Jan. 3,i92i Fusarium-Wilt of Tobacco 531 



no credit (20X0), which is the minimum and equals o per cent resistance, 

 or 100 per cent susceptibility. On the other hand, if, out of 20 plants 

 in a flat, 5 were dead, 5 badly diseased, 5 slightly diseased, and 5 healthy, 

 30 points would result (5 X = o, 5 X 1 = 5, 5 X 2 = 10, 5 X 3= 15, total 30) 

 which is 50 per cent resistance. 



It is only in some such manner, in fact, that resistance could be fairly 

 recorded in figures. Comparative yield of plants would give no better 

 criterion, since a plant might be infected and show no depreciation of 

 yield and might even reach maturity and be badly diseased without 

 appreciably influencing yield. 



The average resistance given is on the basis of only 60 plants, except 

 in a few instances when it is on a basis of only 40 or 20 plants. Though 

 the numbers are small, they are believed to be more significant than 

 could be obtained under field conditions with a greatly increased num- 

 ber of plants, because of the uniformity of the soil and of infestation. 



From these calculations it will be noted that none of the varieties 

 tried were absolutely immune. The most resistant varieties are the 

 Connecticut Havana, Cuban, and Sumatra, with 98 per cent resist- 

 ance. Since the figures are not regarded as significant within about 5 per 

 cent, the Pennsylvania Broadleaf and the Wisconsin binder selection 

 Hi 2074, a strain selected for resistance to rootrot due to Thielavia basi- 

 cola, should be included in this group. The least resistant of the Nico- 

 tiana tabacum varieties is the ordinary White Burley (32 per cent) (Pi. 67, 

 IV). Strangely enough, N. glauca, perhaps the species farthest removed 

 from N. tabacum in similarity, is the least resistant (23 per cent) to 

 F. oxysporum var. nicotianae of all plants tried. The varieties listed have 

 been repeatedly tried out for their resistance to the rootrot of tobacco 

 due to T. basicola (5), and it is interesting to note the correlation in 

 resistance to the two parasites. N. rustica is immune to Thielavia but 

 may be attacked by Fusarium. Shade-grown Cuban, Little Dutch, and 

 Wisconsin selection Hi 2074 are very resistant to Thielavia, but, while 

 Little Dutch is not very resistant to Fusarium, the other two are de- 

 cidedly resistant. The Pryor and Oronoco types are very susceptible 

 to. Thielavia but relatively resistant to Fusarium. The White Burley, 

 which is most susceptible to Thielavia, is also most susceptible to Fu- 

 sarium. A strain of White Burley selected for resistance to Thielavia 

 is also fairly resistant to Fusarium. The F t generation of a cross be 

 tween resistant and susceptible Burley is seemingly intermediate in 

 resistance to Fusarium- wilt, as it is to Thielavia. The figures for the 

 latter are, however, not large enough to be of much significance. The 

 cases cited seem to be sufficient to warrant the statement that the cor- 

 relation between resistance in tobacco to Thielavia basicola and to F. 

 oxysporum var. nicotianae is low. 

 17777°— 21 2 



