ASPECTS OF BIOLOGY AND CONTROL OF A STEM ROT OF CUCUMBER CAUSED BY PENICILLIUM-OXALICUM

In 1989 penicillium stem rot affected 12 crops of protected cucumbers in the Lea Valley, Essex and one on the Isle of Wight; all of the affected crops were growing on rockwool slabs. Generally the disease incidence was low, but in one crop the majority of plants were affected and over 50% died. The casual fungus, identified as Penicillium oxalicum, occurred on stubs left after fruit picking and leaf and side-shoot trimming, and developed to cause stem softening and withering. Blue-green sporulation of penicillium on the main stem was usually surrounded by a ring of brown tissue. P. oxalicum failed to infect undamaged fruit, but was an aggressive pathogen of damaged fruit, with rot progressing more rapidly at 20-degrees-C and above than at 15-degrees-C. Inoculation of damaged cotyledon leaves induced lesions on a range of cultivars, with Cilla and Elka more resistant than others. Young plants of cv. Corona grown at 22-25-degrees-C and 90% r.h. and inoculated with mycelial plugs on the stems at fresh leaf scars developed stem lesions with Canandian isolates but not with UK isolates; in the former case wilting and collapse of the plant sometimes followed. No lesions developed following inoculation on plants grown at 70-80% r.h. P. oxalicum was consistently recovered from within the stem of most plants at the leaf axils two leaves above and two leaves below the inoculation site. Growth on potato dextrose agar of isolates of P. oxalicum from cucumber occurred more rapidly at 20-degrees-C than at 15-degrees-C. Spore germination of most isolates on agar occurred more rapidly at 20-degrees or 25-degrees-C than at either 15-degrees or 30-degrees-C. Isolates of P. oxalicum grew on agar amended with benomyl at 2 and 20-mu-g/ml but showed no growth on agar amended with iprodione at the same rates. In a crop with a high incidence of the disease, sprays of iprodione and imazalil appeared to reduce the rate of disease spread; sprays of benomyl and vinclozolin were less effective. Iprodione and vinclozolin were the most effective of five fungicides applied as paints to stem lesions to limit their development.