604 INSECTS AFFECTING VEGETATION 



physiological one. Climatic conditions have much to do with the 

 presence or absence of this trouble. It is quite common in either 

 .very dry or very wet weather. It is also present during the change 

 from one extreme to the opposite. If the plants are crowded and 

 the growth rank, sufficient food is not available to supply all fruit 

 forms especially those furthest from the source of supply, conse- 

 quently many squares fall. 



Some varieties of cotton seem to be more susceptible than others. 

 Certain soil conditions have a detrimental influence and aggravate 

 this trouble. Even the character of the fertilizer is said to show a 

 marked effect on shedding. During some seasons the loss by this 

 disease is considerable. Under normal conditions there is also a loss 

 which, however, does not reach an alarming stage. 



The symptoms are easily recognized. The parts affected change 

 in color from a deep green to a pale green and finally become yellow. 

 In the majority of cases the forms drop off, yet it is not uncommon 

 to find some attached to the parent plant by a few strands of the 

 peduncle or stalk. 



This disease is a physiological one and cannot be attributed to 

 any one cause. It is rather the result of a series of causes which are 

 constantly changing. Such causes are the unfavorable conditions 

 of climate and soil. 



Physiological diseases have as yet received little attention at 

 the hands of plant pathologists and for that reason it is difficult to 

 give any reliable remedy. We have no control over climatic con- 

 ditions and therefore the hope of a suitable remedy lies in the direc- 

 tion of a thorough study of soil conditions. A system of soil prepa- 

 ration, culture and fertilization with a view to affording the best con- 

 ditions for the development of the plant should do much towards 

 checking this malady. (Miss. E. S. B. 140 B.) 



Cotton Wilt. The cotton plant is subject to wilt at an early 

 date. When about ten or twelve inches high, it is susceptible and 

 from then on to maturity. Wilt is most abundant in June and 

 July, especially when the rainfall is plentiful in early spring and 

 during the growing season. 



The external characteristics are first apparent when the leaves 

 turn yellow at their margins and between the veins. Later the 

 whole leaf becomes yellow and drops off. In extreme cases of wilt 

 nothing but the bare cotton stalks and branches are left. In less 

 severe cases one or two branches may be stripped of leaves while the 

 rest of the plant is apparently unaffected. Wilted plants may par- 

 tially recover but will always show a stunted or bushy appearance. 



It is by means of the internal appearance of the affected cotton 

 stalk that we establish without a doubt the cause of this disease. 

 The wood in the region of the water conducting vessels that pass up 

 the root and stem is brown. When the stem is cut across with a 

 knife this brown ring is clearly seen and is due to the dense, closely 

 woven threads or hyphas of the fungus which has filled up the vessels, 

 shut off the water supply and killed the plant. This brown ring in 

 the stem is characteristic of this disease. In addition to the above 



