Biology and epidemiology of crown rot infection of strawberry caused by Phytophthora cactorum (Leb. and Cohn) Schroet
The effects of photoperiod, drought stress and cold storage treatments on the susceptibility of strawberry plants (Fragaria ananasa x Duch.) cvs. Tamella, Cambridge Favourite and Red Gauntlet to crown rot caused by Phytophthora cactorum (Leb. and Cohn.) Schroet. were studied. Infection and disease susceptibility were assessed by; a) the time from inoculation to first symptom appearance; b) measurement of crown necrosis; c) measurement of fungal colonisation by a comminution-colony count method developed during the study; d) detached leaf bioassay. The effect of photoperiod was assessed independently from photosynthetically active radiation and long day treatments consistently increased susceptibility; a result contradicting previous findings. Drought treatments applied prior to inoculation generally increased plants' resistance to colonisation and necrosis. Treatments applied after inoculation significantly reduced the level of colonisation capable of causing wilt symptoms. Cold storage treatments increased the susceptibility of plants to infection, colonisation and wilt symptom development. Susceptibility to infection by normally non-pathogenic isolates was also increased by cold treatments although no wilt symptoms resulted from these infections. Plants stored with latent infections developed immediate chronic symptoms when transplanted from cold storage. The mortality of these plants in the cold store also increased with storage time. Cold storage-enhanced susceptibility occurred irrespective of pathogen factors. Large-scale sudden collapse of cold-stored strawberry stands was caused by carriage of oospores through storage, either in soil or in scattered latent infections. P. cactorum produced the cell wall degrading enzymes (CWDEs) exo 1,4--galactanase and endo polygalacturonase in culture. Both enzymes were detected in infected plants. CWDE activities increased with colonisation and symptoms, and levels of PG and galactanase were significantly greater in cold-stored tissues than in controls but their importance in symptom expression and virulence was not conclusively demonstrated.