THE RELATIONSHIP BETWEEN COLONIZATION AND CROWN ROT SYMPTOMS IN STRAWBERRY PLANTS INFECTED WITH PHYTOPHTHORA-CACTORUM

Strawberry tissues infected with Phytophthora cactorum were comminuted and plated in a selective antibiotic agar medium to determine levels of tissue colonisation as indicated by the number of colony forming un its (CFU) recovered per gramme of infected tissue. The number of CFU r ecovered per gramme of tissue increased logarithmically with the amoun t of necrosis in infected crown, leaf and petiole tissues. Under the c onditions of enhanced susceptibility to infection and colonisation cau sed by cold storage treatments, this relationship between colonisation and necrosis was not significantly altered in the susceptible cv. Tam ella. A recovery index was used to determine the effect of infected ti ssues on the recovery of CFU. This indicated that increasing levels of host colonisation stimulated CFU recovery and may partly explain the large increase in CFU g-1 with larger amounts of necrosis. The amount of tissue colonisation was greater in inoculated plants of the suscept ible cv. Tamella than in less susceptible cv. Cambridge Favourite, alt hough the necrotic tissues of the latter contained more CFU g-1, indic ating a greater level of tolerance to colonisation. In cv. Tamella sma ll amounts of colonisation were capable of causing wilt symptoms, alth ough no wilted plants contained less than 200 CFU g-1. Conversely, pla nts containing more than 1000 CFU g-1 always wilted. In the early stag es of infection, low levels of colonisation could be detected in straw berry crowns in the absence of symptoms. Dormant strawberry plants of cv. Tamella were readily infected by P. cactorum zoospore inoculations but, unlike actively growing plants, the majority of infections remai ned latent. These latent infections exhibited little or no symptoms an d CFU recoveries from infected tissues were always below 100 CFU g-1.