THE GARDENERS' MONTHLY 



[April, 



known fact, which would appear as conclusive 

 evidence. 



The theory that the freezing of sap in a healthy 

 branch will cause "blight" is still prevalent. 

 Tender or unripe shoots often suffer or are killed 

 by sudden freezing and thawing in the sun, caus- 

 ing, however, a different effect from the so-called 

 blight. A blighted spot, the size of a hand, often 

 found on the trunks of trees, would be difficult to 

 explain by the "frozen sap" theory. 



I do not believe that sap ever freezes in a healthy 

 tree. Frost extracts moisture from the plant cells, 

 and if the roots do not extend below frost, or 

 where they can supply the deficiency, the bark 

 shrivels, and the tree often dies. 



It was left for the microscope with its modern 

 improvements, and to the accuracy of investiga- 

 tions made with it, to reveal the true nature of the 

 mysterious disease. In a letter to the Gardeners' 

 Monthly (August, 1875), Dr. Hunt says: "I have 

 examined those pear branches, and find that the 

 black color is caused by a fungus. * * * i can. 

 not name the fungus. Repeated observations 

 %nly can determine that question. * * * I have 

 made thin sections of stem, bark, fruit and leaves' 

 and removed excess of black color until I could 

 send daylight into every cell ; and then under 

 * 500 the parasite reveals its presence.' ' 



For the next five years little progress seems to 

 have been made, except that the German and 

 French naturalists, principally Cohn, Magnin, 

 Pasteur, and Frische, continued to publish their 

 experiments and discoveries. In 1880, Professor 

 Burrill announced that "blight" in the pear, apple, 

 and quince was caused by bacteria, the smallest 

 living organism known. He found that they 

 destroy the stored starch grains, causing the same 

 to ferment, leaving the cell structure apparently 

 unharmed. 



With the poisoned sap he inoculated healthy 

 trees, of which over sixty per cent, showed signs 

 of " blight," clearly proving that bacteria is the 

 cause and not the effect of the disease. No counter 

 evidence has been brought against these experi- 

 ments of two years ago. 



About twenty years ago, Derlaine stated that 

 bacteria belonged to the vegetable instead of the 

 animal kingdom, as was the belief up to that time, 

 and only a few years since it has been proven that 

 they attack and destroy living matter. They in- 

 crease by "fission," dividing in the middle, under 

 favorable circumstances, once every hour, and 

 sometimes even oftener. Once an hour would be 

 at the rate of sixteen and a half million in twenty- 



four hours. A few species are also perpetuated by 

 spores, like fungi. The most favorable temper- 

 ature for their rapid development appears to be 

 about 95° Fahrenheit, together with plenty of 

 moisture. 



Prof Burrill is of the opinion that this kind of 

 bacteria (Microcosus amylovorus B.), is rarely 

 found floating in the air, being extremely viscid, 

 and usually mucilaginous, when moist. In this 

 condition they would be readily carried about by 

 insects. The most likely to aid in their dissemina- 

 tion would be the true bugs (Hemiptera), who 

 obtain their food by the use of a sharp beak, with 

 which they puncture the bark to suck the sap, 

 and by coming in contact with the sticky, poison- 

 ous fluid, may carry it from one branch or tree to 

 another. 



The following is Burrill's description of the 

 species: 



" Mycrococus amylovorus, Burrill. — Cells oval, 

 single, or united in pairs, rarely in fours, never in 

 elongated chains ; imbedded in an abundant 

 mucilage, which is very soluble in water; move- 

 ments oscillatory ; length of a separate cell, .00004 

 to .000056 in.; width, .000028 in.; length of a 

 pair, .00008 in.; of four united, about .00012 in." 



It is quite evident that the disease is one of the 

 outer cellular bark, as the bacteria are unable to 

 penetrate through the best cells, and can spread 

 up or down only by working their way through 

 the apparently solid cell walls. There being no 

 such things as sap veins in plants, analogous to 

 blood veins in animals, the spread of the disease 

 from the point of attack must be comparatively 

 slow. 



Soil, situation, exposure, &c., have little or noth- 

 ing to do with the disease. That some varieties 

 are more subject to its attack than others is well 

 known, and has been fully discussed by your 

 society, as well as lists published of tkose most 

 exempt. 



Of the different modes of cultivation, the one 

 that produces a moderate, healthy growth should 

 be preferred to that of excessive growth. It is 

 quite apparent that trees highly stimulated by 

 manure, severe winter pruning, and clean cultiva- 

 tion are most subject to "blight." The orchards 

 uniformly most exempt from "blight" that have 

 come under my observation were those well culti- 

 vated in grass, /. e., the grass kept short by re- 

 peated cutting (never allowing the grass to ripen 

 or go to seed), with occasional, at least biennial, 

 top dressing of barn-yard manure, or other fertil- 

 izers. In short, treated like a lawn. The annual 



