Reaction of Brassica juncea to Australian isolates of Leptosphaeria maculans and Leptosphaeria biglobosa ‘canadensis’

Abstract

Blackleg (causal agents Leptosphaeria maculans and L. biglobosa) is the most significant disease of canola (Brassica napus) worldwide. This was the first survey of Leptosphaeria isolates recovered from Brassica juncea stubble in Australia. Sixty-four L. maculans isolates and 88 L. biglobosa ‘canadensis’ isolates were collected from B. juncea stubble over two years (2005–2006). These isolates were screened over three canola quality B. juncea and three B. napus differentials. As expected, L. biglobosa ‘canadensis’ isolates caused lower disease severity than the L. maculans isolates. However, 13 L. biglobosa ‘canadensis’ isolates were capable of causing ≥50% mean internal infection on at least one of the six differentials. Although these isolates were recovered from B. juncea stubble, disease severity was low when canola quality B. juncea differentials were inoculated. The same differential lines were also screened over two field sites by placing pots containing each of the differential lines onto B. napus and B. juncea stubble from commercial fields. At one site, inoculum in B. napus stubble caused higher internal infection severity than the B. juncea stubble in all differentials except B. napus differential ‘ATR-Beacon’. At the other site, however, inoculum from B. juncea stubble produced more internal infection in one B. juncea differential in 2007 and in four differentials in 2008. This investigation suggests that when B. juncea is widely grown, increased blackleg symptoms could occur and that L. biglobosa ‘canadensis’ isolates can contribute to disease severity.