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

Genetic resistance is the only practical method to control foliar dise ases on forage grasses. Once developed, resistance must be transmitted , without selection, through several generations of seed increase. Thi s study was conducted to determine whether disease resistance would be maintained when disease resistant germplasms were advanced two genera tions without conscious selection for disease resistance. Orchardgrass (Dactylis glomerata L.) germplasm PL-OGDR1 resistant to purple leaf s pot (caused by Stagonospora arenaria Sacc.), and smooth bromegrass (Br omus inermis Leysser) germplasm PL-BDR1 resistant to brown leaf spot [ caused by Pyrenophora bromi (Died.) Drechs.] were used to initiate thi s study. Plants of the germplasm releases, two populations produced by unselected advance, and three cultivars of each species were rated fo r 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 freq uency distribution of disease scores within generations remained relat ively constant for generations. Cultivars were much more susceptible, with disease scores that averaged twofold to threefold higher than res istant germplasms. Results indicated that in the absence of selection pressure, disease resistant populations developed by five cycles of re current phenotypic selection for resistance to orchardgrass purple lea f spot or bromegrass brown leaf spot can be advanced for two successiv e unselected generations without a shift in resistance.