LATE BLIGHT INFECTION OF POTATO VARIETIES IN ESTONIA IN 1920-1991 M KOPPEL Jogeva Plant Breedin. Institute, Jogeva EE 2350, ESTONIA Background and objectives Genetical analysis of isolates of causal agent of potato late blight Phytophthora infestans (Mont.) de Bary have indicated that a new population has displaced the old population in Europe during the 1980s [1]. There are evidences that the Estonian population of P. infestans has changed also in this time. Two P. infestans isolates collected in Estonia in 1983 belonged to new genotypes based on DNA f in-erprint pattern and izosyme pattern [2]. Two highly virulent samples of P. infestans isolated at the end of growing season in 1985 were identified as race 1 0.1 1. at All Union Institute of Plant Protection, Moscow. The changes having taking place in population of P. infestans have increased infection of potato varieties by late blight. This paper characterises the changes in the late blight resistance of potato varieties in Estonia in long-term period, 1920-199 1. Materials and methods Infection of potato haulm by late blight has been registered every year since 1920 in naturally infected untreated field trials at the J6geva Plant Breeding Institute, Estonia. The following varieties were assessed: early varieties (Early Rose RO 1922-195 1, Priekule visagrie R0 1951-1985, Premiere R0 1986-199 1), and maincrop varieties (Jageva kollane R0 since 1940, Olev R 1 R4 since 1948, Sulev Rl since 1952, Ando Rl R4 since 1966, and Sarme Rl R2 R3 R4 since 1980). Potatoes were grown on 5-20 m2 plots in 2-6 replications. The state of disease has been evaluated visually at 7-day intervals according to a key of grades ranaing from 1 to 100 %. Scorings were done on period from mid-July to mid-September. The timings were chosen to cover the general course of the epidemic in certain year. Resistance of a variety was expressed by the date of beginning of permanent infection or by average resistance over scoring dates. Data were transformed using 4253H filter of time series transfortnation of programme STATISTICATM to show trendlines of changes in late blight resistance of varieties over the study period. Results and conclusions Acquired long-term results enable to examine changes having taking place in infection of potato haulm by late blight. Increase of genotypic diversity and higher aggressiveness of new races of P. infestans has caused the decrease of resistance of potato varieties to late blight. Infection of early varieties has increased year by year and infection has begun to start ca 3 weeks earlier. At the same time has shorthened period between infection of early and maincrop varieties. Period for varieties Olev and Sarme has shorthened 4 weeks, for Sulev weeks, for Ando 2 weeks, and for Jaceva kollane one week. Varieties Olev, Sarme and Sulev have lost the race specific resistance due to spread of more complex races of pathogen already at the beginning of infection. Only the variety Ando, which combines major gene resistance with high field resistance was able to resist the late blight epidemics in late 1980s. The biggest changes in date of establishment of infection have occurred in the second half of 1980s. Haulm is started to be fully destroyed earlier in all of assessed varieties. The time when the haulm is completely destroyed arrives three weeks earlier for maincrop varieties, but only one week earlier for early varieties. Simultaneously has shortened the period needed for haulm killing for Jageva kollane 3 weeks, for Sulev 2 weeks, for Olev one week. The period needed for haulm killing was somewhat longer after 1988, in connection of earlier date of establishment of infection by no changes of date of fully destroyed haulm. Sharp increase of average infection is notable for early varieties in mid 1960s, for Sulev in mid 1970s, and for other varieties in mid 1980s. Dramatic increase in late blight infection is noticeable in second half of 1980s. In condition of prevailing new genotypes of P. infestans, infection of late blight starts at the same time in all of varieties, race specific resistance is not effective, and infection develops similarly rapidly in most varieties. Only varieties with high field resistance are able to resist the severe late blight epidemics. References 1. Fry WE, Drenth A, Sp ic Iman LJ, Mantel BC, Davidce LC, Goodwin SB, 199 1. Phytopathology, pp. 13 3 0-13 3 6. 2. Goodwin SB, Cohen BA, Fry WE, 1994. Proc. Natl. Acad. Sci. USA, pp. 1 1 591-11595.