190 BACTERIA IN RELATION TO PLANT DISEASES. 



Negligent pruning in some instances may be responsible for the distribution of disease. 

 There is not much doubt that pear-blight may be distributed from one tree to another by 

 means of infected pruning shears. Since this sentence was written D. H. Jones has done it 

 with a pruning saw. Probably olive-tubercle can be spread in the same way : The Italians 

 think so. Apple gall is favored by carelessly made grafts (Hedgcock). Daisy knot fre- 

 quently appears on the wounded end of cuttings. 



The wounding of the roots of plants in transplanting from the seedbed to the field is a 

 fertile source of infection. The most striking examples of this are the distribution of to- 

 bacco wilt and of tomato wilt. It would be best, if possible, to avoid transplanting into 

 fields subject to these diseases. If such fields must be used, then the seeds should be planted 

 where the plants are to be grown or the transplantings made with unusual care. Any wound- 

 ing of the roots in such soils is highly detrimental because the plants seem to be able to keep 

 out the soil organism so long as the roots are not broken, but when these are injured there 

 is an open passageway into the vascular system of the plant which the parasite is not slow 

 to find. It has been observed (Hunger) that this wilt disease is worst on fields occupied by 

 plant-infesting nematodes, the direct loss due to these worms being a small part only of 

 the actual injury since the wounds they make form an open passageway through which the 

 bacteria enter the plants and destroy them. A great desideratum is some easy means of 

 combating in the soil this type of nematodes. The early removal and destruction of trap 

 crops is supposed to be a partial remedy. On small areas, e. g., in houses or under tents, they 

 may be destroyed by the use of steam. 



Having said thus much about nematodes it remains to say a word about the destruc- 

 tion of insect carriers of disease. We know, from Mr. Merton B. Waite's experiments, 

 that pear-blight is commonly distributed by bees and flies. My own experiments have, I 

 think, settled the fact that the wilt of cucurbits is distributed by Diabrotica vittata. Brenner 

 plated Bad. campestre from an aphis allowed to puncture diseased veins of a cabbage-leaf. 

 Jones has recently plated Bacillus amylovorus from aphides feeding on blighting shoots of 

 the apple. The question is yet, perhaps, an open one whether aphides are responsible for 

 the general distribution of many bacterial diseases. The same is true for various other bugs 

 which have been incriminated, c. g., the squash-bug. Exact experiments are still wanting. 



On general principles, however, it is desirable to keep down the prevalence of biting 

 and puncturing insects both in hothouse and field by the use of suitable insecticides. 



Change of crops is an important means of combating many diseases. This is par- 

 ticularly true of bacterial diseases if we may accept Laurent's idea as in any degree repre- 

 senting what really takes place in the soil. His laboratory experiments led him to believe 

 that non-parasitic forms are often converted into transitory parasites by change of food, i.e., 

 by growing for a time saprophytically on portions of the plant they become able to attack 

 it parasitically. If, on the contrary, the organisms which have attacked plants are com- 

 pelled to live for a time on other foods they lose the parasitic habit, i. c, become non-viru- 

 lent. If this is true, fields which have developed a disease should be plowed so as to aerate 

 the soil and thus hasten the decay of diseased roots and stems. Moreover, if possible the 

 field should be put to other crops for some years. Rotation of crops is an old subject much 

 written upon, but not yet sufficiently impressed upon the multitude of planters. To follow 

 one crop with another of the same sort and that year after year invites disaster. 



We do not yet know much about the proper chemical treatment of soils to put them into 

 the best condition to resist a particular disease, i.e., use of lime, acid phosphates, etc., but 

 there is undoubtedly much to be learned. Of this fact I think there can be little doubt, 

 namely, that bacterial plant parasites die out of some soils more readily than out of others. 



This chapter may well close with a pregnant sentence from Louis Pasteur: 



II est au pouvoir de l'hommc de faire dispairaitre de la surface du globe les maladies parasitaires. 



