. Carnegie Institution of Washington publication. WILT OF CUCURBITS. 213 pure culture on steamed potato or nutrient agar, or in beef-bouillon, and make a few deli- cate punctures into a susceptible plant, e. g., into the blade of a cucumber-leaf or musk- melon-leaf. The period of incubation in the writer's experiments has varied from 3 to 31 days and must depend partly at least on the number of bacilli inserted. Ordinarily when 20 or 30 needle-pricks are made the first signs appear in from 5 to 9 days in the punctured part of the leaf (figs. 56, 60, 63, 74). When young cultures are used on ver
. Carnegie Institution of Washington publication. WILT OF CUCURBITS. 213 pure culture on steamed potato or nutrient agar, or in beef-bouillon, and make a few deli- cate punctures into a susceptible plant, e. g., into the blade of a cucumber-leaf or musk- melon-leaf. The period of incubation in the writer's experiments has varied from 3 to 31 days and must depend partly at least on the number of bacilli inserted. Ordinarily when 20 or 30 needle-pricks are made the first signs appear in from 5 to 9 days in the punctured part of the leaf (figs. 56, 60, 63, 74). When young cultures are used on very susceptible plants such as Cucumis sativus, Cucumis melo, or Cucurbita foetidissima, the disease appears with the certainty and regularity of clock-work. It is more difficult to inoculate squashes successfully, at least with some strains of the organism, and this corresponds to the observed fact that they are more resistant in the field. One winter, on several kinds of squashes the writer experienced repeated failures, using virulent cultures obtained from the cucum- ber. The pricked cucumber-plants and muskmelon-plants contracted the disease; the squashes, both summer and winter varieties, inoculated at the same time, in the same way, and from the same cultures, resisted, or only showed traces of primary wilt. This. Fig. 53.* resistance may be due to some extent to varying degrees of virulence on the part of par- ticular strains of the organism; or to varying degrees of resistance on the part of the host. Possibly the squash bacillus should be regarded as a variety. In the summer of 1905 the question of the identity of the squash-wilt and cucumber- wilt was gone over once more. Inoculations made into four varieties of squashes, using a strain isolated the previous year from a muskmelon, and proved by numerous control- experiments to be virulent to cucumbers, would not infect squashes. A little later in the season the same squashes were readily infected with a strain of the b
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