Environmental requirements and host-specificity of Puccinia eupatorii, a potential biocontrol agent of Campuloclinium macrocephalum in South Africa

Campuloclinium macrocephalum (pompom weed) is considered one of the most important invasive weeds in the summer rainfall regions of South Africa. Two rust fungi, Puccinia eupatorii and P. conoclinii, are recorded on pompom weed in South America where the weed originates. Urediniospores from infected C. macrocephalum leaf material collected in northern Argentina were found to be pathogenic to the South African pompom weed. Microscopic evaluation of urediniospores and teliospores identified this rust fungus as Puccinia eupatorii. Urediniospores germinated at an optimal temperature of 18 °C and at temperature required a minimum dew period of 6 h. Urediniospores germinated 3 h after inoculation on C. macrocephalum leaves with stomatal penetration occurring approximately 12 h after inoculation. After 48 h fungal hyphae were observed radiating out from the point of infection. Host-specificity testing of thirty Asteraceae species, including crop and indigenous plants, was undertaken. Puccinia eupatorii was found to only infect C. macrocephalum, while all other plant species tested were either highly resistant or immune. Prior to the release of the Argentinian isolate of P. eupatorii tested, a rust fungus was discovered on pompom weed in South Africa and identified, through sequencing, to be the same species. Puccinia eupatorii is currently prevalent in most areas invaded by pompom weed in South Africa.