2o6 Bulletin 236. 



9. The Blight Canker in its Relations to Weather Conditions 



It is well known that the " blight " in the twigs of pear and apple 

 trees is most active and severe during a continued period of warm, muggy 

 weather. The same is to be said of the cankers on the limbs and bodies 

 of the trees. Considering that both are (hte to the activity of the same 

 organisms, this is to be expected. It was repeatedly noticed during the 

 past season that the active spread of the cankers was coincident with 

 certain periods of rainy weather. The progress of the disease through 

 the bark is always abruptly checked on the appearance of bright, sunny 

 days. The recurrence of favorable weather may often cause a renewal 

 of activity, the canker spreading and increasing its former extent. This 

 usually results in concentric cracks within the cankered area. (Fig. 53.) 

 This relation between the activity of the canker and the condition of the 

 weather seems to be dependent on two factors : ( i ) The increased 

 moisture content of the soil results in an increased amount of water in 

 the bark and growing tissue of the tree, which, in connection with high 

 temperature, causes the rapid growth of succulent tissue. This con- 

 dition of the host is the most favorable for the growth and rapid multi- 

 pHcation of the bacteria. (2) The atmospheric humidity itself seems in 

 some way directly to affect the activity of the parasite ; for, although a 

 heavy rain followed by bright, warm weather would seem to afford ideal 

 conditions for the rapid growth of the tree, such weather was observed 

 to be much less favorable to the activity of the cankers than damp, cloudy 

 days. Smith has observed^'^ in his work on the asparagus rust that at- 

 mospheric conditions, aside from their indirect effects through the host, 

 exercise a direct influence on the formation of jecidiospores by the para- 

 site. The spores were observed to form only during periods of abundant 

 atmospheric moisture, — conditions most favorable to their germination. 

 The effect of cloudy weather on the canker organism seems to be of a 

 similar nature. The rapid gro\yth and multiplication of the bacteria re- 

 sult in their exudation in large drops (Fig. 51) from the diseased tissue, 

 from which, through the agency of insects, they are carried to other trees. 

 Bright, sunny weather would cause this viscid substance to dry and harden 

 at once. But, as we have seen, the activity of the organism is checked 

 by bright days, and consequently there is no exudation. 



Humidity of the atmosphere is necessary not only for the best de- 

 velopment of the bacteria, but is requisite also for the successful infection 

 of healthy tissue by them. As has been stated before, infection can take 

 place only through a wound of some sort. The moisture in the air pre- 

 vents the rapid drying out of the infection court, thus affording the 

 bacteria sufficient time, under favorable conditions, in which thoroughly 



