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Show 22 Utah Academy of Sciences, Arts and Letters [Vol. XV, initiation of the curly top project and the following procedure was developed and used with the tomato. Seed of the varieties to be tested are drilled as of frost is over. and care is taken to be certain that soil soon as danger type, depth of planting and the application of water is as uniform as it is possible to obtain on the open field plot. As soon as. the plants are in the primary leaf stage of growth, inocula tions of an arbitrary number of plants of each selection begin, by caging a given number of adult viruliferous insects on each plant. Inoculations are made on additional groups of plants of each selection each week for three or four weeks and then at bi-weekly intervals until about eight inoculations have been made. In this manner inoculations are made at different age levels on representative plants of the same selections having the same history. For the purpose of making comparative readings possible. the cycle of disease development from its incipiency to the death of the plant is divided The perfectly healthy plant is considered as being free of into five stages. the disease and therefore is represented on the records as being in a zero stage. The very earliest expression of the disease if classified as stage one, and as the expression of the disease becomes more pronounced and the symp toms most characteristic of the disease begin to take form. the plant is said As the disease reaches its full expression and the to exhibit stage two. characteristic symptoms of the disease are most pronounced. the plant is said to exhibit stage three. Following the full expression of the disease and the accompanying cessation of growth. necrosis sets in and the tissues gradu as ally die and turn brown. Plants exhibiting such necrosis are considered the expressing stage four. Complete death of the plant is considered as Each inoculated plant is given a number and at each disease in stage five. of (made at weekly intervals) the numerical value of the stage . reading disease manifested by the plant is recorded. in At the end of the season the disease stages exhibited by all of the each for reading. totalled be weekly oculated plants of each selection may From these data a comparison of the progress of the disease on the various selections may be made at weekly intervals and the rate of disease develop The last reading of the year will ment in the various selections obtained. the respective selections to the disease give a record of the total response of for the season. of the A coefficient that will convey an idea of the relative resistance the even be though that terms compared may various selections in numerical obtained by di number of inoculated plants of each selection may differ, is inoculated into a figure viding the total disease-stages exhibited by the plants all the death of and plants of the selection indicating complete susceptibility were records if completed on 90 Thus, for which records are complete. and death would susceptibility the complete a season, variety during plants of 450. or Assume, for times 90, have a disease stage numerical value of 5 the 90 plants exhibited disease by the stages last the reading example, that on 450 + 375 = .2. If the totalled 375, the coefficient of resistance would be the coefficient of re disease stage exhibited by the 90 plants totalled 450, would and express complete sistance would be 450 + 450 = 1 or unity, disease the hand stages ex other the on If susceptibility to the disease. resistance of wc:uld be hibited by the 90 plants totalled 0, the coefficient resistance complete would represent In this case infinity 450 + 0 = 00. to infinity for A scale of response ranging from 1 for complete susceptibility complete resistance is employed. A vegetable variety that consistently exhibits . resistance coefficient of is consistently coefficient the I f 50 may be considered commercially desirable. of as excellent and in possession considered be 100 or over, the variety may a pronounced degree of resistance. very a |
