78 PHYSIOLOGICAL RELATIONS 



pretends to be authoritative. It is true that the great majority of 

 fungi described as the causes of plant diseases have not undergone 

 experimental tests, although it will be admitted by most experi- 

 enced pathologists that a large proportion of the claims made are 

 just, beyond all question. Where the spore-bearing parts of a 

 fungus emerge directly from only slightly injured tissues, or in 

 other equally plausible cases, the statement of parasitism made by 

 an experienced pathologist is usually correct. In all cases where 

 decay has set in, or where there is great discoloration of the parts 

 affected, especially in root and stem diseases, leaf spots, leaf 

 burns, etc., experiments are necessary to determine the primary 

 cause of the disease. 



It is often the case that the fruit bodies or the mycelial stages 

 of several different fungi are found associated with a diseased con- 

 dition, and it is necessary to determine either which fungus is the 

 real cause of the trouble, or what part each one may play in the 

 effect produced. All organisms must be isolated, and separate in- 

 fection experiments should be made with each. In such cases, of 

 course, the fungi may be only secondary, appearing more as sapro- 

 phytes on plants which are diseased owing to the action of some 

 more general environmental factor, to the injuries of some insect, 

 or to a mechanical agent. In many instances the fruit bodies of a 

 causal fungus may not be formed until after the death of the plant, 

 as is particularly true of the pyrenomycetous fungi. If. not readily 

 developed in culture, for comparison with those produced in nature, 

 it will be necessary not only to make infection experiments with 

 the pure cultures, but also with the spores produced in the open. 

 In general, controlled infection experiments will be more rigor- 

 ously demanded as our knowledge is advanced. There are propor- 

 tionally few groups of fungi which may be designated saprophytic 

 or parasitic in more than a relative sense. 



2. Infection experiments often enable one to determine the role 

 which may be played in the predisposition to attack by such con- 

 ditions as excessive moisture in the atmosphere or soil, the state 

 of nutrition of the host, etc. Excessive moisture in the soil and 

 the crowding together of seedlings offer advantageous conditions 

 for the outbreak of damping-off diseases, produced by such fungi 

 as Rhizoctonia and Pythium. Moisture on the leaf surfaces favors 



