Growth, pathogenicity and sexual fertility of the African tree pathogen Ceratocystis albifundus

The African fungus Ceratocystis albifundus does not cause disease symptoms on its wide range of native woody hosts. However, on non-native Acacia mearnsii and orchard-grown Protea cynaroides, it represents an economically significant pathogen. Because previous studies exploring the biological fitness of C. albifundus were constrained by small sample sizes, we aimed to determine how commonly used measures of fitness (growth in culture, pathogenicity and sexual fertility status) vary across natural populations. For this purpose, a collection of 58 isolates originating from diverse hosts and geographic locations in South Africa were subjected to growth studies on synthetic culture medium, pathogenicity tests on A. mearnsii saplings, and sequence-based assays of fertility status. We found that these traits were generally not correlated with one another, although isolates from the Summer rainfall region and from native hosts induced significantly shorter lesions on A. mearnsii than isolates from the Winter rainfall region and from diseased A. mearnsii and orchard-grown P. cynaroides tissues. In other words, aggressiveness of C. albifundus to A. mearnsii was significantly influenced by the isolates’ geographic origin and host species, irrespective of their fertility status or growth rates. Additionally, the broad lack of correlation among growth, pathogenicity and fertility suggested that these fitness components are likely underpinned by distinct genetic and molecular mechanisms. Our study thus provides a robust foundation for further exploration of the fitness landscape in this important tree pathogen.