302 



THE AGRICULTURAL NEW! 



r 



September 16, 191L 



FUNGUS NOTES. 



RECENT WORK ON BORDEAUX MIXTURE. 



In considering the poisonous action of copper salts on 

 plant organisms when they are used in the form of sprays, it 

 must be borne in mind that no substance in the solid form is 

 capable of penetrating the walls by which the living protoplasm 

 of practically all plants is protected, and that, consequently, in 

 order to bring about the death of the organism, the copper 

 salt must be soluble in water. Such a soluble salt is copper 

 sulphate, or blue Stone, and it might at first seem that the 

 application of a solution of this substance to diseased plants 

 would be all that is required to kill the fungi causing the 

 disease. This course has several drawbacks. In the first place, 

 it is often found that such a solution, when strong enough to 

 kill the fungi, damages the host plant as well; secondly, it is 

 easily washed off by rain. 



In order, therefore, to diminish its harmful effect on the 

 host plant, and at the same time to increase its adhesiveness, 

 it is mixed, in the preparation of Bordeaux mixture, with 

 lime-water, or water containing slaked lime partly in solution 

 and partly in suspension. As a result of this mixing, an 

 insoluble compound of copper is formed, which may often be 

 mixed with excess of lime. The Bordeaux mixture must con- 

 tain no copper in the form of the soluble sulphate, if injury 

 to the leaves of the sprayed plant is to be avoided. Conse- 

 quently, enough lime is always added to turn all the copper 

 into the solid form. The liquid containing the insoluble 

 copper precipitate suspended in the form of fine particles is 

 then sprayed on the plant, and covers the parts to be protected 

 with a fine film of insoluble copper compounds. The question 

 now naturally arises as to how this insoluble substance is 

 again rendered soluble, as it must be, if it is to bring about 

 the death of germinating fungus spores, which would other- 

 wise infect the sprayed plant. 



There are three theories which have been put forward 

 to account for the manner in which the insoluble copper com- 

 pounds are rendeied soluble; these are: (1) that the copper is 

 brought into solution by the action of the atmosphere, more 

 especially owing to the presence of the carbon dioxide in it; 



(2) that the insoluble compounds are dissolved by some 

 substance or substances secreted by the sprayed leaves; 



(3) that the copper is rendered soluble by some substance 

 secreted by the fungus itself, which consequently brings 

 about its own destruction. 



The first of these theories was supported by I'ickering 

 (see Eleventh Report on the Wuhurn Experimental Fruit Farm, 

 1910); but recently, further work carried out by (3iminghani, 

 and loy Barker and Gimingham, discredits this theory and 

 lends support to the last, namely that of the action of the 

 fungus itself. {Journal of A'jricultmal Science, Vol. IV, |)p. 

 (19 and 76.) 



rickerinj; found that the insoluble substances containing 

 copper formed in the preparation of Bordeaux mixture were 

 partly dis.solved in water containing a large amount of carbon 

 dioxide and that copper sulphate was forinetl in the solution. 

 This naturally led to the idea that the fungicidal action of 

 the mixtine was due to the effect of atmospheric carbon 

 dioxide on the insoluble copper precipitate. Furthermore, 

 he observed that when the mixture contained excess of lime, 

 no copper appeared in the solution until all the lime had been 

 converted into chalk by the action of the carbon dioxide. 

 Consequently, he recommended that care should be taken to 

 prepare Bordeaux mixture without excess of lime, since this 



substance woilld only delay the action of the mixture as 

 a fungicide. 



Gimingham, however, found that if the excess of carbon 

 dioxide was removed from the liquid, the copper was again 

 precipitated in an insoluble form; while as the result of 

 several experiments he finally concluded that it was unlikely 

 that the copper was rendered soluble by the action of the 

 atmosphere. 



The possibility that the copper is rendered soluble by 

 secretions from the sprayed leaves was examined by Barker 

 and Gimingham. They found that a certain amount of 

 soluble copper sulphate was produced by the substances 

 .secreted through minute punctures or alirasions on the 

 surfaces of the leaves. This quantity, however, was not 

 sufiicient to account for the fungicidal action of the 

 Bordeaux mixture; on the other hand, it was enough to cause 

 scorching of the leaves in .spots, and explains why older 

 leaves which have been longer subject to possible damage 

 are more liable to scorching than younger foliage, since 

 undamaged leaves do not appear to secrete the necessary 

 substances. 



The same workers then turned their attention to the 

 possibility that the copper is rendered soluble by substances 

 secreted by the spores or germ tubes of the fungi themselves. 

 They found that spores possessing thin walls, and also the 

 tips of young germ tube.s, do actually appear to secrete small 

 quantities of such substances sufficient in amount to dissolve 

 enough copper to cause their death. The amount of copper 

 dissolved depended, however, on the distance of the spore 

 or germ tube from the particle of copper compound near- 

 est to it; while the fungicidal action of the particle only 

 took place when this distance was very small. Another 

 point determined was that there was no secretion from spores 

 [irovided with a special, thick protective wall, and that these 

 could only be killed after the formation of a germ tube. 

 This makes it still more improbable that the epidermal cells 

 of the sprayed leaves can give rise to the necessary secretion, 

 as these also are furnished with a special thick cuticle in 

 almost all cases. 



These results have an important practical bearing on the 

 application of Bordeaux mixture. In the first place, they 

 indicate that the mixture is of more service as a preventive 

 than as a remedy; this is supported 113- actual results. In 

 cases where fungi have already gained a hold on the leaves 

 before spraying, the hyphae in the i)lant tissues will not come 

 into intimate contact with the particles of copper compound 

 on the sprayed surfaces, and will, therefore, not be affecteJ. 



In the second place, the film of coi)per compounds depo- 

 sited must be even and universal, for, if small untreated areas 

 are left, fungus'spores will be able to germinate on them un- 

 harmed, and the resulting hyphae will penetrate the intcn al 

 tissues underlying the fungicidal film, without coming into 

 close contact with this, and, therefore, without receiving 

 any check. It is because of the advantage gained by uni- 

 formity in the film deposited that a second spraying is parti- 

 cularly beneficial. Excess of lime in the mixture would 

 appear to have but little effect on its action, except in so far 

 as it increases the distances between individual particles of 

 the copper compounds in the film. 



A final point not considered bj' these workers is of impor- 

 tance in the tropics. Freiiuentlj-, extra adhesives must be 

 added to Bordeaux mixture when it is used in places 

 where the rainfall is high. The choice of such adhesives 

 requires care, since they might conceivably form a film over 

 the particles of copper compounds, such as would prevent the 

 secretions of the fungi from exerting their solvent action on 

 those particles. 



