Identification and pathogenicity of Pythium spp. isolated from walnut seedlings in Fars province nurseres

To study the role of Pythiym species associated with walnut root and crown rot, diseased sampled were collected from Fars province’s nurseries. Pieces of infected root and crown of walnut were washed with tap water blotted dry and plated on CMA supplemented with delvocid (10ppm pimarcin), ampicillin (250ppm), rifampicin (10ppm) and PCNB (150ppm). 23 isolates recoverd from various parts of the Fars province, 8 isolates were identified as P. vexans that were isolated from crown and root in Roodbal, Zarghan, Lapoee and Abbarik and Bohrghan. 5 isolates were identified as P. deliense which were isolated from walnut crown nurseries of Bisto due bahman, Roodbal, Sado ou dah and Abbarik. 10 isolates were identified as P. aphanidermatium that were isolated from walnut crown and root in Bohrghan, Zarghan, Lapoee, Sado ou dah and Bisto due bahman. Pathogenicity test of the isolates was carried out on detached branches. In all cases P. aphanidermatium was more pathogenic and aggressive than P. deliense and P. vexans. The reaction of crown and root of 2-month-old Reez Yasouj cultivars to virulent isolates of P. aphanidermatium, P. deliense and P. vexans were evaluated under greenhouse conditions. The inocula were produced for several weeks on vermiculite supplemented with hemp seed extract and applied around each two month-old walnut seedling. Comparative percent dead seedlings and crown and root colonization showed that Reez Yasouj cultivar was infected with these pathogens but in all cases P. aphanidermatium was more pathogenic and aggressive than P. deliense and P. vexans.