BACTERIA AND DISEASE 411 



which flourishes especially in the soft, juicy tissues of the trees. It 

 attacks all above ground parts of the tree, including the blossoms 

 and fruit, gaining entrance mainly by way of the nectaries or 

 honey secreting glands of the blossoms, or by way of the stomata 

 in young leaves, in either case being spread by insects, wind, 

 rain and dew, and man's activities. The bacteria overwinter in 

 cankers, or infections on the woody parts of the trees, and are 

 exuded in gummy masses during the spring when sap flow is 

 resumed. These serve as sources of infection for the new growth. 

 In some years this disease causes little loss, but in other years 

 entire crops and even whole orchards may be wiped out. The 

 Bartlett pear is so susceptible to fire blight that the growing of 

 this variety has been abandoned in some sections due to the 

 disease. During a twenty year period, fire blight destroyed almost 

 one-third of all the full grown pear orchards in California. The 

 best way to control fire blight is to remove infected parts of trees, 

 thus eliminating sources of infection. Various sprays during the 

 blossoming period are also very helpful. 



Bacterial blight of beans occurs commonly in the more 

 humid bean growing areas and reduces the crop yield by about 

 5%. It is caused by several kinds of rod bacteria which over- 

 winter in the seed or in the soil. It usually forms blackened, dead 

 areas on the leaves, stems and pods, thus interfering with normal 

 growth of the plant or normal development of the fruit. During 

 the growing season the disease is spread by drops of rain or dew 

 being blown from infected to disease free plants, and by man's 

 cultivating and picking operations. Control of this blight is best 

 effected by the use of disease-free seed produced in the dry parts 

 of the country where the disease is rare, and by avoiding cultiva- 

 tion or picking when the foliage of the bean plants is wet with 

 rain or dew. 



There are many other bacterial plant diseases, among them 

 the destructive potato scab which causes a loss estimated at 

 between five and twenty four million bushels of potatoes a year, 

 and CROw^N gall (fig. 25 3 A) which is very destructive on apples, 

 roses, raspberries, and other woody plants. Black rot of 

 CRUCiFERS attacks most of our important members of the Mustard 

 Family such as cabbage, cauliflower, turnip, radish, mustards 



