Necessity of Replicated Measurements for Selection of Alfalfa Plants Resistant or Susceptible to Stem Inoculation bySclerotinia trifoliorum

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