Diseases and Pests of Cultivated Plants in 

 British Columbia, and their Control. 



THE NATURE OF DISEASE. 



O accurately define the term " disease " as applied to plants is very 

 difficult. Usually we think of disease as an abnormal condition result- 

 ing in weakness and often in death. In the case of plants, we have, 

 however, a less fully unified organism than is .the case with animals, 

 and it is possible to have local injury, and even death, of groups of 

 cells without apparent injury to the vitality of the plant as a whole. 

 A good example of this is the bitter-pit or fruit-pit of the apple, which results from 

 the death of groups of cells in the fruit, often without any indications of an abnor- 

 mal or unhealthy condition in the tree itself. The seeds in such a fruit may also 

 be quite normally developed, so that the amount of " disease " viewed from the 

 standpoint of the life and continuity of the plant is practically negligible. Since, 

 however, the commercial value of the fruit is seriously reduced, bitter-pit is a very 

 serious disease from the cultivator's point of view, however little the life of the 

 plant itself may suffer. 



NON-PARASITIC DISEASES. 



Plants placed in unsuitable soil, subjected to unfavourable climatic conditions, 

 or otherwise placed in an unfavourable environment, become unthrifty and sickly, 

 and may die as a direct result or fall an easy prey to some parasite. Just where 

 such a condition may lie considered as passing into actual disease it is hard to say. 

 Moreover, the full effects of the different factors in the environment on the pro- 

 cesses going on in the plant are not very well known. One variety of plant will 

 thrive where another will succumb. Even in the same variety one individual will 

 get along much better than another. Of the constitutional differences which make 

 this possible we know next to nothing. For these reasons few troubles not due to 

 the action of parasitic organisms will be considered here. As a result of horticul- 

 tural practice and experiment, varieties of plants suitable for certain soils and 

 climates can be selected with reasonable assurance of their fitness, but this, and 

 also the proper treatment necessary to keep the plant in a vigorous condition, falls 

 within the province of the horticulturist. One or two of the more definite diseases 

 of non-parasitic nature, such as bitter-pit and water-core, will, however, be con- 

 sidered in their proper place. 



PARASITIC DISEASES. 



Most of our destructive plant-diseases are due to the action of organisms which 

 attack the plant in such a way as to abstract from it the food necessary for its own 

 growth and development. In doing this they also introduce poisonous or injurious 

 substances which have a profound effect on the life and growth of the plant. 

 According to the nature of the attack, the plant may be killed outright or in certain 

 members, or may be seriously weakened, stunted, malformed, or otherwise rendered 

 unproductive. The parasites capable of bringing about these changes may be classi- 

 fied under the following four heads : 



(a.) Flowering plants. 



(b.) Fungi, including slime-moulds. 



