Phenotypic and Genotypic Variation of Puccinia helianthi in South Africa.

Sunflower (Helianthus annuus L.) is the third largest grain crop by area planted in South Africa (SA). The annual yield is negatively affected by sunflower rust caused by Puccinia helianthi Schw. (Phe). Four Phe races were described in SA in the middle 1990s, but since then, no new race descriptions have been conducted. This has resulted in an information gap on the current Phe population, making it difficult to explain increased disease incidence and loss of resistance in previously resistant hybrids. To address this, 114 Phe field isolates along with 23 historic isolates were phenotyped using the international set of 11 sunflower differentials containing the R1, R2/R10, R3, R4a, R4b, R4c, R4d, R5, Pu6, and Radv resistance genes. Three new Phe races were identified, bringing the total number of South African races recorded to seven. No avirulence was detected attributable to the R1 gene, with the R4d and Radv genes remaining effective. Four main genetic lineages were detected with no obvious correlation between phenotype and genotype. The detection of three genetic lineages consisting exclusively of field isolates collected post-2006 suggested the possible recent entry of exotic introductions into SA. This, combined with the fact that one lineage consisted exclusively of the most virulent race Phe7721, confirmed a clear shift in the Phe population that could explain the increased virulence and occurrence of the disease in SA.