November 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
in water and forms colorless transparent easily fusiblc crystals. It is found in Searles Lake, Calif. A related compound is sulphohalite, 3NsnSO~.NaCINaF, occurring as pale greenish-yellow octahedrons which are very easily fusible, and have a refractive index of 1.455. This mineral is found in borax or Searles Lake. Double salts of aluminum sulfate and aluminum fluoride have een shown ,to exist, including A12(SO4)a.4AIF~.12HaOand Alp?h.AlF*.15H20 & (II,I8). One of these compounds, often referred to as aluminum fluorosulfate (AlF2)2S0,.12H20, is formed when calcium fluoride and aluminum sulfate are heated in boiling water. By rapid evaporation of the solution it forms a sirupy concentrate, and by slow evaporation a crystalline powder can be obtained. The compound is soluble in water, but when dried a t 300” C. it changes to an insoluble form. A solution of aluminum fluosulfate will react with hydrofluoric acid and sodium sulfate, forming chiolite or cryolite depending on the amount of sodium used. The compound K2BeF4. Ah( SOi)a,24Hz0 has been reported (12).
Arsenic trilluoride sulfur tetrachloride, 2AsFa.SCl4, was p r e pared by Ruff (IO), It forms yellow crystals which attack glass slowly and decomposes carbon tetrachloride, carbon disulfide, alcohol, ether, benzene, and petroleum ether. It is probably decomposed by water into sulfurous and thiosulfuric acids. Potaasium difluodithionate, &&Od?~.3Hpo, rubidium difluodithionate, Rb1S206F2.3H~0,and cesium hydroxyfluoditbionate, Cs&Os(OH)F.HeO, are formed by adding hydrofluoric acid to the saturated solution of the dithionate, and cooling (368).These salts are unstable, and when exposed to air quickly decompose with the evolution of water and hydrofluoric acid, leaving a residue of the dithionpte. When heated they give off water, hydrogen fluoride, and sulfur dioxide, leaving a residue of potassium sulfate.
(1) (2) (3) (4)
2227
LITERATURE CITED Anderson, H. H., J. Am. Chem. Soc., 69, 2496-7 (1947). Baumgarten, P., Ber., 73B, 1397-8 (1940). Baumgarten, P., and Hennig, H., Ibid,, 72B, 1743 (1939). Booth, H. S., and Cassidy, M. C., J.A m . Chem. Soc., 62,2369-72 (1940).
(5)
(6) (7) (8) (9)
Booth, H. S., and Herrmann, C. V., Zbid., 58, 63 (1936). Booth, H. S., and Mericola, P.C., Zbid., 62, 640-2 (1940). Booth, H. S., and Seabright, C. A., Zbid., 65, 1834-6 (1943). British Intelligence Objectives Sub-committee, BIOS Final Rept. 1595, Item No. 22. Centnersawer, M., and Strenk, C., Bw., 56B, 2249 (1923); 58B, 914 (1926).
(10) Denbigh, K. G., rtnd Whytlaw-Gray, R., J . Cliem. SOC.( L o d o n ) , 1934, 1346. (11) Ehret, W. F., and Frere, F. J., J. Am. Chrm. SOC.,67, 68-71 (1946).
(19) (20) (21) (22)
Ephraim, F., “Inorganio Chemistry,” 6th ed., p. 596, New York, Interecience Publishers, 1948. Fischer, J., and Jaenckner, W., 2.azlgew. Chem., 42, 810 (1929). Lange, W., and Askitopoulos, K., Ber., 7 l B , 801-7 (1938). Luohsinger, W., dissertation, Breslau, 1936. Meslam. M., Bull. 8oc. chim. France (3), 15, 391 (1896). Moiasan, H., and Lebeau, P., Cmpt. rend., 132,374 (1901). Nikolaev, N. S., J. Chem. 2nd. (U.S.S.R.),14, 1087-98 (1937). Ruff, O., Ber., 37B, 4620 (1904). Ruff, O., et aE., J W , , 47B, 046-56, 65G60 (1914). Sabatier, P., Compt. r d . , 112, 862 (1891). Schumb, W. C., Trump, J. G.,’ and Priest, G. L., Isu. 1 4 : ~ ~ .
