Usefulness of blast resistance genes
Identification of functional blast (caused by Pyricularia grisea) resistance genes for a particular region is a pre-requisite for their meaningful deployment. In a mini-network programme, we evaluated some of the blast resistance genes in six locations spread over in five different States (Cuttack in Orissa; Jagadaplur and Ambikapur in Madhya Pradesh; Almora in Uttar Pradesh; Jorhat in Assam and Suttur in Karnataka) under Uniform Blast Nursery conditions during the wet season of 1998. Local susceptible cultivars either Karuna (Cuttack), HR 12 (Jagadalpur, Ambikapur, Almora and Suttur) or Mahsuri (Jorhat) were used as spreader row. Uniform incidence of leaf blast occurred in all the test locations in the seedling stage of the crop. Leaf blast reaction was scored when the disease was at its peak using a 0-5 scale. Plants rated 1-3 were considered resistant, and those rated 4-5 were considered susceptible.
The susceptible check cultivars, Karuna, HR 12 and Mahsuri and the two recurrent parents, Kalinga III and Vandana being used at CRRI for improving their blast resistance were highly susceptible to blast in all the locations. Another susceptible cultivar Co 39 carrying resistance gene Pi-a (Co 39-gene) in which background, the near-isogenic lines with blast resistance genes have been developed succumbed to the disease in all the test locations excepting at Jorhat in Assam where it was resistant.
The effectiveness of resistance genes individually varied between locations. The genes, Pi-1(t) was effective in Jagadalpur, Ambikapur, Jorhat and Suttur and Pi-2(t) in Jagadalpur, Ambikapur and Jorhat, while Pi-3(t), Pi-4a(t) and Pi-4b(t) were not effective in general in all the locations tested. However, F-124-1 presumably carrying Pi-4a(t) showed variations in different locations. In all probabilities, this line might carry some additional resistance genes. Cultivar Moroberekan possessing many resistance genes was resistant in five out of six locations. However, among RILs derived from Moroberekan and Co 39, RIL 10 [Pi-12(t)] showed resistance only in Suttur and RIL 29 [(Pi-7(t)] succumbed to the disease in all the locations tested. Interestingly RIL 45 and 249 presumably carrying Pi-5(t) showed differential reaction to pathogen population at Jorhat and Suttur, while RIL 77 was susceptible at all sites tested suggesting that this RIL may not have the Pi-5(t) gene. LTH near-isogenic lines which were susceptible at Cuttack varied in their reactions at other locations, particularly in the east (Jorhat) and in the southern (Suttur) regions.
Of the four gene combinations evaluated, the one with Pi-1(t) and Pi-4(t) was susceptible to blast at Jagadalpur, Almora and Jorhat. The combinations with Pi-1(t) and Pi-2(t) and Pi-2(t) and Pi-4(t) were effective in all the six locations tested. Our results clearly depicts the usefulness of resistance genes in different locations.
Marker-assisted selection (MAS)-based breeding strategies for improving resistance to blast
Breeding material arising from the crosses made (Kalinga III/C101A51(Pi-2(t)), Vandana/C101A51, Kalinga III/O. minuta (WHD-IS-75-1-127(DM360)derivative, Pi-9(t)) and Vandana/O. minuta derivative) with the objective of pyramiding blast resistance genes (Pi-2(t), Pi-9(t)) were advanced via backcross method. BC3F1 with Kalinga III and Vandana background presently (wet season 2000) available will be further advanced via MAS backcross scheme for obtaining advanced backcross populations.