Ann. appl. Biol. (1973), 73, 189-196

Printed in Great Britain

Glasshouse tests of chemicals for control of potato common scab

BY A. H. McINTOSH Department of Insecticides and Fungicides, Rothamsted Experimental Station,

Harpenden, Herts.

(Accepted I 5 August 1972)

S U M M A R Y

Quintozent: is the only chemical used successfully in practice to control soil-borne Streptomyces scabies, the cause of potato common scab. However it may be carcinogenic, and the aim of this work was to find a substitute for it.

About IOO chemicals, most of which had known fungitoxic action or were related to such chemicals, were tested in the glasshouse by growing potato plants in soil with which the chemicals had been mixed, usually at 50 ppm. Many of the chemicals failed to decrease the incidence of scab, or decreased the yield of tubers, or both. The most effective chemical was captafol, which was as effective as quintozene and, unlike quintozene, did not decrease yield. Captan and folpet, which are very closely related to captafol, were ineffective.

I N T R O D U C T I O N

Potato common scab, caused by soil-borne Streptomyces scabies (Thaxt.) Waksman & Henrici, can be controlled in various ways: by cultural methods (Wood & Tveit, 1955 ; Vruggink, 1970), by applying manganese sulphate (McGregor & Wilson, 1966), quintozene (de Bruin & Heuver, 1971) or very large amounts of sulphur (McCreary, 1967) to the soil before planting, or by irrigation during the growing season (Lapwood, Wellings & Rosser, 1970; Forsund, 1971). These and other methods of control were discussed by Labruybre (1971), who pointed out that the only chemical method that has had any practical success is soil treatment with quintozene before planting. How- ever, quintozene is now suspected of being carcinogenic (Searle, 1966) and its future in plant protection, in Britain at least, is uncertain.

This paper reports results of glasshouse tests of about IOO other chemicals for possible control of scab, mostly by soil treatment before planting.

M A T E R I A L S A N D M E T H O D S

A few Chemicals were made by conventional methods, but most of those tested were supplied by manufacturers, either technically pure or as dusts or wettable powders.

The general method of testing the chemicals, on potted glasshouse plants growing in

A. H. MCINTOSH 190 treated soil, was described by McIntosh (1970). The chemicals were thoroughly mixed in a food-mixer with dry sieved scab-infested soil from Woburn Experimental Farm. All chemicals were tested in soil from a field called Great Hill Bottom, and a few in soil from Schoolfield as well, at 50 ppm of a.i. This corresponds to about IOO lb/ acre or I 12 kg/ha, and is a higher rate than is normally used for quintozene in the field, but its use makes for easier detection of chemicals with only a slight effect on yield. Lower rates were used if 50 ppm proved to be obviously harmful to the plants. In a few tests, the method was modified so that chemicals could be tested for possible down- ward-moving systemic action. For this, plants were grown in untreated scab-infested soil from Great Hill Bottom. The soil was protected from direct and indirect spray, and the plants were sprayed to run-off with aqueous solutions or suspensions five or six times at about 4-day intervals during the 2nd, 3rd and 4th weeks after potting, i.e. during the time when the very young tubers were susceptible to infection by S. scabies.

In each test there was one nil treatment and about six treatments with different chemicals, including quintozene as a soil treatment to give an internal standard; there were 15 replicate plants per treatment. At harvest, tubers were weighed and conventional scab indexes calculated (Large & Honey, 1955; Lapwood & Dyson, 1966). The index for the nil treatment was usually in the range 25-35, i.e. 25-35 % of the tuber surfaces was disfigured by scabs. Because the amount of scab in the nil treatment varied from test to test, the 'scab incidence' for each chemical treatment was calculated as the percentage of that in the corresponding nil treatment. Yields were expressed in the same way.

Analysis of variance of each test gave values of P, measuring the significance of the differences, in scab or yield, between the figures for each chemical and the correspond- ing nil treatment. Quintozene nearly always decreased scab incidence at P < 0-001 in single tests. Chemicals which were ineffective ( P > 0.05) in single tests were not usually re-tested. More effective chemicals were tested several times on different occasions.

R E S U L T S

Tests by spraying foliage Table I shows results of spraying tests with four chemicals, compared with quinto-

zene as soil treatment. The only chemical which seemed to decrease scab incidence by spraying was DL-ethionine, but its effect was very slight. None of these chemicals decreased yield.

Tests by soil treatment The following chemicals did not affect scab at 50 ppm of a.i. or at the rate shown,

in Great Hill Bottom soil. They are listed by common name where possible ; asterisks show those which decreased yield.

