Spread ofDothistroma septosporain plantations ofPinus radiatain Victoria between 1979 and 1988

Summary Pine needle blight on Pinus radiata, due to infection by Dothistroma septospora, was first recorded in Victoria at one location in the north-eastern highlands of the State in 1979. For the first two years afterwards the disease was confined to the same small area, then in 1981/82 it appeared at a very low level in areas up to 50 km distant, notably in several P. radiata valley sites and in plateau country where mist collected. The infection intensity increased rapidly after March 1983 and severe defoliation appeared on trees in these sites in 1984. Moderate to severe needle-cast developed much more slowly in adjacent plantations. By 1988 most of the plantations in north-eastern Victoria were infected but the amount of needle-cast was extremely variable. Topography, stand and site features were found to affect disease expression. By 1987, 1505 ha (16%) of the plantations surveyed had more than 25% of their crowns affected. Two types of disease spread were observed within trees. In Type 1, infection commenced in the lower-most branches and moved progressively up the crown. In Type 2, infection started in the inner, lower-most branches and moved up the inner crown and subsequently needle-cast spread outwards along the branches. Type 1 was usually seen just prior to and for two to three years after canopy closure. Type 2 was most evident on exposed trees before canopy closure and in older plantations. Attempts at estimating disease hazard in Victoria by comparisons with areas in New Zealand experiencing similar mean monthly temperatures and rainfall were found to be unreliable, possibly because of the large diurnal temperature variations in Victoria and the effect of topography and stand structure on the microclimate.