The soilborne oomycete Phytophthora capsici causes root, crown, and fruit rot of economically important vegetable crops in the Cucurbitaceae and Solanaceae families. P. capsici is a persistent problem due to long-lived oospores in soil and plant debris. The effects of host resistance and fungicides on P. capsici and susceptibility of Brassica spp. to the pathogen were investigated. Eight commercial pepper cultivars and experimental breeding entries (collectively termed entries) were evaluated for field resistance to P. capsici with or without fungicides in 2014 and 2015. The susceptible cultivar, Camelot X3R, had > 90% wilt and plant death in the untreated plot each year. All other entries had < 10% of plants with root rot symptoms in 2014, however, ‘Aristotle’, ‘AP4835’, ‘13SE12671’, and ‘AP4841’ displayed 10 to 30% wilt and plant death in 2015. Using fungicides reduced disease incidence and improved yield compared to the untreated plot but there was no entry x fungicide interaction. Marketable yield for untreated ‘Paladin’ was significantly higher than other entries in both years. Fruit size for ‘13SE12671’ was largest among entries; significantly larger than ‘Camelot X3R’, ‘AP4839’, ‘AP4841’, and ‘Aristotle’ in 2014 and ‘Camelot X3R’, ‘AP4839’, ‘AP4841’ and ‘Paladin’ in 2015 (P = 0.05 ).Brassica cover crops are recommended as biofumigants to reduce soil infestation and have not been reported as hosts for P. capsici. Ten Brassica spp. vegetable and biofumigation cover crops were grown in the greenhouse in P. capsici infested potting medium. Disease incidence, severity, and foliar fresh weight were recorded, and roots of symptomatic plants were sampled. All Brassica spp. tested displayed disease symptoms. ‘Bronco’ cabbage, ‘Pacific Gold’ mustard, and ‘Groundhog’ radish (P < 0.05) had significant reductions in fresh weight. P. capsici was re-isolated from the roots of all Brassica spp. tested. ‘Spineless Perfection’ zucchini and ‘Cougar’ straightneck squash considered to be less and more susceptible to root and crown rot, respectively, were investigated for differences in root and crown physical factors; the histology of crown infection by P. capsici was also investigated. The pH, titratable acidity, and crude fiber of healthy root and crown tissue were not significantly different between cultivars (P > 0.05). However, dry matter (%) was highest for ‘Cougar’ (P = 0.05). Whole mounts and histological sections of healthy and infected crown tissue revealed that vascular bundles and metaxylem vessels were most abundant in crowns of ‘Spineless Perfection’. Twelve to 48 hours 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’. Tyloses, 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 were apparent in ‘Cougar’. Additional squash and pumpkin (Cucurbita pepo) cultivars were evaluated in greenhouse studies for resistance to root and crown rot. Straightneck, crookneck, scallop, and acorn squash cultivars (C. pepo ssp. ovifera) were significantly more susceptible (P < 0.0001) to root and crown rot than zucchini, marrow, and pumpkin (C. pepo ssp. pepo).
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