Characterisation of pathogenicity ofPhytophthora parasiticaisolates by stem and detached‐leaf inoculations in four tobacco cuitivars

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Abstract
The response of four cuitivars of tobacco (Nicotiana tabacum) to eight Australian and exotic isolates of Phytophthora parasitica and one of P. cryptogea was studied using stem and detached‐leaf inoculations. With both techniques cuitivars differed in their susceptibility to pathogen colonisation, which was strongly dependent on the cultivar‐isolate interaction. The ability to cause expanding lesions on inoculated stems differentiated some tobacco isolates that were particularly aggressive from less aggressive tobacco isolates, and from isolates from tomato and carnation. Virulence on cultivar ‘NC2326’ identified two of the tobacco isolates as race 1, one from Australia (9201) and one from Cuba (309). P. cryptogea induced some foliar systemic necrosis in three cuitivars. Four P. parasitica isolates, one from carnation and two from tobacco, exhibited a similar toxicity only on the cultivar ‘Samsun’. On wounded detached leaves both non‐tobacco and tobacco isolates initiated lesions. The length of lesion reflected their relative aggressiveness, but not their virulence, on tobacco. On the basis of their pathogenicity in both stem and leaves, tobacco isolates were not separated from non‐tobacco isolates, but significant differences of both aggressiveness and virulence were observed among P. parasitica isolates.