Estimates of heterosis and combining ability for resistance of maize to Colletotrichum graminicola.

LIM, S. M. , and D. G. WHITE. 1978. Estimates of heterosis and combining ability for resistance of maize to Colletotrichum graminicola. Phytopathology 68:1336-1342. Various heterosis constants for resistance in maize to leaf discolored 75% or more) and s.c. a. effects for lesion length blight and stalk rot caused by Colletotrichum graminicola from adult plants. Overall heterosis effects accounted for 29 were estimated from 45 F, diallel crosses and their ten to 43% of the disease severity of the entry sum of square (s.s.). parental inbreds. Leaf blight reactions, disease severity, and Certain susceptible inbreds contributed more heterosis for lesion lengths were evaluated on seedlings (fiveto six-leaf resistance in single crosses than resistant inbreds. However, stage) in the greenhouse and on adult plants in the field 2 wk crosses between resistant inbreds were more resistant to C. after mid-silk. Stalk rot reactions, number of internodes graminicola for both leaf blight and stalk rot than were those discolored 75% or more, and total number of internodes involving intermediately resistant or susceptible inbreds. Of discolored were evaluated in the field following artificial inbreds used in the study, Pa9I and TI II were the most inoculation. Seedling blight reactions in the greenhouse were resistant to leaf blight. The significant g.c. a. effects for significantly correlated with leaf blight reactions of adult resistance were usually found from resistant inbreds. Based plants in the field, but leaf blight was not correlated with stalk on predominant additive gene effects and partial dominance rot. All constants, parental inbreds (Vi), overall heterosis (hij), for resistance, we believe that combinations of inbreds the average heterosis (fi), the parent heterosis (hi), general resistant to leaf blight with inbreds resistant to stalk rot may (g.c.a.) and specific combining ability (s.c.a.), and mean be used to produce hybrid corn resistant to both leaf blight squares (m.s.) were significant for all disease evaluations and stalk rot. except fi effects for stalk rot reactions (number of internodes Additional key words: Zea mays L., anthracnose, disease resistance, heterotic effects, genetic constants, combining ability. The maize anthracnose caused by Colletotrichum through cheesecloth. The conidial concentration was graminicola (Ces.) Wiles. has been known primarily as a estimated with a hemocytometer and adjusted to 3.5 X leaf spot disease in the United States. Recently, this 10/ml. Stalk rot inoculum was prepared by washing disease has been observed more frequently throughout a conidia from the culture surface and adjusting the wide range of maize-growing areas and it often causes not suspension to 2 X 10 conidia/ml. only a severe leaf spot but a stalk rot resulting in preForty-five F1 diallel crosses and their ten parental mature plant kill and in lodging (7, 13, 15). Severe inbreds were tested. Ten inbreds representing a range in infection on the upper portion of the maize stalk, susceptibility to C. graminicola were crossed in all including the tassel, has been observed in Central Illinois possible combinations. Inbreds used were Pa9 1, TI 11, for several years (5). The increase in importance of maize C123, Oh07B, Mo 17, Va26, R177, H95, B73, and Mo940. anthracnose has prompted studies on the distribution, Three pots were planted with ten kernels per pot for pathogenicity, and survival of C. graminicola and on the each entry, and seedlings were thinned to five plants per reactions of maize to this fungus (11, 12, 14). pot at the twoto three-leaf stage. Seedlings at the fourto In this paper we report the effects of heterosis and five-leaf stage were sprayed with the spore suspension combining ability on resistance to maize anthracnose. until runoff. They then were placed in moisture chambers for 16 hr. A randomized complete block design with three MATERIALS AND METHODS replications was used to arrange pots in the greenhouse benches. An isolate of C. graminicola was obtained from maize Plants were rated for disease reaction 14 days after leaves infected in the field and cultured on oatmeal agar at inoculation using the Horsfall-Barratt grading scale (3). room temperature. Leaf blight inoculum was prepared by The readings were converted with Elanco Conversion blending sporulating cultures with distilled water in a tables (Eli Lilly and Co., Indianapolis, IN 46206) into Waring Blendor and filtering the resulting suspension percent leaf area infected. Also, the length of clearly 00032-949X/78/000242$03.00/0 expressed lesions on the fifth leaf of five plants per pot Copyright © 1978 The American Phytopathological Society, 3340 were measured. Pilot Knob Road, St. Paul, MN 55121. All rights reserved. During 1976, a set of parents and all possible F1 crosses,