Inorganic compounds Organo-tin compounds anhydrous ammonia (450 ppm) bis(tripheny1tin)sulphide copper oxychloride dibutyltin diacetate sulphamic acid (300 ppm)" fentin acetate"

191 Tests of chemicals against potato scab Organo-phosphorus compounds

diethyl phthalimidophosphonothioate

edifenphos triamiphos (6 ppm) *

(DOW M2452)

Dithiocarbamates mancozeb maneb thiram zineb

I -chloro-2,4-dinitrobenzene 2,4-dinitrophenyl thiocyanate dinocap sultropen

z,+Dinitro compounds

Chlorinated nitrobenzenes 3,4-dichloronitrobenzene* 2,6-dichloro-4-nitrophenol* dicloran" 2,3,5,6-tetrachlorodinitrobenzene 2,3,5,6-tetrachloro-4-nitroaniline 2,3,5,6-tetrachloro-~-nitroanisole

Other chlorinate,d aromatic compounds chloroneb" chlorthal-methyl" ~,4-dichlorobenzyl akohol dichlorophen hexachlorobenzene hexachlorophene pentachloroaniline ~,~,~,~,6-pentachlorobenzyl alcohol 2,3,5,6-tetrachloroaniline 2,3 ,5,6-tetrachloroanisoleX I ,2,3,4-tetrachlorobenzene" 2,3,5,6-tetrachlorophenol* tetrachlorophthalic anhydride tetrachloroisophthalonitrile (proposed

common name : chlorothalonil)

Phthalimides captan N-(4-chlorophenylthiomethyl)phthalimide

(Bayer 66109; proposed common name: thiochlorfenphim)

folpet tetrachlorophthalimide

Pyridines 2-bromopyridine 2-chloro-6-trichloroniethylpyridine (N-Serve ;

z,6-dichloropyridine 3,5-dichloropyridine 4-nitropyridine- I -oxide pyridinitril

13 ppm)

Other heterocyclic compounds (N only) benomyl 4-t-butyl-7-decylamino-3,4,5,6-tetrahydro-

zH-azepine-chlorohydrate (Geigy I 6306) dimethirimol (I 3 ppm) 0

I ,4-di-(z,2,z-trichloro- I -formamidoethyl)- piperazine (Cela W 524; proposed common name : triforine)

ethoxyquin glyodin 8-hydroxyquinoline sulphate mecarbinzid triarimol (5 pprn)"

Other heterocyclic compounds (0, S, ON, OS, SN)

carboxin (10 ppm)" dithianon dodemorph benzoate (10 ppm) drazoxolon " griseofulvin oxycarboxin (10 ppm)" quinomethionate thiabendazole tridemorph (10 pprn)"

Other N and NS compounds benquinox isobutylamine s-butylamine dichlofluanid I -(3 ,~+-dichloroanilino)-~ -forrnylamino-2,~,2-

trichloroethane (Imugan; proposed common name : chloraniformethan)

phonate (Dexon)" p-dimethylaminobenzenediazo sodium sul-

dodine m-ethionine" guanoctine sesquisulphate mebenil (10 ppm) DL-methionine I -phenylthiosemicarbazide phenylthiourea" quinazamid tetradifon tetrasul thiophanate thiophanate-methyl

Miscellaneous compounds 2-bromofluorene caffeic acid cinnamic acid

A. H. MCINTOSH

Table I. Relative amounts of scab on Majestic tubers af1.r foliage spraying or soil treatment

Compound

Xi1 DL-Methionine 2-Pyridinethiol-I -oxide,

Na Salt Cela W 524 (triforine) m-Ethionine Quintozene

NO. of r

Route Rate tests

4 - -

Spray 0'2 yo I Spray 0'2 "/o 2

Spray 0.025 y* I

Spray 0'2 yo 2

Soil 5 0 PPm 4

Scab incidence

% P" -Ap--.-

- I 0 0 109 N.S. 103 N.S.

91 N.S. 69 0.03 41 < 0'001

* Difference from scab incidence in nil treatment was either not significant (N.S.) or significant at value of P shown.

