Grasses as a refuge for Fusarium circinatum L. – evidence from South Africa

Fusarium circinatum L. is an important pathogen in countries that grow and manage Pinus species. Approximately 50% or 600 000 ha of South Africa’s commercial plantations are planted to Pinus spp. and some of these are threatened by this fungus. Contaminated plants, planting trays, soil and water can all act as sources of inoculum. In this study, we considered the role of grasses as a possible source of inoculum for F. circinatum-associated disease of Pinus in South Africa. Isolates of F. circinatum were collected from grasses in the understories of pitch-canker affected stands of Pinus radiata D.Don and Pinus patula Schiede ex Schltdl. & Cham. in the Western Cape and Limpopo provinces, respectively. The mating types and microsatellite-based genotypes of the isolates were then compared with those in collections of this pathogen associated with pitch-canker on P. radiata and P. patula in the respective regions. The results showed that the F. circinatum isolates from grass originated from inoculum produced from stem and branch cankers on the trees above the grasses. The discovery of F. circinatum from five grass species in Limpopo increases the total number of known grasses associated with the fungus to nine in South Africa and seventeen globally. All of the F. circinatum isolates recovered from grass in South Africa displayed levels of aggressiveness to P. patula seedlings that were comparable with those of an isolate used for routine screening of commercial planting stock. The data also suggest that grass might influence the expression of disease caused by F. circinatum on Pinus. This was because a specific genotype of the pathogen originating from grass was less aggressive on P. patula seedlings than its counterpart from diseased Pinus. Taken collectively, the results indicate that phytosanitary practices for the management of F. circinatum should include grass as a significant source of inoculum, and this should be an important quarantine consideration, both nationally and internationally.