4 MASS. EXPERIMENT STATION BULLETIN No. 283 



through tlie .stomata, and tliis is the only iiiothod of infection seen by the writer. 

 I'sing the xylol test of Molisch (31), it appears that the stomata of the youngest 

 leaves of cucumbers are not open. This fact, and the relative difficulty with 

 which such leaves are wetted, is believed to exjilain the resistance of young 

 cucumber leaves. 



Fertilizers 



It has been reported (28) that applications of i)otassic fertilizers decrease 

 the susceptibility of some plants to certain fungous diseases. Blattny (5) ob- 

 served indications of this effect of potash, and of lime, on downy mildew of hop; 

 and, according to Quanjer (35), downy mildews of cabbage, grape, and lettuce 

 may be more severe in soils deficient in potassium. 



In order to learn whether or not potash and lime have any effect on the sus- 

 ceptibility of cucumbers to downy mildew, cucumber plants were grown from 

 seeds to the fruiting stage, in soil (an infertile and fairly acid sandy loam) treated 

 as indicated in Table 2. The plants were grown in two-gallon crocks with treat- 

 ments in triplicate. All plants were inoculated with P. cuhensis, and the per- 

 centages of leaves which became infected within five weeks after inoculation 

 are recorded in Table 2. The susceptibility of cucumber to downy mildew was 

 not affected significantly by potash or lime (or a complete fertilizer) as applied 

 to this soil. 



Table 2. — Effects of Potash and Lime on Susceptibility of 

 Cucumber to Downy Mildew 



Soil treatment (per acre) Percentages of leaves infected 



Check (no lime, potash, or fertilizer) 50 



Potassium carbonate, 600 lbs 46 



Potassium carbonate, 1200 lbs 67 



Hydrated lime, 3 tons 52 



Hydrated lime, 4 tons 61 



Hydrated lime, 3 tons and potassium carbonate, 600 lbs 52 



Hydrated lime, 4 tons and potassium carbonate, 1200 lbs 45 



Complete fertilizer (5-8-7), 1500 lbs 42 



Complete fertilizer, 1500 lbs. and hydrated lime, 3 tons 45 



Complete fertilizer, 1500 lbs. and hydrated lime, 4 tons 40 



Complete fertilizer, 1500 lbs. and potassium carbonate, 600 lbs 41 



Complete fertilizer, 1500 lbs. and potassium carbonate, 1200 lbs 59 



Complete fertilizer, l.WO lbs., hydrated lime, 3 tons, and potassi\un carbonate, 000 lbs. 49 



SOURCES OF THE PRIMARY INFECTION 



Search for Oospores and Other Wintering-over Stages of the Fungus 



in the Field 



Many species of the Peronosporaceae live through the winter by means of 

 oospores, and a number of investigators (13, 17, 21, 11, 36, 10, 4, 14) have, there- 

 fore, looked for oospores of P. ctibensis. Rostowzew (36) is, however, the only 

 one who believed that he found them. But he did not establish, as by their ger- 

 mination and infection of cucumber with them, that they were indeed oospores 

 of this fimgus. 



The writer has repeatedly looked for oospores in cucumber leaves infected 

 with or killed by P. cubensis. This has been done in the fall, before and after 

 the death of the leaves, and also in the following spring and summer, after the 

 dead diseased leaves had passed through the winter (or through two winters) 

 buried in soil (outdoors, in a greenhouse, or in a laboratory). Oospores of one 



