Plant Diseases 433 



cabbage field from year to year, alternating with crops that are 

 not affected by this fungus. 



Black knot. This disease is frequently seen in plum and 

 cherry orchards and is made conspicuous by the presence of 

 greatly thickened portions of twigs and branches. The spores 

 of the fungus causing it seem to be carried by the wind, and 

 infection takes place through the bark. As soon as the hyphae 

 of the fungus penetrate the inner tissues, the twigs begin to en- 

 large and the outer bark is broken. On the swollen surface in 

 the spring and early summer one may see with a magnifying 

 glass hyphae producing spores. 



During late summer another type of small, rounded fruiting 

 body develops on the knots, which last over the winter. In the 

 spring these bodies contain numerous sacs, each with eight spores 

 inside it. These constitute a second means of spreading the 

 disease. Where the disease is very prevalent, as in the Atlantic 

 coastal states, it is difficult to control because it affects the wild 

 as well as the cultivated cherries. The removal and destruction 

 of all knots by burning has been found effective, and may best 

 be done during the winter when the trees are leafless and before 

 the winter spores are ripe. 



The smuts. The smuts, described in the last chapter, are , 

 very destructive to cereals, but effective methods of control have 

 been found. Corn smut differs from the other smuts in that its 

 spores are shed from the lesions on the plant and remain over the 

 winter on the soil. Hence the only methods of control available 

 are the rotation of crops and the removal and burning of infected 

 plants from the field as soon as they are noticeable. 



Among the small cereals there are two types of smut, one of 

 which results in the total breaking down of the grain and glumes 

 into fine powder and spores. These are the so-called " loose- 

 smuts." Other species of smut do not produce as complete a 

 destruction of the seed coats and glumes. These are known as 

 the " covered smuts." Experiments have shown that the covered 



