3.4.21
INFLUENCE OF ISOLATE AGGRESSIVENESS ON COMPONENTS OF RESISTANCE TO STAGONOSPORA NODORUM IN TWO POPULATIONS OF SOFT RED WINTER WHEAT

DE FRASER, JP MURPHY and S LEATH

North Carolina State University, Raleigh, North Carolina NC27695-7616, USA

Background and objectives
Stagonospora nodorum Berk., the causal agent of cereal leaf and glume blotch, causes significant yield losses annually on a worldwide basis [1]. Variation in severity associated with environmental conditions and isolate aggressiveness complicates selection of disease resistant genotypes. An understanding of the relationship between isolate aggressiveness and host response may provide the basis for an improved selection process. The objective of the study was to use isolates with predetermined levels of aggressiveness to measure differences in components of resistance among individuals in two segregating wheat populations, under both field and controlled environments.

Materials and methods
Isolates used in these experiments were selected, on the basis of resistance components, as the most and least aggressive isolates from a collection of >100 North Carolina field isolates. Isolate treatments consisted of the most and least aggressive isolates separately, together and a water sprayed control. Two wheat populations were used in both field and controlled experiments. Population A consisted of F4:5 lines derived from a resistant x susceptible cross. Population B consisted of random genotypes in F3, F4, F5 and F6 generations and was representative of a southern US wheat breeder's nursery.

Controlled environment: experimental design was 4x50x2 factorial design with 3 replications. The independent variables were 4 isolate treatments, 50 genotypes and 2 populations. Plants were inoculated at the two-leaf stage and the dependent variables recorded were incubation period, number of lesions and percent leaf area diseased (%LAD) five days after inoculation.
Field environment: experimental design was a 4x100x2 factorial design with 3 replications at each of 3 locations. The independent variables were 4 isolate treatments, 100 genotypes and 2 populations. Plants were arranged in a 10x10 grid of hill-plots, evenly spaced on 30 cm spacing. Inoculations were done at growth stage 37 of the Zadoks scale and the %LAD was recorded at 3 bi-weekly intervals after inoculation.

Results and conclusions


Incubation period and number of lesions were significantly different among isolate treatments in both populations under controlled conditions (P<0.0001). Genotypic differences within populations were also significant (P<0.05), except for incubation period in population A. Comparing incubation period, the most and least aggressive isolates resulted in a similar separation of resistant and susceptible individuals in population A. For population B, overall number of diseased individuals was higher. Incubation period was generally shorter with the most aggressive isolate, but greater numbers of individuals became infected when inoculated with the least aggressive isolate. In mixed inoculations of most and least aggressive isolates, the separation of resistant and susceptible individuals was the same for both populations. Additional components of resistance from controlled conditions and results from 1997/98 field studies in relation to the epidemiology of S. nodorum will be discussed.

References
1. Eyal Z, Scharen AL, Prescott JM, van Ginkel M, 1987. The Septoria Diseases of Wheat: Concepts and methods of disease management. Mexico, D.F.: CIMMYT, 52 pp.