322 Diseases of Truck Crops 



Late Blight 

 Caused by Phytophthora infestans (Mont.) De Bary. 



Late blight is a disease which is restricted to some 

 parts of the United States. As it thrives best in 

 States where the midsummers are moist and cool, 

 it is common in the Northern States. Farther 

 south or west, it is unknown or it occurs sporadically, 

 causing little damage. 



Symptoms. Late blight attacks both the foliage 

 and the tubers in the field, or the tubers alone in 

 storage, the disease appearing when the plants have 

 passed the flowering stage. 



On the leaves the trouble is first manifested as 

 purplish black or brownish black areas on the lower 

 side (fig. 61 b). It attracts attention only when the 

 upper leaves are attacked and blackened. At first 

 the infected leaves become watersoaked and pale, 

 then they wilt and blacken. On examining an in- 

 fected leaf during a dewy morning, a delicate growth 

 of the fungus is perceptible as a fine powdery 

 bloom on the under side. 



When the tops are badly blighted, the tubers too 

 will show evidence of disease. In early stages the 

 infection becomes perceptible as brownish to pur- 

 plish discoloration of the skin with a softening of the 

 inner tissue (fig. 61 c). In dry, well drained soils, 

 the progress of the disease underground is slow, and 

 at harvesting dry rot may be in evidence. Infected 

 tubers when stored in cool, dry cellars may pass the 



