TWO EPIDEMICS OF POTATO BLIGHT AND ROT. 1 73 



infection the blight may be well started on the leaves and a 

 heavy rain wash the spores down onto the tubers in the soil, and 

 decay at once starts up, or if the tops are producing an abun- 

 dance of spores when the crop is dug, the weather cloudy or 

 rainy, the atmosphere humid, and the tubers after being show- 

 ered with millions of spores from the blighting leaves are at 

 once picked up and stored in large bins without being allowed 

 to dry off infection is almost sure to result as in the fall of 1909. 



WEATHER CONDITIONS AND THE DISEASE. 



Rain, dew, wind and insects are the chief agencies for dis- 

 tributing the disease. So far as known the only way that the 

 crop of any one year is first infected is from planting diseased 

 tubers in which the fungus has remained semi-dormant over 

 the winter. Hence the first blow to be struck in the fight 

 against this disease is to plant nothing but healthy, sound 

 tubers. 



Late blight revels in moist, cloudy weather and, contrary to 

 the general notion, in relatively cool or moderate temperatures.* 

 The spores are produced in greatest abundance in rainy or 

 cloudy weather and are extremely susceptible to drying and 

 hence one need never fear an outbreak in dry, hot, sunshiny 

 weather. Wet weather is almost sure to bring it on unless 

 spraying is kept up during the continuance of such weather. 

 In fact there is no other disease of our common field or garden 

 crops where a careful observer can predict with more accuracy 

 its probable appearance or absence. From a financial stand- 

 point this is a point that no progressive potato grower should 

 overlook. While spraying operations should not be delayed 

 till favorable conditions for blight appear, how thoroughly they 

 are followed up may well be governed by weather conditions 

 if one thoroughly understands the significance of the latter. 

 In this latitude late blight seldom occurs to any extent before 

 the last of July, hence the main fight against it must be con- 

 ducted throughout August and September, even up to the very 

 day the crop is dug or the plants are killed by frost. .\s will 



* "Humid, still days, with a temperature of about 73° F. Above 78° 

 F. and below 50° F. there is practically no germination of the spores." 

 Frascr, Samuel, The Potato, p. 114, Orange Judd Co., N. Y. (1908). 



