Identification of fungal causal agents of grapevine trunk diseases in Fars province

Grapevine trunk diseases are destructive fungal diseases that have considerably increased in the recent decades worldwide. More than 130 fungal species were associated with grapevine trunk diseases worldwide; however, the pathogenicity tests have not been conducted for some fungal species. From 2018 to 2020, grapevines with stunted growth, yellowing, branch dieback, and discoloration of the wood tissues symptoms were observed during field observation in the Fars province of Iran. This study aims to isolate, identify and characterize the fungal agents that cause these diseases using a combination of the morphological and molecular approaches.

Grapevines with stunted growth, yellowing, branch dieback, and discoloration of the wood tissues symptoms were used for the isolation of fungal strains. Small pieces of infected wood tissues taken from the margin of symptomatic and approximately healthy parts were surface sterilized and plated onto potato dextrose agar and malt extract agar media amended with ampicillin. Fungal colonies were purified using single spore and hyphal tip methods. Isolates were identified using morphological characteristics as well as sequencing of different gene regions. The pathogenicity of the isolates was tested in greenhouse conditions using one-year-old seedlings of grapevine. To complete Koch's Postulates, fungal agents were re-isolated from inoculated plants and identified.

166 fungal isolates were obtained and identified using the morphological characteristics as well as sequencing of different gene regions. The isolates were identified as Botryosphaeria dothidea, Cytospora chrysosperma, Fomitiporia mediterranea, Kalmusia variispora, Macrophomina phaseolina, Phaeoacremonium minimum, and Phaeoacremonium parasiticum. Among the species, P. minimum (28.31%) and C. chrysosperma (5.42%) had the most and least frequency, respectively. Pathogenicity tests using one-year-old seedlings in glasshouse showed all tested isolates can cause grapevine trunk diseases and produced white rot (in F. mediterranea) or wood necrosis (in other species) on inoculated tissue, developing from the point of inoculation. 

This study's results showed that grapevines in Fars province are infected with fungi causing trunk diseases, which were previously reported from other provinces of Iran and other countries. According to our knowledge, it is the first report of all species, except P. minimum and P. parasiticum, causing grapevine trunk diseases in Fars province. More studies are required to better understand the causal agents of grapevine trunk diseases in other parts of Iran and introduce chemical or natural components that can reduce the economic losses caused by grapevine trunk diseases.