232 



THE CANADIAN HORTICULTURIST. 



PLUM AND CHIi:RRY TREE BLACK 

 KNOT. 



This season has been favorable for the 

 development of the disease of the plum 

 and cherry trees known as black knot. 

 This is an old, long-standing trouble of 

 the orchards, and has been the subject 

 of much discussion in the agricultural 

 and horticultural papers for many years. 

 The " diseases," so-called, of plants are 

 now divided under three heads : First, 

 those caused by insects, as the various 

 galls, &c. ; second, those of fungous 

 origin, like the rusts, smuts, rail- 

 dews, &c.; and third, those troubles that 

 are organic, as far as they are under- 

 stood. It is safe to define the third 

 class as including those diseases that 

 do not belong to the first and second 

 classes — the plants are " out of sorts." 

 The black knot was long believed by 

 entomologists to be of insect origin, 

 and they seemed to have a very strong 

 a,rgument. The eggs and young of in- \ 

 sects were usually found within the 

 substance of the knot, and their presence 

 was strong evidence of the cause being 

 insects. All sorts of distortions, like 

 galls on willows, oaks, &c., were known 

 to be the work of insects — gall-flies, 

 &c. Within the past ten years, the 

 black knot has been carefully studied 

 by several experts in fungi, and under 

 the higher powers of the microscope 

 the cause of the peculiar distortions 

 became evident. The black knot is now 

 well demonstrated as belonging to the 

 second class of diseases or disorders, and 

 is therefore of fungous origin. Among 

 other things, it was shown that the 

 same kind of insect was not always 

 present ; in fact, no insects or eggs 

 were found in the early stages of the 

 knot. But instead, the substance of 

 the infested part is found to contain a 

 multitude of small threads or filaments 

 of a fungus. 



It may be well to state that a fungus 

 is a plant of a very low order, and with 

 a very simple structure. Among the 

 most familiar members of this group of 

 flowerless plants are the various moulds 

 that grow on bread, cake, cheese, &c., 

 and make sad havoc in the houskeeper's 

 pantry in midsummer. The toad-stools 

 and the mushroom are larger examples 

 of the class in question. The black knot 

 fungus is known to botanists as Sphceria 

 morbosa, and attacks the young branches 

 of the plum and cherry trees in the 

 spring. By the first of June the in- 

 fested parts have swollen considerably, 

 and soon after these portions have 

 cracked longitudinally in one or more 

 places. The surface thus exposed is 

 soft or spongy, and quickly turns to an 

 olive-green colour. This colour is due 

 to the formation of a multitude of 

 minute spores that form on the tips of 

 plants, extending from the surface. 

 The knot continues to increase in size 

 until past midsummer, and frequently 

 the branch becomes bent to one side, or 

 otherwise distorted, owing to the ir- 

 reorular orrowth of the fungus. It is 

 seldom that the knot extends equally 

 on all sides of the branch. 



As the season advances, a second 

 form of spore is formed, and it is within 

 the substance of the knot. The surface 

 spores above mentioned are quickly 

 grown and serve to spread the disease 

 from branch to branch. A spore has 

 the same ofiice to fill as a seed, but 

 differs in structure. Spores are, in 

 short, the seeds of flowerless plants. 

 The internal spores of the black knot 

 are formed slowly, and designed to carry 

 the life of the plant through the winter. 

 They germinate in the spring. Very 

 many fungi have two or more kinds of 

 spores, some for quick propagation and 

 others for the preservation of the 

 species. 



The fact that insect eggs and "worms" 

 are usually found in the substance is 



