Artificial Inoculation of Wheat for Selecting Resistance to Stagonospora Nodorum Blotch

Phaeosphaeria nodorum (E. Muller) Hedjaroude (anamorph = Stagonospora nodorum (Berk.) Castellani & E.G. Germano) causes Stagonospora nodorum blotch (SNB), a disease of wheat (Triticum aestivum) leaves, stems, and glumes. SNB occurs persistently, but with varying degrees of severity, in the eastern U.S. soft red winter wheat region. Acceptable levels of partial resistance are available in adapted soft red winter wheat lines, yet many widely grown varieties are susceptible (4). Progress in breeding for SNB resistance in this region has been modest (23) due to a lack of continuous selection pressure, low heritability of resistance, and sporadic natural epidemics (9,10). P. nodorum has both sexual and asexual stages, and populations from diverse geographic regions have high levels of gene and genotypic diversity (11,12) and a balance of mating types (6,24) consistent with random mating. Uncertainty remains about the relative importance of seed infections, pycnidiospores, and ascospores from previous crop debris in establishing SNB epidemics (6,11,12,17,22). The pathogen may be seedborne in a high percentage of wheat seed samples, and the percentage of seed that is infected can vary considerably (3,7,17,21,27). An effective and practical artificial field inoculation technique would facilitate breeding for resistance to P. nodorum (8,10), but little is known about the best timing for such inoculations. Williams and Jones (27) found that severity of seed infection by P. nodorum was greatest when inoculations were performed at 1⁄2to 3⁄4head emergence, and declined steadily when plants were inoculated at later growth stages. Wainshilbaum and Lipps (25) found that the growth stage of winter wheat significantly affected its susceptibility to P. nodorum, with leaf blotch severity increasing with increasing plant age at time of inoculation. This effect was observed at a range of temperatures (19, 24, and 29°C) in controlled environment chambers. Researchers have considered the number of isolates advisable for artificial inoculation of breeding materials (2,15,16). In general, P. nodorum interacts quantitatively with wheat (8,13), and major genes for virulence or resistance have not been reported in this pathosystem. Significant cultivar-by-isolate interactions have been found (1,2,14,19,20), although the magnitude of the interaction was generally small relative to the main effects, and nonsignificant interactions have also been observed (16). Fraser et al. (10) observed a masking of host genetic variation after inoculation with a single isolate in one site-year. Concluding that field inoculations with selected P. nodorum isolates at boot stage did not distinguish more and less resistant host cultivars better than natural inoculum, they suggested investigation of naturally infected wheat straw as an inoculum source. Currently, soft red winter wheat in some breeding nurseries is inoculated at boot stage (Zadoks GS 40-49) with P. nodorum conidial suspensions. However, many breeding programs rely solely on natural epidemics. The objective of this study was to assess the efficacy of various artificial inoculation methods. Three methods of artificial inoculation (P. nodorum–infected wheat straw applied in the fall, P. nodorum conidia applied in the fall, and P. nodorum conidia applied in the spring) were compared with each other and with natural inoculum. The criteria for evaluation of an artificial inoculation method were its capacity to discriminate among cultivars, its accuracy relative to natural inoculum in ranking cultivars, its representativity of the average inoculation environment in the experiment, and its practicability for breeding programs. The study also allowed us to examine the influence of disease severity on yield and test weight.