Erysiphe trifolii is able to overcome er1 and Er3, but not er2, resistance genes in pea

Until recently, Erysiphe pisi was thought to be the only causal agent of powdery mildew in pea, but recent studies showed that other species such as Erysiphe trifolii and Erysiphe baeumleri can also cause this disease. Three genes, er1, er2 and Er3, conferring resistance to E. pisi have been reported so far in pea. Previous studies showed that E. trifolii and E. baeumleri were able to overcome er1 resistance, but whether er2 and Er3 were effective against E. trifolii was not known. In this study, pea accessions carrying these three genes were evaluated for resistance to E. trifolii under controlled conditions at 20 and 25 A degrees C. In addition, these accessions were also evaluated under field conditions in Spain and in India. Analysis of the ribosomal DNA (rDNA) internal transcribed spacer (ITS) sequences showed that E. trifolii was the causal agent of powdery mildew symptoms in lines carrying er1 in Spain and that this pathogen was also present in India. Our results showed that E. trifolii was able to overcome er1 and shows that this pathogen can also overcome Er3 resistance in some conditions. In contrast, er2 provided high level of resistance against E. trifolii in all conditions and locations studied. Temperature affected the expression of Er3 against E. trifolii, but not of er1 or er2. The pea accession JI2480, containing er2, was highly resistant and JI2302 containing er1 was susceptible to E. trifolii at both temperatures, whereas P660-4 containing Er3 was resistant at 20 A degrees C but susceptible at 25 A degrees C. The present study also identified sources of resistance effective against both E. pisi and E. trifolii.