Autecological studies on Penicillium expansum

This thesis reports an autecological study on an apple rotting isolate of Penicillium expansum. The study was concerned with the survival and growth of the fungus in the soil, in the rhizospheres of apple, broad bean, maize and tomato, and in plant litter. P. expansum spores did not germinate in natural soil but could remain viable (68%) for up to one year at least. Spores germinated and developed to form conidia microcyclically in soil sterilized by autoclaving. Spores germinated in natural soil after the addition of nutrients, in the form of malt extract broth, indicating that nutrient deficiency might be one factor determining spore dormancy. Evidence was also obtained to show that non-volatile diffusible inhibitors of spore germination were present in natural soil. Heat treatment of natural soil at 80°C destroyed the inhibitory effect whereas treatment at 60°C had no effect although it destroyed most fungi and bacteria isolatable on Malt extract agar or Modified Hutchinson's agar. If nutrient deficiency were the inhibitory factor the release of nutrients from the micro-organisms killed by treatment at 60°C should have stimulated germination. Thus the activities of other micro-organisms in the soil, probably through the heat labile inhibitors they produced, were the main factors inhibiting germination. Some of the fungi which survived at 60°C for 6h were inhibitory to the growth of P. expansum in culture. The experiments on the survival of spores and of spore germination and growth in the rhizospheres of apple, broad bean, maize and tomato showed that spores could germinate and the fungus could grow in the rhizospheres of all three plant species. P. expansum was observed to grow occasionally closely attached to the root surfaces of broad bean, maize and tomato. It was never observed to invade the internal root tissues of tomato. The other two species were not investigated for the colonization of the internal tissue. The removal of the inhibition of spore germination in the rhizospheres could have been due to the effect of root exudates either serving as nutrients or as inactivators of inhibitory factors. The presence of P. expansum in the rhizospheres of tomato plants can affect plant growth to a limited extent. Inoculum in the rhizosphere slightly reduced stem height and total leaf area but had no effect on total dry matter, the number of leaves or the development of reproductive structures. Colonization by P. expansum of root and leaf litter from apple in the soil in the presence of other soil micro-organisms was very low. However it could survive in apple root tissues if the apple root was colonized before addition to the soil