Studies on the phylloplane microflora of Pinus radiata D. Don and its interaction with the fungal pathogen Dothistroma pini Hulbary.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
1. The epiphytic microflora of Pinus radiata needles was investigated. Bacteria occurred in the order of 106 per gram fresh weight of needles, yeasts 104 and moulds 103 per gram. 2. Bacterial numbers were higher on shaded aspects and near the bottom of the tree. Numbers also increased as the needles aged although no change was observed as the trees aged. Bacteria were most numerous in late Spring and numbers decreased during the Summer months. However little variation in numbers was noted between the three localities studied. In contrast the population of moulds decreased during Spring increasing to a maximum in Winter. 3. The most common bacteria belonged to the plant coryneform group, followed by lactic acid bacteria and Gram-negative rods (pseudomonads, flavobacteria and paracolons). Low numbers of cocci were also isolated in most studies. Most yeasts on the needle surface belonged to the family Cryptococcaceae. The most regularly occurring sporing moulds were Cladosporium and Penicillium. Sterile mycelial moulds were also numerous. An initial study of the phylloplane microflora as an interacting population was made both by removing part of the natural microflora and by introducing micro-organisms from an outside source. The subsequent readjustment of the populations suggested that interactions between micro-organisms on the needle surface played an important role in determining the composition of the phylloplane population. 4. During the development of needles the epiphytic microflora was initially composed entirely of Aureobasidium pullulans. This was followed by a mixture of Gram-negative bacteria, yeasts and moulds, and finally Gram-positive bacteria became more numerous until the composition of micro-organisms in the phylloplane was that of the mature needles. 5. It is suggested that the most important factor governing the number of micro-organisms in the phylloplane is the availability of nutrients leached on to the needle surface. Under the conditions studied, physical factors such as temperature and humidity had direct effects within the limits set by the availability of nutrients, and indirect effects where they may have influenced the leaching of nutrients from the plant. 6. Preliminary studies were made of the interactions between epiphytic micro-organisms and Dothistroma pini. In culture some saprophytic flavobacteria and pseudomonads inhibited the growth of D. pini. Studies on detached needles suggested that several bacteria from the phylloplane reduced germination of D. pini conidia and that some flavobacteria may retard the subsequent growth of the germ tubes. Later observations suggested that growth of mycelium over the needle surface was not markedly reduced. However field trials on seedlings demonstrated that those sprayed with some saprophytic flavobacteria showed a reduction in symptom expression.