The Dry Root-Rot of the Bean 1021 



Even more marked than the effect of soil temperature on the disease 

 is the effect of varying amounts of soil moisture. As already stated, 

 the pathogene F. martii phaseoli is not a wilt-producing organism although 

 the mycelium frequently invades the vascular system. The effect of the 

 fungus is rather to kill and dry up the tissue as it progresses up the root, 

 thus greatly reducing the root system of the plant. The roots that remain 

 healthy or are able to carry on their normal functions are the surface 

 roots. If there is an abundance of water in the soil, these surface roots 

 can supply the plant with sufficient moisture. As the percentage of soil 

 moisture decreases, however, this ability of the surface roots to supply 

 moisture also decreases. A drought of ten days or two weeks will bring 

 forth from the growers the statement that their beans are " going back." 

 The expression is descriptive. The symptoms of the disease become very 

 evident in a relatively short time. If a prolonged dry spell occurs at 

 the time of blossoming and pod production, it is very injurious to the 

 diseased plants. The yield, without doubt, will be reduced over fifty 

 per cent. On the other hand, a very high moisture content of the soil 

 aids materially in the dissemination of the fungus. Moisture also causes 

 a more rapid progress of the disease in the host, since the diseased parts, 

 which are usually dry, absorb water very readily. In this way sufficient 

 moisture is supplied to the pathogene. A continuous rainy season, how- 

 ever, does not cause a noticeable reduction in yield of diseased plants. 

 During the season of 1915 a field of White Marrow beans near Wyoming, 

 New York, was under the observation of the writer. The plants on this 

 field were severely infected, and at podding time fifty per cent or more 

 of their root system was destroyed. Nevertheless a yield of 24 bushels 

 to the acre was produced. Unquestionably this was due to the fact that 

 the surface roots, which are more numerous in damp than in dry soil, 

 were able to supply the plant with water. During the dry season of 1916 

 the writer observed numerous bean fields affected with the dry root-rot 

 which did not produce over two or three pods to a plant. In this season 

 there was sufficient moisture directly after planting time to cause a succulent 

 growth in the plant and a rapid progress of the disease. Throughout 

 the dry period that followed, the plants were checked in their growth 

 and few pods were formed. 



