CONTROL OF RED SPIDER AND POWDERY MILDEW 

 ON GREENHOUSE CUCUMBERS' 



By W. D. Whitcomb, Assistant Research Professor of Entomology 



and 

 E. F. Guba, Assistant Research Professor of Botany 



Introduction 



Tlie red spider-, sometimes called the greenhouse mite, and powdery mildew''' 

 are common pests of greenhouse cucumbers in Massachusetts. Few growers 

 in this state can l)oast that they have grown this crop for two successive sea- 

 sons without suffering damage from one or both of them. In 1917 it was 

 estimated by VinaP that the red spider caused annual losses amounting to 

 $1.50,000 in the Boston market gardening district alone. Powdery mildew is 

 often equally serious and its presence at tunes has proved an important ob- 

 stacle in the profitable production of greenhouse cucumbers. The presence of 

 both pests on the vines at the same time is common and the need for a com- 

 bined treatment is obvious. Likewise, a knowledge of the compatibility of 

 insecticides and fungicides used in the greenhouse for combating these and 

 other pests is extremely important to prevent injury to the vines. 



Host Plants 



The cucumber, especially when grown in the greenhouse, is one of the 

 fa^'orite host plants of both red spider and powdery mildew. Greenhouse to- 

 matoes are also attacked by this mite although they are seldom seriously 

 injured. The red spider also feeds on a great nuiny greenhouse flowers, espe- 

 cially violets, carnations, chrysanthemums and sweet peas. The list of out- 

 door plants known to be infested by it includes about two hundred wild and 

 cultivated plants ranging from aibor vitae to chickweed. 



Cucumlier powdery mildew occurs on squash, pumpkin, melon, and other 

 cultivated plants of the gourd family as well as the wild balsam-apple and 

 the bur cucumber. It is also definitely known to attack plantain and sun- 

 flower. The common powdery mildews on other plants such as lilac, rose and 

 clover, although similar in general appearance, are different from cucumber 

 powdery mildew and cannot live on plants of the gourd family. 



Description and Growth 



Effective control of plant pests demands not only the application of specific 

 treatments but also the adaptation of those treatments to local and some- 

 times varied conditions. This can be satisfactorily accomplished only by a 

 knowledge of the appearance, growth, and habits of the pest and an under- 

 standing of the conditions under which it thrives. 



1 The information in this bulletin is based on the results of experiments conducted by 

 the writers at the Market Garden Field Station, Waltham and in commercial greenhouses 

 in pastern Massachusetts. 



-■ Tetranychus telarius Linn. (Syn. T. hiiiiaculatus Harvey). The red spider belongs to 

 a 'group of Spiders called Mites. 



3 Enisivhe cichoracearum UC. 



*' Vinal Stuart C, The Greenhouse Red Spider Attacking Cucumbers and Methods for 

 Its"Control. Mass. Agr. Expt. Sta. Bui. 179, 1917. 



