254 Transactions British Mycological Society. 



It often happens that varieties of cultivated plants which are 

 specially resistant to disease are poor in quality or in cropping 

 power. The plant pathologist and the cultivator here look for 

 the assistance of the expert plant breeder to make the necessary 

 desirable combinations. As is well known, this has already been 

 done with many annual plants, especially the cereals, with 

 great success, and one looks forward to the time when similar 

 developments will take place with perennial plants such as fruit 

 trees, although success in this direction will necessarily be slower. 

 Again, it does not follow that because a plant is immune from 

 one disease that it escapes attack from other diseases. Thus, 

 Lord Derby apples which are resistant to ordinary canker are 

 liable to serious damage by blossom-wilt, and Pershore plums 

 which are practically immune from silver-leaf are often attacked 

 by the fungus Fames pomaceus — which, however, is fortunately 

 far less destructive than Stereum purpureum. It is, therefore, 

 a counsel of perfection to advocate the selection of varieties 

 which are not affected by disease, and so means have to be 

 devised and set in operation for attacking fungoid and insect 

 pests as they appear. 



In human illnesses, medical means are often applied to effect 

 a cure. Thus some drug is taken, or injected into the blood, which 

 either exerts a stimulative action enabling the body to throw 

 off the malady, or which, by some directly poisonous effect, 

 kills the parasitic organisms that are the cause of the disease. 

 In plant pathology, however, medical treatment by internal 

 application can only rarely be applied with any hope of success, 

 chiefly, as already stated, because the higher plants possess 

 nothing comparable to the blood stream of animals, the move- 

 ments of sap in the former being essentially different from the 

 latter. There is, however, a mode of dealing with certain insect 

 and fungoid pests which is of the greatest importance to fruit- 

 growers, and which can be compared in some respects to medical 

 treatment. I refer to spraying with insecticides and fungicides 

 — in the use of which fruit-growers, from the time when the vine- 

 growers of France first began to use copper compounds as a 

 means of protection, have always been the pioneers. As is well 

 known, insecticides are usually most potent when applied just 

 as the pest is emerging from the resting state or at any rate 

 before the insect is abundant in an active condition, but many 

 fungicides must be applied before the appearance of the fungus 

 in an infectious form in order for the leaves and stems to be 

 protected from penetration. Certain pests and diseases, such 

 as aphis in plums and scab in apples, can be entirely, or almost 

 entirely, controlled by spraying. It is not proposed to deal 

 further with the subject of spraying in the present paper, except 



