1716 



ENCYCLOPEDIA OF PRACTICAL HORTICULTURE 



Fig. 1. Late Blight of Potatoes. At this stage the tops appear very much as though frosted. 



and 74 degrees, with an abundance of 

 moisture, it will rapidly change the en- 

 tire top into a moist, putrid mass and 

 sweep over large fields in a few days. 

 (Fig. 1.) 



The damage resulting from the loss of 

 the tops might not always prove serious, 

 since the outbreak rarely comes until late 

 in the season, but the fungus does not 

 stop at this. The greatest loss may re- 

 sult from the attack on the tubers. Any 

 exposed potatoes are pretty certain to be- 

 come infected by the numerous spores 

 formed in the spots on the leaves, and if 

 rains come many spores are washed into 

 the soil and reach many potatoes below 

 the surface. Rot follows and spreads in 

 the hills. At digging time some of the in- 

 fected potatoes are pretty certain to go 

 into storage with the sound ones, and, if 

 storage conditions are not properly 

 watched, the entire lot may rot. 



The tubers, when first affected, show 

 slight depressed areas on the surface; 

 these areas are dark colored and at first 

 penetrate but a short distance beneath 



the surface (Fig. 2). This gradually ad- 

 vances into the interior until the whole 

 tuber is decayed. If the soil is wet, the 

 decay is aided by soft rot caused by bac- 

 teria, and where a field is affected in this 

 way, the putrid odor of decay may be 

 noticeable for some distance. The white 

 tufts of the fungus on which spores are 

 produced come out on the surface of the 

 tubers when the soil or air is humid, in 

 this way greatly aiding the spread of the 

 disease. 



Sometimes dry weather follows a period 

 favorable to the spread of the blight. 

 Under these conditions the fungus ad- 

 vances very slowly in the foliage, produc- 

 ing no spores and seldom spreading to 

 new fields. The tubers which have been 

 infected rot more slowly and take the 

 form of a dry rot. 



Cause 



At the time of the famine in Ireland in 

 1845 many ideas and theories were held 

 concerning the cause of the potato blight. 

 Some said it was a direct visitation of 

 Providence, some ascribed the disease to 



