16 MASS. EXPERIMENT STATION BULLETIN No. 283 



average of 21 infection points per plant on the plants inoculated with conidia 

 not sulfured, but there was no infection of plants inoculated with sulfured coni- 

 dia. In a similar experiment, lettuce jolants were inoculated with conidia of 

 Brcmi'a lactticae which had been in contact with sulfur (dry) for 6 hours at 23° C. 

 No infection resulted, although there was abundant infection of lettuce inocu- 

 lated with (;onidia from the same source but not exposed to sulfur. In these 

 experiments, there was no water on the leaves when the sulfur came in contact 

 with conidia. 



In other experiments, leaves of cucumber plants were dusted with sulfur, 

 and drops of water containing conidia of P. cubensis were placed on them. Under 

 these conditions, infection was not prevented. As before, the behavior of P. 

 cubensis in this respect was compared with that of another downy mildew fungus. 

 Drops of a water-suspension of the conidia of Bremia lactucae were placed on let- 

 tuce plants after the plants had been dusted with sulfur, and under these condi- 

 tions normal infection resulted. Copper-lime dust suiiilarly used prevented all 

 infection bj^ both of these fungi. 



In these and other experiments, partial protection of cucumbers was thus 

 afforded by sulfur if the inoculum, conidia of P. cuhensis, was made ineffective 

 by the sulfur before the inoculum in water was present on a leaf. But if the 

 conidia were in water on a leaf before the sulfur reached them, their germination 

 and the resulting infection were not prevented. It seems evident that more 

 reliance may be placed on sulfur for the control of dowTiy mildew of cucumber 

 in the greenhouse than in the field. It is true that in the greenhouse, leaves of 

 cucumber may be wet at times (as by leaks, overhead watering, or water of 

 guttation) and under these conditions, sulfur will not be effective; but in the 

 field, leaves and conidia may be wet every dewy morning and under these con- 

 ditions, sulfur is ineffective. It is then that copper fungicides are most valuable. 



Earlier investigators are practically agreed that downy mildew of cucurbits 

 as it occurs in the field, is profitably controllable by Bordeaux mixtures. Their 

 conclusions have recently been cited by Weber (loc. cit.) and will not be re- 

 viewed here. Adams' and Weber (loc. cit.) have also found copper-lime dusts 

 effective. 



In experiments by the writer, sulfur and copper-lime dusts were applied to 

 cucumbers in the field at intervals of 8 days beginning both before and after 

 infection by P. cubensis (2 days after the first conidiophores were found on the 

 leaves.) Plots dusted first before infection received two more applications than 

 did plots dusted first after infection. Each plot was 10 feet wide and 100 feet 

 long and plots were in triplicate. Average 3'ields in pounds per unit area, expressed 

 in relative numbers, are recorded in Table 5 for the years 1926 and 1927. 



Sulfur did not increase yields, in fact its use was followed by yields lower than 

 those of check plots. This means that sulfur not only failed to provide protection, 

 but actually injured the cucumber plants to which it was applied. Plots which 

 received early applications of sulfur did, however, yield more than plots first 

 dusted with sulfur after the disease appeared, indicating that sulfur had some 

 fungicidal effect, although not enough to offset its toxicity to the dusted plants. 



Other investigators (53, 29, 15, 48) have also found that sulfur may be in- 

 jurious to cucumbers and other cucurbits, especially muskmelon. Because of 

 the susceptibility of greenhouse cucumbers to injury by sulfur if applied too 



Adams, J. F. Downy mildew caused by Pseudoperonospora cubensis. U. S. Dept. .\gr., Bur. 

 Plant Ind., Plant Dis. Rptr., Sup. 34, pp. 202-203. 1924. (Mimeographed.) 



