CHAPTER 19 



Diseases of Farm Crops and Their Remedies 



By Dr. Mel. T. Cook 

 Plant Pathologist, New Jersey Agricultural Experiment Station 



When any of the various parts of a plant are not doing their work 

 properly the plant is said to be diseased. The disease frequently causes 

 poor growth or poor fruit, or both; and in case of our cultivated plants, an 

 unsatisfactory crop. 



The most important causes of plant diseases are fungi, bacteria, slime 

 moulds, parasitic flowering plants, insects, mites, nematodes, unsatisfactory 

 soil, too much or too small amount of moisture, unfavorable temperature, 

 gas fumes and smoke. Some plant diseases occur for which there are no 

 satisfactory explanations. 



Plant diseases may be detected by characteristic symptoms which 

 readily distinguish the disease upon the healthy plants. The most common 

 of these symptoms are: (a) a discoloration of the foliage and sometimes 

 of the new growths; (&) wilting, frequently followed by yellowing and 

 browning; (c) dropping of the foliage; (d) the formation of spots on foliage, 

 stems or roots; (e) perforation of the foliage commonly called "shot hole;" 

 (/) variegation of the foliage commonly called mosaic; (g) the "damping 

 off" or dying which is especially common on seedling plants; (h) the blight 

 or dying of leaves, twigs or stems; (i) the dwarfing of parts; (j) the increase 

 in size of parts; (k) formation of galls, pustules or corky growths; (I) 

 cankers on fruit, stems or roots; (m) abnormal fruits; (n) the formation 

 of masses of small shoots called "witches' brooms ;" (o) the curling of leaves ; 

 (p) the formation of leaf rosettes; (q) abnormal root growths commonly 

 known as hairy root; (r) exudations of gums, resins, etc.; (s) the rotting of 

 fruit, stems or other parts; and (t) sunburn of fruits and foliages. 



Some diseases of the soil, such as "damping off," are very severe in 

 seed-beds and in greenhouses, and can be controlled by sterilizing the soil. 

 Diseases that occur in the soil in fields are frequently overcome by a rota- 

 tion of crops, by improved drainage and sometimes by stimulating the 

 plants with suitable fertilizer. 



Many diseases are controlled by spraying, but in most cases spraying 

 is used for the protection of plants against disease and not for curing them; 

 therefore, it is a kind of insurance and must always be supplied in advance 

 of the appearance of the disease. Spraying cannot be conducted in a 

 satisfactory manner unless the grower is sufficiently familiar with the disease 



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