Overwintering form of the causal agent of shot hole disease in Khorasan Razavi, Iran.

Shot hole disease of stone fruit trees caused by some plant pathogenic fungi is a major constraint to stone fruit production worldwide where the trees are grown. Identification of the causal agents of the disease and their overwintering forms in stone fruit trees of Khorasan Razavi was necessary for disease management programs. Buds, twigs, fallen leaves and fruits were collected from the infected peach, apricot, nectarine and almond trees in winter 2007. The samples were superficially disinfested in 1% sodium hypochlorite for 2-3 min and then in 70% ethanol for 45 sec. Two to three fragments of 4x4 mm from each tissue were separately cultured on 2% water agar and potato dextrose agar (PDA), and purified on PDA. Just a pathogenic fungal species, Wilsonomyces corpophilus was isolated from the infected buds and twigs. No microorganism was isolated from the fallen leaves and fruits collected from underneath of the infested stone fruit trees. Pathogenicity of the fungus was examined on detached shoots of current year of four varieties of stone fruit trees. Fungal discs were placed under the bark of the bud base. Control shoots were similarly treated with sterile PDA discs. Inoculated shoots were placed in a humid growth chamber at 25 degrees C. Fungal hyphae appeared at 30 days post inoculation. Control shoots were asymptomatic. Pathogenicity intensities or lesion lengths were significantly different among the four varieties tested. A completely randomised design with five replicates was employed to measure the number of spores in infested buds and twigs of each variety of stone fruit tree. The samples were sliced and placed into a glass tube of centrifuge containing 3 ml of sterile distilled water. They were mixed on a vortex mixer for 30-40 min and centrifuged at 3000 rpm for 5 min. Pelleted material from each sample was suspended in 500 microl of sterile distilled water and the spores were counted using a hemocytometre. Results revealed that the fungus overwinters as hyphae and conidia in the infected buds, and as hyphae and globular chlamydospores in twig lesions.