A Study of Meadow-Crop Diseases in New York 51 



summer dissemination of the conidia which are very light and easily blown 

 about. To keep clover plants uninfected in the greenhouse in the autumn 

 is difficult because wind-blown conidia filter in through the ventilators and 

 doors. Moisture is of less importance in the development of this disease 

 than in some others because the conidia of E. polygoni will germinate 

 on a dry glass slide in a humid atmosphere almost as well as in a drop of 

 water. 



Control 



Much of the prevailing opinion regarding control of clover powdery 

 mildew is summed up in a statement by Young (1923: 160) when he says, 

 " Practically nothing can be done to control clover mildew under field 

 conditions." Mains (1923:308) holds that "control of a disease of the 

 nature of mildew under the cultural conditions necessary for clover is 

 extremely difficult." 



Unless the fungus occurs in the seed, as Crawford (1927 : 6) has indicated 

 for the pea mildew, then exclusion measures would be useless. Crawford 

 obtained control of the primary infections by treating the seed in hot 

 water at 56° C. for twenty minutes. Even in that case secondary infections 

 soon overwhelmed his plants and the disease became as bad as if he had not 

 treated the seed. 



The epiphytic nature of powdery mildew renders it highly sensitive to 

 chemicals applied to the plant. In the greenhouse on two occasions 

 severely diseased red-clover plants in pots or flats were divided into two 

 lots, one of which was dusted twice at two-weeks intervals with Kolodust. 

 Within a month the dusted plants were entirely free of infection and 

 showing a dark, healthy green as contrasted with a sickly yellow and brown 

 appearance of the undusted lot. 



Sulfur dust has been almost universally recommended for the eradication 

 of powdery mildews of various suscepts. The use of this material in such 

 a way extends back many years into phytopathological history. What 

 appears to be the first attempt at controlling the disease on clover in the 

 field was reported by Barss (1926) in Oregon. He states that two county 

 agents sprayed clover fields with lime sulfur, 1\ gallons in 100 gallons of 

 water for each acre of clover, and found that the mildew disappeared in 

 five days. There was an increase of 2\ bushels of seed to the acre at a 

 cost of about $1 an acre. Fifteen pounds of sulfur dust to the acre also 

 gave satisfactory control. Barss says, " Hot weather is believed to be 

 essential in getting a thorough control with sulfur dust since the heat causes 

 the sulfur to volatilize and give off fumes which kill the fungus and its 

 spores." This is improbable, as is pointed out in the discussion of the 

 Heterosporium leaf spot of timothy, where it is shown that temperature 

 seems to exert little effect upon the fungicidal action of sulfur. It is more 



