60 MAINE AGRICULTURAL EXPERIMENT STATION. I9OI. 



by fire. The rapid spread of the disease, which is caused by a 

 parasitic fungus, is dependent in large measure upon certain con- 

 ditions of moisture and heat. A daily mean temperature of from 

 J2° to 74 F. for any considerable time, accompanied by moist 

 weather, furnishes the best conditions for the soread of the para- 

 site. On the other hand, if the daily temperature exceeds yy° 

 for a few days, the development of the disease is checked. This 

 fact explains why the fungus seldom occurs to any serious extent 

 in sections where the mean daily temperature exceeds y/° for 

 any length of time, and probably why it appears later than the 

 so-called early blight. The late blight not only stops the growth 

 of the tubers but causes them to rot. Bordeaux mixture 

 (formula 2) will prevent this. 



LEAF DISEASES, ETC., RESEMBLING BLIGHT. 



Leaf burn or scald sometimes occurs and mav be . confused 

 with early blight. The tips and edges of the leaves turn brown 

 and these discolored areas soon become hard and brittle. The 

 burning or scalding may occur at any time and is the result of 

 unfavorable conditions surrounding the plant. Long continued 

 cloudy and damp weather followed by several hot and bright 

 days is apt to result in the burning of the foliage. Leaf burn 

 may also occur as the result of protracted drv weather. 



Leaf poisoning or burning may occur where Paris green is 

 applied to potatoes and frequently it can not be distinguished 

 from early blight by any ordinary examination. It sometimes 

 happens, therefore, that farmers are led to believe that their 

 potatoes are affected with early blight and other diseases when 

 the trouble has been brought on by themselves through the im- 

 proper use of Paris green. Injuries resulting from the use of 

 this substance are very apt to occur where flea beetles have eaten 

 the foliage. The arsenic attacks the tissues at such points, and 

 as a result more or less circular brown spots are produced, hav- 

 ing for their centers the holes eaten out by the flea beetles. By 

 combining the Paris green with Bordeaux mixture or with lime, 

 these injuries may be avoided. 



