STEWART'S DISEASE OF SWEET CORN (MAIZE). 



91 



due to the presence of an oxidizable substance in some bundles and its absence in others, or 

 simply to the fact that more air (oxygen) has been able to enter some vessels than others, 

 the dark stain depending on an oxidation. A similar phenomenon occurs in the sugar-cane 

 attacked by Cobb's disease. Reddeningof the vascular bundles or white striping of the leaves 

 (a sign met with in sugar-cane attacked by Bacterium vasciilanim) does not occur in this 

 disease. The slime of the disease also sometimes occurs in roots which appear sound exter- 

 nally. This is especially true of parts of roots near their union with badly diseased stems. 

 As a rule the roots are not badly diseased. 



The writer has observed no tendency of the yellow slime to break through and ooze on 

 the surface of the plant, except from the inner face of the inner husks, where such oozing is 

 quite common, and more rarely from the inner face of the leaf-sheath. Stewart found this 

 yellow slime oozing over the kernels in plants killed by the disease. The main axis of the 

 maize plant has a firm siliceous covering, and even at the base of the stem, which is usually 

 the first part of the stem to become diseased, there is a thin layer of unbroken, hard, sound 

 tissue separating the sick interior from the air and soil, 

 even when the plants have been diseased for some time 

 (10 weeks or more) and the vascular bundles are occu- 

 pied by the bacteria the whole length of the stem. 

 This is true even when the lower nodes and internodes 

 have become quite brown and gummy and most of the 

 leaves have succumbed to the disease. Very rarely, in 

 the middle part of the stem, in badly diseased, soft 

 plants, ooze may come from some restricted spots on 

 the internodes. As the disease progresses, the bac- 

 terial slime is sometimes found in the parenchyma 

 between the bundles either as water-soaked patches or 

 as bright yellow spots, but large cavities are not com- 

 mon. According to Stewart : 



Fields of sweet corn affected with the disease are very 

 uneven, particularly at the time when the ears are forming. 

 Plants in various stages of the disease are intermingled with 

 apparently healthy plants of different sizes. It is a com- 

 mon thing to find diseased plants in the same hill with 



healthy ones which may continue in good health to the end Fig. 39.* 



of the season. * * * Usually, the small plants are the first 

 to succumb to the disease, which fact suggests that the disease may be the cause of their slow growth. 



ETIOLOGY. 



This disease is caused a yellow polar-fiagellate schizomycete, Bacterium siewarti EFS. 

 The organism was discovered in 1895 by Mr. F. C. Stewart, who in 1897 published the first 

 account of the disease. He claimed it to be the cause of the disease because, whenever he 

 examined stems of maize affected by this disease, he found this organism present in the vas- 

 cular system in enormous numbers, and, apparently or actually, to the exclusion of all other 

 organisms. The evidence based on constant occurrence is certainly all that could be desired. 

 The disease and the organism are always associated. Stewart also attempted infections, but 

 his results were not conclusive, as he himself admits. The reasons for this failure were 

 partly because his experiments were conducted in a locality where the disease was naturally 

 prevalent, so that the controls became infected, and partly because his inoculations were not 

 made at the best time and in the most natural manner. Possibly also in some cases he may 

 have inadvertently infected his checks with the material used for his inoculations. 



*Fig. 39. Bacterium stewarti. Cross-section of sweet-corn stem, showing bacterial ooze, 

 occupied. A natural infection from Mr. Pieters's trial plots. July 21, 1903. X3. 



Nearly every bundle 



