Validation of consensus quantitative trait loci associated with resistance to multiple foliar pathogens of maize.

Maize production in sub-Saharan Africa incurs serious losses to epiphytotics of foliar diseases. Quantitative trait loci conditioning partial resistance (rQTL) to infection by causal agents of gray leaf spot (GLS), northern corn leaf blight (NCLB), and maize streak have been reported. Our objectives were to identify simple-sequence repeat (SSR) molecular markers linked to consensus rQTL and one recently identified rQTL associated with GLS, and to determine their suitability as tools for selection of improved host resistance. We conducted evaluations of disease severity phenotypes in separate field nurseries, each containing 410 F2:3 families derived from a cross between maize inbred CML202 (NCLB and maize streak resistant) and VP31 (a GLS-resistant breeding line) that possess complimentary rQTL. F2:3 families were selected for resistance based on genotypic (SSR marker), phenotypic, or combined data and the selected F3:4 families were reevaluated. Phenotypic values associated with SSR markers for consensus rQTL in bins 4.08 for GLS, 5.04 for NCLB, and 1.04 for maize streak significantly reduced disease severity in both generations based on single-factor analysis of variance and marker-interval analysis. These results were consistent with the presence of homozygous resistant parent alleles, except in bin 8.06, where markers were contributed by the NCLB-susceptible parent. Only one marker associated with resistance could be confirmed in bins 2.09 (GLS) and 3.06 (NCLB), illustrating the need for more robust rQTL discovery, fine-mapping, and validation prior to undertaking marker-based selection.

Pythium delawarii--a new species isolated from soybean in Ohio.

A new species of Pythium isolated from soybean in Ohio is described. Pythium delawarii sp. nov. is characterized by globose internally proliferating sporangia, aplerotic oospores and diclinous antheridia that make broad lengthwise contact. Sporangia produce conspicuous papilla and germinate indirectly by producing zoospores via a vesicle and proliferate internally or the sporangia germinate directly with either one or more germ tubes. Pythium delawarii is pathogenic on soybean causing damping-off of seedlings. This oomycete can grow at 10-34 C with an optimum of 28 C. The sequence of the ITS1, 5.8S and ITS2 region of the rDNA did not match the sequence of any known Pythium species but was similar to P. citrinum, P. litorale and P. sterilum. P. delawarii can be distinguished from these three species based on the presence of aplerotic oospores and diclinous antheridia and the absence of hypogynous antheridia. Therefore biological, morphological and molecular data support the recognition of a new species.

Heritability and Components of Resistance to Cercospora zeae-maydis Derived from Maize Inbred VO613Y.

ABSTRACT Gray leaf spot (GLS), caused by the fungus Cercospora zeae-maydis, is one of the most important foliar diseases of maize. This study was undertaken to estimate heritability of C. zeae-maydis resistance and examine the relationship between previously identified resistance loci and certain components of resistance including incubation period, lesion number, and maximum lesion length. Partially inbred progenies arising from hybridization between maize inbred lines VO613Y (high level of partial resistance) and Pa405 (susceptible) were examined in Ohio and South Africa. Heritability estimates of resistance were calculated based on severity and incubation period values. The range of heritability estimates based on severity was broad, with values ranging from approximately 0.46 to 0.81 (mean = 0.59). Estimates of mean heritability for incubation period were lowest (0.18), indicating that this component would likely be unsuitable for selection of germ plasm intended for deployment in diverse regions. Length of GLS lesions was significantly affected by host genotype, with resistant genotypes having shorter lesions from one site in Ohio during two seasons. Genotype also had a significant effect on incubation period and lesion number; the lower values for these components also were associated with resistant genotypes. The combined action of these resistance components resulted in lower overall disease severity.

Distribution and Pathogenic Characterization of Pyrenophora tritici-repentis and Stagonospora nodorum in Ohio.

