156 Bulletin 27^. 



An epidemic of this disease means not only the loss of the season s 

 crop but also the loss of entire trees (Fig. 5), or at least much of the 

 bearing wood (Fig. 11), which is the product of years of growth and 

 the replacing of which will require years more. Aside from the actual 

 money loss there is the discouragement which always comes with such 

 an experience. The grower with the prospect of a repetition of such a 

 disaster before him is loath to invest more time and money in a crop 

 subject to such losses. Many men in this and other states have given 

 up pear-growing chiefly because of the ravages of this disease. The 

 disease often appears also in nursery'stock, where in the actively growing 

 shoots of the young trees it spreads rapidly. The disease broke out 

 in the nurseries about Dansville, N. Y., in 1908, in many cases wiping 

 out entire blocks of apples, pears, and quinces. Of the sixty odd growers 

 of nursery stock in that section, scarcely one escaped heavy losses 

 from the epidemic. 



Symptoms. It seems hardly necessary to describe the signs or symp- 

 toms by which this disease is to be recognized. The growers generally 

 know them only too well. Figures 6 to 9 indicate fairly well the appear- 

 ance of the disease on limbs, blossoms, twigs and fruit. The most 

 striking symptom to be recognized by the grower is the twig or limb 

 with dead, brown or black leaves clinging to it (Fig. 18), contrasting 

 sharply with the dark green foliage in summer and the naked branches 

 of the trees in winter. In no other disease of our fruit trees do the leaves 

 cling so tenaciously to the dead branches. The effect is not unlike 

 that resulting from scorching by fire or injury by frost. It is from 

 this appearance that the name Fire Blight has been appropriately 

 applied. 



The cause of the disease. The disease has long been known to be 

 due to a species of bacteria {Bacillus amylovorus (Burr) De Toni) which, 

 when introduced into the growing parts of the tree, for example the 

 blossoms or young shoots, multiply rapidly, feeding upon the tissues 

 and secreting poisonous substances that kill the affected part. The 

 bacteria which are responsible for this destruction are very minute 

 rod-shaped plants (Fig. 12), so small that 2,000 of them end to end 

 would be required to span an inch. They are, of course, microscopic 

 and can be seen only when magnified several hundred times under 

 the microscope. They are covered with fine hair-like flagella by means 

 of which they swim about in the juices of the diseased tissues of the 

 host. During moist weather they ooze forth in sticky, milky masses 

 from diseased twigs, blossoms or cankers. They multiply by dividing 

 into two at the middle (Fig. 12), growing to normal size and dividing 