(23)
Stevenson, D. P.. and Russell, H., J. Am. Chem. SOC.,61,3264-8
(24)
Traube, W., Hoerena, J.. and Wunderlioh, F., Ber., 52B, 1272
(12) (13) (14)
(16) (16) (17)
(18)
CHEM.,41, 1348-61 (1949). (1939). (1919).
(26) Wells, A F., “8tructural Inorganic Chemistry,” p. 83, London,
Oxford Press, 1945. Werland, R. F., and Alfa, J., 2.anorg. Chem., 2 1 , 4 3 (1899). (27) Yost, D. M., and Russell, H., “Systematic Inorganic Chemistry of the Fifth-and-Sixth Group of Nonmetallic Elements,” pp. 297-309, New York, Prentioe-Hall, 1944. (26)
REOEWED April 18, 1060.
SULFUR IN FUNGICIDES M. M. BALDWIN Battelle Memorial Institute, Columbus, Ohio Although only a small percentage of the sulfur produced is used in the manufacture of agricultural fungicides, this element holds a key position in the fungicide industry. Varioum types of sulfur compounds are considered a0 fungicides. Besides being the most extensively used fungicide in ita elemental form, sulfur also is an active aonstituent of the dithiocarbamates and playa an indirect
role, through copper sulfate, in the preparation of copper fungicides. Sulfur and ita compounds are used to control dinewes of fruita, vegetables, and grasses. The mechanism of the fungicidal action of sulfur is discusaed brieny. Each of the various mechanisms proposed by different investigators may account for the toxicity of sulfur and its compeunds under diflerent operative conditions to fungi.
T
Comparisons are made in Table I from the standpoint of the sulfur industry. These figures show that only a relatively small percentage of the total amount of sulfur produced finds its way into fungicidal uses. This is not too surprising in view of the fact that almost three fourths of the sulfur in this country is converted to sulfuric acid, one of the most fundamental materials in the chemical industry. Even if only the nonacid uses of sulfur are considered aa a basis for comparison, the utilization of sulfur in fungicides amounts to only about 9% of that total. Nevertheless, around 81,500 long tons are an appreciable quantity of material. On the other hand, from the standpoint of the fungicide industry, sulfur overshadows other basic materials in this field. Estimated comparative figures are shown in Table 11. Not only is more elemental sulfur used than any other type of agricultural
HIS paper presents a summary of the position of sulfur and its compounds in the field of agricultural fungicides. Elemental sulfur is one of the oldest fungicides known, and sulfur compounds are among the most recently developed toxiaants to fungi. It can be said that sulfur is one of the fundamental elements of fungicidal materials. An indication of its prominent position during the development of fungicides is given by Horsfali in the frequent references to sulfur-containing fungicides in his list of landmarks in fungicide history (f7).
RELATIVE POSITION OF SULFUR F”0ICIDES It is difficult to obtain any very firm or clear-cut statistica on the production of fungicides. However, estimates can be made that give indications of the current position of sulfur fungicidw relative to the sulfur industry and to the fungicide industry.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
2228
Table I. United States Sulfur Activity, 1947 Sulfur Mined From other sources Exported Net domestic supply Nonacid use Fungicidal use (estd.)
Long Tons 4,441,000 (14) 747,000 (14) 1 356 000 (14) 4:237:000 (14) 910,000 ( 8 ) 81,500
Table 11. Sulfur in Fungicides, Estimated Production in 1941 Fungicide Elemental sulfur (including dust, wettable, and lime sulfur) Copper chemicals (sulfate basis) Dithiocarbamates Mercury compounds Miscellaneous
Pounds of Fungicide
Pounds of Sulfur
175,000,000 105,000,000 4,000,000 150,000 1,000,000
166,000,000 12,000,000 1,200,000
285,150,000
179,200,000
.... ....
fungicide, but sulfur is also e constituent of basic copper sulfate, the dithiocarbamates, and tetramethylthiuram disulfide. In addition, i t plays an indirect rale through copper sulfate in making available most of the copper used for fungicides in the form of Bordeaux mixture. According to Table 11, sulfur is involved in almost two thirds of the fungicide production of the United States.
