112 



FUNGOUS DISEASES OF PLANTS 



Symptoms. The external and internal symptoms of this disease 

 are readily noted and distinctive. The affected plants die by wilt- 

 ing and drying, the water supply being cut off. Usually the leaves 

 wilt one after another and the plant may live a month, but in some 



cases where the plants affected 

 are a foot or less in height si- 

 multaneous wilting of the leaves 

 may result, and the plants may 

 die within four or five days 

 of the first appearance of the 

 disease. There is no discolora- 

 tion, decay, or other complicat- 

 ing symptoms. 



The internal evidence of dis- 

 ease is equally clear. Upon cut- 

 ting the stem lengthwise, the 

 " fibro vascular-bundles appear," 

 according to Stewart, " as yel- 



FIG. 26. CROSS SECTION^OF STALK OF low streaks in the white paren- 

 chyma ; but in the stems of 

 plants that have been dead for 

 some time some of the bundles 

 may be black instead of yellow. If the stem is cut crosswise and 

 the cut surface exposed to the air for about five minutes, a yellow 

 viscid substance exudes in drops from the ends of the vessels." 

 Except for the greater accuracy of poured plates, pure cultures, 

 which are essential, might be made by direct inoculation into tubes. 

 The appearance of diseased bundles in cross and longitudinal sec- 

 tions is illustrated in Figs. 26 and 27. 



Pathology. The organism is confined to the fibrovascular bun- 

 dles exclusively, and appears to infest only the vessels. There is 

 no disorganization of the tissue, and the pathological effect is there- 

 fore due, in large part, doubtless, to cutting off the transpiration 

 stream. If there are secondary effects felt in the protoplasm of 

 rather distant living cells, and brought about by diffusion of inju- 

 rious excreted substances, it has not been demonstrated, so far as 

 I am aware, in the case of any bacterial disease of plants. Field 

 corn and pop-corn are resistant, but inoculation experiments with 



SWEET CORN, SHOWING BUNDLES occu- 



graph by F. C. Stewart) 



