A
Potential Source of Resistance to Summer Black Stem.
K.D. Glover, P.C. St. Amand, D.Z. Skinner, and D.L. Stuteville.
Kansas State University and USDA-ARS; Manhattan, KS
Summer Black Stem, incited by Cercospora medicaginis (Ellis and Everh.), is a common and chronic alfalfa production constraint. Chocolate-brown leaf spots with irregular margins are the most obvious symptoms (Fig. 1).
Relatively low levels of resistance to this disease are present in many alfalfa varieties.The objectives of this research were to: 1) evaluate the performance of KS211 using a growth chamber screening procedure, 2) determine whether number of inoculations significantly altered test results, 3) determine consistency of screening results among tests, and 4) derive a broad-sense heritability estimate for Summer Black Stem resistance using tetraploid varieties ranging from resistant to susceptible. Twelve entries were included in a split-plot randomized complete block design with inoculations as main-plots and cultivars as sub-plots. Each test contained four blocks and five tests were performed.
KS211 was found to be significantly more resistant than all but three entries (P = 0.01). Four inoculations did not present significantly higher disease pressure than one inoculation. Therefore, all main plots were considered equal in subsequent analyses and the number of blocks within a test increased from four to eight. Pearson product correlations and Spearman rank correlations among tests were very high (> 0.90 ) when testing cultivars with significantly different resistance levels. Correlations among tests were low to moderate (< 0.50) when tests included a large number of cultivars where resistance levels were not significantly different. A broad-sense heritability estimate of 0.747 was calculated using 12 cultivars as the reference population. The moderate broad sense heritability estimate suggests that increasing levels of resistance to Summer Black Stem should be possible. Results indicate that KS211 is a good source of resistance to Summer Black Stem.
Figure 1. Leaf spots from artificial inoculation of Cercospora medicaginis on tetraploid alfalfa.