SULEVR F"G1CIDES The relationship of sulfur to the fungicides that depend upon it is shown in Figure 1. This scheme includes some of.the other sulfur compounds that are not used in very large quantities at present as compared to the materials already mentioned. No toxicity to fungi is being ascribed to the sulfate sulfur-it is of indirect importance in its association with the active ingredients.
Vol. 42, No. 11
respect to the toxicity of sulfur to fungi but also to its adherence to plant foliage. A number of investigators have looked into this relationship (6, 8, 13, SI). In general, the finer the particle size, the more effective sulfur is with respect to its toxicity to fungi and its adherence to plant foliage. Most of the ground sulfurs used today are around 325 mesh or finer. Over 95% of the particles of flotation sulfur paste are not larger than 3 niicrons in diameter, according to Sauchelli (94). Suitable degrees of fineness are obtained by milling mined sulfur, emulsification of molten sulfur as in the Grimrod process, heating mixtures of sulfur with Bentonite, and by using flotation sulfur obtained from the recovery of the element from hydrogen sulfide in coal and petroleum gases. Finely ground sulfur is suitable for dusting purposes without any additives, but wetting agents are needed for the preparation of suspensions for spray purposes. Wetting agents recornmendcd for this use from time to time include flour, dextrins, calcium caseinate, glue, resins, skimmed milk, Bentonite, sulfite liquor, and various synthetic wetting agents The principal agricultural applications of sulfur as a fungicide are listed in Table 111, ~ t sgiven by McCallan (22). Chester (3) lists, in addition, its use on soybean downy mildew, rose-leaf diseases, and iris leaf spot. Most of the elemental sulfur is used to combat diseases of fruit trees, particularly apples and peaches. The finer grades are applicable to both crops, but, according to Groves (IS),the larger particle preparations do not give adequate control of apple diseases. Sulfur is safest to use a t relatively low temperatures, since injury to foliage tends to occur in the 90" to 105' F. range ( 4 ) The principal inorganic compositions that contain sulfur are the limesulfurs. These compositions have been prepared by a variety of methods from lime and sulfur mixtures, These methods include: adding wa%r to the dry mixture and using the heat from slaking of the lime to accelerate the reaction between calcium hydroxide and sulfur to give self-boiled lime-sulfur; boiling a mixture of lime, sulfur, and water by the application of external heat; and evaporating the water from the boiled mixture to give a dry mix.
SULFATE
The chemistry involved in this reaction has been the subject of considerable investigation but is not completely understood ( 7 ) . It is believed that the principal constituenb of the reacted mixture are: (1) sulfoxylate, sulfite, and thiosulfate, (2) sulfide, and (3) polysulfidea. Of these, the polysulfides, particularly the pentasulfide, are considered to be the most active fungicidally.
