276 



STATE BOARD OF AGRICULTURE. 



but this is not a necessary accompaniment of the disease for this symptom to- 

 gether with the bad smelling head may be due to the work of other bacteria which 

 are living on the tissue already weakened by the true black rot organism. In 

 the early stage of the disease, the leaves show a withered, dried appearance along 

 the margin followed by a yellowing in time. The small ribs that lead to the 

 mid rib are usually blackened first and ultimately the larger ones and the mi^ 

 rib succumb. On cutting across the stem of an infected leaf or sick stalk, one 

 can see the blackened ends of the fibrous strands, known as the fibro vascular 

 bundles, which lead from the stalk out into the leaf and supply it with water 

 and soil foods. A microscopic examination of these vascular bundles will show 

 the tiny tubes of which they are composed to be alive with germs. As soon as 

 these food and water channels are destroyed, the blade of the leaf is no longer 

 able to get the subsistence required and dies. Diseased leaves usually fall pre- 

 maturely, leaving a long naked stalk with a tuft of leaves at the tip. The old 

 leaf scars will show the ends of blackened strands corresponding to the diseased 

 fibro vascular bundles in the leaf. 



Fio. 5. — Cabbaces showing: effect of artificial inoculation. Middle plant vigorous and healthy (unin- 

 oculated). Plants 1 and 3 inoculated with Black Rot perms. Note dwarfed, one sided manner of 

 growth. Russell. (Wisconsin Station Bulletin No. 65.) 



It has been found* that the most common method of infection in the field is 

 through the very small water pores scattered over the blunt teeth on the margin 

 of the leaf. It is by these water pores that a part of the soil moisture that is 

 taken up by the root system escapes from the plant as water vapor but if the 

 surrounding atmosphere is very moist, there will be no evaporation and the water 

 will be seen to accumulate in tiny droplets just over the pores. However, if the 

 soil is very dry, even though the air is moist, "we do not have these water beads 

 formed. They can be seen frequently in the early morning on the surface of 

 the leaves and are frequently mistaken for dew. It must be borne in mind 

 that the air is always filled with dust and numerous bacteria and among these 

 there are almost certain to be the germs of black rot, especially if the wind is 

 blowing over a sick field carrying with it fragments of rotting plants and in- 

 fected soil. These germs are in the beads of moisture, referred to above, and 

 finding a comfortable lodging place and abundant food and water supply, they 

 multiply very rapidly and being able to move about, soon find their way down 

 through the water pores into the veins of the plant. Here they continue to mul- 

 tiply and work toward the stalk, leaving behind them the blackened veins and 

 withered blades. The germs may also gain entrance by means of the bites of 

 gnawing insects and again by way of the broken roots at the time of transplant- 

 ing. 



* Bulletin 65 Wis.; Exp. Sta. Farmer's Bulletin 63 U. S. Dep't Agr. 



