ROLE OF SCLEROTIA IN THE AGGRESSIVENESS AND PATHOGENICITY OF SCLEROTIUM CEPIVORUMYEHIA A.G. MAHMOUD, REDA H. SAMMOUR, ABDEL-ZAHER M.A. MUSTAFA AND REEM A. ALHOZEIMAbstract Twenty-six Egyptian isolates of Sclerotium cepivorum Berk., were collected from eleven Egyptian governorates. Pathogenicity was tested against five onion cultivars grown in local (Giza 6, Giza 20), regional (Baladi from Saudi Arabia), and international (yellow onion from USA and Ailsa Craig from UK) areas. The study revealed that S. cepivorum isolates Kf11, As1, Is, Mf, So1, and So2 had high virulence against all five onion cultivars with ratings of 5, while isolates Kf1, Kf2, Kf3, and Kf4 were rated the lowest in virulence based on disease severity. S. cepivorum sclerotia with high and low pathogenicity of four strains were observed at high and low magnifications using electron microscope analysis to reflect the sclerotia surface and layers. A study of the most virulent S. cepivorum isolate (Is) using electron microscopy showed regular spherical sclerotia, with one pointed end at the germination point. The surface was observed to be rough with many cracks, and contained a thick, multilayered cell wall with small dark spherical precipitates. The inner components of the sclerotia included dense, sharply defined cytoplasmic organelles with a rough outer surface, and a large nucleus with dense chromatin. The avirulent S. cepivorum isolate Kf3 was observed to have irregular sclerotia, and a non-pustular surface with distinctive deep grooves. The Kf3 isolateâs sclerotia contained less dense inner components; the cytoplasmic organelles were smooth and lacked an outer surface while the nucleus was small and granulated and had lighter chromatin than observed in the Is isolate. It can therefore be concluded that S. cepivorum aggressiveness may rely upon content of pigments such as melanin and thickness of cell wall. |