RECOVERY BY EXCISION. 191 



RECOVERY BY EXCISION. 



It would be difficult to say when excisions were first practiced for the protection of 

 plants. The method is probably as old as gardening itself. It is a common practice in 

 China. It has been, however, purely empirical. 



The systematic practice of excision of plant-parts with a particular end in view, that 

 end based on a scientific knowledge of the habits of the parasite to be overcome, is com- 

 paratively modern. 



Wakker seems to have been the first scientific man to try this for a bacterial disease of 

 plants. Knowing that the movement of the bacteria was downward in the vessels, he re- 

 moved the leaves of hyacinths showing the beginnings of bacterial disease at their apex, 

 and thus prevented the infection of the bulbs. 



Dr. Russell and the writer both showed that infection of cabbage-heads by Bacterium 

 campestre could be prevented by removal of leaves only recently out of the water-pore stage 

 of infection, the movement of the bacteria in this case also being downward. 



For details on excision experiments made by the writer to control wilt of cucurbits, 

 see page 276. By the very prompt removal of affected leaves or branches valuable plants 

 may sometimes be saved, but usually the wilt is not discovered until the bacillus has entered 

 the main stem and then excisions are of no avail. 



The most successful example of protection by pruning known to the writer is the winter 

 treatment of fire-blight of the pear, devised by Merton B. Waite. Mr. Waite discovered 

 that Bacillus amylovorus winters over in pear-trees, but only in a small proportion of the 

 whole number of the trees attacked, one tree, let us say, in fifty, or one in a hundred, the 

 exact proportion is immaterial. From the gummy exudate out of patches of bark on these 

 trees, through the agency of insect-carriers (bees, flies, etc.) the bacteria are distributed 

 again the following spring to other trees in the orchard and the blight renewed first, 

 usually, as blossom-blight. Mr. Waite has not been able to find any other source of the 

 early spring infection. These cases of "hold over blight," as he has called them, can be 

 detected by sharp eyes and removed while they are in a dormant condition; and, theoreti- 

 cally at least, if all were thus removed, there would be no source of infection the following 

 spring, and consequently no new blight. The correctness of this hypothesis was first tested 

 on a considerable scale in Georgia, where Mr. Waite succeeded in saving a large pear- 

 orchard which had been threatened with complete destruction. Every visible vestige of 

 the winter-blight was removed from these pear-trees several years in succession, and the 

 disease ceased to be a cause of anxiety. In fact, the only cases that appeared thereafter in 

 this orchard were scattering examples traceable to an occasional case of hold-over blight 

 which was missed during the winter examinations, and to such infections as wandered into 

 the borders of the orchard from the small neglected orchards of careless neighbors. The 

 experiment was considered to be a brilliant success, both by the pathologist and by the owner 

 of the orchard. 



In more recent years Mr. Waite and his assistants have endeavored to apply this 

 method on a large scale to the pear-orchards of California. When he was urged to under- 

 take the task, about one-fourth of the orchards of the State had already been destroyed by 

 this disease, and the remainder were threatened with destruction, the disease being widely 

 disseminated, although of recent appearance, and very aggressive. To be asked to do for 

 a whole State what he had done before for a single large orchard might well appal any man. 

 The loss of these orchards meant, however, a money loss to the State of California of per- 

 haps ten million dollars. Mr. Waite, therefore, energetically undertook to save them. The 

 task, however, was a gigantic one since it involved critical inspection of every pear-tree 

 in the State, and the removal of all diseased parts during the winter season. It involved 

 also the education of the pear-growers of a whole State, the combating of much ignorance, 



