Abstract
SUMMARYTwo races of Bremia lactucae are described; race W.1, which can attack various lettuce varieties now being grown commercially but cannot attack the resistant lettuces bred at Wellesbourne, and race W.2, which can infect not only the market varieties but also some of the W.1‐resistant lines. Race W.2 was shown to arise directly from race W.1, possibly by mutation.The length of incubation time from inoculation to sporulation of the mildew in lettuce leaves decreased as the number of spores applied in the inoculum increased. This fact was used to assess the concentration of race W.2 spores in naturally occurring spore populations, by comparing the effects on W.1‐resistant plants of graded dilutions of spores from the field with those of known concentrations of race W.2. These tests showed that race W.2 was most prevalent where W.1‐resistant lettuces had been grown for some years. Race W.2 spores were found, however, where no W.1‐resistant lettuces had been grown, and it appears that race W.2 may be a normal constituent of most Bremia populations. The importance of these findings in relation to future breeding for downy mildew resistance is discussed.
Published Version
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