STABILITY OF RESISTANCE TO LEAF DISEASE IN ORCHARDGRASS AND SMOOTH BROMEGRASS GERMPLASMS

Genetic resistance is the only practical method to control foliar diseases on forage grasses. Once developed, resistance must be transmitted, without selection, through several generations of seed increase. This study was conducted to determine whether disease resistance would be maintained when disease resistant germplasms were advanced two generations without conscious selection for disease resistance. Orchardgrass (Dactylis glomerata L.) germplasm PL-OGDR1 resistant to purple leaf spot (caused by Stagonospora arenaria Sacc.), and smooth bromegrass (Bromus inermis Leysser) germplasm PL-BDR1 resistant to brown leaf spot [caused by Pyrenophora bromi (Died.) Drechs.] were used to initiate this study. Plants of the germplasm releases, two populations produced by unselected advance, and three cultivars of each species were rated for disease reaction in two greenhouse inoculation tests. Mean spot size and disease coverage scores were low for all generations of resistant germplasms, and there were no differences among generations. The frequency distribution of disease scores within generations remained relatively constant for generations. Cultivars were much more susceptible, with disease scores that averaged twofold to threefold higher than resistant germplasms. Results indicated that in the absence of selection pressure, disease resistant populations developed by five cycles of recurrent phenotypic selection for resistance to orchardgrass purple leaf spot or bromegrass brown leaf spot can be advanced for two successive unselected generations without a shift in resistance.