WLFATE
Figure 1. Relationship of Sulfur in Fungicides In considering the various fungicides that contain fungicidally active sulfur in somewhat greater detail, i t is interesting to note that the oldest material, elemental sulfur, is still in rather a dominant position, in spite of the rather extensive development that has been going on in this field during the past few years. This does not mean, however, that while the newer fungicides were being developed advances have not been made in the technology of using elemental sulfur 89 a fungicide. Early in the application of the element, i t was used in the sublimed form-flowers of sulfur. It was soon realized that the size of the sulfur particles had considerable bearing on its performance aa a fungicide. Particle size waa found to be a factor not only in
Table 111. Pdncipd Uses for Sulfur Fungicides in the United States CrQP
& ' ~ ~ ~ Peanut
Grape Home gardens and orchards
gCherry zp
Shade trees and ornamentals
Disease or Use Scab
Brown rot and scab Leaf spot Powdery mildew General disease control Leaf curl. rust Scab and'leaf blight Leaf spot and brown rot
........ Soil fungicide amendment
% of Total 62
17 7 3 3 2 1
1
1 3
November 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
Although lime-sulfur is more toxic to fungi than sulfur alone, it is not so safe for the plank nor so easy to use. The current tendency is to replace lime-sulfur with wettable and flotation sulfur. According to Chester (3),limesulfur uses included dormant or growing-season sprays for apple leaf and fruit diseases, brown rot of stone fruits, and cherry leaf diseases. The principal present use is for dormant sprays against apple scab and peach leaf curl. Carbon disulfide has some direct use in controlling fungus by its application as a general fumigant. However, its principal current use aa a fungicide is indirect through the increasing application of the dithiocarbamates (6,if?,86). These compounds are prepared by the reaction of carbon disulfide with amines in alkali solutions. Typical reactions may be represented by the following schemes :
R
R
\
NH
/
+ CS2 + NaOH
S
This type of compound was first introduced to the fungicide industry as the disodium salt
II
NaSC-NH-CH&HpNH-CSNa
H2C-~H4-s Zn Zinc ethylenebisdithiocarbamate (zineb)
R Sodium dithiocarbamatm
B
R -SNa
/
+ MX,
&
R
LR’
1.
Heavy metal dithiocarbamates
and NH2-CI12CH2-NH2
+ 2CS2 + 2NaOH + S
S
NaS-&-NH-CHp-CH2-NE-&SNa Disodium et hyleriebisdithiocarbamates S
li
&
NaS -NHCHZCHZ-NH-
-SNa
H
is probably formed. This mixture is used more extensively now than the straight disodium salt. The zinc salt also may be prepared directly and supplied as such. The bisdithiocarbamates have been used successfully on potatoes to control blight and are finding rather general use on vegetable crops and for some fruit crop diseases. Encouraging results have been obtained recently with manganese ethylenebisdithiocarbamate, and a cyclohexylamine complex of the zinc salt is reported to show promise. If a soluble dithiocarbamate is subjected to mild oxidizing conditions, two dithiocarbamate groups combine to form a tetraalkylthiuram disulfide-for example,
+ MS, --+
S
2 CHjN-!-HCHs
H&-I!J-&-S Hz --N-C-S
I/
disodium ethylenebisdithiocarbamate (nabam). Although this salt, which is very water soluble, undergoes some change after application which results in a relatively tenacious and insoluble deposit, it still does not have entirely satisfactory weathering qualities. Heuberger and Manns (16) found that the practical effectiveness of the disodium salt was markedly improved by the addition of zinc sulfate and lime. The zinc salt S
/
R
S
S
\N-b-SNa
----f
2229
>hl
Heavy metal ethylenebisdithiocarbamates Variations in structure of these compounds have been studied. Goldsworthy, Green, and Smith (18) investigated various metal salts of dimethyl-, diethyl-, and dibutyldithiocarbamic acids. The dimethyl compounds have the highest fungicidal efficiency. The best coinbination of toxicity to fungi and solubility and nonphytotoxic properties is found in the zinc and ferric salts, and these are available commercially.
