Variability among isolates of Fusarium udum and the effect on progression of wilt in pigeonpea

Vascular wilt (caused by Fusarium udum) is the most important disease of pigeonpea. Breeding strategies are needed to develop durable resistance against this pathogen. Knowledge of the genetic and pathogenic variability among isolates of F. udum is essential for effective deployment of resistance. Thirteen isolates of F. udum, collected from three regions of India were studied using cultural characteristics, molecular variability and pathogenicity. The isolates of F. udum showed variability in aerial and radial mycelium growth, colour of mycelia, substrate pigmentation, length and septation of macro- and microconidia. Amplified fragment length polymorphism (AFLP) analysis of the isolates determined seven phylogenetic groups. Pathogenesis and progress of wilt on susceptible pigeonpea cultivars varied and pathogenic isolates from the same phylogenetic group exhibited similarities in timing of infection and colonization. Non-pathogenic isolates formed separate clusters in the phylogenetic tree. The timing of fungal invasion, vascular clogging, drooping of shoots, and wilt establishment were demonstrated for the first time in pigeonpea. Vascular clogging occurred with susceptible cultivars at 78-84, 54-60, 48-60 and 36-48 h post inoculation (HPI) when inoculated with isolates of F. udum from AFLP groups 1, 3, 6 and 7, respectively. Subsequently, susceptible cultivars inoculated with isolates from the same groups exhibited wilt at 144-168, 120-144, 96-120 and 72-96 HPI, respectively. This study contributes to characterising vascular wilt of pigeonpea.