1 92 BACTERIA IN RELATION TO PLANT DISEASES. 



the persuasion of many obstinate, doubtful individuals. For this purpose not enough money 

 was available. As a result of the first year's work (winter of 1905-6) the disease was checked 

 in many places, but the surgeons were too few and the cooperation of all the growers could 

 not be obtained. The blight, therefore, still prevailed in many places. In succeeding years, 

 by more vigorous efforts and a general campaign of education, many of the more intelligent 

 pear-growers now assisting in the work of eradication, the disease has been so restricted 

 as to afford a further demonstration of the immense value of systematically conducted 

 winter-excisions.* Mr. O'Gara has carried on a similar campaign of eradication against 

 blight in the apple and pear orchards of Oregon (Rogue river valley) with the very best 

 results in the protection of the magnificent orchards which there also were threatened with 

 destruction. Even in localities in California where practically everything has been swept 

 away by blight single growers of great energy and intelligence have saved their pear orchards 

 by systematic removal of blighting wood. The pear orchard of 6,000 trees owned by Mr. 

 Reed at Marysville is a good example. This is now almost or quite the only orchard of any 

 size left in that region, the others having been destroyed by the blight. 



It is not too much to say, therefore, that excision is a remedy of prime importance in 

 case of pear-blight, and that the disease can be held in check thereby. The principal 

 difficulty lies in obtaining that unity of action throughout a community which is absolutely 

 essential to full success. 



In case of the olive-tubercle equally vigorous treatment would probably yield equally 

 good results. This statement is based on hothouse observations of inoculated plants metas- 

 tasizing freely. In 6 or 8 months from the first appearance of a primary tubercle the 

 organism causing the disease may pass downward through the stem in young and vigorous 

 plants a distance of 1 8 to 36 inches or more, rupturing to the surface at intervals in the form 

 of deep seated secondary tubercles. It is therefore of the utmost importance for the 

 removal of this disease to prune early and severlyso as to reach beyond the unseen advancing 

 bacteria. 



GERMICIDES AND INSECTICIDES. 



The surface sterilization of plants or parts of plants has been treated in Vol. I, so far as 

 regards its use in connection with the isolation of pure cultures. To what has been said there 

 may be added some remarks on the newer germicides and on the reasons for the occasional 

 ineffectiveness of mercuric chloride. Respecting the latter it is a well-known fact that or- 

 ganic substances of various sorts throw this salt out of solution fixing it in their outer layers. 

 Consequently in many cases its penetrating power is slight, and even when a rather strong 

 solution is used the deeper layers of a substance may not be reached and consequently some 

 bacteria may escape. For this very reason it is well adapted to the surface disinfection of 

 thin tissues such as leaves from the interior of which we wish subsequently to make a 

 culture. The exposure, however, must be very short. 



It has also been shown in recent years that a killing quantity of mercuric chloride may 

 be rendered ineffective by a subsequent soaking of the treated bacteria in water, i.e., the 

 poison can be washed out of the outer layers of the bacterial membrane and sometimes 

 growth will then take place. These facts must be borne in mind whenever mercuric chloride 

 is used. The writer still uses it as described in Vol. I. It is not likely that it always fully 

 sterilizes the surfaces treated, but long experience has shown that it does do so to an 



*In a letter dated June 6, 1908, from Auburn, California, Mr. P. J. O'Gara wrote to me as follows: 

 "The work in California on pear-blight control is showing excellent results wherever there has been united effort 

 in eradication. To show you how hard it is for me to get a culture of the organism, I traveled all one day through the 

 best pear section of Placer County and only succeeded in getting one case. Another interesting fact is that there are 

 more pear trees growing in this county (Placer) and adjoining counties than there were three years ago. The local 

 nurseries have not been able to supply the demand during the past year or two, all of which speaks very well for our 

 office." 



