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United States Department of Agriculture

Agricultural Research Service

Title: Expression of Resistance by Tetraploid and Diploid Lucerne Genotypes to Phytophthora Medicaginis Is Not Influenced by Inoculum Level Or by Temperature

Authors
item Irwin, J - UNIV OF QUEENSLAND-AUSTR
item Maxwell, Douglas - UNIV OF WISCONSIN-MADISON
item Smith, Richard

Submitted to: Australian Journal of Agricultural Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 14, 1996
Publication Date: N/A

Interpretive Summary: Alfalfa varieties considered to have resistance to the fungal disease Phytophthora do not always perform well under conditions of high disease pressure in the field. To determine whether resistance expression remains stable under different infection intensities, the influence of different levels of disease inoculum, the period of plant exposure to these levels, and temperature on disease expression were determined in controlled environments. Generally, expression of resistance by resistant plants remained stable under all these conditions. This research suggests that factors other than the disease organism, P. medicaginis, may be responsible for the poorer than expected performance under field conditions. These factors may include other disease organisms, environmental effects, or the percentage of resistant plants in a variety. It would appear that the percentage of resistant plants in some varieties currently classified as resistant to P. medicaginis is not sufficient to provide protection under severe epidemics of the disease. Therefore, to get high, consistent forage yields, farmers should utilize persistent varieties of alfalfa with high levels of resistance to all important diseases including Phytophthora.

Technical Abstract: Phytophthora-resistant lucerne cultivars do not always perform well under conditions of high disease pressure in the field. To determine whether resistance expression remains stable under different infection intensities, tetraploid and diploid lucerne genotypes, genotypically defined for their reaction to Phytophthora medicaginis, were clonally propagated, 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 deg C and 24/30 deg C) on disease expression was determined in controlled environments. Generally, expression of resistance by resistant genotypes remained stable under these conditions. Biotic or abiotic factors other than P. medicaginis may be responsible for the poorer than expected performance under field conditions in some instances, or the percentage of resistant plants in some cultivars currently classified as resistant is insufficient to provide buffering against productivity reductions under severe epidemics. Further research is needed to clarify the situation.

Last Modified: 9/22/2014
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