Study of the eyespot disease of cereals caused by Pseudocercosporella spp.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The eyespot disease of cereals was found to be prevalent across Southland and also in areas of Otago and Canterbury. Surveys between 1984-1986 showed the disease was more severe with frequent lodging in the 1984/85 season. Wheat, barley, rye and triticale crops were sampled and isolates of Pseudocercosporella obtained. Isolates of fast-even morphology remained the predominant type. Slow-feathery isolates increased in proportion during this time and were not significantly higher on barley than wheat, as is the case in the U.K. Electrophoretic analyses of esterases present in Pseudocercosporella provided evidence for the separation of fast-even and slow-feathery morphological types but not species. Variation in minor bands was detected between isolates from both within N.Z. and between N.Z. and Europe. In vitro resistance to the fungicide benomyl, was found in the Southland Pseudocercosporella population at all tested concentrations, ranging from 0.2 - 200 μg a.i. ml-1. Proportions of isolates resistant to 2 μg a.i. ml-1 remained similar over the sampled seasons. Isolates produced the same responses in shaking liquid-culture as they did on solid agar media. Resistance was detected in both sprayed and unsprayed crops. Insensitivity to the demethylation-inhibiting (DMI) chemical, prochloraz, was detected at all tested concentrations ranging from 0.1 - 200 μg a.i. ml-1. Both hyphal growth and sporulation was suppressed in only a proportion of subcultures from an isolate, however proportions of insensitive subcultures were consistent for each isolate. DMI-insensitive responses were numerous with small differences in ECSO values and occurred in isolates distributed widely within the wild population. Negatively-correlated cross resistance between benomyl and MDPC was not found in Pseudocercosporella, hence MDPC would be of no use for controlling benomyl-resistant isolates. The presence of acenaphthene did not induce its requirement in isolates of Pseudocercosporella. A fungicide spraying trial undertaken in Southland demonstrated that in the presence of benomyl-resistance, prochloraz gave better eyespot control than benomyl. The use of mixtures and efficient spray application is discussed. N.Z. slow-feathery and fast-even isolates of Pseudocercosporella were shown to have the expected association with R-type and W-type pathogenicities described of overseas isolates. Wheat cultivars screened for response to eyespot were highly susceptible except for Bounty, which contains Capelle-Desprez resistance, and Takahe. The barley cultivars were also susceptible to both fungal pathotypes, and of the rye cultivars, Dominant was the most resistant. The triticale cultivars had varying responses, with Karere being susceptible to both pathotypes, Lasko susceptible to R-types and Salvo and Aranui moderately resistant to both pathotypes. A range of scores was obtained for wheat breeding lines which were screened, but all were quite susceptible. A source of resistance was identified in a line of Hordeum bulrosum L. Epidemiology trials undertaken in Southland and Canterbury showed a dependence of inoculum dispersal on rain-bearing wind. Fast-even isolates appeared to spread more quickly, however slow-feathery isolates became better established in plots inoculated with isolates of both morphological types. The natural infection time is suggested to be late winter/early spring in Canterbury. An unsuccessful attempt was made to detect a perfect state of the fungus on stubble and to induce it in culture. A trial evaluating yield in barley following eyespot infection was undertaken in Southland and a significant reduction in head weight was obtained.