64 Diseases of Economic Plants 



So diverse and numerous were the early theories as to the 

 nature and cause of bUght, knd so fruitless were the discus- 

 sions in earlier years that the Western New York Horticul- 

 tural Society passed resolutions to prohibit further reference 

 to the subject until entirely new facts were forthcoming. 

 Among the numerous theories may be mentioned those 

 depending upon electrical or atmospherical influences, 

 freezing of the bark, too long culture of particular varieties, 

 freezing of the roots, too high culture, insects, fungi, and 

 lack of nutrition. One of the most prominent of these was 

 Downing's "frozen sap theory," according to which the 

 disease is due to the freezing and thawing of the sap, resulting 

 in loss of vitality and development of poisons in the tissues. 

 It is unnecessary to discuss any of these theories further than 

 to say that none of them explains the facts. 



In 1878 Burrill discovered bacteria in the diseased twigs, 

 and by transferring to healthy twigs some of the exudate 

 from diseased tissue bearing these bacteria was able to 

 cause the disease. Arthur later grew the bacteria in pure 

 cultures and with these cultures produced the disease. It 

 was thus proved beyond question that bacteria cause the 

 blight. 



It has been demonstrated that the germs cannot gain 

 access through healthy, mature bark, but will readily pene- 

 trate into wounds, or into floral parts. 



This blight bacillus deposited upon the flower, or upon 

 tender shoots, gains entrance and rapidly grows downward 

 through the wood and bark, 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 blight are thus 

 distinguished : one very dangerous, the other no longer offen- 

 sive. It is the limbs still in condition of ''active blight" that 

 carry the pest over winter. 



