BLACK ROT OF CRUCIFEROUS PLANTS. 



309 



on particular water-pores which were extruding fluid. Once past the incubation period, the 

 downward progress of the disease is comparatively rapid (i cm. or more per day), especially 

 if the weather is warm and the soil is moist enough to induce a vigorous growth of the host- 

 plant. The progress of the disease in cool weather and in plants making a slow growth is less 

 rapid (plate 2). On September 7, 1897, during weather very favorable to the progress of the 

 disease, the writer examined a cabbage-plant at Racine, Wisconsin, which bore 170 separate 

 water-pore infections all spreading rapidly. As yet there was no disease of the root, stem, 

 or interior of the well-formed head, nor was there any black stain in the base of any of the 

 petioles. These infections, therefore, probably took place not much earlier than the first 

 of August. Such a plant might be expected to be badly rotted in stem and head in course 

 of another six or eight weeks. 



Brenner states that when he inoculated cabbage-plants on a single leaf-tooth many 

 other groups of water-pores subsequently contracted the disease, the organism being pre- 

 sumably carried along the moist 

 surface of the leaf-margin to 

 the other groups of water-pores 

 by capillary attraction. As 

 noted in 1898, the writer saw one 

 large cabbage-plant which bore 

 more than 400 distinct marginal 

 leaf-infections, while many other 

 plants in the same field and in 

 other fields showed from 50 to 

 150 such separate infections. 

 Sometimes nearly every serra- 

 ture on a leaf would be infected. 

 The writer has dissected hun- 

 dreds of cabbage-leaves which 

 were attacked at the time of 

 observation only on their mar- 

 gin, and other hundreds in 

 which the organism was already 

 well into the middle of the leaf 

 or had already entered the stem, 

 as determined by cutting. An 

 extensive marginal infection 



obtained by spraying, with the Fig. 107.* 



entire absence of infection by 



way of the ordinary stomata is shown in fig. 10. For an early stage of water pore infection 

 see fig. 1300 and Vol. I, fig. 87; for a late stage with disorganization, this volume, fig. n. 



The leaf surface of many crucifers is covered with a waxy bloom repellant to 

 water, and this preserves the leaf from injury when submerged for some hours, and also 

 undoubtedly makes it difficult for the bacteria to enter through the ordinary stomata. 

 At least, infections through such stomata have not been observed. Russell's observations 

 and experiments agree with those of the writer. 



In case of leaf-infections by way of the water-pores the general progress of the dis- 

 ease is downward in the spiral vessels of the leaf (Vol. I, fig. 76, 77, 78). These are com- 

 monly filled densely, many of them at least, by the rapid multiplication of the organism. 

 From the blade of the leaf the bacteria pass into the leaf -traces of the petiole (fig. 109, no) 



*Fic. 107. Cross-section of a small portion of a blackened kohlrabi bundle, showing cavity due to presence of 

 Bacterium campestre. Section from enlarged edible part of plant. Material collected at Miami, Fla., March, 1904. 

 x 500. Slide 293 b 18. 



