Pyrenophora teres f. maculata, the cause of Pyrenophora leaf spot of barley in South Africa

Abstract

Net blotch caused by Pyrenophora teres is a serious disease of barley in many cereal production areas world-wide, including the Western Cape province of South Africa. The pathogen occurs as two forms, P. teres f. teres, which produces net-blotch symptoms, and P. teres f. maculata which produces leaf spots. Pyrenophora japonica and P. hordei, which have also been reported in South Africa, also produce spots on susceptible barley cultivars. Using RAPD markers, spot-forming isolates from the South African population were found to be relatively uniform. Single ascospores were obtained from pseudothecia after in vitro mating had occurred between a verified P. teres net-blotch isolate from Denmark and a representative Pyrenophora leaf spot isolate from South Africa. Using amplified fragment length polymorphism (AFLP) and RAPD markers, recombination was demonstrated in the progeny which had DNA banding patterns different from the two parental isolates. Pathogenicity trials also confirmed that recombination had taken place during mating. Inoculations were conducted on differential cultivars susceptible to the net-blotch or leaf spot forms. The two parents induced typical net-blotch or leaf spot symptoms whereas the progeny mostly induced a jagged spot symptom on each cultivar. Fungicide sensitivity tests using the ergosterol biosynthesis inhibitors triademinol, bromuconazole and triticonazole showed that, due to recombination, some progeny could have increased resistance to these fungicides. Due to mating and subsequent recombination between a net blotch isolate of P. teres and a representative leaf spot isolate, it was concluded that the latter was P. teres f. maculata. These results contrast with the earlier belief that Pyrenophora leaf spot isolates in the Western Cape are P. japonica and P. hordei.