122 Transactions British Mycological Society and fruits with the fungus. The final stage will be the intercomparison of the most promising new or improved spray fluids by field trial. The continuation of this work, begun in November, 1934, was made possible by the aid of a grant from Imp erial Chemical In- dustries, Ltd., to whom the writers record their ind ebt edness. [Some of the apparatus used in this work was exhibited during the meeting, and the method of washing the spray deposit on the slides was demonstrated.] REFERENCES (I) HERANGER , SERGE (1935), Rev. Vitic., Paris, No.2 119, 93. (2) P ALMITER, D. H. (1934), Phytopathology , XXIV, 22. II. THE EVALUATION OF PROTECTIVE FUNGICIDES WITH SPECIAL REFERENCE TO APPLE SCAB VENTURIA INAEQUALISI By R. W. MAR SH Long Ash/on Research Station III. PHYSICO-CHEMICAL ASPECTS By H UBERT MARTIN Long Ashton Research Station T HE status of physico-chemical methods in the laboratory examina- tion of any particular fungicide is determined by the extent of know- ledge of the mode of action of that and related substances. Once the fungicidal properties of the substance have been correlated with specific chemical and physical properties, laboratory testing is re- duced to the evaluation of those properties. From the results obtained , the performance of the substance under field conditions can be fore- told and an examination by biological methods is rendered un- necessary. For example, standardization by biological methods of lime sulphurs can be replaced by the analytical determination of content of polysulphide sulphur. It is evident that the trustworthiness of the conclusions derived from the physico-chemical examination of the substance depends upon experience of the field performance of related compounds. For this reason, it is most important that the biological tests of any new product should have collateral physico-chemical tests designed not only for the purpose of defining the new product but for the eventual elucidation of its mode of action. The usefulness of such supplementary laboratory tests is illustrated by the following results obtained in an investigation of the protective fungicidal efficiency of [ See these T ransactions, vol. xx, 304-9 .
Transcript
122 Transactions British Mycological Society
and fruits with the fungus. The final stage will be the intercomparisonof the most promising new or improved spray fluids by field trial.
The continuation of thi s work, begun in November, 1934, wasmade possible by th e aid of a grant from Imperial Chemical Industries, Ltd., to whom th e writers record th eir indebtedness.
[Some of the apparatus used in this work was exhibited during themeeting, and the method of washing the spray deposit on the slideswas demonstrated.]
REFERENCES( I ) HERANGER , SERGE (1935), Rev. Vitic., Paris, No.2 119, 93.(2 ) P ALMITER, D. H. (1934), Phytopathology , XXIV, 22.
II. THE EVALUATION OF PROTECTIVE FUNGICIDESWITH SPECIAL REFERENCE TO APPLE SCAB
VENTURIA INAEQUALISI
By R. W. MARSHLong Ash/on Research Station
III. PHYSICO-CHEMICAL ASPECTS
By H UBERT MARTINLong Ashton Research Station
T HE status of physico-chemical methods in the laboratory examination of any particular fungicide is determined by the extent of knowledge of the mode of action of that and related substances. Once thefungicidal properties of the substance have been correlated withspecific chemical and physical properties, laboratory testing is reduced to the evaluation of those properties. From the results obtained,the performance of the substance under field conditions can be foretold and an examination by biological methods is rendered unnecessary. For example, standardization by biological methods oflime sulphurs can be replaced by the analytical determination ofcontent of polysulphide sulphur.
It is evident that th e trustworthiness of the conclusions derivedfrom the physico-chemical examination of the substance dependsupon experience of the field performance of related compounds.For this reason, it is most important that th e biological tests of anynew product should have colla teral physico-chemical tests designednot only for the purpose of defining the new product but for theeventual elucidation of its mode of action. The usefulness of suchsupplementary laboratory tests is illustrated by the following resultsobtained in an investigation of the protective fungicidal efficiency of
[ See these T ransactions, vol. xx, 304-9.
