The fungus Bipolaris sorokiniana causes brown spot, a barley (Hordeum vulgare) disease, worldwide. Identification of the fungus is based mainly on the morphology of its conidia, whose size and septation may be altered by many factors. This research studies the effect of growing substrates on the size, septation, and morphology of conidia, as well as the relationship of inoculum density to disease intensity. The various substrates included six culture media, seeds, and fresh leaves of barley, wheat (Triticum aestivum), rye (Secale cereale), and triticale (Triticum secalotricum). Conidia formed in culture media (68,2 × 21,9 mm; 5,7 pseudosepta) and on seeds (78,3 × 20,4 mm; 7,2 pseudosepta) were shorter, wider, and with less septa than those from leaf lesions (92,9 × 18,2 mm; 7,7 pseudosepta). The effect of the inoculum density (ID) on disease intensity (DI) was tested by applying spore suspensions (2.5 x 10³, 5.0 x 10³, 10.0 x 10³, 15.0 x 10³, and 20.0 x 10³ conidia/ml) to plants of the barley cultivar BR-2. The ID/DI relationship was represented by a quadratic model equation, in which the maximum values of 183 lesion/leaf and 79% disease severity were obtained with 16.500 and 14.000 conidia/ml, respectively. The number of conidia required for one leaf lesion was estimated in 50 to 90.