Table 2. Relative yields and amounts of scab on Majestic tubers after soil treatments at 50 ppm or the rates shown (Great Hill Bottom soil on&)

Treatment

Nil Dichlone Chloranil z,3 4, j-Tetrachloronitrobenzene Furcarbanilf PH jc-829 Fluorene Pentachloropyridine a-Pyridinethiol-I -oxide, Na salt Dinocap phenols11 Quintozene Tecnazene Binapacryl ~,~,~,6-Tetrachloro-~-nitrophenol Captafol Dinobuton, 50 ppm Dinobuton, 25 ppm Dinobuton, 12.5 ppm

NO. of (--

tests

28

4 2 2 2 2 2

4 4 5 28

7 2 2

S 2 I I

Yield

% I 0 0 I 0 0

109 91 88

124 65 73

I 08 87 93 81 46 98 94 67 80 98

P" -

N.S. N.S. N.S. N.S. < 0'01 < 0'001 < 0'001 N.S. N 0'001 < 0'001 < 0'001

< 0'001 N.S. N.S. < 0'001 - 0'01 N.S.

Scab incidence?

I 0 0

73 61

5 3 5 0 47 46 43 38 32 28 26 23

58

22 I 0 26 59

" Difference from yield in nil treatment was either not significant (N.S.) or significant at value of

t All treatments decreased scab incidence at P < 0.001. f Furcarbanil is the proposed common name for z,5-dimethyl-3-furanilide, coded by BASF United

9 PH 50-82 is the Philips-Duphar code for (2,4,5-trichlorophenyl) sulphonylmethyl thiocyanate

1 1 Coded by Murphy Chemical as MC281o; sometimes known as DNOP (Kirby, Frick & Gratwick,

P shown.

Kingdom as BAS 3191 F (Pommer, 1971).

(Foursoff, Elings & Meijer, 1969).

1966).

Table 2 lists the chemicals which decreased scab incidence at P < 0.001 in the combined number of tests shown, in Great Hill Bottom soil. Some of the figures differ slightly from those given in a previous summary (A. H. McIntosh, 1971 b) because of re-testing.

From the results of a few tests in Schoolfield soil (not shown in Table 2) it was

Tests af chemicals against potato scab I93 clear that this gave as much scab in the nil treatments as Great Hill Bottom soil. How- ever, the scab in Schoolfield soil was not so effectively controlled by some of the chemicals. Thus quintozene (in three out of eight tests), chloranil and dinocap phenols (two tests each), furcarbanil, PH 5*82 and fluorene (one test each) failed to decrease scab significantly ; but pentachloropyridine and captafol (one test each) were as effec- tive in Schoolfield as in Great Hill Bottom soil. None of the other chemicals listed in Table 2 was tested in Schoolfield soil.

D I S C U S S I O N

Tests by spraying foliage With the exception of DL-methionine, which was included for comparison with

DL-ethionine, the chemicals used in the spraying tests are reported to have downward- moving systemic action in plants (Kaars Sijpesteijn, Rombouts, van Andel & Dekker, 1958; Zentmyer, Moje & Mircetich, 1962; Ost, von Bruchhausen & Drandarevski, 1971). However, none of them except DL-ethionine had any action on scab even after thorough and repeated spraying. DL-Ethionine was ineffective at 50 ppm in soil treatment tests (p. 191)~ and its slight action in the spraying tests is probably unimpor- tant. In similar tests, Labruykre (1971) found that spraying with streptomycin had no effect on scab.

Tests by soil treatment Most of the chemicals were tested because of their known general fungicidal or

bactericidal action, because of their ability to inhibit S. scabies in vitro (Schaal & Johnson, 1955) or because they were chemically related to such compounds. However, two chemicals, sulphamic acid and 2-chloro-6-trichloromethylpyridine (N-Serve), were suggested by two indirect methods for controlling scab. The first of these methods, the application of large amounts of sulphur to the soil, brings the pH down to 5 - 5 or less (Dippenaar, 1933; Labruyhre, 1971), which is unfavourable for the in- fection of plants by ordinary strains of S. scabies. Sulphamic acid also lowers soil pH, but it was too harmful to the plants to be of any practical value. In one glasshouse test, sulphamic acid at 300ppm decreased the pH of the potting soil from 7.0 to 6.6, but also decreased yield by 60%, with only a slight effect on scab. In the second indirect method (Huber & Watson, 1970)~ application of N-Serve to soil at about 0.5 ppm gave some control of scab in the field; but in one glasshouse test, N-Serve at 5 and 13 ppm of' soil had no effect on scab.

Under field conditions a slight decrease in yield, as is commonly found after using quintozene as soil treatment (de Bruin & Heuver, 1971; Labruyhe, 1971; A. H. McIntosh, I ~ ~ I U , 1972) may be acceptable if scab control is good enough, but ob- viously no treatment that seriously decreases yield has any chance of practical success.

