DISEASES. 



Diseases of plants, for convenience of consideration, may be grouped 

 as follows : First, those caused by (") insects and other animals, 

 {b) fungi, (c) bacteria, (d) slime moulds, and [e) flowering plants; and, 

 second, the unfavorable action of soil, drought, heat, wind, lightning, 

 frost, and sun scald. Sometimes two or more of these causes may operate 

 to bring about a diseased condition of the plant. In particular we may 

 note that plants which have been rendered unhealthy by excessive mois- 

 ture, excessive dryness, or imperfect access of light and air, are more 

 liable to attack from insects and fungi. It is notalways easy to tell when 

 a plant is in a diseased condition, for the condition of a healthy plant 

 varies between fixed limits, and it is only when these limits are exceeded 

 in either direction that the life of the plant is threatened, and there is 

 disease instead of health. We sometimes group the causes which bring 

 about diseases in plants as external and internal, but it is very probable 

 that the so-called internal causes will be found to be nothing more than 

 external causes acting- in an indirect manner. 



Young plants are, as a rule, more liable to attack by fungi and 

 insects than older plants, because their tissues are softer and their cuticle 

 thinner. 



In the following pages attention will be given only to those diseases 

 in plants which are caused by insects, fungi, and slime moulds. 



The losses produced every year by the action of insects and fungi 

 on vegetable crops are very large, amounting in Ontario to probably one- 

 half million dollars. While it may be true that some of these losses 

 cannot be averted, yet it is also true that a large percentage of them can 

 be prevented by proper treatment. On account of the low prices pre- 

 vailing in the vegetable markets, the method becomes a very important 

 factor in the control of these pests. In some cases the cost of treatment 

 would amount to as much as the market price of the vegetable. It is 

 our object in the following pages to recommend such treatments as have 

 been found effective and practicable in vegetable gardens cultivated on a 

 commercial basis. 



A few words about the habits of fungi and insects. Fungi are a 

 group of lower plants (without green-coloring matter) that produce spores 

 instead of seeds. The body of a fungus may be very simple, composed 

 of a few threads, or it may be more complex, composed of many threads 

 matted together. The fungus derives its nourishment from the cells of 

 the plant which it attacks. Sometimes the threads live on the surface 

 of the plant {e.g., the Powdery Mildews), but more frequently they live 

 within the plant, either between or within the cells. Fungi produce, 

 roughly speaking, two kinds of spores — summer spores, capable of 

 developing threads as soon as they are set free; and resting, or winter 

 spores, requiring a period of rest before germinating. Spores are carried 

 by sueh agencies as wind and water, and, coming into contact with a 

 suitable host plant, they develop threads which may enter through wounds- 

 la BULL. 150 



