PLANT DISEASES AND INSECT PESTS 161 



the vines and roots, and often causes serious loss in the 

 tubers by causing them to rot. A thin sUce cut across the 

 stem end of tubers affected usually shows a dark ring or 

 diseased portion just under the skin. This disease causes 

 a great loss in yield and from tubers rotting. It may be 

 entirely controlled by selecting clean seed and by rotation 

 of crops. Clip off the stem ends of tubers to be used for 

 seed, and discard all that show the disease, or at least cut 

 off all the diseased part. Select for seed only disease-free 

 tubers, and then treat with corrosive sublimate solution the 

 same as for scab. Plant only clean seed on clean land. 



Potato blight causes serious losses in the potato crop 

 nearly every year. There are two distinct bhghts that 

 affect potatoes. They are called early and late blight. 

 Both diseases affect the leaves of the growing plants, and 

 reduce the crop of tubers by partially or completely destroy- 

 ing the leaves. Late bhght also causes the tubers to rot; 

 so it is more serious than early bUght. Both of these blights 

 are controlled by spraying the vines with Bordeaux mix- 

 ture. See page 149. The potato vines must be sprayed 

 from three to five times. Begin spraying when the plants 

 are six to eight inches high, and repeat every ten to fifteen 

 days until the potatoes are ripe. 



Cotton Diseases. — There are two serious common diseases 

 of the cotton plant : wilt and root rot. Cotton wilt is some- 

 what similar to flax wilt. Some plants and some varieties 

 of cotton seem to resist the disease better than others. 

 The selection of resistant varieties and the rotation of crops 

 are the most effective means of control. Root rot affects 

 cotton quite seriously on heavy soils. Deep plowing, drain- 

 age and rotation of crops help to control this disease. 

 Questions: 



1. Name some important diseases that affect potatoes. 



2. How are the potato bhghts controlled? 



3. Describe the making of Bordeaux mixture? 



4. Name two diseases of cotton and remedies for them. 

 Arithmetic: 



1. If an acre of potatoes yields 160 bus., how much is the loss 

 per acre, if the value of each bushel is reduced 5c. on account of scab? 



2. If a normal yield of potatoes is 160 bus. and the yield is re- 

 duced 25% on account of wilt, what is the loss per acre, if potatoes are 

 worth 40c. per bushel? 



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