442 INSECTS AFFECTING VEGETATION 



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. It is 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 in a 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 bac- 

 terial 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 care- 

 ful study of the same in pure cultures on various media. 



(c) Production of the characteristic signs and lesions of the dis- 

 ease by inoculations from pure cultures into healthy plants. 



(d) Discovery of the organism in the inoculated, diseased plants, 

 re-isolation of the same, and growth on various media until it is de- 

 termined beyond doubt that the bacteria in question are identical 

 with the organism which was inoculated. 



While these methods and rules are stated with special reference 

 to bacteria as the cause of disease, they apply with equal force to 

 the proof of cause in the case of any given parasitic fungus. These 

 methods require rigorous and exact work in the isolation and subse- 

 quent culture of the parasite upon sterile media, followed by equally 

 careful inoculation work using these pure cultures as a source of the 

 organism. 



CULTURE PROOF NOT ALWAYS POSSIBLE. 



While in all cases of bacterial diseases where the body of the 

 organism is so little different from that of the bacteria of decay, 

 fermentation, etc., these rigorous proofs are required before the dis- 

 ease is listed as of proven bacterial origin, we do not find it necessary 

 in practice to reprove again the case as against frequently occurring 

 species of fungi associated with particular plant diseases. This 

 does not make it less necessary to prove all cases as to parasitic cause, 

 although the practicability in any single laboratory of pathology is 

 admittedly one of narrow limits. 



ENZYMATIC DISEASES OF PLANTS; CHLOROSIS OR PANACHURE. 



To this form of physiological breakdown, induced however, by 

 specific causes recently determined, we attribute some very wide- 

 spread and injurious diseases which belong under the head of chloro- 

 sis. Peach yellows, possibly peach rosette, frenching or mosaic dLs- 



