PLANT DISEASES AND THEIR TREATMENT 209 



spring months, beginning as soon as the first leaves are 

 opened. 



There is another leaf-blight which attacks pears, quinces, 

 and apples and makes the leaves look as if they had been 

 scorched by fire. The leaves do not fall off as in the case of 

 the other leaf -blight. This blight is called fire-blight or twig- 

 blight. There is no remedy but to cut out the affected twigs 

 and burn them. 



Another serious disease which may be called a blight is the 

 black knot of plum and cherry trees. The twigs and limbs 

 swell at various points, becoming larger than the surrounding 

 parts. The swellings become very black by winter. The 

 next spring the swellings continue to increase at their edges 

 until the twig or limb dies. There is no effective remedy. 

 Cutting off the attacked limbs some distance from the swelling 

 is probably best. Painting the knots with kerosene is also 

 helpful. Spraying with Bordeaux mixture will tend to pre- 

 vent the starting of new swellings. 



Potatoes are attacked by two bad blights. One is the early 

 blight and the other is the late blight. The early blight ap- 

 pears as circular yellow spots near the edges of the leaves. 

 As time goes on, the spots increase in size, become brown in 

 color, the leaves roll up, the stems become affected, and the 

 plant dies. Dry weather seems to be favorable to this disease. 

 This blight is not fully understood yet, but spraying with 

 Bordeaux mixture has been beneficial. The late blight is 

 helped by warm, moist weather and soon destroys a potato 

 plant. This blight appears as irregular-shaped brown spots 

 anywhere on the leaves and spreads rapidly, soon killing the 

 plants. In the case of this blight the potatoes in the ground 

 also rot. This blight usually appears later in the season than 



