Molecular phylogeny and pathotyping of Fusarium solani: a causal agent of Dalbergia sissoo decline

Sissoo (Dalbergia sissoo), commonly known as shisham, is amongst the finest woods of South Asia, but wilt' disease has caused a rapid decline in this species. The cause of the disease remains uncertain. The aim of this study is to identify the causal agent of the disease and characterize isolates made from diseased trees, based on genomic data and variations in virulence. Samples of infected roots, stems and the ooze exuded from infected trees were obtained from plants showing symptoms in different geographical regions of India for the isolation of microorganisms. Isolates were used to inoculate healthy plants. Based on the morphological characteristics, genus- and species-specific PCR, and in silico analysis of 5.8S rDNA-ITS regions, of the 38 fungal isolates, 24 and 14 were identified as Fusarium solani and Fusarium sp., respectively. In a pathotyping study, eighteen F.solani isolates, isolated from roots and stem parts of symptomatic plants, induced typical wilt symptoms when inoculated through soil and roots on D.sissoo seedlings of 1-15months in age. The population of F.solani was the highest in infected roots and the lowest in parts of stems, gradually decreasing with height, and was isolated constantly up to approximately 40% height of the seedling. F.solani isolates used in inoculations were successfully re-isolated from the rhizosphere, infected roots and wilted stems, as confirmed using isolate-specific DNA fingerprints. Molecular phylogenies based on rDNA-ITS sequences showed that the 38 isolates fell into 2 groups. Group I comprised of F.solani isolates from D.sissoo and F.solani sequences in the NCBI GenBank database, whereas group II included Fusarium isolates other than F.solani. These results are helpful in developing integrated control measures for this highly variable pathogen and to establish a base for future population studies.