1714 



ENCYCLOPEDIA OK I'UAOTICAL llOlfncUl/rUUE 



weight of potatoes, and acconliuKly the 

 yield, continues to increase at a remark- 

 able rate for every week added to the nor- 

 mal growth of the top. Premature ripen- 

 ing from any cause certainly results in re- 

 duction of yield. 



As the plant slowly succumbs, owing to 

 the death of the roots, it falls over and 

 can be more easily detected. Such plants 

 pull easily. The roots are easily broken 

 and are often covered with a white or 

 pinkish fungous growth. The main root 

 is discolored; frequently this brown dis- 

 coloration can be followed along the un- 

 derground stems into the stem end of the 

 tubers. In this way the fungus enters 

 the potatoes, and, under favorable condi- 

 tions, it produces the dry stem-end rot. 

 If rot is not produced the fungus can live 

 over winter in the tubers and so spread 

 with the seed potatoes to new fields. Such 

 Infected tubers can be detected if a thin 

 slice is cut off at the stem end. If the 

 fungus is present the vascular or fibrous 

 tissue is brown and this tissue spreads so 

 that a ring or a portion of a ring of the 

 brown spots appears on the cross-section 

 at different depths, always following near 

 the surface of the potato. This brown 

 discoloration may follow the fibers back 

 only a very short distance or it may ex- 

 tend very nearly to the apical end of the 

 tuber; in either case it indicates the 

 presence ot the disease. In case of rot 

 the fungus does not confine its attack 

 to the vascular tissue but slowly spreads 

 from it through the surrounding tissue, 

 finally lireaking it down. As it comes to 

 the surface it breaks through and pro- 

 duces dense tufts of the delicate white 

 fungus. 



Cause 



The cause of this disease is a soil fun- 

 gus ( Funarium oxysporum)*. It is closely 

 related to the organisms causing the west- 

 ern tomato blight and the wilt of water- 

 melons. It can live for years as a sapro- 

 phyte, depending on dead organic matter 

 In the soil for food. When potatoes are 

 planted on infested soil the fungus again 

 assumes Its parasitic habit and produces 

 disease of the living |)laiit. 



• Several other spefles of FiiHarla aip linown 

 to caune decay of potato tutu-is. 



Like the fungus causing watermelon 

 wilt, this fungus forms small spores 

 which are produced in the vessels of the 

 plant, and larger curved spores on the 

 surface. These larger spores will with- 

 stand long periods of drought or cold and 

 germinate when conditions are favorable. 

 They are scattered by wind and insects 

 or at digging time by the implements used 

 in the field. 



Control 



The fact that this fungus is a persistent 

 soil organism, capable of producing dis- 

 ease whenever potatoes are planted in soil 

 where it is present, makes it necessary to 

 use precaution in keeping the fields free 

 from it. 



All seed used should be carefully in- 

 .spected. It would be a simple matter if, 

 at the time potatoes are cut for seed, a 

 thin slice be cut at the stem end and any 

 tubers that show brown discolorations of 

 the fibers be discarded. In case infected 

 seed is found it will be advisable to use 

 in addition, the formalin seed treatment 

 as tor potato scab. This will dispose of 

 spores which may have collected on the 

 surface. It will not, however, serve to 

 disinfect potatoes which show the brown 

 discoloration of the vascular tissue. 



Decaying potatoes often carry this fun- 

 gus to the compost heap, either directly 

 or through being fed to stock. Such prac- 

 tices are to be avoided. Also avoid cul- 

 tivation or travel from diseased fields 

 onto new land or land where the disease 

 does not exist. 



Wliere the fungus is once established 

 in a field, practice long periods of rota- 

 tion, preferably to cereal crops. It may 

 be found possible to develop a potato that 

 will be resistant to this disease. There 

 is evidently a varietal difference in this 

 respect, and in time a strain may be de- 

 velor)ed through i)rocesses of selection 

 and breeding which will prove highly re- 

 sistant. 



Itiblio!;rai)hy for the Fusariiim Wilt 

 Disease 



(The better references indicated by**) 

 **]904. V. S. I)e|)artment of Agricul- 

 lure, Hureau Plant Industry Bulletin h5. 



