342 MASS. EXPERIMENT STATION BULLETIN 260 



practices was evolved to provide proper conditions for the control of leaf- 

 mold. Experimentsi were conducted for comparison of "good" and "bad" 

 growing conditions as related to control of the disease. Proper manage- 

 ment resulted in decreasing leaf infection about 70 per cent and yield 

 of fruit was increased 18 per cent. 



Downy Mildews of Cucumber and Lettuce. (W. L. Doran). The first 

 appearance of downy mildew on cucumbers in the field was recorded on 

 August 19, only a few days later than for the last five years, — and this 

 despite the extremely dry summer, in which neither onion nor potato 

 mildew was observed in this region. The disease was no less severe in 

 some fields where cucumbers had not been grown for at least three years 

 previously than on fields where cucumbers were affected with the disease 

 last year. 



Lettuce mildew rarely occurs in the field here except in cool weather 

 in fall. Attempts to maintain the fungus on infected plants (Belmont 

 variety) set in the field in May failed, probably because of the warm, dry 

 weather. Observations indicate that high air temperature constitutes an 

 important natural defense against this disease in the field. 



In the greenhouse, work continued on the relation of management and 

 the use of fungicides to the control of these diseases. Experiments to 

 determine whether potash and lime applied to the soil tend to decrease 

 susceptibility of cucumber plants to attack by downy mildew produced 

 only negative results. 



Pseudoperonospora of cucumber and Bremia of lettuce were success- 

 fully grown for several weeks on detached cotyledons of host seedlings 

 floated in a 5 per cent solution of sucrose in Petri dishes. It is thus 

 possible, by occasional transfers, to maintain these fungi when conditions 

 in the greenhouse do not favor their growth. 



Study of over-wintering of downy mildew of cucumber added nothing 

 to previous knowledge of this phase. 



Eradication of Nematodes in Greenhouse Soils. (L. H. Jones). Work 

 previously reported has shown that a combination of acetic acid with cal- 

 cium cyanide will eradicate nematodes. Of considerable importance is 

 the fact that two treatments with this combination must be used, with 

 an interval of about seven days between the treatments. The role of the 

 acetic acid in the combination chemical is apparently that of a carrier 

 for the lethal cyanide. The effect is to maintain the fairly volatile cyanide 

 gas in the soil much longer than it would ordinarily persist if the acetic 

 acid were not used. If this same principle could be applied by usyig a 

 dry chemical in place of the liquid acetic acid the idea would have a better 

 opportunity for adoption in agricultural practice. Paradichlorobenzene, 

 the P. D. B. used in eradicating peach borers, seems to fulfill the same 

 r61e as acetic acid. It is a dry chemical and when mixed with calcium 

 cycnide the combination remains a true mixture. There is no caking and 

 once a uniform mixture is attained it remains as such. One treatment with 

 this combination reduces the numbers of nematodes considerably, but era- 

 dication is not obtained unless two treatments are used with an interval 

 of one week between applications. The use of the paradichlorobenzene 

 with calcium cyanide has been tried by varying the proportions of each 

 chemical and also the amount applied. In general the results indicate that 