ABSTRACT To determine the distribution of Stagonospora nodorum and Pyrenophora tritici-repentis on wheat in Ohio, flag leaves with lesions were collected from wheat-producing counties in 2002 and 2003. Counties were arbitrarily grouped into seven regions. Log-linear analysis of pathogen presence within regions indicated that the presence of S. nodorum was independent of the presence of P. tritici-repentis. A logistic analysis revealed that the occurrence of both pathogens varied by region in one or both years. The aggressiveness of S. nodorum isolates was determined by inoculating two susceptible genotypes with a subsample of isolates from each region from both years. S. nodorum isolates obtained from northeast Ohio, with fewer wheat fields, were less aggressive than those from other regions. Isolates obtained from west-central Ohio, surrounded by regions with high wheat production annually, were significantly more aggressive than those obtained in the remaining five regions. Isolates from the five other regions did not differ significantly (P > 0.05) in aggressiveness. Races 1 and 2, and a few race 3 isolates, of P. tritici-repentis were detected in Ohio. The distribution of P. tritici-repentis races 1 and 2 was not associated with any region, although the prevalence of race 1 was three times greater than race 2. The rarer race 3 was associated with three dispersed regions. Results indicate that S. nodorum was the major wheat leaf-blotching pathogen. There were no positive or negative associations of S. nodorum and P. tritici-repentis or individual races of P. tritici-repentis in any of the tested regions, which indicates that neither pathogen can be used to predict the presence of the other. The isolated northeastern corner of Ohio appeared to contain isolates of S. nodorum with unique characteristics and potentially only one race of P. tritici-repentis, indicating that this area may be genetically isolated from the remaining tested areas of the state.

Reaction of Commercial Soft Red Winter Wheat Cultivars to Stagonospora nodorum in the Greenhouse and Field.

Thirteen cultivars were evaluated in the greenhouse and field for reaction to Stagonospora nodorum leaf and glume blotch. Coker 9663 exhibited consistent resistance across both greenhouse and field tests. In field plots, all cultivars tested had relatively high percentages of S. nodorum in harvested seed. Strong correlations between reactions from greenhouse and inoculated plots indicated that resistant and susceptible genotypes could be identified using either procedure. The amount of S. nodorum resistance expressed depended on the host organ assessed, but Coker 9663, and OH708 in inoculated field plots, had both leaf and glume blotch resistance. Cultivars reacted similarly in inoculated and naturally infested plots based on mean leaf or glume blotch severity, whereas severity was inconsistent between the greenhouse and naturally infested plots. Results indicated that leaf and glume blotch resistance could be identified in naturally infested plots in epidemic years, but that cultivar reactions should be confirmed in inoculated plots or greenhouse tests due to possible interference from other leaf blotch pathogens.

Effects of Choline, Betaine, and Wheat Floral Extracts on Growth of Fusarium graminearum.

Fusarium head blight has been more severe when infection occurs during anthesis, indicating that floral organs may be important infection courts. Choline acetate and glycinebetaine have been extracted from wheat and reported to be growth stimulants of Fusarium graminearum. They are hypothesized to enhance infection and tissue colonization. Growth of F. graminearum was examined on media amended with extracts from floral parts of nine wheat genotypes with various Fusarium head blight resistance levels. Results indicated no significant effect of anther, palea, or lemma extracts on radial growth when compared with unamended controls. Effects on spore germination and hyphal growth of F. graminearum by choline, betaine, and an equimolar mixture at concentrations ranging from 0.01 to 1,000 µM also were examined. Spore germination was not significantly (P ≤ 0.05) affected by choline, betaine, or a combination of the compounds compared with unamended controls. Radial hyphal growth also was not consistently affected (P ≤ 0.05) by choline or betaine when compared with controls. Equimolar mixtures of the two compounds showed significant slight reduction in growth rate at higher concentrations when compared with controls. The reduction in growth rate was due to higher concentrations of betaine. Results of this study indicate that endogenous compounds in floral parts may not be associated with wheat resistance to F. graminearum.

Evaluation of Inoculation Methods to Determine Resistance Reactions of Wheat to Fusarium graminearum.

Reliable greenhouse assays are needed to differentiate types of resistance in wheat to Fusarium graminearum. Genotypes with known field reactions were evaluated for resistance type using four greenhouse inoculation techniques. Percentage of spikelets with symptoms per spike (severity) and percentage of inoculated spikelets per spike developing symptoms were assessed 7, 10, and 14 days after inoculation (DAI). Genotypes were evaluated using disease assessments 14 DAI and area under the disease progress curve (AUDPC). Significant genotype-inoculation technique interactions for disease assessments indicated that genotypes responded differently to greenhouse inoculation techniques. The central floret injection technique used to assess resistance to spread within the spike (type II resistance) did not indicate a resistant response in genotypes with low field disease severity (putative type II). Atomizing macroconidia onto spikes, used to assess resistance to primary infection (type I resistance), indicated a resistant response in one of five genotypes with low field disease incidence (percentage of spikes with symptomatic spikelets) (putative type I). The inoculation techniques in the greenhouse were unable to differentiate between different types of partial resistance. Results indicated disparity in the ability of greenhouse inoculation methods to identify genotypes with partial resistance as observed in the field. It was concluded that atomizing conidia onto the spikes and assessing disease severity did not differentiate between resistance types. Additionally, a one-time assessment 14 DAI was determined to be as informative as multiple assessments and calculating the AUDPC.