10 The Colorado Experiment Station 
greenhouse by applying the culture to the unbroken epidermis, it is 
altogether possible that stomatal infections take place under field 
conditions. The leaflets often exhibit yellowish, watery areas along 
the margin and the larger veins when there is no evidence of the 
tiouble in any other part of the plant; again, the tiny petioles suc¬ 
cumb to the disease independently of either the stem or the attached 
leaflets. Water pore and stomatal infection similar to that des¬ 
cribed for the black rot of cabbage may explain these cases. 
There are doubtless other ways in which infection can take 
place, but the methods described above, especially the inoculation 
through the split epidermis, seem to be the most common. It is 
possible that added observations of another season will give us more 
light upon this point and so rather than draw any final conclusion 
as to the one way in which inoculation takes place in the field, we 
prefer to leave the question open. 
GREENHOUSE experiments. 
Ihe constant occurrence of characteristic white colonies, in 
such a large percentage of our plates, was sufficient to make us 
suspicious that the micro-organisms making up such colonies were 
the immediate cause of the disease. However, the crucial test of 
a pathogenic organism is its power to reproduce the given disease 
when introduced in pure culture into its normal host. Accordingly, 
we have fulfilled this requirement by making a large number of in¬ 
oculations upon alfalfa plants under greenhouse conditions, and by 
this means we have been able to establish Ps. medicaginis, n. sp. 
beyond the remotest shade of possible doubt, as the unquestionable 
cause of the trouble. We have reproduced the infection in from 
five to seven days with practically its characteristic field symptoms, 
and we have been able to follow its progress through the different 
changes up to the blackening and complete destruction of the stem 
after six weeks. 
In order to determine whether the infection was communicated 
to the plants through the roots, twelve pots were prepared with sick 
soil containing quantities of the diseased stems. Fifteen germin¬ 
ated alfalfa seeds, which had been sterilized previously in a 1-500 
mercuric chloride solution, were planted in each of the above pots. 
A good, vigorous stand was obtained. The possibility of frozen 
stems was eliminated by growing the alfalfa in the greenhouse and 
the danger from dust infection was reduced to a minimum by keep¬ 
ing the surface soil in the pots moist. These plants are now sixteen 
months old and up to the present time not a single stem in any of 
the twelve pots has shown any sign of the disease. From these 
results, we can say with a reasonable degree of certainty, that the 
disease is not, primarily, a root trouble, and if the roots do become 
