Powdery Mildew Caused by Erysiphe pisi on Alfalfa in Idaho and Oregon

Molecular and cytological responses of Medicago truncatula to Erysiphe pisi

SUMMARY Powdery mildew is an economically important disease in a number of crop legumes; however, little is known about resistance to the disease in these species. To gain a better understanding of the genetics of resistance and plant responses to powdery mildew in legumes, we developed a pathosystem with Medicago truncatula and Erysiphe pisi. Screening accessions of M. truncatula identified genotypes that are highly susceptible, moderately resistant and highly resistant to the fungus. In the highly resistant genotype, fungal growth was arrested after appressorium development with no colony formation, while in the moderately resistant genotype a small number of colonies formed. Both resistant and moderately resistant genotypes produced hydrogen peroxide and fluorescent compounds at pathogen penetration sites, consistent with a hypersensitive response (HR), although the response was delayed in the moderately resistant genotype. Very little hydrogen peroxide or fluorescence was detected in the susceptible accession. Microarray analysis of E. pisi-induced early transcriptional changes detected 55 genes associated with the basal defence response that were similarly regulated in all three genotypes. These included pathogenesis-related genes and other genes involved in defence, signal transduction, senescence, cell wall metabolism and abiotic stress. Genes associated with the HR response included flavonoid pathway genes, and others involved in transport, transcription regulation and signal transduction. A total of 34 potentially novel unknown genes, including two legume-specific genes, were identified in both the basal response and the HR categories. Potential binding sites for two defence-related transcription regulators, Myb and Whirly, were identified in promoter regions of induced genes, and four novel motifs were found in promoter regions of genes repressed in the resistant interaction.

Reactions of Accessions in the Annual Medicago Core Germ Plasm Collection to Erysiphe pisi

Reaction to Erysiphe pisi was evaluated in the annual Medicago core germ plasm collection of 199 USDA Plant Introduction accessions from 33 species. Seedlings at the first trifoliolate leaf stage were inoculated with conidia and maintained in growth chambers for 21 days at 20°C, 70 to 90% relative humidity, and a 12-h photoperiod. Beginning 4 days after inoculation, plants with the first sign of powdery mildew were recorded and removed daily. Results for each accession were presented as the percentage of plants mildew-free 21 days after inoculation and also as area under the disease progress curve (AUDPC) values expressed as standardized proportion/days. AUDPC values were useful to account for length of latent period (number of days from inoculation until first sign of mildew). Thirty-two accessions from 14 species included plants mildew-free 21 days after inoculation. Mildew-free plants in those 32 accessions ranged from 100% in M. heyniana PI 537136 and M. truncatula PI 292436 down to 1% in six accessions. All plants in 87 accessions, which included all accessions of 11 species, had powdery mildew 4 days after inoculation and thus expressed no resistance. The remaining 80 accessions included slow-mildewing plants with latent periods of 5 to 14 days. This slow-mildewing trait may be a useful source of resistance to E. pisi.