288 MASS. EXPERIMENT STATION BULLETIN 246 



FumUjation with Naphthalene 



In the past, no funiigant has effectively controlled the red spider without 

 injury to the plants. Recently, however, exi^eriinents both in Europe and 

 this country have shown that three or four treatments with vaporized naph- 

 thalene at the rate of IJ to l ounces of crystals to each 1000 cubic feet were 

 very effective and safe for this purpose with high temperature and humidity. 

 During these experiments, however, it was found that the fruits absorbed the 

 odor and taste of the naphthalene. This almost entirely disappears after 

 several hours in fresh air but the danger of selling tainted fruit to the con 

 sunier is considered so important that the treatment is not recommended to 

 cucumber growers. 



Specific Measures for Powdery Mildew 



Greenhouse Management 



The proper management of the greenlumse air as a means of avoiding 

 attacks of powdery mildew should receive consideration. Healthy plants are 

 never quite as healthy after they ha\e been sprayed or dusted, altiiough tiie 

 detrimental effect of the materials applied is seldom realized. An atmos- 

 phere unfavorable for mildew infection and consistent with the best growth 

 of the plants is the natural method of culture and should supersede the use 

 of fungicides. 



Moisture saturated air in the greenhouse is conducive to the development 

 of mildew. The moisture content of the greenhouse air depends on the use 

 of heat and ventilation and may be adjusted to prevent favorable conditions 

 for infection. The relative humidity of the air can be accurately determined 

 only from a hygrometer, a satisfactory and economical type of which is illus- 

 trated in Fig. 1. This instrument consists of a dry thermometer, wet ther- 

 mometer, and glass tube of water for supplying the wet bulb with moisture. 

 The relative humidity is determined by ai)plying the thermometer readings to 

 a simple table. When the air is saturated the temperature of both the wet 

 and dry thermometers is the same. It is desirable to keep the relative hu- 

 midity of the greenhouse air below 90 per cent. This can be done with heat, 

 ventilation, or both. Side ventilation should not be used at night or during 

 damp weather. If at such times ventilation is desired, it should be from the 

 top and from the side on the top opposite the direction of the wind. Water- 

 ing should be done on bright days before noon with ventilation and if the air 

 is. not sufficiently dry by night, heat should be used. Stuffy air conditions 

 should be corrected by adequate ventilation. Vestibules at the entrances are 

 helpful in preventing conditions favorable for infection of the ])lants at these 

 places. 



Dusting with Sulfur 



Sulfur is the most effective fungicide for controlling mildew, and dusting 

 is the most practical and economical method of application. Only sulfur of 

 extreme fineness should be used. Some of the commercial sulfur dusts of this 

 type sold in Massachusetts are Kolo dust. Grape Dust and Tricked Sulfur. 

 The active principle in each of these materials is sulfur but its content varies. 

 Non-dusting sulfur, such as is ordinarily used for burning, lacks the fluffy, 

 })owdery qualities of a fine dust, and siiould not lie used for dusting pur- 

 poses. The desirable grades of dust referred to in this bulletin are prepared 

 especially for dusting plants and may be secured of dealers specializing in 



