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.