Technique for Evaluating Fungicidal Properties 123
certain copper derivatives, Potato Blight being used as the testorganism. As a working hypothesis it was assumed that the efficiencyof the spray will be dependent upon the retention on the foliage of acopper-containing deposit from which the active fungicide is slowlyformed. It follows that the protective efficiency is determined bothby retention factors and by availability factors, and to separate thesetwo sets of factors analytical studies were made of the sprayed potatofoliage and the total amount of copper present at intervals afterapplication was determined. Simultaneously, estimates of blightinfestation were made in the field, with the results recorded in Table I.
The partial correlation between protective efficiency and retentionis shown and differences attributable to availability can be determined.For example, comparing the relative efficiencies of cupric oxide andof cuprous oxide, the indications are that cupric oxide is better retained whereas cuprous oxide has the greater availability. In the dry
season of 1934 the latter factor was predominant and cuprous oxideproved the superior protector; in the wet 1935 season, the greateramount of cupric oxide retained overshadowed its lower availabilityand cupric oxide proved the better fungicide. Conclusions of thischaracter must obviously be confirmed by further trial and they areinstanced only for the purpose of demonstrating the value of physicochemical tests as supplements to field trials. It is not, at this stage,suggested that they are essential and it may be possible to subject anunknown product xrz to field trials and, if found satisfactory, toadmit the product to the category of fungicides suitable for recommendation. Physico-chemical criteria play no part in such a schemebut, once the field trials are replaced by laboratory biological tests,physico-chemical considerations again require attention.
The simplest definition of a laboratory trial is one in which one ormore of the factors affecting fungicidal efficiency which are variablein the field trial are maintained constant. This definition may be
124 Transactions British Mycological Society
illustrated by a brief description of Salmon's method of assessingdirect fungicidal properties, which employs the hop Powdery Mildewas the test fungus. By using hop plants vegetatively propagated fromone parent plant susceptible to the mildew and .by selecting fortreatment leaves and mildew patches in the same stages of activegrowth, the biological condition of the fungus and host plant isstandardized and variations in their condition which may affect theresponse of the fungus to the fungicide are diminished. Further, thephysical properties of the spray are modified so that it is able todisplace air from and to wet the conidiophore mass. Finally, asufficient amount of spray is applied to achieve complete wetting.In interpreting to practice the results obtained by this technique, itmust be remembered that, as variations in the biological conditionof the fungus and host plant have been eliminated, the results are ofrelative and not absolute value. Further, that the results are applicable only to sprays which have wetting properties sufficient towet the conidiophores when applied in heavy amounts in the field.
The accuracy of the interpretation of the results of laboratorytests is dependent upon the correctness of the allowances made forthe influence of variations in the factors held constant in the trial.This theme has been developed in a previous discussion u ) and .it isonly necessary to emphasize that the missing variables are alwaysfound to involve physico-chemical factors. The neglect of suchcriteria, which was conceivable in field trials, becomes highlydangerous in laboratory tests.
From the biological aspect, the laboratory trial is generally designed for the examination of the toxicity of the material to fungi.Fungicidal efficiency, although primarily dependent upon actualtoxicity, is det ermined by other factors, of which the amount ofmaterial applied and retained by the fungus or plant surface may beselected as an example. In all laboratory methods of evaluatingfungicidal properties, standard procedure is adopted for the application of the spray or dust under test. Moore and Montgomery spreada definite volume of spray over a standard area of surface; Marsh andSalmon both apply the spray under standard conditions to the surfaceheld at right angles to the direction of the spray; Marsh spraying fora definite time, Salmon spraying copious amounts of a spray of goodwetting properties. What assumptions do these various proceduresinvolve and how are the results comparable with those of practicalspray methods?
With the surface held at right angles to the direction of the sprayit has been found (2) that, with solid-liquid systems akin to thoseencountered in spray practice, the whole of the spray applied is retained up to a certain amount dependent upon the particular liquidsolid system. Irrespective of the spray or surface, the amount re-
Technique for Evaluating Fungicidal Properties 125
tained is equal to the amount applied until the volume present issuch that the excess runs or drains off the surface. By the techniqueused by Marsh, therefore, standard amounts of spray are applied perunit area provided that the time of spraying be so selected that norun-off or drain-off of the spray occurs. The technique thus producesthe same end-point as that of Moore and Montgomery except that thelatter method makes no apparent provision for the possibility thatexcess of spray may be applied to the surface. In Salmon's method,excess of spray is deliberately applied and the amount retained willbe determined by the wetting properties of the particular spray undertest. But a requisite of the technique is that the wetting properties ofthe spray shall be sufficient to permit complete wetting. Under suchconditions it has been shown that approximately constant amountsof spray are retained.
