18 MASS. EXPERIMENT STATION BULLETIN No. 283 



In each of the six years from 1925 to 1930 inclusive, downy mildew was looked 

 for in cucumber fields of Hampshire County, Massachusetts, at intervals of 3 

 days beginning July 20 and continuing until the disease appeared. Downy 

 mildew was first found here on August 15 in 1925, August 4 in 1926, August 6 in 

 1927, August 9 in 1928, August 18 in 1929, and August 11 in 1930. For six years, 

 therefore, the disease always appeared here for the first time between August 4 

 and August 18, and this without any apparent relation to the date of planting 

 or the weather of the season. On the basis of these observations, it is probable 

 that field-grown cucumbers here need to receive treatment with a copper fungi- 

 cide for protection against downy mildew l)egiiuiing aI)out July 25, but not 

 before. 



Frequency of Applications 



The intervals of time between ai)i)lications of a fungicide to plants are usually 

 determined by the rate at which the plants grow and the rapidity with wliich 

 the material is lost from the leaves as by weathering. It is frequently recom- 

 mended that fungicides be applied "often enough to keep new growth covered." 

 However, as has been shown, the youngest leaves of cucumber do not become 

 infected with downy mildew, although these temporarily resistant leaves become 

 susceptible as they grow older. P. cuhensis is, under favorable environmental 

 conditions, reproduced and disseminated very rapidly. On August 5, 1926, 

 downy mildew was found on less than 2 per cent of the cucumber plants in a field 

 of about 1 acre; on August 10 it was present on a little more than 50 per cent of 

 the plants; and on August 15 every plant in the field was infected. It is evident 

 that the intervals of time between applications of fungicides in such cases should 

 not be long. Clayton (9) recommendetl that fungicides be applied to cucumbers 

 in the field twice a week. In Massachusetts, however, adequate protection has 

 been obtained with a copper fungicide applied 5 times at intervals of 8 days; 

 and the disease was profitably prevented by Weber (loc. cit.) when applications 

 of Bordeaux mixture or of copper-lime dust were applied at intervals of 1 week. 

 This is considered to be often enough in this State. 



Concentration of Bordeaux Necessary 



Downy mildew of cucumber was controlled in the field by Bordeaux mixture 

 3:3:50 without injury to plants, and that is the concentration here recommended 

 for use in the field. There may, however, be severe injury and stunting of the foli- 

 age of greenhouse cucumbers sprayed with Bordeaux mixture of this formula. 



Many fungicides as used in the field are applied at a greater concentration 

 than is actually required to prevent spore germination and infection. This 

 practice may be necessary in the field, however, because of the gradual loss or 

 removal of the effective constituent by rains or weathering. In greenhouses, 

 the likelihood of such loss is at a minimum. With this consideration in mind, 

 and because of the possibility of injury to greenhouse cucumbers by copper 

 fungicides, the effects of each of several Bordeaux mixtures on infection of 

 greenhouse cucumbers by P. cuhensis was investigated. 



Greenhouse cucumber plants (30 plants receiving the same treatment) were 

 sprayed 3 times at intervals (jf 1 week using Bordeaux mixtures of the following 

 formulas: 3:3:50, 3:10:50, 2:2:50, 2:10:50, or 1:1:50. Although there was in this 

 case no real burning of foliage, the growth of plants sprayed with Bordeaux 

 mixtures containing more than 0.25 per cent copper sulfate was retarded. All 

 plants (including checks) were inoculated with P. cuhensis 2 days after the 



