3i8 



FUNGOUS DISEASES OF PLANTS 



spreads to the stem and leaves. Entrance to the tubers is gained, 

 therefore, as a rule, through the stems upon which they are 

 borne. The vascular system of the host plant is discolored, al- 

 though frequently the tubers are not seriously injured externally 

 until after they are gathered. In storage, however, the fungus 

 progresses rapidly, blackening the vascular ring. At this stage 

 the disease is only made apparent in the tubers by cutting them 

 crosswise ; still it may be so serious as to render them unavailable 

 for table purposes. Later on there may be considerable drying of 

 the tubers, or soft rots due to secondary organisms may ensue. 



The fungus. 

 The mycelium 

 produces micro- 

 conidiaand macro- 

 conidia (Fig. 150) 

 abundantly in arti- 

 ficial cultures, also 

 some chlamydo- 

 spores. On boiled 

 potatoes small 

 greenish sclerotia 

 are developed, but 

 no ascogenous 

 stage has thus far 

 been connected 

 with this species. 

 Control. This fungus lives apparently -for a considerable time 

 in the soil, and a rotation of crops is essential whenever it be- 

 comes of serious importance. Again, the use of pure seed only 

 should be allowed. If necessary, inspect by cutting a large num- 

 ber of the tubers which are to be used for this purpose. All 

 diseased and discarded tubers should be burned and not returned 

 to the land. Seed tubers which may have come in contact with 

 conidia should be treated as for potato scab. 



The sleepy disease of tomatoes which has been attributed to 

 Fusarium Lycopersici Sacc. may also be produced by the fungus 

 above described, although this point has not been demonstrated 

 experimentally. 



Fig. 150. Fusarium oxysporum: Mycelium, Conidia, 

 and chlamydospore 



