DURATION OF DISEASE. 



67 



by Bacillus tracheiphilus, the progress of thedisease is rapid after the incubation period has 

 passed. In squashes, on the contrary, the resistance is much greater, and it may be several 

 weeks after a vine shows the wilt before it entirely succumbs. At night, or in moist weather, 

 it becomes turgid, to again collapse with the reappearance of sunlight and dry air. In the 

 growing season, pear-blight is usually a rapid disease, but in 

 the cool weather of autumn and winter there is frequently an 

 almost balanced activity between host and parasite, result- 

 ing in what is known as "hold-over" blight. In this way 

 the disease is carried from one growing season to the next. 

 Vascular diseases, such as those of sweet corn and sugar- 

 cane, already mentioned, kill the plant very gradually, if it 

 is of good size when infected or when the constitutional signs 

 first appear, but after the vascular occlusions have reached 

 a certain volume the destruction of the plant is speedy. In 

 maize, which has reached this stage, the leaves dry out 

 within a few days, and the green stem then shrivels. In 

 case of the olive-tubercle, the tree as a whole does not, so 

 far as we know, become infected but only particular parts 

 of it, yet there may be wide metastasis especially in young 

 trees. Individual knots live for several months, and fre- 

 quently portions of them for several years, the knot enlarg- 

 ing from some particular part which has not been injured 

 beyond the power of cell-division. Terminal twigs girdled 

 by tubercles are frequently starved and die, but not very 

 promptly. Knots and cankers due to bacteria are generally 

 of slow progress and correspondingly long duration. The 

 life of a shoot of chrysanthemum, sapped by a big tumor 

 due to Bact. tumefaciens, varies from 6 months to a year or 

 more. Often the plants live many months. Peach trees 

 attacked by crown-gall generally live for several years. 

 Galled apple trees may live indefinitely. In the recently 

 discovered tuberculosis of the sugar beet due to Bacterium 

 beticolum {Vide Crown Gall, etc., Bull. 213) decay is rather 

 prompt. 



FINAL OUTCOME. 

 Plants, like animals, are affected to very different 

 degrees by the various bacterial parasites. This must be 

 apparent from what has been said under duration of the 

 disease. In the animal world there are protracted bacterial 

 diseases and rapid ones, diseases terminating fatally or end- 

 ing in recovery. The same is true of plants. The simplest 

 cases, perhaps, are the stomatal infections resulting in leaf- 

 spots and fruit-spots. The leaves are more or less disfigured, 

 and the fruit may be destroyed or so spotted as to be unsal- 

 able, but generally it is beyond the power of the organism 

 to destroy the plant, or even to render it wholly unfruitful 



Fig. 19.* 



In bacterial blights, such as 

 that of the mulberry or pear, much larger portions of the plant may be destroyed, twigs 

 or even large branches, and yet it may recover. In a majority of cases, after running a 



*Fig. 19. Coconut budrot of Eastern Cuba. Outer enveloping leaf sheaths removed to show condition of inner 

 undeveloped leaves sound below, rotted above. Tree No. 1 1. Bud itself not dead, but enveloping sheaths rotted. 

 Color of decayed part was a mixed gray and brown. Photographed by the writer at Baracoa, Cuba, April 20, 1904. 

 One-third natural size. 



