DISEASES AND PESTS OP PLANTS. 



15 



Fig. 1. 



(a.) Crown-gall on nursery stock. 



(6.) Hairy root on young apple-tree. 

 SCAB (Venturia irwcqualis). 



A generally distributed pest, its importance, however, being dependent on cli- 

 matic conditions. In the Island and Lower Mainland it is a factor to be constantly 

 reckoned with, as also at many points in the Interior. In the Dry Belt it has 

 usually been considered a negligible disease, but it has recently caused much loss at 

 Veruon and Kelowna. The rainfall in these districts in a wet season is sufficient to 

 provide the requisite conditions for a bad attack of scab. 



The disease is due to a fungus which may attack the leaves, flowers, fruit, and 

 twigs. On the leaves it forms dark olive-green spots which may be circular or 

 irregular in shape. They are usually more irregular and less clearly defined on the 

 lower surface. Under a lens each spot may readily be seen to consist of the radiating 

 branched threads (hyplice) of the fungus. From these, summer spores (conidia) 

 are produced, which in turn give rise to new infections. The assimilating power of 

 a leaf so attacked is much reduced, and if the attack is severe, premature yellowing 

 and fall of the leaf results. Serious loss may result from the disease attacking the 

 flower-stalks, causing the flowers to fall off instead of setting fruit. On the fruit 



