CLIMATE, POWDERY MILDEW, SAINFOIN RESISTANCE AND YIELD

Associations between dry matter yield (DM), 120 powdery mildew epidemics, six climatic variables, and resistance index were assessed during three growing seasons in 40 sainfoin accessions. In contrast to a two-year greenhouse study, powdery mildew developed on all sainfoin accessions in field experiments. During three growing seasons, plant reactions to powdery mildew varied by accession and study year. Only one accession, Oshnavieh, was rated tolerant to powdery mildew. According to contingency and correspondence analyses, high DM yield in sainfoin crops corresponded to 17-23 days with average relative humidity (RH) below 50%, 13 days with maximum temperatures above 25 degrees C, 4-7 days with average temperatures within 15-20 degrees C and RH above 50%, powdery mildew severity lower than 40%, and 0-3 mm rainfalls during four weeks before disease assessments. Greater disease severity (> 40%) was associated with lower DM yield levels (<= 0.150 t/ha), fewer (12) days with average RH below 50%, 23-28 days with maximum temperatures above 25 degrees C, no exposure to daily maximum temperatures above 35 degrees C, 14 days with average temperatures within 15-20 degrees C and RH above 50%, fewer (2-4) days with average temperatures within 20-25 degrees C and RH below 50%, and total rainfall as high as 13 mm. Such field-scale epidemiological findings provide useful information to develop accurate predicting models for sainfoin powdery mildew.