14 CIRCULAR NO. 121, BUREAU OF PLANT INDUSTRY. 



THE OVERWINTERING OF THE ORGANISM. 



During 1912 search was made to fii\d, if possible, some plant 

 other than the sugar beet which was harboring the leaf-spot organ- 

 ism, and upon which a winter stage of the fungus might occur, but 

 no such plant was found. There is still, however, the possibility of 

 the existence of such a host. 



A detailed examination was also made of the beet balls of each 

 variety of beet seed that the American Beet-Sugar Co. had on hand 

 for the season of 1912 at Rocky Ford, Colo. This examination did 

 not reveal any spores of the Cercospora fungus which were capable 

 of germination. The work was carried on in a way which was posi- 

 tive enough to admit of the assertion that no leaf-spot occurred m 

 this vicinity durmg 1912 as a result of the beet seed used. How- 

 ever, while Cercospora spores capable of germuiation did not appear 

 upon the beet balls of the 1911 output, it can not be concluded that 

 every year's output will give like results, smce at present there seems 

 to be a possibility that the disease must be carried upon the seed 

 into localities which are entirely isolated from sugar-beet growing 

 sections. Recent reports of Cercospora heticola from Europe indicate 

 that the spread of the disease from this source is slight. 



The soil as a harbormg agent of the leaf-spot fungus is of importance 

 because of the manure, beet leaves, and crowns that are to be found 

 in it. The possibility that manure carries the leaf-spot organism 

 over from one season to the next is very slight, if, indeed, it can be 

 considered to be a factor of any importance in the light of recent 

 experunents. The indications are that the organism is entirely killed 

 by its passage through the alimentary tract of cattle, pigs, and sheep. 

 It is at least certam that its growth is greatly inhibited. Of course, 

 where beet-top ''hay" is fed in such a manner that it is not entu'ely 

 eaten up, this means of getthig rid of the disease would be at fault. 



It has been proved repeatedly during the last year, and thus far 

 this year, that the leaf-spot organism remams alive upon the old beet 

 tops, includmg the leaves, petioles, and crowns, which are left from 

 the previous year's crop. It makes no difference where these tops 

 are left; they are capable of producing infection. 



FIRST APPEARANCE OF INFECTION. 



Since, then, Cercospora is carried over from one year to the next 

 upon the old beet tops wliich are left in the field and which later dis- 

 integrate in the soil, the first ap})earance of the leaf-spot in the fol- 

 lowing season should come on the first-formed leaves of the beet 

 plants or those nearest the soil where the source of infection occurs. 

 This is actually what ha{){)ens. In last season's work it was found 

 to be the rule that in fields which were known to have had leaf-sj)ot 



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