Mode of gene action for resistance to Rice Blast Disease among the commercially released resistant rice varieties in Uganda

Abstract

Rice blast, caused by Magnaporthe oryzae, is a significant disease that threatens rice production in Uganda. Resistant varieties have been identified; however, their resistance diminishes within 2 to 3 growing seasons. Thus, it is essential to persist in identifying resistance, particularly within local varieties. The objective of this study was to examine the mode of gene action for resistance to rice blast disease among the local commercial resistant sources in Uganda, with a view to identifying the genetic mechanisms underlying resistance and informing breeding strategies for the development of blast resistant rice varieties adapted to Ugandan agro-ecological conditions. Seven resistant genotypes, comprising two lines exhibiting monogenic resistance and five local varieties, were crossed with five susceptible locally adapted varieties using a North Carolina design II (NCDII). The F2 populations of the 26 crosses were established in both the screenhouse and field utilizing a randomized complete block design (RCBD). The findings indicate that the inheritance of resistance to rice blast in these genotypes is predominantly additive. The crosses and general combining ability (GCA) of the male resistant parents exhibited high significance (P<0.001); whereas both specific combining ability (SCA) and GCA of the female susceptible parents were not significant. The observed genetic similarities among female parents in susceptibility transmission, coupled with the non- significant SCA effects, underscore the predominance of additive gene action in the inheritance of rice blast resistance. The high broad sense (0.85) and narrow sense (0.74) heritability estimates, along with a Baker’s ratio of 0.86, further confirm the reliability of additive effects in resistant × susceptible crosses. The segregation ratios ranging from single dominant gene (3:1) to complex epistatic interactions (9:7, 13:3, and 27:37) suggest that resistance is controlled by one to three genes, depending on the cross. This genetic insight enables breeders to select appropriate parental combinations and apply marker-assisted selection (MAS) to efficiently incorporate and stabilize resistance traits across generations, thereby addressing the core research objective of developing blast-resistant varieties with long-term field durability. These findings are critical for guiding breeding strategies aimed at introgressing durable resistance into elite Ugandan rice cultivars.