Wide variation in aggressiveness and growth in South African <i>Fusarium circinatum</i> isolates with geographical origin as the primary determinant

Fusarium circinatum is a globally important fungal pathogen that causes pitch canker on a wide range of Pinus species. In this study, we considered mycelial growth in culture and aggressiveness of a South African population of F. circinatum isolates. The specific aim was to determine how these phenotypes are correlated with one another and other isolate characteristics, including mating type, host species and geographic origin of the isolates. The study involved the selection of 102 isolates of F. circinatum from five provinces of South Africa based on genotype, mating type and original host from which they were isolated, after which pathogenicity tests were carried out on Pinus patula seedlings. Mycelial growth in culture was also determined for all the isolates at three different temperatures. In the pathogenicity tests, most of the isolates produced significant lesions. While seven had low levels of aggressiveness, other isolates tested were as or more aggressive than a standard set of isolates routinely used for resistance/ tolerance screening. Most F. circinatum isolates grew best at 25 °C, followed by growth at 20 °C and 30 °C. A notable correlation between aggressiveness and the geographical origin was identified. Moreover, the study highlighted a moderately positive relationship between the growth rate of F. circinatum isolates and their reported aggressiveness. The extensive data gathered on the aggressiveness of F. circinatum will offer a broad range of isolates for future comparative studies using other genotypes of F. circinatum and will also aid in the consideration of F. circinatum isolates used for tolerance screening studies.