RED SPIDER AND POWDERY MILDEW 



291 



3 to 5 rows of plants (Fig. 3,c). At every 5 to 10 walking paces the exten- 

 sion tube of the cluster should be raised overhead. One to two pushes of the 

 jDlunger with the duster in an obliquely vertical position will cover about 500 

 square feet with dust. If mildew is encountered on the lower sheltered leaves, 

 light horizontal discharges of dust should be made but the nozzle should never 

 be placed directly upon the leaves. The same general directions apply to a 

 house of young cucumbers. The dust should be directed into the air above the 

 plants and allowed to settle on the foliage. In exercising these precautions 

 the danger of sulfur injurj- to the vines is reduced to a minimum and there 

 is no waste of material. 



Tlie practice of using more sulfur dust than is necessary to control mildew 

 has been responsible for injury. The results of numerous experiments have 

 shown excellent control in 40x280 foot houses from one application with as 

 little as 5 to 6 pounds of dust. More tlum this amount should not be used 

 even if mildew is generally present in the house. Proportionately smaller 



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(a-) (b) (;c) 



Fig. 3. Courses of travel with duster in differeutly constnicted greenhouses. 



quantities should be used in smaller houses. The reasons for suggesting these 

 precautions are (1) powdery mildew is very susceptible to extremely small 

 doses of sulfur under warm air conditions; (2) under similar conditions ex- 

 cessive doses of sulfur cause either burning, hardening or stunting of the 

 leaves; (3) sulfur dusting materials are highly concentrated. 



The effectiveness of sulfur in controlling mildew is dependent upon sulfur 

 gas which is generated above 70° F., and in increasing amounts with higher 

 temperatures. By proper management of the ventilator.s, temperatures fa- 

 vorable for the liberal production of gas may be obtained in bright weather. 

 The closing of the ventilators also offers a means of concentrating the gas 

 in the greenhouse. Since the gas given off from sulfur is fungicidal, contact 

 of the sulfur particles with mildew is not necessary, to obtain control. After 

 dusting, the ventilators should be managed, if necessary, to provide tempera- 

 tures of at least 80 to 8-5 °F., for a few hours each day until the fungus has 

 been eradicated. 



