172 



FUNGOUS DISEASES OF PLANTS 



by the vine. Pods badly affected may wilt and die, and the fungus 

 may penetrate to the seed. 



Moist seasons are most favorable to the production and spread 

 of this disease. Sturgis has determined an interesting relation of 

 this fungus to insects. Bees and other insects visiting the blos- 

 soms of the beans may come in contact with the basal portion 

 of the style and the basal portion of the ovary, corresponding to 

 the two ends of the pod. Observation indicates that it is at these 

 points primarily that the fungus begins its work. Rain is also 

 effective in rapidly spreading the spores. 



The fungus. The mycelium is irregular in diameter, siphon- 

 aceous when young, and often empty and septate when old. The 

 conidiophores are produced in great numbers. They are upright, 

 considerably branched near the bases and longer than those of 

 other species of the genus. They form on the surface a matted 

 mass, and it is possible that threads of the mycelium proper may 

 also develop superficially. The conidia, produced much as in the 

 previous species, are large, measuring generally 28-42 x 17-27/1, 

 with a distinct germinal papilla. Fresh spores germinate readily, 

 and generally by the production of biciliate, fusiform zoospores. 

 Germination may also occasionally proceed by means of a germ tube. 



Oospores of this fungus were not found until 1905. According 

 to Clinton, " Judging from the experience of the past year, the 

 oospores should be looked for toward the end of the season and 

 in the seeds of the pods badly infected with the mildew." It is 

 believed that the production of oospores is frequently interfered 

 with by the rapid growth of saprophytic fungi. The oogonia are 

 inter- or intra-cellular, rather thick- walled, smooth, and generally 

 19. 5-22. 5 //, in diameter. 



By carefully removing the seeds from infected pods, Clinton 

 was able to grow this fungus in pure cultures on such seeds, and 

 likewise on nutrient media containing agar, corn meal, etc. Both 

 conidia and oogonia were produced. 



Control. On a small, scale spraying experiments with Bordeaux 

 mixture have been successful. It is important, however, that only 

 seed from clean pods should be used. Rotation of crops is required 

 wherever oospores are produced, and under such circumstances, 

 also, the destruction of all diseased parts is equally valuable. 



