Potvdery Mildew of Clover 307 



DIFFERENCES IN THE SUSCEPTIBILITY OF CLOVER 

 TO POWDERY MILDEW.^ 



E. B. Mains, Purdue Agricultural Experiment Station. 



The powdery mildew of clover has aroused considerable interest this 

 year (1922), because it has been widespread and abundant. This 

 disease has heretofore been rare in America and in consequence the 

 general whitening of clover by mycelium and spores of this fungus has 

 caused more or less alarm among the farmers, from whom numerous 

 inquiries have been received. The disease made its first noticeable ap- 

 pearance in the fall of 1921, and during the spring and summer of 

 1922 spread rapidly. According to the Plant Disease Survey", the 

 mildew first made its appearance upon the first crop of clover in the 

 South, and was first reported from Louisiana, April 10. With the 

 advance of the season, it spread northward and by June 28 was reported 

 in Maine and Minnesota. The writer saw the disease more or less preva- 

 lent practically everywhere on his trips through South Carolina, Vir- 

 ginia, Tennessee, Kentucky, Indiana, Wisconsin, and Michigan during 

 May, June, and July. According to the Plant Disease Survey', the first 

 record of this disease in the United States was from Prof. Sheldon who 

 reported its inconspicuous occurrence in the vicinity of Morgantown, W. 

 Va., in 1908. In 191.5, reports were received from Idaho, Washington, 

 and Oregon, and in 1916 and 1917 from Utah. 



There is some question as to the identity of this mildew, since, as 

 far as the writer is aware, no perithecia have been found accompanying 

 the disease, at least in the eastern United States. Miss V. K. Charles 

 (I.e. 3) found perithecia en collections from Oregon, Washington, Mon- 

 tana, Colorado, and Idaho, but not from any of the eastern states. 

 Since all clover mildew has been considered as belonging to the species, 

 Erysiphe Polygoni D.C., it is very likely that the mildew so prevalent in 

 the eastern United States is this species and that the proper condi- 

 tions for perithecia formation have not occurred in this region. 



Although it is generally agreed that clover mildew has been very 

 severe, there seems to be considerable difl'erence of opinion among 

 pathologists as to the damage caused (I.e. 2, 3), some believing very 

 little damage is caused while others believe considerable loss is suffered. 

 The reasons given for attributing loss to this disease, are the death of 

 the lower leaves (Fromme) causing hay to be slightly off color and of 

 poorer quality, the reduction in tonnage by death of lower leaves and 

 also the proportionally higher percentage of stems (Edgerton), and the 

 greater shattering and powdering of the leaves in hay from diseased 

 fields (Elliot). This is one of the diseases of plants for which it is 

 hard to obtain any accurate estimate since it does not kill the plants 

 attacked nor reduce the yield to such a marked extent but that the loss 



^ Contribution fiom the Department of Botany, Purdue University Agricultural Ex- 

 periment Station. 



= U. S. Dept. Agr. Plant Disease Survey Bull. Vol. 6, No. 1, pp. 8-14. 1922. 

 3 U. S. Dept. Agr. Plant Disease Survey Bull. Vol. 6, No. 3, pp. 53-55. 1922. 



"Proc. 38th Meeting, 1922 (1923)." 



