Characterizing the Dynamics of Virulence and Fungicide Resistance of Phytophthora capsici in Michigan Vegetable Fields Reveals Loci Associated with Virulence.

The oomycete Phytophthora capsici is a destructive pathogen infecting more than 50 plant species and is one of the most serious threats to cucurbit production. Phytophthora blight caused by Phytophthora capsici can affect all plant growth stages, and fungicides and cultural controls are used to limit losses. Dissecting pathogen virulence and fungicide resistance can provide insights into pathogenic mechanisms and inform effective management practices to control P. capsici. In this study, we assessed virulence, mefenoxam sensitivity, and genetic diversity of nine P. capsici populations collected from Cucurbitaceae, Solanaceae, and Fabaceae host families in Michigan from 2002 to 2016. We developed 992 simple sequence repeats (SSRs) in the P. capsici genome and identified 60 SSRs located within or close to RXLR-class (Arginine-any amino acid-Leucine-Arginine) effectors and 29 SSRs within or close to effector CRN (CRinkling and Necrosis) family protein, which represent 62 RXLR and 34 putative CRNs. Population structure analysis shows that mefenoxam resistance was not associated with the year of collection, host type, or location, but there were significant differences in virulence among the populations. Using the general linear model and mixed linear model-based association analyses with all effector-related SSR markers, we identified four SSR markers significantly associated with at least one of the virulence-related parameters. Of these, one (Pce_SC18) was in a predicted CRN effector and had high identity with the putative PhCRN37 effector in the pathogen Plasmopara halstedii, which can be further verified for virulence identification in P. capsici.

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.

Transport and Retention of Phytophthora capsici Zoospores in Saturated Porous Media.

Phytophthora capsici is an important plant pathogen capable of infecting several major vegetable crops. Water-induced P. capsici transport is considered to be a significant contributor to disease outbreaks and subsequent crop loss. However, little is known about factors controlling P. capsici zoospore transport in porous media, thus impeding our understanding of their environmental dispersal and development of filtration techniques for contaminated irrigation water. This study investigated the transport and retention of P. capsici zoospores in saturated columns packed with iron-oxide-coated sand (IOCS) or uncoated sand in Na(+) or Ca(2+) background solution at pH 7.7 ± 0.5 or 4.0 ± 0.3, in combination with XDLVO interaction energy calculations and microscopic visualizations. Significantly more encysted zoospores were retained in IOCS than in uncoated sand, and at pH 4.0 than at pH 7.7, which likely resulted from increased electrostatic attraction between zoospores and grain surface. At pH 7.7, up to 99% and 96% of the encysted zoospores were removed in IOCS and uncoated sand, respectively, due to a combination of strong surface attachment, pore straining, and adhesive interactions. Motile biflagellate zoospores were more readily transported than encysted zoospores, thus posing a greater dispersal and infection risk. This study has broad implications in environmental transport of Phytophthora zoospores in natural soils as well as in cost-effective engineered filtration systems.

Pathogenicity of Phytophthora capsici to Brassica Vegetable Crops and Biofumigation Cover Crops (Brassica spp.).

The soilborne oomycete Phytophthora capsici causes root, crown, and fruit rot of many vegetable crops in the Cucurbitaceae and Solanaceae families. P. capsici is a persistent problem in vegetable fields due to long-lived oospores that survive in soil and resist weathering and degradation. Vegetable crops in the Brassicaceae family have been considered nonhosts of P. capsici and are planted as rotational crops in infested fields. Brassica spp. are also grown as biofumigation cover crops to reduce inoculum levels of P. capsici and other soilborne pathogens, and this use has increased concurrent with restrictions on soil fumigation. Oriental mustard (Brassica juncea), oilseed rape (B. napus), and oilseed radish (Raphanus sativus var. oleiferus) contain high levels of glucosinolates and are widely recommended for biofumigation and as cover crops. The objective of this study was to evaluate vegetables and biofumigation cover crops in the Brassicaceae family for susceptibility to P. capsici. Brassica spp. used as vegetable crops and for biofumigation were grown in P. capsici-infested potting soil in the greenhouse and disease incidence and severity were recorded. In greenhouse trials, infection by the pathogen reduced the fresh weight of all Brassica spp. tested and resulted in plant death of 44% of plants of B. juncea 'Pacific Gold'. P. capsici isolates exhibited differences in virulence (P < 0.0001), and were reisolated from the roots of all Brassica spp. included in the study. The biofumigation cover crop Pacific Gold mustard may not reduce populations of P. capsici in soil and, instead, may sustain or increase pathogen levels. Further research is necessary to test this possibility under field conditions.