Estimates of Heterosis and Combining Ability for Resistance of Maize to Colletotrichum graminicola

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Abstract
Various heterosis constants for resistance in maize to leaf blight and stalk rot caused by C. graminicola were estimated from 45 F1 diallel crosses and their 10 parental inbreds. Leaf blight reactions, disease severity and lesion lengths were evaluated on seedlings (5-6 leaf stage) in the greenhouse and on adult plants in the field 2 wk after mid-silk. Stalk rot reactions, number of internodes discolored 75% or more and total number of internodes discolored were evaluated in the field following artificial inocualtion. Seedling blight reactions in the greenhouse were significantly correlated with leaf blight reactions of adult plants in the field, but leaf blight was not correlated with stalk rot. All constants, parental inbreds (Vi), overall heterosis (hij), the average heterosis (.hivin.h), the parent heterosis (hi), general (g.c.a.) and specific combining ability (s.c.a.), and mean squares (m.s.) were significant for all disease evaluations except (.hivin.h) effects for stalk rot reactions (number of internodes discolored 75% or more) and s.c.a. effects for lesion length from adult plants. Overall heterosis effects accounted for 29-43% of the disease severity of the entry sum of square (s.s.). Certain susceptible inbreds contributed more heterosis for resistance in single crosses than resistant inbreds. Crosses between resistant inbreds were more resistant to C. graminicola for both leaf blight and stalk rot than were those involving intermediately resistant or susceptible inbreds. Of the inbreds studied Pa91 and T111 were the most resistant to leaf blight. The significant g.c.a. effects for resistance were usually found from resistant inbreds. Predominant additive gene effects and partial dominance for resistance indicate that combinations of inbreds resistant to leaf blight with inbreds resistant to stalk rot may be used to produce hybrid corn resistant to both leaf blight and stalk rot.

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