VARIATION IN SUSCEPTIBILITY AND TOLERANCE WITHIN AND BETWEEN POPULATIONS OF TUSSILAGO-FARFARA L INFECTED BY COLEOSPORIUM-TUSSILAGINIS (PERS) BERK

Quantitative variation in susceptibility to infection by the rust Cole osporium tussilaginis, and in reduction in growth owing to the infecti on, were determined in 30 clonally propagated genotypes of Tussilago f arfara sampled from three geographically isolated sites, Arnhem (The N etherlands), Lancaster and Sheffield (both UK). Populations, and genot ypes within populations showed no significant variation in intensity o f disease after artificial inoculation under ideal conditions with a f ungal isolate collected from the Lancaster population. By contrast, si gnificant differences between and within populations were observed aft er natural secondary infection by spores produced from the artificiall y inoculated plants. Genotypes differed considerably in the reduction in growth caused by the infection, but this was not correlated with th e extent of symptoms shown by the plants. Low resistance was associate d with high tolerance, as in the Lancaster and Sheffield populations, while high resistance was associated with low tolerance, as in the Arn hem population. The evolutionary significance of these associations is discussed.