CHOOSING CROP CULTIVARS AND CULTIVAR MIXTURES UNDER LOW VERSUS HIGH DISEASE PRESSURE - A CASE-STUDY WITH WHEAT

Four winter wheat cultivars and all possible equiproportional mixtures among them were grown under low and high severity of Septoria blotch (caused by Mycosphaerella graminicola and Leptosphaeria nodorum) and eyespot (caused by Pseudocercosporella herpotrichoides) in each of 3 years. On average, mixtures provided only a small reduction of Septoria blotch relative to their component pure stands (27, 9, 15% for the three seasons), however, mixtures containing both very resistant and very susceptible cultivars demonstrated larger reductions. Reductions of eyespot severity due to mixing averaged only 6% in the one mixture evaluated, and were statistically non-significant. The two most resistant cultivars yielded less than the two more susceptible cultivars under low disease pressure, while the opposite relationship between yield and disease resistance was found under high disease pressure. There was considerable variation among mixtures for yield, and the yield of a mixture could generally not be predicted a priori. Some mixtures demonstrated high yields over years and under both low and high disease pressure. For western Oregon and Washington, the best strategy may be to choose a cultivar with intermediate yield potential and disease resistance or to grow a cultivar mixture with proven yield performance.