330 OHIO EXPERIMENT STATION: BULLETIN 214 
effects in nitrogen fixing, exerted by the root nodule bacteria of 
leguminous plants. The well known beneficial effects of the root 
nodule bacterium upon clover has made rotation in clover an agricul- 
tural necessity. The species or forms of root nodule bacteria 
required on alfalfa, cowpeas, vetches, etc., have become recognized 
as factors of consequence in our efforts at seeding and new species 
of legumes on the farm. 
A less understood relation between certain fungi which develop 
as mycorrhiza upon the roots of some deciduous trees and notably 
on conifers may not be passed. Herein we may find an explanation 
of rotation in forest species when reforestation crops are to be 
grown. 
THE PROOF OF PARASITIC CAUSE IN PLANT DISEASE 
The mere presence of a fungus, determined by the microscope 
in diseased tissues of the plant, does not prove the case against the 
organism found. Itis not easy at all times to be certain whether 
discovered spores belong to this or that organism, or group of organ- 
isms, although with certain groups as the anthracnoses, species of 
Fusarium, etc, the spore forms give somewhat clear evidence. The 
differences between parasitic and saprophytic fungi are not always 
simple matters admitting of ready determination; further, we must 
bear in mind that after a parasite has caused death or even minor 
lesions ina plant, the organisms of decay may be expected to appear 
to do their great work as the scavengers of the world. The fungi or 
bacteria found in a dying plant may be both saprophytic and para- 
sitic, or these may be only saprophytic. 
The methods of proof of parasitic cause in the bacterial diseases 
of animals including man have been extended to the study of bacterial 
diseases of plants and finally to the diseases caused by parasitic 
fungi. These methods consist of a group of rigorous exact rules 
which are stated by Dr. E. F. Smith in the following terms: 
(a) Constant association of the organism with the disease. 
(b) Isolation of the organism from the diseased tissues and careful study of 
the same in pure cultures on various media. 
(c) Production of the characteristic signs and lesions of the disease by inoc- 
ulations from pure cultures into healthy plants. 
(d) Discovery of the organism in the inoculated, diseased plants, re-isola- 
tion of the same, and growth on various media until it is determined beyond 
doubt that the bacteria in question are identical with the organism which was 
inoculated. 
Smith—Bacteria in Relation to Plant Diseases. Vol. 1, p. 9. 
