Characteristics of Resistance to Phytophthora Root and Crown Rot in Cucurbita pepo.

Root and crown rot incited by Phytophthora capsici causes considerable annual losses in squash-producing regions in the United States. 'Spineless Perfection' zucchini and 'Cougar' straightneck squash (Cucurbita pepo L.), partially resistant and susceptible to root and crown rot, respectively, were investigated for differences in root and crown physical factors and the histology of crown infection by P. capsici. The pH and titratable acidity of healthy root and crown tissue from tissue extracts were not significantly different between cultivars (P = 0.05). Crude fiber content (%) of blended and oven-dried root and crown tissue from healthy plants was similar between cultivars. However, dry matter (%) was highest for Cougar (P = 0.05). Colonies of P. capsici grown from mycelial plugs in root exudates collected from each cultivar were similar in diameter. Whole mounts and histological sections of healthy and infected crown tissue revealed that vascular bundles and metaxylem vessels were more abundant in crowns of Spineless Perfection than Cougar. Twelve to 48 h post inoculation (hpi), mycelia in the crown of each cultivar was limited to the cortex and hypodermal tissue. By 72 hpi, hyphae were observed in the cortex and endodermal tissue of Cougar and were concentrated in the phloem and parenchyma cells surrounding vascular bundles. Mycelia were limited to the outer cortex in Spineless Perfection. Mycelia and occluding material were present in the majority of metaxylem vessels of Cougar but not Spineless Perfection at 92 hpi; dissolution of parenchyma cells surrounding vascular bundles was apparent in Cougar. The vascular occlusions observed in Cougar may be responsible for plant wilting, a common disease symptom. Additional straightneck, crookneck, scallop, and acorn squash (C. pepo ssp. ovifera), and zucchini, marrow, and pumpkin (C. pepo ssp. pepo) cultivars were evaluated in a greenhouse study for resistance to root and crown rot. Cucurbita pepo ssp. ovifera cultivars were significantly more susceptible than C. pepo ssp. pepo to root and crown rot (P < 0.0001). Growing C. pepo ssp. pepo cultivars may be beneficial in an integrated Phytophthora management program.