Association of the pitch canker pathogen Fusarium circinatum with grass hosts in commercial pine production areas of South Africa

The pitch canker pathogen, Fusarium circinatum, has major impacts on production in pine nurseries and plantations in South Africa. Thus far, efforts to reduce local spread have focused on rouging of infected pines and sanitation to eliminate local sources of inoculum. Although the host range of F. circinatum was thought to be limited to pines and Douglas-fir, recent studies in California indicate that this fungus is capable of infecting grasses as a symptomless endophyte. Consequently, it is possible that grasses represent a reservoir of inoculum that influences the occurrence of disease in South African pine nurseries and plantations. The objectives of this study were to survey a wide range of grass species in both nurseries and plantations in South Africa for the presence of F. circinatum. In all, 22 species of grass were sampled at a nursery in Mpumulanga and in a plantation on the Western Cape. Isolates obtained from grasses were identified based on morphological criteria and DNA sequence data. Fusarium circinatum was recovered from vegetative tissues of four grass species including Briza maxima, Ehrharta erecta var. erecta, Pentameris pallida and one species that could not be identified. All isolates were pathogenic to pines and comparable in virulence to a known F. circinatum isolate that was included as a positive control. These studies indicate that grasses may constitute inoculum reservoirs that could facilitate persistence and dissemination of the pathogen in nurseries, and provide a means for the pathogen to move between widely separated pine stands, where grass hosts occur in intervening areas.