ARS | Publication request: Ersiphe trifolii-a newly recognized powdery mildew pathogen of pea.

Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 5, 2010
Publication Date: N/A

Interpretive Summary: Powdery mildew of pea is an important disease worldwide. Managing the disease is through application of fungicides and breeding and planting resistant cultivars. A mystery often surrounds pea breeding program for powdery mildew resistance: Some resistant pea lines selected in the greenhouse are susceptible to powdery mildew in the field. Such inconsistent resistance performances of pea lines could be due to different disease pressure in different environmental conditions and/or different pathogen populations or even different pathogen species occurring in different environments. This study was carried out to examine powdery mildew samples of pea collected from greenhouses and production fields in a three-year period. Results of differences in DNA sequences and reproducitve structures showed that there are two distinct species infecting pea: Erysiphe pisi and E. trifolii. Erysiphe trifolii was previously not known to infect pea. Observations also showed that the two pathogen species cause different levels of symptoms on stems and leafs of different pea genotypes. The discovery of two distinct species infecting pea helps explain inconsistent resistance performances of pea lines. In addition, this study has identified several alternative hosts of pea powdery mildews.

Technical Abstract: Population diversity of powdery mildews infecting pea (Pisum sativum) in the US Pacific Northwest was investigated in order to assess inconsistent resistance performances of pea genotypes in different environments. Phylogenetic analyses based on ITS sequences, in combination with assessment of morphological characters, defined two groups of powdery mildews infecting pea (Pisum sativum). Group I included most field samples and some greenhouse samples, and its ITS sequences exhibited 99% similarity to those of Erysiphe pisi in GenBank. E. pisi is normally considered as the powdery mildew pathogen of pea. Group II consisted of most of the greenhouse samples and two field samples, and ITS sequences from this group exhibited 99% similarity to those of E. trifolii. Morphological examination determined that the samples in group II conform to the descriptions of E. trifolii. E. trifolii is not previously known as a pathogen of pea. The existence of two distinct powdery mildew species infecting pea in both greenhouse and field may complicate with the breeding programs and possibly explains the putative "breakdowns" of resistance in previously resistant pea breeding lines. In-vitro pathogenicity studies identified several potential alternative hosts of E. trifolii and E .pisi.