NECESSITY OF REPLICATED MEASUREMENTS FOR SELECTION OF ALFALFA PLANTS RESISTANT OR SUSCEPTIBLE TO STEM INOCULATION BY SCLEROTINIA-TRIFOLIORUM

The variation in lengths of necrotic tissue following replicated stem inoculations with Sclerotinia trifoliorum was characterized on 25 plan ts in each of four alfalfa populations. The plants were ranked for res istance based on measurements in 12 replications, and the five most re sistant and the five most susceptible (selection pressure of 20%) plan ts in each population were identified. Subsets consisting of 11, 9, 7, and 5 replicates were systematically or randomly selected by a comput er program from the original 12 replicates, and the plants were ranked again and selected for resistance. The probability of reselecting pla nts previously identified as either one of the five most susceptible o r resistant plants in each population was determined for each size sub set. With five replicated inoculations, there was an 83% probability o f 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.