346 GENERAL PLANT PATHOLOGY 



transpired exceeds that absorbed by the roots wilting occurs. Wilting 

 may result, if the normal ascent of the sap is interfered with by the 

 growth of fungi into the water-conducting tissues, the entrance of bac- 

 teria into the woody vessels of the plant, whereby they are literally 

 plugged with such organisms, or some injury which cuts off the ascend- 

 ing current of water. Damping-off is a form of- wilt in which an oomy- 

 cetous fungus enters the collar of seedling plants, or where a Rhizoctonia 

 species invests the roots of the growing plants and interferes with the 

 regular water absorptive processes. 



4. iVecro5M.— Necrosis is the mortification, or death, of the tissues. 

 The term is usually applied to the death, or loss of vitality, of one part of 

 a plant, while the other parts remain alive. When the fungus, Fusa- 

 rium trichothecoides , is inoculated into Green Mountain potato tubers, 

 in about three weeks' time it will be found that a portion of the tuber, 

 usually the central part directly beneath the point of inoculation, has 

 undergone necrosis. The surface of the potato tuber becomes sunken 

 through the death and collapse of the starch containing cells and the 

 lesions may involve half of the tuber. The black rot of the navel 

 orange is due to a fungus, AUernaria citri, which gains entrance to the 

 fruit through slight imperfections about the navel end. A black 

 decayed area is found under the skin. This decay does not spread im- 

 mediately through the entire fruit, but remains for weeks- as a small 

 black necrotic area with a mass of the fungus present. The decayed tis- 

 sue does not always extend to the surface, but remains beneath the skin. 

 Necrosis often follows the action of frost in killing the cortex cells of 

 fruit trees in patches with a blackening of the tissues. Fire bhght may 

 be the cause of necrosis, for the cambium which is killed dries up in 

 black patches. 



5. Dwarfing. — A reduction in the size of a plant is very often asso- 

 ciated with disease. This may be true of the whole plant, or some 

 particular organ only may be dwarfed. Apples are frequently reduced 

 in size by the attack of the scab fungus, sometimes not reaching one- 

 fourth the size, and the same is true of apples affected by the cedar rust. 

 Dwarfing of the whole plant may be a symptom of malnutrition. It 

 may be evidence of a poor soil, or the repeated maiming, or nipping off of 

 the buds by cattle, or purposely by man, as is the case with the minia- 

 ture trees of the Japanese. Dwarfing, or nanism, may be the result 

 of climate, as is the normal case with alpine plants. Prostrate forms of 



