STEWART'S DISEASE OF SWEET CORN (MAIZE). 



95 



leaves have been saved in alcohol for microscopic study. The bases of many of these same leaves 

 were still green and certainly contained bacteria in then- bundles. If bacteria are present in the tips 

 of these leaves it would not prove infection by way of the water-pores, but if they are not present, it 

 would mean that the leaves were infected from the stem or at least not first at their apex; in other 

 words, the movement of the bacteria in the foliar bundles might be assumed to be upward and not 

 downward. This last inference I regard as probable for several reasons, particularly because the 

 leaves are well up on the stem and because the stem-bundles are so full of bacteria that we must sup- 

 pose the infection of the plants to have taken place several weeks ago and considerably lower down. 

 I am inclined to think that the drying up of these leaves is due to secondary infection and to the very 

 general plugging of the vascular system of the stem.* If my observations and inferences are correct, 

 the primary infection took place when the corn was small and through the water-pores of the first 

 leaves, which have long since fallen. The condition of the extreme base of the stem in these plants 



Fig. 41. t 



points strongly to this conclusion. It is sound externally but within it is more badly diseased than 

 any other part. Here the connective tissues are brownish and gummy, and there are small cavities, 

 i. e., the bacteria have been present for a considerably longer time than farther up the stem, have 

 escaped from the bundles, and have stained the tissues. Sections of two such stem-bases have been 

 preserved in alcohol for further study (fig. 42). None of the roots were diseased externally and all 

 seem to be sound within, but not enough were examined to venture any positive general statement. 



Higher up the stem only an occasional bundle is stained brown. On longitudinal sections the 

 affected bundles are distinctly yellow against the background of white pith. One characteristically 

 diseased stem was saved dry and another was sectioned and put into alcohol. 



Under the compound microscope the slime from these maize plants appeared to be all one thing, 

 whether pale yellow or bright yellow, and subsequent experiments confirmed this inference. For 



These inferences were confirmed by a study of the leaf-tips already mentioned. On microscopic examination it 

 turned out that in more than one-half of the dead leaf- tips (13 out of 25) bacteria were not present. 



fFio. 41. Cross-section of stems of inoculated sweet corn, showing Bacterium stewarti oozing as a yellow slime 

 from the vascular bundles. Plants inoculated in the seedling stage in the hot-house on the tips of the leaves and grown 

 in the field (Potomac Flats) nearly to maturity, before the disease developed. Photographed Oct. 7, 1902. X 6. 



