Potato leafroll virus: evaluation of resistance in potato cultivars

Benton (1976-1981) in New Brunswick. Bagnall, R. H. , and Tai, G. C. C. 1986. Potato leafroll virus: Evaluation of resistance in potato The different cultivars were planted in cultivars. Plant Disease 70:621-623. single-row plots, 25 plants to an 8-m plot with 32 plots to a full row. Each third row Thirty-six potato cultivars tested in the field for resistance to potato leafroll virus (PLRV) at was planted with PLRV-infected tubers Fredericton, NB, from 1972 through 1981 were separated into four resistance groups by Scott- of the cultivar Saco, highly resistant to Knott cluster analysis. Susceptible groups A (47.4-55.0% infection) and B (26-39.8%) and moderately resistant group C (11.6-24.8%) differed significantly from the resistant group D potato viruses A and X (24)and S (4)and (0-4.4%) in a supplementary Duncan's multiple range test. An additional 21 commercial cultivars moderately so to PVY (6). Two replicated and numerous seedlings from the Fredericton breeding program, which were in the trials for 5-9 of but independently randomized blocks the 10 yr, could be classified by comparison with these four groups. Comparisons with official were planted each year. Two buffer rows Dutch lists indicate that under higher selection pressure, group D cultivars could be split into were located on each side of the field, with several further categories. Through development and use of high-quality group D cultivars, PLRV a 3-m buffer plot at each end. A fungicide could be reduced drastically in the northeastern seed-growing areas (Maine and New Brunswick). (mancozeb) was used to control the late Even the moderate resistance of group C cultivars could temper the infrequent PLRV epidemics blight fungus, and an insecticide (carbothat occur in this area, but displacement of the PLRV-susceptible Russet Burbank, widely grown furan) was used against flea beetles and for french fry processing, is a formidable task for breeders. Colorado beetles, though seldom after mid-July. Spread of PLRV, which occurs The search for resistance to the potato A new epidemic of PLRV occurred in at Fredericton largely after 1 August, was leafroll virus (PLRV) began before the the Northeast during 1972-1975 followed left to natural infestations with aphid virus nature and transmission by aphids by another sharp decline (5,22). The first vectors. Harvest was by machine, using a was widely understood (10,14,15), but it author has speculated on the possible role distinctively colored cultivar as a marker was a sudden increase in PLRV in the of a climatic cycle in PLRV epidemiology in a 1-m space between plots. Twenty-five seed-growing areas of the northeast- (4), and if such an explanation is valid, we tubers were taken at random from each Aroostook County, Maine (7,16,17,25), can expect PLRV to surge periodically plot and eye-indexed (6) in the greenhouse and adjacent areas of New Brunswick into the northeastern seed-growing areas during late winter and early spring when (5)-during the period 1937-1945 that from a reservoir further south. Thus, PLRV symptoms showed best. gave impetus to breeding for PLRV cultivars with resistance to PLRV would We regularly recovered PLRV from resistance in North America. During the be of long-term value, but such cultivars indexed plants showing leafroll symptoms 1940s, American breeders and patholo- would need high quality to retain a place by aphid-transfer (Myzus persicae Sulz.) gists made progress in terms of resistance in the trade. More particularly, breeders to plants of Physalisfloridana Rydb. , as (9,19), but by 1955, Folsom (8) reported face the formidable task of displacing described by MacKinnon (12). Tubers that the most promising PLRV-resistant Russet Burbank, a cultivar susceptible to from such plants were saved and used in seedlings were horticulturally "substan- PLRV but widely grown for french fry the greenhouse the following year to dard. " Improvement in quality was processing. produce PLRV-infected plants for apparently hampered by a rigorous In the breeding program at Fredericton, comparison purposes. Also, we infected elimination of seedlings even slightly enhanced resistance to potato viruses is greenhouse-grown plants of the more susceptible to the virus. PLRV declined now sought through selection and use of resistant cultivars by means of heavy sharply after 1946 (20,2 1). Nevertheless, resistant parental stocks possessing infestation with PLRV-infective M. the work continued and the PLRV- desirable commercial qualities. New persicae to demonstrate that none of resistant cultivars Cascade (l 1), Penobscot seedlings are first subjected to several these was a symptomless carrier of the (18), and Abnaki (1) were eventually years of horticultural selection. Survivors virus. (Since 1984, we have tested released. None is widely grown (2,3). are entered in the PLRV resistance trial, apparently resistant cultivars by ELISA. ) There was interest in PLRV resistance where they are classified in resistance Analysis. Typically for the years 1972 in Canada at this time, and some breeding groups or categories by comparison with through 1981, 40 commercial cultivars was done at Fredericton (13), but none of a set of standard cultivars. The standards, and 60 Fredericton seedlings (F seedlings the offspring was released to the trade. themselves, have been subjected to [6]) were included in the trial. During this assessment over a period of years and period, a group of 36 "standard" have been separated into resistance cultivars, 20 commercial and 16 F groups as we describe in this report. We seedlings, were tested each year. For