Studies on the effects of some microclimatic factors and of spore dispersal on infection of two species of Pinus by Dothistroma pini

Abstract

This thesis presents a review of the literature on Dothistroma pini and examines effects of climatic factors, shoot phenology and spore dispersal as determinants of patterns of infection by this fungus in New Zealand plantations of Pinus radiata and P. ponderosa. Worldwide, epidemics of the disease have occurred in humid temperate and subtropical climates where monthly rainfall exceeds 80mm in four consecutive months with mean temperature above 15°C or where monthly rainfall exceeds 100mm in six consecutive months with mean temperature above 10°C. In the areas in which this study was conducted, distinctive patterns of infection within the landscape, within forest stands, among species and within individual plants have been recognised and are described. It was established by measurement of the duration of leaf wetness periods and leaf temperature that the topography - related patterns of infection are determined in part by the influence of topography upon air movement and upon rates of drying of foliage. Marked differences between P. radiata and P. ponderosa in patterns of infection within trees, within shoots and along the length of needles are described. In P. radiata trees ca.20m tall, infection decreased markedly with height in the tree, with distance from the base of shoots and with distance from the tips of needles. In immediately adjacent P. ponderosa of similar height, infection was more evenly distributes along all three gradients. Shoot extension in P. ponderosa commences in late October, a time when weather conditions are favourable for infection. The limited shoot growth through much of the height of the tree maintains current foliage on most shoots at distances less than 5cm from sources of inoculum in needles of the previous season. In P. radiata growth commences in late August and rapid elongation, which increases with increasing height in the tree, carries new foliage beyond the immediate proximity of inoculum before the onset of the main infection period, at all positions except those low in the crown. As a result of interactions among these factors spore loads upon needle surfaces, along the length of needles and at intervals along shoots are more evenly distributed in P. ponderosa than in P. radiata. Splash-dispersed conidia emanating from a source of infected needles 2m above ground were seldom trapped beyond 2m from the source even during storms with wind speeds up to 64km per hour. Dispersal over long distances is relatively rare and is probably restricted to periods of strong vertical turbulence associated with cyclonic depressions or thermal updrafts. Trials involving removal of lesioned needles on various portions of shoots on 5m tall P. radiata indicated that infection was caused principally by inoculum derived from sources very close to the infection site. Similarly-localised inoculations of shoots at several levels within 5m tall P. radiata and of shoots on deliberately displaced branches demonstrated i) that stromata within 10cm of newly exposed needles are the principle sources of inoculum and ii) that increase in shoot vigour and changes in microclimate associated with increasing height in the tree are not important determinants of the inverse relationship between degree of infection and relative height within trees of P. radiata. This newly obtained understanding of the relationships between infection patterns, shoot phenology, and limited distance of conidial dispersal helps explain, for the plantations of the central North Island, New Zealand, why a single application of fungicide in early summer has controlled Dothistroma blight on Pinus radiata, but not on P. ponderosa.