216 REporRtT OF THE DEPARTMENT OF BOTANY OF THE 
it should be promptly clipped, but not mown closely. Close mowing 
may ruin young alfalfa. The mower cutting bar should be run at 
least four inches above the ground. This is the treatment usually 
recommended for weeds. It is widely practiced in New York. The 
treatment of charlock by spraying with a solution of copper sul- 
phate, so successfully used in oat fields, can not be used in alfalfa 
fields. Alfalfa is injured by the spray. 
FUNGUS’ DISEASES: 
LEAF SPOT.° 
(Pseudopeziza medicagmis (Lib.) Sacc.) 
Leaf spot, also known as rust and blight, is by far the most 1m- 
portant fungus disease affecting alfalfa in New York. In fact, it 
is the only really important fungus disease. It appears in all parts 
of the State where alfalfa 1s grown, on all kinds of soil, in all situa- 
tions and under almost all conditions. Probably, more or less of :t 
may be found in almost any alfalfa field in any season, in dry 
weather as well as in wet. Some severe cases have been observed 
in the midst of an August drought. It may make its appearance 
at any time after about June 1. The lower leaves are the first to 
be attacked. They show numerous small brown spots, turn yellow 
and fall. (See Plate XXV.) In severe attacks so many of the 
leaves fall that the value of the hay is much impaired. 
Fields of all ages and in almost any stage of growth may be at- 
tacked. Newly-seeded fields are sometimes ruined by leaf spot. 
Olver fields may suffer very severely from it but are rarely, per- 
haps never, killed outright. While it may seriously affect the first 
cutting in June, the second and third cuttings are the ones most 
likely to be injured. COverripe plants are especially liable to attack. 
Through extensive dropping of the lower leaves, affected plants pre- 
sent a trimmed-up appearance. The disease may also appear on 
the stems in the form of elliptical black spots I to 3 millimeters in 
length. Usually, but not invariably, leaf spot causes the leaves to 
* Although there are at least three other leaf spot diseases of alfalfa in 
New York it seems best to reserve this name for the common and well- 
known disease caused by Pseudopeziza medicaginis. If it becomes necessary 
to use common names for the unusual diseases caused by Ascochyta, Stago- 
nospora and Cercospora they may be designated as Ascochyta leaf spot, 
Stagonospora leaf spot and Cercospora leaf spot respectively. 
