2 3 6 



FUNGOUS DISEASES OF PLANTS 



spite of the fact that the disease may sometimes be suddenly 

 manifested, yet it is certain that it has a long period of incuba- 

 tion, and works very slowly, the final killing of the plant being 

 effected only when its water supply is almost completely cut off 

 by the filling of almost all of the vessels with the fungous hyphae. 

 Resistance of the varieties of the hosts. In almost any infested 

 field it will be noticed that there are plants of different degrees of 

 resistance toward this fungus. In some plants the fungus is only 

 able to effect an entrance into the roots, and each point of infec- 

 tion may be the point of origin of several rootlets developed in 

 the form of a small tuft. Again, the fungus may extend practi- 

 cally throughout the root system and yet fail to invade the stem. 

 Finally, the whole plant may sometimes be affected. Two plants 

 in the same hill may show great diversity in this relationship. 

 Therefore it may be said that all degrees of resistance may be 

 found. Experiments conducted by planting many varieties across 

 land known to be infected by the disease have shown interesting 

 racial variations. Special resistance has been shown by some of 

 the Egyptian cottons, although they are not in any case wholly 

 resistant. On the other hand, sea island cotton is particularly 

 susceptible to this fungus. The most resistant of the upland cot- 

 tons thus far reported are certain strains of the variety known as 

 Jackson, a limbless sort. The following interesting table (Orton) 

 was prepared in 1 900 : 



TABLE SHOWING VARIETAL RESISTANCE OF COTTONS TO THE 

 WILT DISEASE 



(The figures denote the comparative resistance of the different races 

 on a scale of one thousand.) 



