STUDIES ON A CANKER DISEASE OF CYPRESSES IN EAST AFRICA, CAUSED BY MONOCHAETIA UNICORNIS (COOKE & ELLIS) SACC

Autor: D. Rudd Jones
Rok vydání: 1953
Předmět:
Zdroj: Annals of Applied Biology. 40:323-343
ISSN: 1744-7348
0003-4746
DOI: 10.1111/j.1744-7348.1953.tb01086.x
Popis: A canker disease of cypresses, caused by Monochaetia unicornis, was first observed in Kenya in 1942. Outbreaks appear to have followed a spread from south-east to north-west, the direction of the south-east monsoon, and the disease has now been recorded in nearly all the cypress-growing areas of Kenya, Tanganyika and Uganda, in Southern Rhodesia and the Union of South Africa. The oldest known infections occurred in 1937. The fungus grows in the bark and sapwood, forming cankers which spread rapidly up and down the stem or branch, more slowly round, frequently causing girdling. Cankered trees produce large amounts of resin and a periderm may be initiated in advance of the fungus. Infections occur most commonly at main branch crotches, but cankers also start at the tree collar, on the stem and at minor branch crotches. The fungus exists in nature in three strains having differing levels of virulence, growth rate in culture, size of acervuli and conidia, and retention of viability in conidia. The distribution of one strain appears to be limited by ecological conditions. Acervuli ripen and conidia are liberated from 3 to 8 weeks after the start of the rains. Infections start during the rains and misty weather, the slimy spores being dispersed for short distances only. Conidia may retain their viability for up to 28 months. The local spread of the disease in Kenya has been traced and differing levels of virulence east and west of the Great Rift valley, associated with differences of climate, have been observed. Trees 2 years old are the most susceptible and older trees are only infected in the rapidly growing parts. Cupressus macrocarpa is more susceptible than C. lusitanica, the latter species being heavily infected only where it is growing close to diseased C. tnacrocarpa. Occlusion of cankers occurs in both species but is more effective in halting the spread of the fungus in C. lusitanica. It is suggested that the disease originated from a single focus and may represent the mutation of a strain of Monochaetia unicornis parasitic on cypresses from a strain only parasitic on an indigenous host, Juniperus procera. The fungus may, however, have been imported from abroad.
Databáze: OpenAIRE
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