Necessity of Replicated Measurements for Selection of Alfalfa Plants Resistant or Susceptible to Stem Inoculation by Sclerotinia trifoliorum . MYINT AUNG, Applied Research Division, Gyogone, Insein, Rangoon, MYANMAR (Burma). DENNIS E. ROWE and ROBERT G. PRATT, USDA-ARS, Forage Research Unit, Mississippi State, MS 39762-5367. Plant Dis. 78:14-17. Accepted for publication 22 September 1993. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1994. DOI: 10.1094/PD-78-0014.

The variation in lengths of necrotic tissue following replicated stem inoculations with Sclerotinia trifoliorum was characterized on 25 plants in each of four alfalfa populations. The plants were ranked for resistance based on measurements in 12 replications, and the five most resistant and the five most susceptible (selection pressure of 20%) plants in each population were identified. Subsets consisting of 11, 9, 7, and 5 replicates were systematically or randomly selected by a computer program from the original 12 replicates, and the plants were ranked again and selected for resistance. The probability of reselecting plants previously identified as either one of the five most susceptible or resistant plants in each population was determined for each size subset. With five replicated inoculations, there was an 83% probability of reselecting three to five of the most resistant or most susceptible plants, and a 55% probability of reselecting four or five of the most resistant or susceptible plants. These results indicated that at least five replicated measurements of each plant with the stem inoculation method are needed for selection of resistance to S. trifoliorum