Digital image analysis is an objective and nondestructive method potentially capable of providing accurate and precise estimates of disease resistance components. This study was conducted to quantify components of partial resistance to crown rust through the analysis of sequential digital images of inoculated leaves of adult oat plants, and to compare components found in two sources of resistance unrelated genetically. Uredinium density, relative infection frequency, latent period, days to first pustule appearance, uredinium size, and disease progress rates were assessed on three oat lines (RS-line 3W-C2R-9-3b, MN-841801, Starter) in two greenhouse experiments. Resistant lines had fewer and smaller uredinia, and these appeared later than in the susceptible check. Relative infection frequency, latent period, and uredinium size were equally important components in the expression of the partial resistance to crown rust, and the two sources of resistance could not be differentiated by any of the variables studied. The analysis of sequential digital images of diseased leaves produced precise estimates of partial resistance components and disease progress rates.

the absence of significant genetic interactions between host and pathogen genotypes (Parlevliet, 1985), the use Crown rust (caused by the fungus Puccinia coronata Cda. f. sp.of this type of resistance in crop cultivars should prevent avenae Eriks.) is a major disease of cultivated oat (Avena sativa L.). Partial resistance is a form of incomplete resistance characterized by a major shifts in the composition of the pathogen populareduced rate of epidemic development and is potentially more durable tion increasing the durability of the disease resistance than complete race-specific resistance. Four rapid cycles of selection (Geiger and Heun, 1989). for partial crown rust resistance were conducted in an oat recurrent Recurrent selection is a population improvement proselection population after the completion of the seventh cycle of cedure that increases the frequency of desirable alleles selection for grain yield. The objectives of this research were to: (i) through repeated cycles of selection and systematic redetermine the effectiveness of rapid cycle recurrent selection for parcombination. This breeding method has been widely tial crown rust resistance; (ii) assess the indirect effect on grain yield, used for improving quantitative traits in cross-pollinated flowering date, and plant height; and (iii) estimate phenotypic and crops. The expression of partial resistance to fungal genetic correlations between traits and broad-sense heritabilities. Recurrent selection parents and check cultivars were evaluated in six pathogens is usually controlled by several genes with environments. Four cycles of selection for partial resistance reduced additive gene action (Veillet et al., 1996; Geiger and the area under the disease progress curve (AUDPC) 42%. This in- Heun, 1989;Parlevliet, 1978). Although these attributes creased resistance was not reflected in higher grain yields under modsuggest that recurrent selection could be an appropriate erate crown rust epidemics, but it produced a 4.7% grain yield gain and effective breeding method for improving disease per cycle in an environment with a severe rust infection. Selection resistance in plants, it has received only limited attention for partial resistance indirectly delayed the flowering date by 2 d. in self-pollinated crops. Recurrent selection has been Entry mean broad-sense heritability estimates were intermediate (41used successfully to increase the resistance in soybean 64%) for AUDPC. Our results show the usefulness of rapid cycle [Glycine max (L.) Merr.] to Phytophthora rot (caused recurrent selection as a population improvement procedure capable of effectively increasing the level of partial resistance to crown rust by Phytophthora megasperma Drechs. f. sp. glycinea in a high-yielding oat population.Kuan and Erwin) (Walker and Schmitthenner, 1984), in barley (Hordeum vulgare L.) to leaf rust (caused by Puccinia hordei Otth.) (Parlevliet and Van Ommeren, 1988;Reinhold et al., 1993), and powdery mildew J.E.