Abstract
In natural field conditions, hybrids of sunflower (23 vars.) developed no or less (by 65%) collar rot infections by Sclerotium rolfsii Sacc. than composite var. Kironi. Under induced epiphytotic condition hybrids (var. Hysun 33) developed 91% less collar rot infection than the var. Kironi. Older plants (60-90 days age) developed infections quicker (8-10 days) and larger sized lesions than that in younger ones (10-45 days age). Sclerotial size of sunflower isolate of S. rolfsii was larger than isolates from some other hosts like amaranth, Indian spinach and brinjal. Sunflower plants developed less (by 57%) collar rot in dry (Rabi season) and less-irrigated conditions (by 56%). Soil amendments with inorganic fertilizers (N, P, K) proved ineffective while organic manuring and soil liming proved promising in controlling collar rot. Application of cowdung (8-10 t/ha), groundnut oilcake (300 kg/ha) and lime discouraged the infection (1.67%) by S. rolfsii. Liming to raise soil pH to 6.5 or 7.0 completely inhibited the infection of S. rolfsii. Thick epidermis and cuticle and small-sized cortical cells were found as the anatomical features of resistance in sunflower plants. Higher contents of Ca-oxalate, phenols, ascorbic acid and sugars in the stem were detected as components of varietal resistance to S. rolfsii.