Evaluation of winter squash and pumpkin for resistance to fruit rot

Phytophthora capsici annually threatens production of cucurbit and solanaceous crops. Long-lived oospores produced by the pathogen incite primary infection of susceptible plants when conditions are wet. Limiting the rot of winter squash and pumpkin (Cucurbita sp.) fruits is difficult due to the long maturation period when fruits are often in direct contact with infested soil.

Genetic resistance to fruit rot is not widely available within Cucurbita sp.; however, age-related resistance (ARR) to P. capsici fruit rot develops in specific cultivars during maturation.

The objective of this study was to evaluate the fruits of 12 cultivars of Cucurbita pepo, Cucurbita moschata, and Cucurbita maxima for ARR to P. capsici using a mycelial-plug inoculation method. All Cucurbita pepo and Cucurbita moschata cultivars displayed ARR; 7 days postpollination (dpp) fruits were susceptible, limited lesion development occurred on fruits 22 dpp, and lesions did not develop at 56 dpp.

Disease developed on both Cucurbita maxima cultivars tested at 7, 14, 22, and 56 dpp. Firmness of fruit exocarps was measured with a manual penetrometer. Exocarp firmness of all cultivars increased during maturation; however, there was no correlation between firmness and disease incidence among cultivars at 22 dpp (R2 = −0.01, P = 0.85). When fruits of cultivars expressing ARR at 22 dpp were wounded before inoculation, fruit rot developed.

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