S CIA CHa .-2e -f >-b-S-S-&-N< CHa CHa Tetramethylthiuram disulfide (thiram)
-
It is a point of conjecture as to how much, if any, of the dithiocarbamate deposits on foliage undergoes this reaction on weathering. Tetramethylthiuram disulfide, however, has not found any appreciable use in foliage application. Rather, it is used extensively as a seed disinfestant, particularly for sweet corn and some of the other vegetable seeds. It is also used for legume and peanut seed treatment, and to control some turf diseases (21). Another type of sulfur-containing fungicide that may be mentioned here is the polyethylene polysulfides, -[-CH2-CH2-S,ln-- (zdenotes 2, 3, 4, or 5). The polyethylene pentasulfides were reported (96)to be more toxic at a concentration of 2 pounds per gallon, as a protective spray against apple scab, than micronized sulfur at 10 pounds per 100 gallons, under conditions of high and frequent rainfall. However, i t appears that these materials may find their best acceptance as fungicidal stickers (IS),rather than as primary fungicides. MECHANISM OF FUNGICIDAL ACTION
The iron derivative appears to be used principally in controlling apple diseases, except scab, tobacco blue mold, grape black rot, and cranberry fruit rots (22). The zinc salt haa been shown to be effectiveagainst the anthracnose diseases of tomato and against early andlate blight of potato (92 1.
The bisdithiocarbamates are a more recent development. Their fungicidal activity was discovered by Dimond, Heuberger, and Horsfall (6)and these compounds have been extensively studied by Barratt and Horsfall ( 1 ) . From the viewpoint of the orgsnia etructure of the molecule, the ethylene group is preferred.
The action of a fungicide on a fungus spore may be considered as a two-step process. First, the fungicide has to come into contact with the vital processes of the fungus, and secondly, it has to have some property which may be called inherent toxicity that interferes with these processes to a sufficient drgree to kill the fungus spores (fungicidal effect) or at least prevent their growth (fungistatic effect). It is difficult to see how elemental sulfur could exert its known effectiveness as a solid, and so there has been considerable conjecture and experimentation to explain its activity. Thme explanations have included the following factors: volatility of sulfur, formation of sulfur dioxide, formation of hydrogen sulfide.
2230
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
and formation of water-soluble sulfur compounds such as pentathionic acid. The work of several investigators (9,90,87)has shown that sulfur volatilizes a t appreciable rates under outdoor conditions prevailing during the summer in the temperate zone. However, there does not appear to be agreement as to the toxicity-to-fungi properties of sulfur vapor (10,20). There is some evidence that sulfur dioxide is toxic to the spores of certain species of fungi (%O),and pentathionic acid formed from the interaction of sulfur dioxide, water, and sulfur, has been proposed as the toxic agent from sulfur (32). This proposal has been questioned by several workers, including Wilcoxon and McCallan (89). These investigators demonstrated that the toxicity of pentathionic acid and sulfuric acid is the same and, therefore, attributed the toxicity to hydrogen-ion concentration. This conclusion was supported by their finding that the neutral salts were inactive. However, this still allows the formation of acidic materials from sulfur deposits to be a possible factor in the toxicity to fungi and perhaps phytotoxicity of the sulfur deposits. Wilcoxon and McCallan (30) have demonstrated that hydrogen sulfide results from the interaction between plant materials and sulfur. The hydrogen sulfide produced was highly toxic to some of the spores tested and not so toxic to others. Fungus spores are known to produce hydrogen sulfide, and Wilcoxon and McCallan postulated further (30) that the sulfur vapor may be reduced within the spores to form toxic amounts of hydrogen sulfide. The production of B toxic material by the spores themselves is an interesting point. Different fungus species vary in their capacity for hydrogen sulfide production and in their susceptibility to it. For example, Horsfall(l9) points out that a Glomerella cingulatu spore liberates enough hydrogen sulfide into the air to kill two much larger spores of Sclerotiniu f ~ u c t i c o l abut , would have to produce 50 times as much to kill itself. The Sclerotinia spore itself can produce enough hydrogen sulfide to kill