284 MASS. EXPERIMENT STATION BULLETIN 246 



Favorable Conditions for Growth and Spread of the Pests. 



Conditions suitable for growing greenhouse cucumbers are generally favor- 

 able for the growth and reproduction of the red spider. Its natural migra- 

 tion from leaf to leaf and from plant to plant in search for food, thus in- 

 creasing the size of the original infested area, is normally very slow but in 

 hot, dry weather when multiplication takes place rapidlj', colonies will quickly 

 outgrow the leaves on which they are feeding and spread to surrounding- 

 plants. Artificial dispersion resulting in the sudden appearance of isolated 

 colonies, remote from the original infestation, is much more important. Work- 

 men engaged in pruning, training, picking, and otherwise caring for the vines, 

 carry the spiders on their tools and clothing from one plant to another and 

 are the commonest agency responsible for the unnatural spread of this pest. 

 In the suHuner, however, many of them gain entrance to the greenhouse by 

 being blown through open ventilators or carried on clothing from infested 

 out-door plants growing nearby. 



When powdery mildew once appears in the greenhouse its spread may be 

 rapid, 'ihe liglir spores are readily carried by currents of air. The workers 

 liecome sprinkled witli them, especially when brushing against mildewed vines, 

 and arc therefore a conunon means of spreading the fungus from one plant to 

 another and from house to house. Spores from out-door hosts are also a 

 conmion and hnportant source of infection of tiie housed plants. 



Spore germination and infection occur in damp air. The appearance of 

 powdery mildew in the houses is associated with cool, dull, or damp weather 

 when pipe heat is used sparingly or not at all. Powdery mildew may de- 

 velop in any season of the year providing favorable moisture conditions pre- 

 vail. Outbreaks of the fungus accompany improper watering practices and 

 lack of attention to ventilation; drafts of cool or damp air through ventilat- 

 ors, doors, and broken glass; stuffy air pockets not adequately provided with 

 ventilation; and lack of .sufficient heat. In every instance moist air, not 

 properly altered by ventilation or heat, is the basis of growth and develop- 

 ment. Potted plants when grown under any of the above conditions become 

 mildewed. 



Control. 



Repressive Measures for Red Spider and Poivdery Mildew. 



Between-crop Treatment of Greenhouses. 



In order to grow healthy plants the greenhouse must be free from para- 

 sites whicii affect those plants before the new crop is set. This is true in the 

 case of red spider when cucumbers or other host plants follow cucumbers but 

 in the case of powdery mildew, only when cucumbers follow cucumbers, since 

 this is the only host plant grown commercially under glass in Massachusetts. 

 If the previous crop was attacked, between-crop-treatment is necessary and 

 in any case it is advisable. By treating the greenhouse before the old vines 

 are removed, the pests are killed without danger of spreading them beyond 

 the effect of the treatment. 



The coimiionest effective treatment is to burn sulfur at the rate of one- 

 third of a pound for each 1000 cubic feet of space with the ventilators closed 

 for at least 24 liours. The fumes from burning sulfvir are deadly to plant 

 and animal life. Some growers object to the practice of burning sulfur be- 

 tween crops l)ecause of the corroding action which the acid fumes of sulfur 

 have on unprotected metal pipes, posts, and wires, and its bleaching action 



