14 



FARMERS BULLETIN 



moist with dew or rain, show a delicate, powdery bloom on the under- 

 side. (See fig. 17.) The stems are attacked later, and the entire 

 plant may be destroj^ed in a few days. If weather conditions favor 

 the disease, fields go down within a few days as if swept by fire, and a 

 foul odor characteristic of the disease is very perceptible. 



The blightmg of the foliage is followed by decay of the tubers, 

 owing to spores washed down through the soil from the foliage. If 

 the soil is wet and heavy, there may be a rapid soft-rot caused prin- 

 cipally by bacteria. The typical late-blight tuber injury, however, 

 is a dry rot which develops in the field or after storage, as sunken 

 brown spots near the outside of the tuber. 



•Fig. IC— Map of the United States showing the di'^trilnition of potato late-blight. The sections where 

 the disease is the more prevalent arc indicated by the llea^■ier shading. 



CAUSE OF LATE-BLIGHT. 



Late-blight is due to a fungus ^ which lives as a parasite on the 

 potato plant and some of its relativ^es, notably the tomato. This 

 fungus is itseK a plant formed of slender, moldlike filaments (mycel- 

 ium) which penetrate the potato plant and feed upon it and later 

 produce vast numbers of minute spores or fruiting boilies. These are 

 spread by wind and water to other plants, which may become mfected 

 and produce another crop of spores within five or six days. The 

 tubers become infected by spores washed down through the soil or by 

 contact with blighted tops when digging is being done. 



I Phytophthora infestans (Mont.) DeBary. 



