16 



BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE. 



earliest symptom is a wilting of one or more leaves of the plant, 

 often characteristically localized on one side of the leaf or plant 

 (figs. 9 and 10). Yellowing, browning, and drying up of the in- 

 fected leaves follow, and in severe cases the entire plant is killed. 

 Eoots, stalk, midrib, and veins, as well as suckers, may become in- 

 fected, as indicated by cutting across these parts even in early stages 

 of the disease. 



Cause. — This wilt disease is due to a fungus which belongs to the 

 large group of Fusariums, rather common fungi, members of the 



same group produc- 

 ing similar but not 

 identical diseases in 

 various other 

 plants. The par- 

 ticular species con- 

 cerned with tobacco 

 wilt has been named 

 Fusarium oxyspo- 

 rwm var. nicotianac. 

 The strands of this 

 fungus exist in 

 abundance in the 

 vessels of infected 

 plants and are re- 

 sponsible for the 

 wilting. The or- 

 ganism lives over 

 winter in the soil 

 and may possibly 

 exist there in the 

 absence of tobacco 

 for three years. In- 

 fection, therefore, 

 takes place from 

 the soil, and as far 

 as known only 

 through wounds on 

 the roots or basal 

 portion of the 

 plant. 



Conditions favor- 

 ing the disease. — 

 Fusarium wilt ap- 

 pears to be most 

 common in rela- 

 tively warm sea- 

 sons, when the soil 

 temperatures re- 

 main as high as 80° to 90° F. for some time. Infection can occur, 

 however, only on certain especially susceptible strains or varieties, 

 among which White Burley and Maryland Broadleaf are most 

 susceptible, cigar types of tobacco being generally very resistant to 

 infection. The extent of infestation in the soil, as well as the 

 amount of wounding, together with rainfall, naturally also plays an 

 important part in the severity of the disease. 



Fig. 10. — Fusarium wilt of tobacco. Quito commonly this 

 disease is localized and in the early stages affects only the 

 leaves on one side of the plant. 



