Commercial Hard Squash Cultivars Exhibit Differences in Susceptibility to Phytophthora Crown Rot

Crown rot and fruit rot, caused by the oomycete Phytophthora capsici, limit yield and quality of hard squash grown for processing. In a two-year field study, we compared four Butternut types (Cucurbita moschata), two Delicious varietal Hubbard types (Cucurbita maxima), and six Kabocha types (C. maxima) for crown rot resistance, and fruit characteristics relevant to processing including mesocarp soluble solids, percent dry matter, and average fruit weight in 2019 and 2020. To evaluate crown rot, plants were inoculated in replicated field trials. The C. moschata cultivars ‘Dickinson’, ‘Buckskin’, ‘New England Cheddar’, and ‘Ultra Butternut’ had significantly less plant death for both years (<15%) than other cultivars. Significantly fewer 'Thunder' plants died in 2020 (59.6%) compared to the Kabocha cultivars 'Sweet Mama' (>94.2%), 'Delica' (>92.3%), and 'Sunshine' (>90.4%) and had a lower rAUDPC value (≤22.4) in both years. In non-inoculated field trials, mature fruits were assessed for fruit characteristics. Of the resistant C. moschata cultivars, only ‘Ultra Butternut’ exhibited similar °Brix than ‘NK 580’ in both years and had comparable or greater dry matter and fruit weight. Kabocha cultivars with moderate crown rot susceptibility (i.e., ‘Thunder’) exhibited higher °Brix and dry matter and smaller fruit weight compared to ‘NK 580’ each year. Integrated management programs can be advanced by selecting cultivars with crown rot resistance. Further evaluation of desirable yield and quality characteristics of the resistant cultivars could accelerate acceptance of these cultivars by growers and processors.