Treatment.
Fortunately this disease can be effectually checked by the use of Bordeaux spraying mixture.
Damping-off Disease.
Another source of loss was in the damping-off of young plants. The fungus which causes this disease lives in the surface layer of the soil and girdles the plants at the surface layer of the ground, causing them to wilt and fall over. The trouble can be largely avoided by proper drainage and stirring the surface layer, thus aerating and drying the soil.
The Wilt Disease.
By far the most destructive and dangerous disease remains to be described. It made its appearance about the first week in July, causing the leaves to turn yellow and dry up; the seed stem and berries also dried up and died before reaching maturity. This was the disease which caused the greatest loss; whole plantations often being destroyed in a week. Neither the Bordeaux spraying mixture nor lime dust seemed to check its ravages.
I have succeeded in isolating the fungus which is the cause of this destructive disease and have grown it in the laboratory in pure cultures for nearly five months. Cultures were made by scraping the dark spots on diseased stems with a sterile needle and inoculating sterilized bean pods or plugs of potato with the spores scraped from the stem. In two or three days a white, fluffy growth appears on the bean pod which rapidly spreads until it is covered with a growth which resembles a luxuriant mould. I have also isolated this fungus and made cultures from the soil taken from diseased beds.
The fungus belongs to the genus Fusarium and is probably identical with the fungus which is so destructive in causing the wilt of cotton, watermelon and cowpeas, and which has been carefully studied by Smith and Orton of the United States Department of Agriculture.
Treatment.
It will be seen from this brief description of the fungus that it is an exceedingly difficult disease to combat. Living from year to year in the soil it enters the plants thru the roots and spreads upward thru the water-conducting channels. It does not once appear on the surface until the plant is beyond recovery. Obviously we cannot apply any substance to kill the fungus without first killing the plant it infests.