three of its kind and, apparently, continues to produce hydrogen sulfide after i t is dead. This and the fact that heating to 55’ C. inactivates the hydrogen sulfide-producing system suggest that enzymatic action is involved. However, Marsh (23) has shown that susceptibility of fungi to hydrogen sulfide does not necessarily correspond to their ausceptibility to sulfur. The presence of sulfur in a compound is neither a necessary nor sufficient condition that the compound be a fungicide. The presence of sulfur in copper sulfate is purely incidental to the fungicidal properties of this compound. Copper is the active agent. Furthermore, the simple combination of copper with sulfur in cupric sulfide is not very toxic to fungi. Here, the sulfur not only ties up its own potential activity, but also that of copper. In the field of organic compounds of sulfur as fungicides, activity varies considerably with molecular structure. Apparently,
8
U
one of the most active groups is - C 4 in the dithiocarbamates. But even this group must be in the right structural environment. It has already been pointed out in the preceding discussion of dithiocarbamates that toxicity-to-fungi activity depends, to a considerable extent, on the size of the alkyl radicals ( I d ) . This group also.occurs in the xanthates, but they are not so outstanding in their fungicidal properties as the dithiocarbamates (11). In addition to the variation in response of a particular species to different sulfur compounds, there is also the variation in effectiveness of a particular compound in controlling different fungi. Why do these variations exist? They are related to the twostep process cited at the beginning of this section-inherent toxicity and availability to vital processes of the fungus spores. It is believed that, if the site of tosicity-to-fungi activity, in relation to spore structure, can be established with respect to the molecular structure of sulfur fungicides, the manner in which these fungicides act would be much more clearly understood. This information should lead to the design of improved sulfur
Vol. 42, No. 11
fungicides. Work along this line is now in progress at Battelle Memorial Institute. The question as to the inherent toxicity of sulfur fungicides is still more difficult. Horsfall (18) has proposed that redox systems are a factor and has drawn a parallel between fungicidal action and rubber vulcanization. He and his associates have been investigating this parallel.
FUTURE OF SULF’UR IN FUNGICIDES Because of its low cost, elemental sulfur will be difficult to dislodge from those fields in which it is now giving rather adequate protection against fungus diseases. The use of the dithiocarbamates appears to be expanding as indicated by the production figures of 3,991,000 and 8,100,000pounds for 1947 and 1948, respectively (88). Organic sulfur compounds appear to offer the most likely opportunity for new developments. For example, Carbide & Carbon Chemicals Corporation has recently announced two experimental fungicides, indicating that they are complex cyclic organic compounds containing sulfur and nitrogen. The current hearings before the Food and Drug Administration, relative to the need for using pesticides and the toxicological hazards of insecticide and fungicide residues, are injecting a note of uncertainty in the future of some fungicides. However, there is no appreciable doubt that fungicides are required for the production of food supplies, and there should be little question that elemental sulfur residues offer no health hazards. LITERATURE CITED (1)Barratt, R. W., and Horsfall. V. G., Conn. Agr. Erpt. Sta. BulI. 508 (1947). (2) C h m . Eng., 56,113(February 1949). (3) Chester, K.S.,“Nature and Prevention of Plant Diseases,’’ p. 470, Philadelphia, Pa., Blakiston Corp., 1947. (4) DeOng, E. R.,“Chemistry and Uses of Insecticides,’’p. 81,New York, Reinhold Publishing Corp., 1948. (5) Dimond, A. E.,Heuberger, V. W., and Horsfall, J. G.,Phutopathology, 33, 1095-7 (1943). ( 6 ) Feichtmeir, E. F., Ibid., 39, 605-15 (1949). (7) Frear, E.H., ”Chemistry of Insecticides, Funpicidea, and Hcrbicides,” 2nd ed., p. 236, New York, D. Van Nostrand C o . , 1948. (8)Goodhue, L. E., J . Econ. Entomol., 31, 41&14 (1938). (9) Goodwin, W.,and Martin, H., Ann. Applied Biol.. 15, 62:? 37 (1928). (10)Ibid., 16,93-103 (1929). (11) Goldsworthy, M. C., Carter, R. H.. and Green, E. L.. Phuto~ t h o l o g y32,497-504 , (1942). (12) Goldsworthy, M. C., Green, E. L., and Smith, M. A., J . Agr. Research, 66,277-91 (1943). (13) Groves, A. B.,V
