16 BACTERIAL DISEASES OF PLANTS 



unbroken roots will thrive in a soil deadly to one that has been 

 root-pruned. I have myself observed this. We may suppose 

 that substances attractive to the particular bacteria diffuse into 

 the soil from the broken roots, following which they enter the 

 plant. Resistant plants may be supposed to diffuse indifferent 

 or repellant substances. All infections must be chemotactic. 



More interesting perhaps are those diseases which begin in 

 natural openings, i.e., in places where the protective covering 

 of the plant gives place to special organs such as nectaries, water- 

 pores, and stomata. 



All the pome fruits subject to fire-blight are liable to blos- 

 som infection. The bacteria multiply first in the nectaries of 

 the flower and pass down into the stem by way of the ovary 

 and pedicel. Blossom blight of the pear is a very conspicuous 

 and common form of the disease, as everybody knows. Thou- 

 sands of blighted blossom-clusters may be seen in any large 

 orchard subject to this disease. Blossom-blight arises from 

 "hold-over" blight (see Figs. 282 and 283), the visiting insects 

 acting as carriers. 



In the black rot of the cabbage due to Bacterium campestre, 

 the majority of the infections begin in the water-pores. These 

 are grouped on the margins of the leaf at the tips of the ser- 

 ratures. From this point the bacteria burrow into the vas- 

 cular system of the leaf and so pass downward into the stem 

 and upward into other leaves. 



In the black spot of the plum, due to Bacterium pruni almost 

 or quite all of the leaf and fruit infections are stomatal. A 

 large proportion of them are also stomatal in the leaf-spot of 

 cotton due to Bacterium malvacearum, the leaf-stripe of sorghum 

 and broom-corn due to Bacterium andropogoni, the leaf-spot of 

 carnations due Bacterium woodsii, and other leaf -spots. ^ 



TIME BETWEEN INFECTION AND APPEARANCE OF THE DISEASE 



As in animal diseases, the period of latency may be very 

 short or surprisingly long. Some time must be allowed the 



1 The writer first called attention to stomatal infections in 1897, having demon- 

 strated their existence experimentally. See "Bacteria in Relation to Plant 

 Diseases," Vol. 2, pp. .39, 56, 57; Pis. .3, 4, and Figs. 11, 12, 15, 16, 17. 



