Noble Indices for Degree of Host Plant Resistance against Alternaria Blight in Rapeseed-mustard Genotypes

Experiments were conducted at Genetics and Plant Breeding Research Farm of N. D. University of Agriculture and Technology, Kumarganj, Faizabad (U.P.) to evaluate 15 promising genotypes of rapeseed-mustard against Alternaria blight caused by Alternaria brassicae (Berk) Sacc. and A. brassicicola (Schw) Wiltshire. It is the most destructive and major disease problem under eastern Uttar Pradesh in Indian condition causing both the quantitative and qualitative losses. The evaluation criteria for host resistance against the disease considered were the number and size of spot, number of conidia per spot on both the vegetative (leaf) and reproductive (pod) parts were regularly recorded at periodical intervals starting from the disease appearance till the physiological maturity of each genotypes. The indices which were considered for in built resistance were delayed appearance of visible symptoms, minimum number of spot and their size, number of conidia per spot as well as the minimum leaf defoliation. The disease appeared first in genotype T-9 (44 DAS) which was delayed in GSL-5 (53 DAS). The number of spot/10cm2, size, sporulation capacity in terms of conidia per spot varied invariably among the genotypes and significantly in some cases. The appearance of disease symptoms in the form of blighted spots varied invariably (44 to 53 DAS) in different genotypes. The delayed appearance of disease symptoms (53 and 52 DAS), lower number of spot / 10cm2 on leaf (3.07 and 3.65), pods (3.74 and 4.14), smallest spot size on leaf (3.54 and 3.05 cm), pods (1.22 and 1.27 cm2), number of conidia per spot on leaf (1230 and 1630), pods (130 and 150) were visualised in genotype GSL-5 and Pusa Aditya,respectively indicating a certain pattern and may be considered as attributes of disease resistance. Further, this fact was strengthened with minimum leaf defoliation (32.03 and 35.55%), PDI on leaf (12.35 and 14.16), pods (7.73 and 7.93) and ultimately the AUDPC calculated on leaf (486.70 and 551.90) and pods (282.85 and 291.05). Accordingly both the genotypes were grouped in moderately resistant (MR) category. The correlation coefficient (r) among parameters attributing to resistance behaviour worked out proved helpful in drawing the conclusion. Because very limited source of durable resistance are available, it may be suggested to exploit the two genotypes i.e.GSL-5 and Pusa Aditya for breeding programme against Alternaria blight.