582 TEXTBOOK OF BOTANY 



winter as mycelia inside the grain. Spores may also survive the winter 

 on the seed. The spores on the surface are easily killed, but it is difficult 

 to kill the hyphae inside the seed without injuring the embryo. The 

 fungous hyphae in the wheat seed are killed when the seeds are im- 

 mersed in water at a temperature of 129° F. for ten minutes; the hyphae 

 in barley seeds may be killed by similar treatment at 127° F. Neither 

 the wheat nor the barley embryos are injured if these directions are 

 accurately followed. 



Plant diseases caused by bacteria. It is sometimes difficult to discover 

 whether a disease is caused by bacteria or by fungi since both are 

 often present in the same lesion. It is of course necessary that this dis- 

 covery be made before effective remedial measures can be worked out. 

 Most bacterial diseases of plants are very difficult to control. Among 

 the better measures of control are: rotation of crops, use of disinfected 

 seed, prompt and complete destruction of diseased plants or parts of 

 plants, proper care of wounds made by storms and pruning, seed 

 sterilization, and the planting of less susceptible varieties. The fire blight 

 of pear is probably the best known of these diseases, and the methods 

 suggested for its control will be discussed. 



Fire blight. Fire blight is one of the most destructive of the diseases 

 of pome fruits. Its most frequent symptoms appear as blighted twigs, 

 flowers, and leaves. Infection takes place in the flowers, young shoots, 

 and leaves. Any diseased part of the plant may exude a sticky fluid con- 

 taining the bacteria. Rain may distribute the bacteria to other parts of 

 the plant. Insects, however, are the most important carriers of the 

 bacteria from one plant to others. 



Fire blight is difficult to control. To decrease the possibilities of in- 

 fection of apple and pear trees, tender sprouts should be kept removed 

 from the trunk and larger branches during the early part of the grow- 

 ing season. In addition, all blighted twigs should be removed from 

 the pear orchard at least twice weekly during the active blight season. 

 Blighted branches found in the fall of the year should also be removed. 

 Other suggested methods of control include the application of sprays 

 at blossoming time and the planting of varieties of pears and apples 

 less susceptible to the disease. 



Clubroot of cabbage. The organism which causes this disease in cer- 

 tain members of the mustard family, such as cabbage, turnips, and 

 radishes, is a simple mold. Enlarged, deformed, and club-shaped roots 

 are evidence of the progress of the disease. The organism enters through 

 root hairs or tlirough wounds, and digests the tissues of the host as the 