In the glasshouse tests, quintozene gave a 7 yo decrease in yield (Table z), but this was significant in only six out of the twenty-eight single tests. It can be assumed that chemicals giving yield decreases with values of P as low as 0.001 even in only a few tests are unlikely to be acceptable for field use, viz. fluorene, pentachloropyridine, dinocap phenols, tecnazene and binapacryl. Dinobuton, although the most effective

I94 A. H. MCINTOSH chemical, was one of the most damaging, decreasing yield by about one-third at 50 ppm; lower rates were less damaging but less effective.

The other chemicals in Table 2 did not significantly decrease yield. However, 2,3,4,5- tetrachloronitrobenzene and 2,~,~,6-tetrachloro-~-nitrophenol are closely related to quintozene and tecnazene, and may also be carcinogenic (Searle, 1966). Dichlone, chloranil, furcarbanil, PH 50-82 and a-pyridinethiol- oxide were probably not effective enough to be considered for practical control.

Captafol was outstanding in controlling scab in Great Hill Bottom soil, and seemed to be as effective in Schoolfield soil ; it was at least as effective as quintozene, and did not decrease yield. It thus contrasted with the other very closely related phthalimides captan and folpet, neither of which had any effect on scab. Captafol may be intrin- sically more toxic to S. scabies than the others, or be more stable or available in soil, or some combination of these. The stability and availability in soil of a fungitoxic chemical, or of its fungitoxic derivatives, may be at least as important for successful disease control as the intrinsic fungitoxicity; if the action is fungistatic rather than fungitoxic, stability may be decisive. The mode of action of captafol on S. scabies and its relative stability in soil are certainly not clear. Etter (1966) showed that it may be either fungistatic or fungicidal to several species of fungi. Captafol, as measured by GLC, was much less stable in soil than captan (Foschi, et al. 1970), but was more persistent as measured by bioassay (D. L. McIntosh, 1971). These two findings may not be incompatible: the soils were different, and in the experiments of Foschi et al. captafol may have been converted chemically or microbiologically to a fungitoxic derivative which did not analyse as captafol.

Whatever the reason for the difference between captafol and the other phthalimides, the relative rates of inactivation of chemicals must vary from soil to soil; similarly, different strains of S. scabiesfound in different soils (Labruykre, 1971) may vary in their susceptibility to different chemicals. This could explain why some chemicals were less effective in Schoolfield soil than in Great Hill Bottom soil. Captafol, which was very effective in Woburn soil, may not be the best in others.

With these reservations, captafol seems from glasshouse tests to be the most acceptable substitute for quintozene for control of potato scab by soil treatment. It also gave good control in field trials at Woburn (A. H. McIntosh, 1971 a, 1972); these will be reported later.

I thank Miss A. L. Gathergood and Mrs I. Covell for their help and Dr A. H. M. Kirby and the following firms for providing materials: Albright & Wilson, BASF United Kingdom, Bayer Agrochem, Boots Pure Drug, Cela, Ciba-Geigy (U.K.), Cyanamid of Great Britain, Dow Chemical (U.K.), Duphar-Midox, Du Pont (United Kingdom), Farm Protection, Hickson & Welch, Hoechst Chemicals, ICI (Mond Division), Lilly Research Centre, May & Baker, Merck, Merck Sharpe & Dohrne, Monsanto Chemicals, Murphy Chemical, Olin Chemicals, Pan Britannica Industries, Philips-Duphar, Plant Protection, Uniroyal.

I95 Tests of chemicals against potato scab

R E F E R E N C E S

DE BRUIN, T. E. J. & HEUVER, M. (1971). Quintozeen (PCNB) voor de bestrijding van gewone schurft en rhizoctonia bij de teelt van pootaardappelen. Bedrijfsontwikkeling (Akkerbouw ed.) 2, 41-46.

DIPPENAAR, B. J. (1933). Environmental and control studies of the common scab disease of potatoes caused by Actinomyces scabies (Thaxt.) Guss. Sci. Bull. Dep. Agric. For. Un . S. Afr . no. 136.

ETTER, G. E. (1966). Difolatan-nouveau fongicide organique de synthilse. Meded. Landb- Hoogesch. OpzoekStns Gent 31, 837-857.

F0RSUND, E. (197 I). Streptomyces scabies and Spongospora subterranea observed on potatoes grown in different types of soil irrigated at different levels and times. Potato Res. 14, 334-335.

