Michigan ranks second in the nation in the diversity of vegetables that are produced with California ranking first. With a total revenue of $192 billion in 2021, Michigan ranks seventh nationally. The vegetable industry is threatened by diseases caused by the oomycete pathogens Phytophthora capsici and Phytophthora infestans. P. capsici has a wide host range and may cause total crop loss. P. capsici-susceptible hosts represent 67% of the total vegetable production in Michigan and include crops in the Cucurbitaceae, Fabaceae, and Solanaceae families. While P. infestans has a narrower host range than P. capsici, it can cause significant loss to vegetables in the Solanaceae family. This project aimed to identify commercial cultivars of hard squash and tomatoes resistant to P. capsici and P. infestans, respectively, to improve disease management for Michigan's conventional and organic vegetable growers. In a two-year field study, we compared 12 commercial cultivars of hard squash, four Butternut types (Cucurbita moschata), two 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. To evaluate crown rot, the plants were inoculated with P. capsici in replicated field trials. The C. moschata cultivars had significantly less plant death for both years (<15%) than other cultivars. Within the C. maxima 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, dry matter, and smaller fruit weight than ‘NK 580’ each year. In 2018 and 2019, tomato cultivars were tested under growth chamber, greenhouse, and field conditions. Plants were inoculated with an isolate of P. infestans clonal lineage US-23. In the growth-chamber study, the lowest disease severity at the final assessment (<20%) was observed in ‘Matt's Wild Cherry’ and ‘Tomato Stellar,’ with significantly less disease than all other cultivars. The rAUDPC data indicated these cultivars were significantly less susceptible than all others except for 'Mountain Magic' which was similar to ‘Matt’s Wild Cherry.’ In greenhouse experiment 1, ‘Mountain Magic,’ ‘Tomato Stellar,’ and ‘Mountain Merit’ had the least foliar disease severity (0 – 8.0%) for each observation date. For the field experiment, eleven of the cultivars included in the study had foliar disease severity <5% on the final observation date. According to the rAUDPC data, ‘Iron Lady’ and ‘Defiant’ had the lowest disease severity but were similar to ‘Lemon Drop.’ ‘Lemon Drop,’ ‘Cherry Bomb,’ and ‘Fantastico’ were similar; ‘Plum Regal’ was similar to ‘Cherry Bomb’ and ‘Fantastico.’ In greenhouse experiment 2, ‘Iron Lady’ and ‘Defiant’ had the lowest disease severity but were similar to ‘Lemon Drop,’ according to the rAUDPC data. Results from this study can assist growers in selecting cultivars with genetic resistance which can be employed in conjunction with biorational and conventional fungicides.
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