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

Abstract

Strains A and B of Monochaetia unicornis produced cankers following inoculation into Cupressus arizonica, C. benthami, C. lusitanica, C. macrocarpa, C. sempervirens, C. torulosa, Cupressocyparis leylandii, Chamaecyparis lawsoniana, Juniperus procera and Thuja orientalis. Strain D was parasitic only on Juniperus procera. Monochaetta unicornis grew well on agar media prepared from bark and sapwood extracts of Cupressus species and Juniperus procera but only slowly on Podocarpus gracilior bark agar.Test trees of five species of Cupressus, grown commercially in East Africa, were alternated with artificially inoculated trees of C. macrocarpa in field plots. The subsequent development of natural infections in trees of the test species confirmed field observations that C. macrocarpa was the most susceptible.Plots (each of 340 trees) of the five species were inoculated with mycelium. In 4 months, the most extensive cankers had developed in C. benthami, and this species also had the largest number of active cankers and the most rapid height growth. When examined again 4 months later, C. macrocarpa had the most extensive cankers and 60% of them were still active as compared with 20% in C. lusitanica and 6% in C. benthami. Delayed inoculation following wounding confirmed these results and in the dry season the number of successful infections was always highest in C. macrocarpa.Periderm was formed in the bark and sapwood of the Cupressus species in response to wounding, the process being slower in C. macrocarpa and C. torulosa than in other species. Secondary growth was most rapid in C. macrocarpa. It is suggested that rapid height growth coupled with rapid secondary growth in this host favour the growth of the pathogen rather than the host reaction, and C. macrocarpa is consequently very susceptible. During the rains the fungus develops as rapidly in species having a slower secondary growth rate but in the ensuing dry season such infections dry out and the cankers are occluded.