Powdery Mildeiv of Clover 309 



species belonging to 154 genera as hosts. He gives the following species 

 of clover as hosts: Trifolium agrarium, T. alpestre, T. arvense, T. fili- 

 forme, T. hybridum, T. incarnatum, T. invohicratum, T. longipes, T. 

 lupinaster, T. ■mediiini, T. minus, T. nionantlium, T. montanum, T. moran- 

 iJium, T. paucifloruDi^ T. prutense, T. procumhens, T. repens, and T. 

 rubens. That E. Polygoni must consist of a large number of specialized 

 races is indicated by the researches of Neger and Salmon', the former 

 being unable to infect Trifolium repens, Vicia sepium or Hypericum 

 montanum with conidia from T. incarnatuni, and the latter 

 finding that the mildew from T. pratense would not infect 

 T. agririuni, T. repens, T. medium, T. montanum, T. incar- 

 natuvi, T. filiforme, Lotus corniculatus, Melilotus arvensis, Medicago 

 satix^d, Lnpimis hitens or Pisum sativum, but infected T. pratense heavily. 

 According to this, the mildew on T. pratense is a race restricted to that 

 species of clover. Likewise the mildew on T. incarnatum represents an- 

 other closely restricted race. Observations by various pathologists dur- 

 ing the past year as reported in the Plant Disease Survey agree with 

 these results. 



There are evidently few or no observations upon varietal or indi- 

 vidual differences in the reaction of T. pratense to the specialized race 

 of mildew which exists on it. Dr. A. .J. Pieters (I.e. 2) has observed 

 at Arlington, Virginia, that many of the American varieties of red 

 clover were affected while most of the European strains were free or 

 nearly so. In the fall of 1921, the writer noticed that considerable differ- 

 ences existed between the species of Trifolium as to susceptibility to 

 mildew, and also that similar differences were to be found between vari- 

 eties of T. pratense. The plants upon which these observations were 

 made were started in pots of sterilized soil in the greenhouse and were 

 transplanted to the field in late summer. The plants were set about 

 ten inches apart in rows two feet apart, so that there was no difficulty 

 in observing individual differences. By October 13, when notes were 

 taken, the plants were well established and in good condition. The notes 

 given in the following table represent the reaction of individuals and 

 varieties under field conditions to natural infection of the powdery 

 mildew. 



' Salmon. Ernst S. On Specialization of Parasitism in the Erysiphaceae. Beihefte 

 Bot. Ccntralbl. 14 ;261 -31 ."i. 190o. 