Salmon was concerned with direct fungicidal properties for whichpurpose only sprays of good wetting properties are suitable. The othertechniques under discussion are applied to sprays of indifferentwetting properties such as are normally used as protective fungicides.With such sprays, the amounts retained upon foliage will be limitedby the maximum amount retainable, an amount which has beenshown to be greatly influenced by the physical properties of the spray.Thus upon a cellulose nitrate surface, the maximum retainable weightof water is of the order of200 mg. per sq. in.; that ofa solution of aneffective wetting agent is of the order of 25 mg. per sq. in.
There is, however, a compensating set of factors, for the conditionthat the surface shall be held perpendicular to the direction of thespray is but rarely met in practice. If the surface be tilted to the sprayit is found that only a proportion is retained, the proportion increasingwith the wetting properties of the spray. When applied in limitedamount, therefore, a spray of wetting properties similar to those ofwater will be retained in less amount than a spray of good wettingproperties provided that, in neither case, does drip occur.
These considerations concern simple aqueous solutions, and withheterogeneous sprays, such as suspensions and emulsions, otherphysical factors come into play. Enough has been said to show thecomplexity of the apparently simple problem of how to apply thespray in the laboratory test. The exact reproduction of the spraydeposit obtained in practice is not necessary provided that the effectof the factors held constant in the laboratory test be remembered intranslating the results to practice. For this reason there is much to besaid for the simplest technique which will give consistent results andof which the significance in relation to field technique is understood.
The method of application is but one example of the physicochemical considerations involved in laboratory tests, but it is enoughto illustrate the contention that, if the results of biological toxicity
126 Transactions British Mycological Society
trials are to be applied to practice, consideration be paid to thephysico-chemical properties of the spray or dust.
REFERENCES(I) MARTIN, H. (1932), Ann. appl. Biol. XLX, 263.( 2 ) EVANS, A. C. & MARTIN, H. (1935), J. Porno!' XIlI, 261.
IV. FUNGICIDES SUITABLE FOR FOOD PRESERVATION
By R . G. TOMKINSLow Temperature Research Station, Cambridge
INNUMERABLE substances possess fungicidal properties. Whether ornot any of these substances can be usefully employed to prevent thegrowth of fungi in any particular instance depends not on theirtoxicity alone, but also on other properties such as solubility, taste,colour and smell. Consider, for example, the fungi cides which areemployed in a variety of ways in food preservation. Hypochloritesand formalin are used to sterilize storage rooms and grading machinesand packing sheds. Benzoic acid or sulphur dioxide is added to certainfoods to preserve them. Oranges are sometimes dipped in a solutionof borax or sodium bicarbonate in order to reduce the incidence ofrotting by green rot. Carbon dioxide is sometimes added to ships'holds to delay the onset of mouldiness and sliminess of meat carriedin them. It has been suggested that certain fungicides might withadvantage be added to the paper in which fruit is wrapped to preventthe onset or the spread of rotting. The compounds used in thesevarious ways all possess the property of retarding or preventinggrowth, but otherwise the properties which make them useful arevery different.
Because usefulness depends on the combined effects of a variety offactors, it is generally agreed that the advantage of using a fungicidein any particular instance can be tested only by direct experiment.Preliminary laboratory tests of substances can be undertaken to find:
(I) Their action on germination and growth of fungi or theirpower of killing, i.e. their fungicidal action.
(2) Their solubility, volatility, and other properties which may beof importance.
From the results of such tests, it is usually possible to decide whatsubstances are obviously unsuitable and what substances may besuitable for further trial for the purpose for which they are wanted.
LABORATORY TECHNIQUE FOR EVALUATING FUNGICIDAL PROPERTIES
The type of test undertaken to find how fungicidal a substanceis, is usually of the simplest order. This is to be regretted, forcertain qualitative differences in the manner in which growth is re-
