Scoring systems for evaluating rice varietal resistance to bacterial blights (BB): lesion size by growth stage [Philippines]
1988
Baw, A. | Mew, T.W. (International Rice Research Inst., Los Banos, Laguna (Philippines))
Lesion development was measured at different growth stages on 22 rice cultivars grown in pots in the greenhouse and infected with BB [bacterial blight] isolate PXO61 or rice 1. Planting was staggered to synchronize inoculation at different growth stages. Total leaf area was measured using a leaf area meter at 11, 14, and 17 d after inoculation. Lesion were cut from individual leaves, the lesion area measured, and percentage infected leaf area calculated. The cultivars were classified into three groups. Group 1 (including TN1) was highly susceptible at 20 d after sowing (DAS), but the lesion areas gradually decreased from 30 to 50 DAS and became constant from 50 to 80 DAS. In group 2, lesion area decreased from 20 to 40 DAS but the plants remained susceptible. Lesions decreased abruptly at 50 DAS, and were less than 5% at 80 DAS. In group 3, lesion area did not exceed 10% even at 20 DAS. These results show that even in a susceptible variety, lesion development decreases with plant age. Maximum lesion percentage in a susceptible variety such as TN1, IR8, or IR24 seldom reaches more than 50% of leaf area. This suggested that scoring disease by plant age requires different observation periods. Lesion area in most of the cultivars increased when the plants were younger and decreased as the plant became older.
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