This study was concerned with factors affecting the development of potato gangrene caused by Phoma exigua var. foveata, with particular reference to the assessment of cultivar resistance to the disease. The factors considered were isolate pathogenicity, tuber inoculum level, tuber damage, method of inoculation and assessment, incubation temperature and tuber tissue and damage resistance. In addition the influence of isolate and cultivar on the transmission of inoculum from seed to progeny tubers was assessed.
The importance of using isolates of high pathogenicity to detect differences in resistance among cultivars was emphasised. Variation in pathogenicity among isolates was associated with isolate source and history, but there was no evidence of consistent cultivar x isolate interactions of an order to suggest physiological specialisation in this pathogen. In considering the relative contribution of isolate and cultivar to variation in disease development the contribution of isolate was much less than that of cultivar. The relative resistances of cultivars depended to some extent on inoculation technique and assessment method. This indicated the existence of two major components of tuber resistance, namely damage and tissue resistance. There was also evidence that the
relative susceptibilities of cultivars could differ for the tuber cortex and medulla: thus some cultivars mayhave lesions with a small surface area but an appreciable depth of penetration whereas others show wide, shallow rots. The actual level of gangrene depended upon the interaction between cultivar, tuber damage and tuber inoculum density. High inoculum doses coupled with inoculation techniques which allowed the expression of cultivar damage resistance were the most useful in discriminating among cultivars. However, with some assessment methods cultivar differences were obscured at high inoculum densities. Incubation temperature also influenced the course of disease development. At higher incubation temperatures (lO°C) lesions were arrested in all cultivars tested whereas at lower temperatures (4°C) cultivars showed differences in tissue resistance reflected in the degree of lesion retardation but rot development continued in all cultivars. In attempts to gain further evidence of tissue resistance factors gamma irradiation studies were carried out. Irradiation of tubers reduced their resistance to gangrene to an extent dependent upon the irradiation dose and the delay, after irradiation, between wounding and inoculation. There was evidence that wound periderm formation was of minor importance in tissue resistance and that irradiation—induced susceptibility
3.was not associated with tuber cell death. Transmission of P. exigua var. foveata from seed to daughter tubers was shown to be affected by isolate and cultivar but further work is required in this area.