RED SPIDER AND POWDERY MILDEW 281 



Greenliouse red spiders live almost entirely on the under side of leaves 

 where each female lays about 70 spiierical, pearly white eggs (Plate I,al) 

 which are so small that they are scarcely visible to the naked eye. These eggs 

 gradually turn to a dark straw color and in four or five days a round, color- 

 less, six-legged spider larva hatches from each of them (Plate I,a2). After 

 feeding for one to three days, the young red spider, now light green in color 

 with bright carmine eyes, attaches itself to the leaf for a rest and transfor- 

 mation jieriod. While in this condition it appears to he dead but in about 2t 

 hours an eight-legged spider nymph emerges from the old skin and there- 

 after it always has eight legs. Following each of these molts, as they are 

 called, it feeds ravenously and inflicts the greatest damage by piercing the 

 leaf cells and sucking the green matter from them, thus causing the char- 

 acteristic yellow or dead areas. Female spiders repeat this process of growth 

 twice before the adult, about one-third the size of a pin head, develops (Plate 

 I,a3). The adults are usually green or yellowish red in color but occasionally 

 rusty red specimens are seen. The smaller, thinner, male red spiders have 

 but two of these transformation periods. 



As soon as the mites become established they spin silken webs o^er the 

 parts of the leaf where they are feeding and in severe infestations these webs 

 extend from leaf to leaf and to the supporting wires or trellises. The average 

 time required for the growth of female red spiders from the laying of the egg 

 to the development of the adult is eight to ten days and for the males seven 

 to nine days. Reproduction is practically continuous in the greenhouse, and 

 in the South at least seventeen generations in one year have been observed 

 out-of-doors. 



Cucumber powdery mildew is a parasitic fungus. Such a fungus is a simple 

 plant, in some respects similar to green plants but dilTering largely in that 

 it maintains a dependent existence as a parasite on green plants. The white 

 powdery webs (plate I,b) on the leaves and young stems, which suggest the 

 name, consist of a dense network of tiny threads from which arise sinqile 

 stalks each bearing a chain of spores or reproductive bodies. Except for the 

 short, swollen tubes or "suckers" which penetrate the surface cells of the leaf 

 to attach the fungus and to absorb food and moisture, mildew grows entirely 

 on the surface and all visible signs of it may readily be rubbed away with 

 the fingers. When the spores are mature they are shed from the stalks, and 

 drafts of air or slight disturbances to the vines free them from the leaves. 

 Those that fall on cucumber or other host plants germinate under favorable 

 conditions producing suckers and a dense thread-like web bearing a new 

 supply of spores. The nudtiplication of the fungus in this manner causes a 

 rapid spread of the disease. Mildew weakens or stunts the plants and a 

 severe attack reduces the yield and quality of the fruit. 



Most powdery mildews produce winter spores in small black capsules late 

 in the season but very little is known of this stage of cucuml^er powdery 

 mildew. It is believed, however, that the fungus lives through the winter out- 

 of-doors in this form and also as mycelium or sjjawn. In the greenhouse the 

 fungus survives from one crop of cucumbers to the next. 



Cucumber powdery mildew should not be confused with the more serious 

 downy mildew which forms a brown web on the under side of cucumber leaves 

 causing yellowish spots and later dead leaves. This disease ajipears in the 

 late sununer and autumn months and because its attack is sudden and its 

 SDread rapid, it is much more dreaded by growers than powdery mildew. 



