Air temperature is one of the most important factors influencing the spread of potato viruses in Poland. High correlations of leaf-roll infection with accumulated air temperatures for the period I January-30 April, and of infection with virus Y with accumulated air temperatures for the period I January-Io July were obtained. On this basis the expected areas of different incidence of potato virus diseases were delimited in Poland. The correlation of mean infection for 4 years and mean accumulated temperatures for 15 years was higher than for the particular years, and the proportion of the total variance accounted for by regression exceeded 60%.The differences in the spread of both the viruses are discussed.
I N T R O D U C T I O NThe occurrence of aphids as vectors of viruses is largely dependent on the climatic region and, in a given climate, on the weather in particular years. According to Broadbent (1962), the factor most influencing aphids in moderate climates is temperature. The influence of temperature on the development of apterous aphid populations has been demonstrated by Fenjves (1945), Broadbent & Hollings (195 I ) , Barlow (1962a, b), Broadbent (1962) and Ulrich, Neitzel & Scholz (1963). Optimum development occurred within the range 20-25' C. Above this temperature, reproduction declined and ceased completely at 30" C. Under prolonged high temperatures the populations disappeared.Temperature affects the occurrence of winged forms in the first place via the host plants. Kennedy, Lamb & Booth (1958) and Weismann, Macko & Fekete (1960) showed that the decisive factor is the rise of osmotic pressure, generally associated with high temperature and drought. The last moulting of the nymphs and the emergence of mature winged individuals depends on the transgression of a certain temperature threshold (Broadbent, 1949 ;Weismann, 1960;Cockbain, 1961). Flight was much more dependent on temperature than on air moisture (Broadbent, 1949;Johnson, 1952). The optimum temperature for flight lies within the limits 22-26' C., and as the temperature rises above 26", flight activity declines; however in Aphis fabae
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