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Gray leaf spot (GLS), caused by Cercospora zeae-maydis and Cercospora zeina, is one of the most important diseases of maize worldwide. The pathogen has a necrotrophic lifestyle and no major genes are known for GLS. Quantitative resistance, although poorly understood, is important for GLS management. We used genetic mapping to refine understanding of the genetic architecture of GLS resistance and to develop hypotheses regarding the mechanisms underlying quantitative disease resistance (QDR) loci. Nested association mapping (NAM) was used to identify 16 quantitative trait loci (QTL) for QDR to GLS, including seven novel QTL, each of which demonstrated allelic series with significant effects above and below the magnitude of the B73 reference allele. Alleles at three QTL, qGLS1.04, qGLS2.09, and qGLS4.05, conferred disease reductions of greater than 10%. Interactions between loci were detected for three pairs of loci, including an interaction between iqGLS4.05 and qGLS7.03. Near-isogenic lines (NILs) were developed to confirm and fine-map three of the 16 QTL, and to develop hypotheses regarding mechanisms of resistance. qGLS1.04 was fine-mapped from an interval of 27.0 Mb to two intervals of 6.5 Mb and 5.2 Mb, consistent with the hypothesis that multiple genes underlie highly significant QTL identified by NAM. qGLS2.09, which was also associated with maturity (days to anthesis) and with resistance to southern leaf blight, was narrowed to a 4-Mb interval. The distance between major leaf veins was strongly associated with resistance to GLS at qGLS4.05. NILs for qGLS1.04 were treated with the C. zeae-maydis toxin cercosporin to test the role of host-specific toxin in QDR. Cercosporin exposure increased expression of a putative flavin-monooxygenase (FMO) gene, a candidate detoxification-related gene underlying qGLS1.04. This integrated approach to confirming QTL and characterizing the potential underlying mechanisms advances the understanding of QDR and will facilitate the development of resistant varieties.

Identity of quantitative trait loci (QTL) governing resistance to fusarium head blight (FHB) initial infection (type I), spread (type II), kernel infection, and deoxynivalenol (DON) accumulation was characterized in Chinese wheat line W14. Ninety-six double-haploid lines derived from a cross of W14 · ÕPion2684Õ were evaluated for FHB resistance in two greenhouse and one field experiment. Two known major QTL were validated on chromosomes 3BS and 5AS in W14 using the composite interval mapping method. The 3BS QTL had a larger effect on resistance than the 5AS QTL in the greenhouse experiments, whereas, the 5AS QTL had a larger effect in the field experiment. These two QTL together explained 33%, 35%, and 31% of the total phenotypic variation for disease spread, kernel infection, and DON concentration in the greenhouse experiments, respectively. In the field experiment, the two QTL explained 34% and 26% of the total phenotypic variation for FHB incidence and severity, respectively. W14 has both QTL, which confer reduced initial infection, disease spread, kernel infection, and DON accumulation. Therefore, marker-assisted selection (MAS) for both QTL should be implemented in incorporating W14 resistance into adapted backgrounds. Flanking markers Xbarc133 and Xgwm493 on 3BS and Xbarc117 and Xbarc56 on 5AS are suggested for MAS.

Stringent standards of water quality have prompted many horticultural enterprises to limit pollutant discharge associated with nutrient and pesticide applications. Collecting and recycling effluent is a method that has been implemented by many operations to contain pollutants; however, plant pathogens may be spread through recycled effluent. In this study, Phytophthora and Pythium spp. present in a water-recycling irrigation system at a perennial container nursery in southwestern Virginia were characterized using filtering and baiting techniques with two selective media. Members of Phytophthora were identified to species, whereas Pythium spp. were identified to genus only. Pythium spp. were recovered more frequently and in greater numbers than Phytophthora spp. Phytophthora capsici, P. citricola, P. citrophthora, P. cryptogea, P. drechsleri, and P. nicotianae were recovered in filtering assays. Only P. cryptogea and P. drechsleri were identified from baits placed on the surface of the irrigation reservoir, whereas P. cactorum, P. capsici, P. citricola, P. citrophthora, P. cryptogea, and P. drechsleri were recovered at depths, specifically at 1 and 1.5 m. This research provides data for development of detection technology and management practices for plant pathogens in irrigation water and may lead to improvements in conventional assay protocols.

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Existing crop monitoring programs determine the incidence and distribution of plant diseases and pathogens and assess the damage caused within a crop production region. These programs have traditionally used observed or predicted disease and pathogen data and environmental information to prescribe management practices that minimize crop loss. Monitoring programs are especially important for crops with broad geographic distribution or for diseases that can cause rapid and great economic losses. Successful monitoring programs have been developed for several plant diseases, including downy mildew of cucurbits, Fusarium head blight of wheat, potato late blight, and rusts of cereal crops. A recent example of a successful disease-monitoring program for an economically important crop is the soybean rust (SBR) monitoring effort within North America. SBR, caused by the fungus Phakopsora pachyrhizi, was first identified in the continental United States in November 2004. SBR causes moderate to severe yield losses globally. The fungus produces foliar lesions on soybean (Glycine max) and other legume hosts. P. pachyrhizi diverts nutrients from the host to its own growth and reproduction. The lesions also reduce photosynthetic area. Uredinia rupture the host epidermis and diminish stomatal regulation of transpiration to cause tissue desiccation and premature defoliation. Severe soybean yield losses can occur if plants defoliate during the mid-reproductive growth stages. The rapid response to the threat of SBR in North America resulted in an unprecedented amount of information dissemination and the development of a real-time, publicly available monitoring and prediction system known as the Soybean Rust-Pest Information Platform for Extension and Education (SBR-PIPE). The objectives of this article are (i) to highlight the successful response effort to SBR in North America, and (ii) to introduce researchers to the quantity and type of data generated by SBR-PIPE. Data from this system may now be used to answer questions about the biology, ecology, and epidemiology of an important pathogen and disease of soybean.

Gray leaf spot of maize (Zea mays L.), caused by Cercospora zeaemaydis Tehon & Daniels, is a limiting factor in some areas where continuous minimum tillage practices are followed. The inheritance of reaction to this pathogen was studied using a diallel of 14 elite inbreds. The 91 single crosses were grown in field nurseries in Montgomery and Wythe counties, Virginia, in 1987 and 1988 using randomized complete‐block designs. Data on gray leaf spot disease ratings, plant lodging, grain yield, and grain moisture were taken during the growing season. Analyses of variance and combining ability analyses were made on these data. Resistance was found to be highly heritable and controlled by additive gene action. Many widely used inbreds (B73, Pa91, Mo17, and A632) were susceptible to gray leaf spot, explaining why many of the commercially available hybrids do not show high levels of resistance. Inbreds that provide high yielding, resistant, and agronomically acceptable hybrids were identified (B68, NC250, Pa875, Va14, Va17, and Va85), and several hybrids between these lines had high levels of resistance, high yield, and good general agronomic characters (B68 ✕ KB1250, KB1250 ✕ Pa875, and NC250 × Pa875). The data suggest that inbred lines are available to produce agronomically acceptable hybrids with high levels of resistance to gray leaf spot. These inbreds could serve as a basis for breeding programs where leaf spot resistance is emphasized.