Ceratocystis albifundus is the most important fungal pathogen of black wattle (Acacia mearnsii) grown in plantations
in southern and eastern Africa. It is a homothallic fungus but also undergoes unidirectional mating type switching. As
a result, the ascospore progeny can be either self-fertile or self-sterile. The only apparent difference between these mating
types is the deletion of the MAT1-2-1 gene in self-sterile isolates. There is some evidence suggesting that self-sterile
isolates grow more slowly than self-fertile isolates, but this has not been tested rigorously. The aim of this study was to
determine whether self-sterile isolates are less fit by examining growth rate, relative germination rate and pathogenicity.
Five self-sterile isolates were generated from each of five self-fertile isolates of C. albifundus and these 30 isolates were
compared. The results showed that the self-sterile isolates grew consistently slower and were less pathogenic than the
self-fertile isolates. The germination ratio of self-fertile to self-sterile isolates from single ascospores collected from the
ascomata of five self-fertile isolates was on average 7:3. This could be a consequence of the self-sterile isolates having a
lower germination rate. This observation, and the lower growth and pathogenicity levels, suggests that self-sterile
isolates are not likely to compete effectively in nature, raising intriguing questions regarding their role and value to
C. albifundus and other fungi having a similar mating system.