies for disease mitigation include growing less susceptible cultivars, planting non-vining cultivars in raised beds with plastic mulch to limit excessive soil moisture in the root zone, employing drip irrigation to reduce pathogen movement in surface water, and improving soil drainage with subsoil tillage and drainage tiles. Cultivars with increased resistance are highly desired for improved disease management.
To date, there are no squash or pumpkin cultivars available that are completely resistant to phytophthora crown and root rot. A previous study of 115 diverse squash and pumpkin accessions found partial resistance present in several accessions, but no accessions with complete resistance were identified (Padley et al., 2008). Additionally, cultivars have been shown to vary in their degree of susceptibility (Camp et al., 2009; Enzenbacher and Hausbeck, 2012; Meyer and Hausbeck, 2012). Crookneck summer squash and acorn squash cultivars (C. pepo ssp. ovifera) are generally more susceptible to phytophthora root and crown rot than zucchini and pumpkin cultivars (C. pepo ssp. pepo) (Krasnow et al., 2017). The range of resistance phenotypes present among squash and pumpkin accessions suggests that resistance is quantitative, controlled by many genes.
The current lack of cultivars resistant to phytophthora crown and root rot, partnered with the longevity of oospores in infested soil and the emergence of fungicide insensitivity, has resulted in severe challenges for cucurbit growers. As a result, breeding for resistance to phytophthora crown and root rot in squash and pumpkin began in 2011 at Cornell University (Ithaca, NY). Using disease assays conducted on seedlings in the greenhouse, we performed several rounds of selection to breed phytophthora-resistant (PR) lines. To determine the practical use of these lines, we concurrently initiated several trials to evaluate the performance of these lines in a field setting and in different greenhouse experiments. In this study, we describe the development of the PR lines and the results of our evaluations, which had the following objectives: 1) evaluate the resistance of the PR lines in trials in the field and the greenhouse, 2) evaluate the horticultural performance of the PR lines, and 3) evaluate the resistance of the PR lines relative to other accessions with partial resistance and against multiple isolates of P. capsici.
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