218 REPORT OF THE DEPARTMENT OF BOTANY OF THE 
as a distinct species. Yellow trefoil (Medicago lupulina) is known 
to serve as a host for P. medicaginis although, according to our 
observations, it is a much less congenial host than alfalfa. The 
writers have observed repeatedly that in fields containing both yel- 
low trefoil and alfalfa the trefoil plants are but slightly affected 
with leaf spot even when the alfalfa plants mingled with them are 
badly diseased. 
Notwithstanding the probability that the seed used may be a 
source of infection it is unlikely that leaf spot can be avoided by 
any method of. seed disinfection. Neither is the liability of dis- 
seminating leaf spot a serious objection to the use of alfalfa soil for 
inoculation. Chester’s experiments,°* and also those made by 
Combs,*® indicate that infection through the air is common. Ap- 
parently, alfalfa leaf spot is one of those widespread, easily dis- 
seminated diseases which it is useless to try to prevent completely. 
The only practicable method of control known is the widely-recom- 
mended one of mowing the plants whenever they turn yellow and 
become so badly diseased that their growth is stopped or severely 
checked. Mowing causes the plants to throw up new shoots which 
may outgrow the disease. Newly-seeded fields should not be mown 
closely — only the tops of the plants clipped off as in the control 
of weeds. Close mowing, if followed by a period of dry weather, 
may kill the alfalfa. When the disease makes its appearance shortly 
before the hay crop is ready to harvest and begins to trim up the 
plants it is advisable to mow the field a few days early in order to 
avoid the loss of leaves which may be considerable if cutting is de- 
layed. Once well started, the disease progresses very rapidly after 
the alfalfa begins to bloom. 
WILT. 
(Sclerotinia libertiana Fckl.) 
June 12, 1899, it was observed that in an alfalfa field on the Sta- 
tion farm some of the stalks were wilted and others quite dead 
and dry. The affected stalks were scattered here and there through 
the field. It was plain that death had come to them rather suddenly 
when they were nearly ready to bloom. Upon tracing the wilted 
shoots back to the root (the plants were large and badly lodged) 
there was invariably found somewhere on the stem a dead, brown 
section 3 or 4 inches long which was plainly the seat of the trouble. 
wt Chesteta 14.90.0039) 
* Combs (17). 
