6oO MICROBIAL DISEASES OF PLANTS 



This may continue until the whole limb becomes involved, but as a rule 

 ' it is only the smaller twigs which are the worst affected. From this 

 it will be seen that.the external blackening cannot be relied upon, early 

 in the season at least, as a guide to the exact location of the disease; 

 however, as the season advances, the plant tissues harden, conditions 

 for germ life become, less favorable, and as a result, by the middle of 

 summer, the active progress of the blight is checked by natural causes, 

 and the blackening overtakes the advancing infection. 



Blight which appears on the water sprouts of large limbs later can 

 usually be accounted for by inoculation by plant lice and the pear twig 

 borer. 



Causal OEGANiSM.^^^According to Jones* Bacillus amylovorus possesses the 

 following characteristicsi Short motile bacillus, rounded ends, iix-i.Sfi by 0.5/j- 

 o.g/t; stains readily with the aqueous stains; Gram-negative.. No spores observed. 



Agar slant and potato, growth moderate, filiform, glistening, smooth, grajash 

 white, semi-opaque, butyrous. Gelatin stab, growth rather, slow, filiform, slight 

 crateriform liquefaction after twenty days. Nutrient, broth, moderate clouding, 

 uniform; if left undisturbed, a delicate pellicle or ring may form which breaks up and 

 sinks with the slightest jar; scant finely granular sediment after ten da3rs. Litmus, 

 nulk, light blue in four days, pinkish in six days, light blue again in twelve days, 

 upper layer blue in eighteen days; soft gelatinous curd six to ten days, with whey 

 on the surface. Cohn's solution, no growth. Uschinsky's solution, no growth. 

 Nitrates not reduced. No indol. Thermal death-point so"- Optimum tern- - 

 perature 23° to 25°. Slight acid production but no gas from dextrose, etc. Starch 

 is not fermented. 



Control. — It is obvious that spraying is useless for a disease of this 

 character, where the germs are located beneath the surface. 



A systematic cutting out of the diseased limbs and twigs wherever 

 and whenever they appear is the only practical method of controlling 

 the blight. It is almost impossible to get all of the diseased material 

 in the summer time when the heavy foliage hides it, but in the fall and 

 winter the blighted branches can be recognized very readily by the tufts 

 of dead leaves clinging to them. It is necessary in removing the dead 

 wood to cut well below the discolored part, 10 to 15 inches, for 

 the bacteria may be considerably in advance of the discolored area. 

 Clean out all old cankers by cutting well into the healthy part and by 

 removing the dried, diseased material. Disinfect the. freshly cut sur- 

 faces of this wound as well as the exposed ends of twigs and h'mbs with 



* Jones, D. H., The Bacterial Blight of Apple, Pear and Quince Trees. Bull. 176, Ontario 

 Asir. College. 