FOSCHI, S., CESARI, A., PONTI, I., BENTIVOGLI, P. G. & BENCIVELLI, A. (1970). Indagine sulla degradazione e movimento verticale dei fitofarmaci nel terreno. Not i z . Mal. Piante 82-83, 37-49.

FOURSOFF, P. I., EL~NGS, H. & MEIJER, G. (1969). PH 50-82, a new fungicide. Proc. 5th Br. Insectic. Fungic. Conf. 3, 737-741.

HUBER, D. M. & WATSON, R. D. (1970). Effect of organic amendment on soil-borne plant pathogens. Phytopathology 60, 22-26.

KAARS SIJPESTEIJN, A, ROMBOUTS, J. E., VAN ANDEL, 0. M. & DEKKER, J. (1958). Investigations on the activity of pyridine-2-thiol-N-oxide as a systemic fungicide. Meded. LandbHoogesch. OpzoekStns Gent 23, 824-830.

KIRBY, A. H. M., FRICK, E. L. & GRATWICK, M. (1966). Greenhouse evaluation of chemicals for control of powdery mildews. VI. Dinitro-s-alkylphenols: The ' dinocap ' misconception. Ann. appl. Biol. 57, 211-221.

LABRUY~RE, R. E. (1971). Common scab and its control in seed-potato crops. Z'asl. landbouwk. Onderz. Ned. (Agric. Res. Rep.) no. 767.

LAPWOOD, D. H. & DYSON, P. W. (1966). An effect of nitrogen on the formation of potato tubers and the incidence of common scab (Streptomyces scabies). PI. Path. 15, 9-14.

LAPWOOD, D. H., WELLINGS, L. W. & ROSSER, W. R. (1970). The control of common scab of potatoes by irrigation. Ann. appl. Biol. 66, 397-405.

LARGE, E. C. & HONEY, J. K. (1955). Survey of common scab of potatoes in Great Britain, 1952 and 1953. Pl . Path. 4, 1-8.

MCCREARY, C. W. R. (1967). The effect of sulphur application to the soil in the control of some tuber diseases. Proc. 4th Br. Insectic. Fungic. Conf. I, 303-308.

MCGREGOR, A. J. & WILSON, G. C. S. (1966). The influence of manganese on the development of potato scab. PI. Soil 25, 3-16.

MCINTOSH, A. H. (1970). A glasshouse method for testing chemicals for control of potato common scab. Potato Res. 13, 241-247.

MCINTOSH, A. H. (1g71a). Rep. Rothamsted exp. S tn for 1970, Part I , 181-182. MCINTOSH, A. H. (1971 b). Control of common scab of potato, Proc. 6 th Br . Insectic. Fungic.

MCINTOSH, A. H. (1972). Rep. Rothamsted exp. S tn for 1971, Part I , 199-200. MCINTOSH, D. L. (1971). Dilution plates used to evaluate initial and residual toxicity of fungi-

cides in soils to zoospores of Phytophthora cactorum, the cause of crown rot of apple trees. Pl. Dis. Reptr 55, 213-216.

OST, W., VONBRUCHHAUSEN, V. & DRANDAREVSKI, C. (1971). Transport of the systemic fungicide Cela W 524 in barley plants (Part I). Pestic. Sci. 2, 219-224.

POMMER, E. H. (1971). The systemic efficacy of a new furane carbonic acid anilide fungicide (BAS 3191 F). Abstr. 2nd int. Congr. Pestic. Chem., Israel p. I I I .

SCHAAL, L. A. & JOHNSON, G. (1955). The inhibitory effect of phenolic compounds on the growth of Streptomyces scabies as related to the mechanism of scab resistance. Phytopathology 45,626-628.

SEARLE, C . E. (1966). Tumor initiatory activity of some chloromononitrobenzenes and other compounds. Cancer Res. 26, 12-17.

Conf. I , 215-218.

196 A. H. MCINTOSH VRUGGINK, H. (1970). The effect of chitin amendment on actinomycetes in soil and on the

WOOD, R. K. S. & TVEIT, M. (1955). Control of plant diseases by use of antagonistic organisms.

ZENTMYER, G. A., MOJE, W. & MIRCETICH, S. M. (1962). Ethionine as a chemotherapeutant.

infection of potato tubers by Streptomyces scabies. Neth. J . PI. Path. 76, 293-295.

Bot. Rev. 21, 441-492.

Phytopathology 52, 34.