EXPRESSION OF RESISTANCE BY TETRAPLOID AND DIPLOID LUCERNE GENOTYPES TO PHYTOPHTHORA-MEDICAGINIS IS NOT INFLUENCED BY INOCULUM LEVEL OR BY TEMPERATURE

Phytophthora-resistant lucerne cultivars do not always perform well un der conditions of high disease pressure in the field. To determine whe ther resistance expression remains stable under different infection in tensities, tetraploid and diploid lucerne genotypes, genotypically def ined for their reactions to Phytophthora medicaginis, were clonally pr opagated, and the influence of different reproducible inoculum levels (0 . 5 and 5 . 0 g dry weight mycelium/kg dry weight potting mix), the period of exposure to these levels (10-60 days), and temperature (16/ 22 degrees C and 24/30 degrees C) on disease expression was determined in controlled environments. Generally, expression of resistance by re sistant genotypes, remained stable under these conditions. Biotic (e.g . Aphanomyces eutiches) or abiotic factors other than P. medicaginis m ay be responsible for the poorer than expected performance under field conditions in some instances, or the percentage of resistant plants i n some cultivars currently classified as resistant is insufficient to provide buffering against productivity reductions under severe epidemi cs. Further research is needed to clarify the situation.