490 
Journal of Agricultural Research 
Vol. XXIII, No. 6 
The illustration of black rust by Honing shows some resemblance to 
Wisconsin leafspot, but the disease does not seem to be identical with 
the latter. The chief points of difference between the two causal organ¬ 
isms may be summarized as follows : 
Bacterium pseudozoogloeae (Honing) 
1. Produces “black rust.” 
2. Apparently no chlorotic halo. 
3. Produces lesions with concentric 
rings. 
4. Size generally 1.5 microns by 0.7 to 1 
micron. 
5. 1 to 2 polar flagella. 
6. Color usually yellowish gray. 
7. Gelatin stab papillate; liquefaction 
napiform to saccate. 
8. Milk coagulum not peptonized. 
9. Litmus milk rendered acid. 
10. Acid with dextrose, lactose, and 
saccharose broth. 
11. Fluorescence yellowish green (in 
gelatin). 
Bacterium melleum, n. sp. 
1. Produces “brown rust.” 
2. Chlorotic halo frequently present. 
3. Lesions usually not concentrically 
ringed. 
4. Generally 1.8 microns by 0.6 micron. 
5. 1 to 7 polar flagella. 
6. Color usually orange-yellow. 
7. Gelatin stab filiform; liquefaction 
stratiform. 
8. Milk coagulum peptonized. 
9. Litmus milk rendered alkaline. 
10. No acid with dextrose, lactose, and 
saccharose broth. 
11. Fluorescence honey-yellow on potato- 
dextrose agar. 
From this comparison it may be seen that the differences are decided 
in many instances and that the likelihood of the Sumatran and American 
leafspot being identical is very remote. 
PRACTICAL CONSIDERATIONS 
The Wisconsin bacterial leafspot or “rust” no doubt occurs annually 
in this State, or at least it has been found for the last five years without 
much difficulty, although not to such an extent as to cause much con¬ 
cern. As already stated, however, it is quite certain that in years past 
it has been the cause of considerable losses and the object of demand 
for control measures. This belief is strengthened by the writer’s recent 
studies of nonparasitic spotting of tobacco, which might otherwise have 
been confused with the bacterial leafrust. The causal organism is not 
believed to be a vigorous parasite, and special conditions are necessary 
for infection without wounding. Aside from a period of rainy or humid 
weather, and possibly a fairly high temperature, we do not know the 
conditions which are necessary for infection, since these two require¬ 
ments in themselves are apparently not sufficient. This conclusion is 
arrived at as the result of environmental studies in controlled tempera¬ 
ture and humidity chambers. 
There is some ground for the belief that plants may be predisposed to 
the disease from internal causes. This hypothesis may be illustrated by 
an observation of the behavior of this disease in field fertilizer plots at 
the Wisconsin Experiment Station. The plots concerned were in du¬ 
plicate and were intended to compare the value of barnyard manuie 
with commercial fertilizers. For some reason not clearly understood, 
the manured plots gave a yield considerably lower than the unfertilized 
plots. Counts were made of the number of infected plants in each plot 
and, as may be seen from Table I, a fairly close correlation existed be¬ 
tween yield, or the fertilizer applied, and the amount of infection. 
