RED SPIDER AND POWDERY MILDEW 285 



on paint. It lias heen coninion experience that tiiese objections hold only 

 where repeated excessive quantities of sulfur are burned. Many experiments 

 have shov^'n that the fumes from burned sulfur kill all stages of the red spider 

 but in spite of this fact many growers report negative results. These are 

 undoubtedly due to leakage and to the ability of some of the mites to escape 

 the fumes by crawling into cracks where they obtain a draft of fresh air. 

 The practice, nevertheless, is considered worth while as a means of destroying 

 parasites and safeguarding the next crop from a potential inside source of 

 attack. 



If the burned sulfur treatment is undesirable, fumigation with calcium 

 cyanide at the rate of one-fourth to one-half pound per thousand cubic feet 

 for 24 hours will kill all red spiders and insects but not mildew. The smaller 

 dosage is sufficient in a gas tight compartment but most greenhouses require 

 the larger amount. After watering freely to provide moisture for the rapid 

 generation of the hydrocyanic acid gas, the calcium cyanide should be scat- 

 tered evenly over the beds and walks. The fumes are very poisonous and 

 breathing them must be avoided as much as possible. When using cyanide 

 in a large house, it is well for two men to work together, one working toward 

 each end from the center. This treatment is eflfective, easily applied, and 

 requires no equipment. 



When properly carried out, fumigation with melted and vaporized naphtha- 

 lene rids the house of red spiders and insect pests more effectively than the 

 burned sulfur treatment. The naphthalene crystals are melted and vapor- 

 ized in shallow pans or dishes over a lamp or kerosene stove at the rate of 

 1 pound for each 1000 cubic feet. During this treatment, the temperature 

 nmst be maintained continuously at about 75 °F. A stove or lamp for each 

 10,000 cubic feet is desirable and the heat from the stoves should be regu- 

 lated so that the melted naphthalene is vaporized without boiling. This 

 fumigation nuist be continued for at least 48 hours and naphthalene should 

 l)e added at regular intervals until the required amount is used. This treat- 

 ment is used generally in England where special stoves for this purpose arc 

 manufactured. It requires special equipment and moderate attention. The 

 naphthalene costs about 17 cents per pound and is about as expensive as 

 cjanide in proportion. 



The choice of these between-crop fumigation treatments depends largelj' on 

 conditions. Only burned sulfur exerts fimgicidal action, but cyanide and 

 naphthalene are slightly more effective against red spider. Therefore two 

 of the treatments may be advisable. 



Hot water or steam treatments of the soil, practiced for controlling soil 

 inhabiting pests in the greenhouse, will kill red sjiider and powdery mildew 

 present on the treated soil but have no controlling effect elsewhere in the 

 house. Soil sterilizing practices, therefore, shoidd be preceded or followed by 

 fumigation with one or more of the other suggested materials. 



Gro-Lclng Clean Plants 



Frequent inspection of the yoimg plants in the })lant house is necessary to 

 safeguard against an early infestation. If pests are present, the plants should 

 be given the reconuiiended control treatments immediately. Before the plants 

 are set in the houses, they should again be carefully examined. When either 

 red spiders or powdery mildew are found at this time, control measures 

 should be applied Ijefore transi)lanting. Only pest-free plants should be set. 



