2Q2 



THE ESSENTIALS OF AGRICULTURE 



abundance of spores, which again may cause widespread in- 

 fection. Late blight can be controlled by spraying the potato 

 plants with Bordeaux mixture two or three times during the 

 season at intervals of from ten days to two weeks, beginning 

 when the plants are about six inches high. Spores falling on 

 sprayed plants are poisoned by the spray before they can grow 

 sufficiently to infect the potato plants. Care should be taken 

 not to plant potatoes infected with the disease. 



381. Apple blotch. Apple blotch (Fi'g. 139) is another typi- 

 cal disease which can be controlled by spraying. It is caused by 



a fungus which occurs 

 on the fruit, twigs, and 

 leaves of the apple, the 

 principal damage be- 

 ing done to the fruit. 

 The blotch appears as 

 a hard brown spot on 

 the fruit, as small can- 

 kers or slightly enlarged 

 cracked areas on the 

 twigs, and as light- 

 brown or yellow spots 

 on the leaves. The 

 disease-producing fungus lives through the winter in the 

 cankers on the twigs and in the spring produces spores which 

 ooze out in great numbers. These are distributed by wind 

 and rain and probably by insects. When they fall on the 

 fruit and germinate, a mycelium is produced which penetrates 

 the skin, again causing blotch. The disease is common in the 

 central Mississippi Valley region and the southern half of the 

 apple belt. Spraying with Bordeaux mixture is an effective 

 preventive. 



Examples of other diseases distributed principally by spores 

 and controlled by the application of sprays are the brown rot 

 of peaches (Fig. 140) and plums, the early blight of potatoes, 

 the scab and bitter rot of apples, and the leaf curl of peaches. 



FIG. 140. Brown rot of peaches 



These peaches grew on unsprayed trees. (After 

 Scott and Ayers) 



