DISEASES OF CULTIVATED PLANTS 603 



portions are farthest away from the source of food supply, the veins. 

 These latter stay green for a longer time. Being deprived to a 

 great extent of the means of sustenance the leaf in a weakened con- 

 dition becomes a prey to various fungi which ultimately destroy it. 

 The leaves fall off and sometimes the stalk is entirely bare. Such 

 plants do not set a top crop as a rule and therefore the yield is seri- 

 ously reduced. The lint is often badly damaged being shorter and 

 inferior in quality. 



The disease is due to unfavorable soil and weather conditions. 

 Thrifty and vigorous plants do not suffer until the food supply is 

 withdrawn. The three important factors to which rust is attributed 

 are (1) lack of humus, (2) lack of potash, and (3) lack of drainage. 

 Heavy rains followed by long droughts cause no injury to the cotton 

 plant if the soil is in a condition of proper tilth and fertility. Let 

 these conditions be reversed and serious outbreaks of rust will follow. 

 Diseases of this class are all called physiological, since they are re- 

 sponsible for the constant interruption of the normal development 

 of the plant. (Miss E. S. IB. 140 B.) 



The remedy for rust lies in changing the unfavorable soil con- 

 ditions and in securing conditions that will support a vigorous 

 growth of the plant. When land has been cropped to cotton for a 

 long term of years it lacks humus or decaying vegetable matter. 

 This kind of soil needs building up and can be done by plowing in 

 a green crop. A crop of cowpeas or beans are especially good. 

 Barnyard manure is beneficial and will do much to prevent rust. 

 Kainit or potash at the rate of 200 pounds per acre will do equally 

 as well as barnyard manure if the latter is not available. Better 

 drainage of wet lands and seepy hillsides will also reduce rust. 



Damping-off or Sore-Shin. The young plants first show signs 

 of weakness in the stem near the surface of the ground. The turgid 

 cells lose the water they contain and become flabby. At this point 

 the stems bend and the plants fall to the ground. Quite often the 

 plants are only partially affected. This is true of the older plants. 

 One side is marred by a canker or ulcerous depression. Unless these 

 diseased spots interfere seriously with the normal functions of the. 

 plant, the latter will partially recover but will never be normal. 



The sterile fungus, Rhizoctonia, is usually considered the cause 

 of this trouble. Its natural habitat is the soil. The disease is some- 

 times due to the anthracnose fungus or other parasites. Such damp- 

 ing-off disease must be clearly distinguished from wilting due to dry 

 soil or dry air. If warm favorable weather prevails, the cotton 

 seedling may outgrow the disease. As the sterile fungus prefers 

 acid soils, and alkaline soils are fatal to its growth it would follow 

 that the addition of lime to the soil would be exceedingly beneficial. 



Shedding of Bolls. Squares, forms and young cotton bolls 

 often fall for other causes than that due to the injury by insects. 

 Not all of the shedding can be attributed to the boll worm or weevil. 

 In many instances no fungus or insect can be found on or in the* 

 affected parts. Such shedding is simply an outward manifestation 

 of improper or faulty nutrition and assimilation. The disease is a 



