28 James G. Horsfall 



Economic importance 



Usually the rusts of clover have been regarded as of little importance. 

 Most of the records in the Plant Disease Reporter read like this, " common 

 but for the most part of only slight importance" (Haskell, 1919:156). 

 It is interesting that, on the contrary, the early references to the disease 

 in America would seem to indicate that it was worse in years gone by 

 than it is now. It is " of great importance " wrote Freeman in 1905 

 (1905:166). Miss Howell (1890 a: 129) stated that it " prevailed to such 

 an extent during the several wet, cool seasons preceding 1890 . . . that 

 it must be regarded as a disease likely to affect seriously, under conditions 

 favorable to its development, an agricultural crop." Pammel (1891:51) 

 in Iowa said that some plants were so severely attacked that the hands 

 became reddened on touching them. Underwood (1888:302) two years 

 earlier estimated that from 5 to 20 per cent of the value of the clover was 

 being destroyed in the vicinity of Syracuse, New York. 



According to Lobik (1922), on the other hand, the disease stimulates 

 the plants attacked, so that a heavier production of both green parts and 

 seed results. Such a statement requires more experimental demonstration. 



With the exception of that on alsike, the diseases do not now seem to 

 be of much economic importance in New York. Rust can be observed in 

 practically every planting of red clover after midsummer and occasionally 

 as early as June 15, but in no case has severe infection been observed before 

 the first, and most important, cutting was removed. White-clover rust 

 does appear to be destructive to the plants in many lawns and waste 

 places. 



Symptomatology 



Although the writer has not yet seen the aecial stage on red clover, 

 Hoffer (1917:326) remarks that " the striking feature of the leaves bearing 

 aecia is that they blacken and wilt in a very short time." He believes 

 that for this reason the aecial stage is commonly overlooked. The aecial 

 lesions, which are hyperplastic on white and alsike clovers, kill the affected 

 leaves only after a considerable time. The pycnia appear first as honey- 

 yellow dots on a much swollen lesion on vein, petiole, or stem of the plant. 

 Aecia, which soon appear, are of about the same color. The swellings 

 are often profound, causing the petiole, for example, to swell to several 

 times normal size. It may also bend at this point. When veins are 

 attacked, the leaf blade becomes greatly distorted. 



The uredinia are practically alike on all three clovers. They occur, 

 as shown in figure 5, as minute pale cinnamon-brown pustules on any 

 exposed part of the plant, calyces, stipules, stems, petioles, and leaves. 

 They occur on leaves first on the lower surface, and then on the upper. 

 When uredinia are scattered widely over the blade, little or no yellowing 



