Fungal diseases of trees in Tanzania with emphasis on the stem decay of the East African camphor tree, Ocotea usambarensis Engl.

Abstract

This study presents the current situation and gives a background on forest disease research and knowledge in Tanzania. Some important disease epidemics are discussed and known fungi causing diseases to indigenous and exotic tree species have been tabulated to show the hosts, the pathogens and the pans of the host affected. The review on the disease situation also provides some information on the stem decay of Ocotea usambarensis, which is reported in detail under the current study. The stem decay of O. usambarensis has been studied in the Usambara and Kilimanjaro mountain rain forests and causative fungi isolated from standing trees and identified. The decay, characterized by butt rot and main stem decay, and which may attack both the heart wood and sapwood, has been diagnosed basing on the Koch’s postulates. The symptoms, signs, infection courts and the ways by which the decay is transmitted from mother trees to young regeneration are also reported. Trees of all age classes and size are susceptible to the stem decay regardless of whether they have developed heartwood or not. Also, sporophores of 14 larger fungi from the families Hymenochaetaceae, Polyporaceae, Ganodermataceae, Schizophylaceae, Corticiaceae and Xylariaceae were collected from various wood material of the tree species. 72 species of fungi were isolated from decay of standing trees, 12 of which were basidiomycetes and 60 were non-basidiomycctes. It was found that the decay is attributed to a number of fungi which infect and colonize the tree through a series of succession stages defined by the niches available in the decaying stems and by the roles of the fungi in such niches. Pioneering fungi of living sapwood were dominated by parasitic and facultative species which include Ophiostoma spp, Ceratocystis spp, Botryosphaeria ribis, Cylindrocarpon destructions, Cylindrodendrum album, Pestalotiopsis sp, Nodulisporium sp, Leptodontidium sp, some basidiomycetes and other fungi which could not be properly grouped because they had sterile mycelium among which was an important pathogenic fungus known here as ‘Sterile mycelium sp 3’. Possible pioneering fungi of the heartwood were the Alternaria sp, Paecilomyces lilacinus, Phoma sp (Coniothyriuni insigne), Penicillium spp and other unidentified conidial species. The secondary and climax fungi were dominated by basidiomycetes including Phellinus senex, and other conidial and sterile mycelium species. Phellinus senex, which has been widely reported earlier as a primary decay fungus of O. usambarensis, participates in the decay and becomes part of the decay-climax flora of the tree. However, it is not a primary decayer because it can not infect sapwood or heartwood which is un colonized by other fungi. Some selected fungi were used in pathogenicity tests and inoculated in sapwood and heartwood of healthy trees. The rate of infection of some fungi was variably affected by the moisture content and pH of the sapwood. Determination of the decay ability of some fungi was also conducted in vitro using wood blocks of O.usambarensis, and the fungi were also tested for the possession of phenoloxidase enzymes that can degrade gallic acid, tannic acid or lignin. Basidiomycetes were the most aggressive in terms of the rate and magnitude of decay but some non- basidiomycetes were also able to degrade the wood significantly. About 70% of the fungi tested possessed enzymes capable of degrading lignin which means they are white rot fungi. As most of these fungi have pH optima similar to that in the trees, this shows they function optimally in the stems and hence explains the formation of hollow stems in decay-affected trees. Factors which may affect fungal growth and their ability to cause decay in standing trees, such as temperature, pH and oxygen stress were evaluated in culture. It was found that environmental temperature and the pH in trees favour the growth of tire fungi, while oxygen stress seems to act as a factor imposing a selection pressure on fungi during succession by favouring tolerant species. Most fungi are mesophilic and the optimum temperature for most of them was within the average found in the forest environment, while their optimum pH was similar to that found in trees. It was also argued that the ability of Phellinus senex to tolerate anaerobic conditions was a major factor making it the main climax species of the decay and which later fruitify on standing trees with heartrot or butt rot. Comparing the flora of fungi attacking trees in the Usambara and Kilimanjaro forests, trees in the Usambara are infected by relatively more fungi. This is likely due to the climatic and edaphic conditions which are less optimal in the Usambara and hence predispose the trees to infection.