BIKNNIAL REPORT, 1927 AND 1928 309 



Germination did not occur at a relative iiuinidity below 98 per cent. In 

 the greenhouse, it was found that a moisture-saturated atmosphere, and 

 especially water of transpiration on the foliage, without adequate ventila- 

 tion, the use of heat with ventilation, and proper watering to avoid condi- 

 tions favorable to infection, gave excellent control of tlie disease. In the 

 spring of 1928, total yields of No, 1 and No. 2 grade tomatoes were in- 

 creased by 2.83 pounds per plant and the value by 81 cents per plant as 

 a result of these management practices. In the house so managed, 2.08 

 per cent of the leaflets showed infection as compared with 4-5.09 per cent 

 in the check house. 



A technical paper entitled "Fungicidal Control of Cladosporium Leaf- 

 Moid Disease on Greenhouse Tomatoes" has been prepared for early 

 pulilic.-ition. 



Powdery Mildew of Cucumbers. Work on this project has been com- 

 pleted and the results published in Bulletin 246. (See list of publications). 

 Downy Mildews of Cucumber and Lettuce. (W. L. Doran). Field ex- 

 periments with sulfur and copper-lime dusts as fungicides for the control 

 of cucumber mildew were conducted in 1927. With 10 applications of 

 copper-lime dust beginning prior to the appearance of the disease, yield 

 of cucumbers was increased 97 per cent over the undusted check, and with 

 7 applications beginning with the first appearance of the disease, the in- 

 crease was 54 per cent. Sulfur failed to control the disease and was toxic 

 to the cucumber foliage. In laboratory experiments sulfur was inferior 

 to coi)per fungicides in preventing infection, but sulfur was more eflFectrve 

 than copper in preventing sporulation of the mildew fungus when applied 

 to leaves after inoculation or during the incubation period. 



The manner of overwintering of Pseudoperonospora cuhensis (B. «Sc C.) 

 Rostow. is not known. Cucumber leaves killed by mildew were examined in 

 September hut xielded no evidence of oospore formation. In 1927 and 

 1928, oosjiore-iike bodies .ajipeared in the decayed tissues of infected 

 cucumber plants buried over winter in the soil. Failure ;ittended attempts 

 both to germinate these bodies and to oljtain infection by inoculating with 

 them. 



In the cour.'t^e of this study it has been found that the youngest leaves of 

 cucumber are not susceptible to infection by downy mildew and that 

 this is due, apparently, to two factors: (1) such leaves are w^et with 

 difficulty, (2) their stomata are closed. 



It was found that in the presence of dew, rain is not necessary for the 

 sporulation of P. cuhensis, germination of the conidia and infection, but 

 that rain is the most important factor in the rapid dissemination of the 

 fungus. 



In the greenhouse, spread of the fungus was completely stopped by re- 

 moving infected leaves when first seen and by maintaining a low^ humidity. 

 Work on Br&mia lactucae Reg., downy mildew of lettuce, has included 

 preliminary studies of host relations, susceptibility of lettuce varieties, 

 longevity of conidia in the soil and spraying of lettuce seedlings for mildew 

 [)revention. 



Bordeaux mixture proved injurious to seedling lettuce. Its effect on 

 the mildew was not definitely determined because of a low^ degree of in- 

 fection. 



Conidia of the fungus failed to remain alive in the soil. 

 Tests with the two varieties grown commercially in Massachusetts, Bel- 

 mont and May King, revealed Belmont as very susceptible and May King 

 as very resistant to mildew. 



