OCCURRENCE OF Exserohilum turcicum F.SP.SORGHI THE CAUSAL ORGANISM OF SORGHUM LEAF BLIGHT IN UPPER EGYPT

In 2005, severe attack of sorghum leaf blight caused by Exserohilum turcicum was observed on grain sorghum G-15(Sorghum bicolar) and Sudan grass (Sorghum sudanense) adjacent to healthy maize plants in Sids Agricultural Research Station (SARS) of Upper Egypt, A.R.C. Therefore, field experiments were conducted in 2006, 2007 and 2008 to determine host specificity type of E. turcicum in SARS on a set of ten sorghum and maize varieties. Two E. turcicum isolates obtained from lesions of infected sorghum plants were tested for their virulence in the greenhouse on seedlings of the same set of varieties used in the field. Colony and conidial morphology, radial growth at four temperature and serological studies of these sorghum isolates were compared with two maize isolates. One colony type was clearly distinguishable between sorghum isolates from nature at SARS. Fungal colonies from sorghum were generally characterized as having dark olivaceous, determinate margin with scant appressed aerial hyphae. Meanwhile fungal colonies from maize had a determinate margin and profuse aerial hyphae about the center of a gray to green-white colony. Sids isolates of E. turcicum attacked only its own hosts, local grain sorghum and Sudan grass, under natural and artificial infections. Our results reveled that sorghum isolates had a fast radial growth in culture than maize isolates at 27°C. At the same time, there were considerable differences in conidial characters among isolates of E. turcicum from the two hosts although they were serologically identical. Conidia of sorghum isolates had a long length range and more septa than of maize isolates (66-145 µm, 6 and 40-92 µm, 4 septate respectively). Based on the occurrence of distinct fungal colony type of E. turcicum isolates from sorghum along with host-specificity, fungal isolates obtained from Sids were classified as forma specialis.