DISEASES OF SPECIAL CROPS 103 



most prominent of these was Downing's " frozen sap 

 theory," according to which the disease is due to the freez- 

 ing and thawing of the sap, resulting in loss of vitaUty and 

 development of poisons in the tissues. It is unnecessary 

 to discuss any of these theories further than to say that 

 none of them explain the facts. 



In 1878, Professor Burrill ^ discovered bacteria in the 

 diseased twigs, and Ijy transferring to healthy twigs some 

 of the exudate from diseased tissue bearing these bac- 

 teria was able to cause the disease. Professor Arthur 

 later grew the bacteria in pure cultures and with these 

 cultures produced the disease. It was thus proved be- 

 yond question that it is these bacteria that cause the 

 blight. 



It has been demonstrated that the germs cannot gain 

 access through healthy mature bark, but will readily pene- 

 trate into any wounded place, or into floral parts. 



This blight bacillus deposited upon the flower or upon 

 tender shoots gains entrance and rapidly grows downward 

 through the cambium, causing the foliage upon the affected 

 twig to die. The disease rarely progresses more than 5-8 

 cm. in one day, though even 0.3 meter is occasionally 

 reached. Ordinarily, sick twigs dry out, progress of the 

 disease soon stops, and the germs in the twig die. Waite ^ 

 found that in some limbs, probably one of several hundred, 

 the disease remains active. Two forms of the disease are 

 thus distinguished: one very dangerous, the other no 

 longer offensive. It is these limbs still in condition of 

 " active bhght " that carry the pest over winter. 



1 Burrill, T. J., Trans. 111. Hort. Soc. for 1878, p. 80. 



- Waite, M. B., U.S. Dept. Agr. Yearbook, 1895, p. 295. 



