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Effect of Diseases on In Vitro Digestibility of Smooth Bromegrass¹

Disease epidemics reduce the forage yield of smooth bromegrass (Bromus inermis Leyss.) but little is known about their effects on in vitro dry matter digestibility (IVDMD). Thirty-one gcnotypes infected with three pathogens through natural inoculation in the field were evaluated for incident foliar diseases and IVDMD percentages. IVDMD percent was negatively correlated (—0.64, significant at the 1% level) with disease percent. A 1% increase in diseased area on the leaf, and age of leaf (as measured by one leaf lower on the culm), decreased IVDMD 0.12% and 0.19%, respectively.

Greenhouse inoculation of 33 genotypes with Drechslera bromi (Died.) Shoem. produced a significant decrease in IVDMD. Inoculation with bromegrass mosaic virus did not significantly reduce in vitro digestibility. Greenhouse inoculation of the same genotypes with Rhynchosporium secalis (Oud.) Davis. resulted in significant differences between IVDMD of inoculated and uninoculated plants; linear regression showed that the effects of increasing disease lesions by 1% would decrease IVDMD by only 0.04%. Since low levels of infection did not appear to have a large effect on digestibility as measured by IVDMD, a medium degree of resistance to foliar diseases should be sufficient for a smooth bromegrass cultivar for pasture use.

Key Words: Bromus inermis Leyss Bromegrass mosaic virus • Drechslera bromi (Died.) Shoem • Genotype • Pathogen • Rhynchosporium secalis (Oud.) Davis • Selenaphoma bromigena (Sacc.) Sprague and Johnson

² Former NDEA fellow (now research assistant, Trojan Seed Co., Olivia, Minn.) and professors of plant science, S. Dak. State U., Brookings, SD 57006.

Received for publication September 7, 1974.

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