290 



THE ESSENTIALS OF AGRICULTURE 



portions of the trees every fall and winter. Cutting out blighted 

 twigs immediately after blossoming is also helpful. 



The black knot of plums and cherries and the black rot of 

 apples are very common fungous diseases which, like the pear 

 blight, may be controlled in part by careful pruning and burn- 

 ing of the affected branches, 

 twigs, leaves, and fruit. 



379. Diseases controlled 

 wholly or in part by crop 

 rotation. Cotton wilt is 

 caused by a fungus which 

 lives in the soil and enters 

 the roots of the cotton plant 

 when the plant is young 

 (Fig. 138). The mycelium 

 growing up into these parts 

 soon plugs the water-carrying 

 vessels of the root and stem, 

 the water supply is thus cut 

 off, and the plant wilts. One 

 kind of spore is produced 

 within the vessels of the 

 cotton plant, and when the 

 plant dies, another kind is 

 formed on the outer bark of 

 the stem. These spores are 

 scattered by winds and in- 

 sects, and help to spread 

 the disease to other plants. 

 This parasite produces two 

 other kinds of spores in the soil, and these help to spread the 

 disease. This fungus may live in the soil from year to year, and 

 cotton planted on such soil may become diseased. In addition to 

 being spread by spores, the fungus is distributed by direct growth 

 through the soil, by being carried from place to place by cultiva- 

 tors, plows, drainage water, the feet of horses and cattle, and in 



FIG. 138. Effects of cotton wilt 



The plant is dying because its water-carrying 



tissues are clogged by the cotton-wilt fungus. 



(After Orton) 



