Potato Res. 13 (1970) 241-247

A glasshouse method for testing chemicals for control of potato common scab

A. H. M c I N T O S H

Rothamsted Experimental Station, Harpenden~ Herts, England

Accepted for publication: 14 May 1970

Zusannnenfassung, Rdsumd p. 245

Summary

Plants of scab-susceptible varieties (Majestic or Marls Piper) were grown in pots in the glasshouse in field soil infested with S. scabies, with or without added chemicaPs; the stolons were confined to the top 4 cm of soil by a buried layer of 'Terylene' net and the plants, when well rooted, were watered from saucers only. Tubers became heavily scabbed in untreated soil.

Results from comparative glasshouse tests and field trials of seven chemicals agreed well on both scab control and yield. The glasshouse method was quick (8 weeks or possibly less) and could be used throughout most of the year.

Introduction

Several glasshouse methods have been devised for infecting pota to tubers with Streptomyces scabies (Thaxt.) Waksman & Henrici, the cause of c o m m o n scab. Most were designed for studying the infectivity of strains of the organism, or the resistance of po ta to varieties to them. In some, plants were grown in rather artificial media such as vermiculite or sand, or in mixtures of these with soil, but not in soil itself (Taylor and Decker, 1947; Dijkstra, 1956; McKee, 1968; Vruggink and Maat, 1968). The 'little po ta to ' method (Noll, 1968), designed for testing scab-control chemicals, is limited to certain varieties.

This paper describes a simple glasshouse method using plants growing in ordinary i~.".:~ted field soil, with or without added chemicals, under the conditions needed for inf:.ction o f tubers by scab. It is closest to those of Taylor and Decker (1947) and Labruy~re (Vruggink and Maat , 1968).

The conditions are that stolons of a susceptible variety should swell, to become small tubers, in warm dry infested soil of pH > 5.5 (Dippenaar, 1933; Lapwood, 1966). Most of these conditions are satisfied by growing plants of a suitable variety in a glasshouse, heated if necessary, in soil from a field infested with S. scabies. The remaining difficulty with pot ted plants is in keeping the soil dry without the plants wilting. However, this can be done by confining the stolons to dry surface soil, the roots being allowed to reach moister soil at the bot tom.

The reliability of the method was assessed by compar ing results f rom glasshouse

241

A. H. M C l N T O S H

tests of seven chemicals with those from field trials of the same chemicals, in two seasons.

Methods

Glasshouse t e s t s

Sandy loam (pH "~ 7) was taken from the top 10 cm of an infested field at Woburn, Bedfordshire, thoroughly air-dried, sieved, mixed and stored till needed.

Small shoots, obtained from tubers of the scab-susceptible varieties M a j e s t i c or M a r i s P i p e r (cf. Mcrntosh and Eveling, 1965), were planted in potting compost in seed boxes for 1-2 weeks, so that the roots grew to 7 cm or more.

Inoculum, for boosting the natural infectivity of the field soil, was made by drying the contents of liquid shake-cultures (Vruggink and Maat, 1968) on to field soil, using about 150 ml/kg of soil.

Booster inoculum (about 50 g/kg) and test chemical (usually 50 ppm) were thor- oughly mixed with field soil in a food-mixer. Clay pots (12 cm diam.) were filled with three layers of soil of equal depth. The bottom layer was potting compost and the middle layer treated field soil; a circle of 'Terylene' net (0.5 mm mesh), large enough to reach up the sides of the pot to the soil surface, separated the middle layer from the top layer, which was also of treated field soil. One rooted shoot was transplanted into the top layer of each pot (Fig. 1).

The pots were suitably randomized in a glasshouse or growth room, with minimum temperature 20~ (day) and 15~ (night). They were watered freely for the first 10 days, but after that were given only 50 ml per pot per day, from saucers; after another 10 days, 50 ml of Hoagland's solution was substituted for water three times a week. During very hot weather extra water was given as necessary.

The tubers, harvested 8-10 weeks after potting, were weighed and graded for scab infection (Large and Honey, 1955; Lapwood and Dyson, 1966) to give yield and mean 'scab index' per pot. Results, from twenty pots per treatment, were assessed by

242

Fig. 1. Rooted potato shoot just after transplanting

Net - Netz - Filet Treated field soil - Behandelte Ackererde- Sol de champ traitd Potting compost - Kompost fiir Topfversuche - Compost de pot

Abb. 1. Bewurzelter Kartoffeltrieb unmittelbar nach dem Umtopfen. Fig. 1. Un germe de pomme de terre bien enrachtd, im- mddiatement aprbs plantation.

Potato Res. 13 (1970)

T E S T I N G C H E M I C A L S FOR C O N T R O L OF P O T A T O C O M M O N S C A B

standard analysis of variance, giving mean yields, scab indices and errors per treat- ment.

Field trials These were done at Woburn in 1968-9 in the field from which soil was taken for glasshouse tests. In each trial there were four or five plots, of minimum size 4 rows x 20 ft (6 m), per treatment. Chemicals were broadcast as dusts, and the plots were rotavated immediately afterwards and planted the same day. After harvest, scab indices were calculated, in the same way as in the glasshouse tests, f rom samples of fifty ware tubers from the middle two rows of each plot (Mc[ntosh, 1969, 1970).

Results and discussion

In the glasshouse tests, the roots quickly penetrated the net to reach the moist nutrient mixture at the bot tom of the pots, but the stolons did not; tubers were formed in the dry soil near the surface and, in untreated soil, were nearly always severely infected by scab. They were often, but not always, most thickly scabbed on the parts that lay closest to the soil surface and to the rims of the pots, where the soil was driest. Adding booster inoculum to already-infested soil may not always be necessary. In one test, without added chemicals, the scab indices were: with booster 20, without 19. Nevertheless, the booster was included in routine tests as an insurance against poor infection. Yields were about 40 g of tubers per pot, but were smaller when the plants were grown from 15-month- old seed in December-January.

One advantage of the glasshouse method is that it uses unsterilized field soil. If vermiculite (Dijkstra, 1956), sand (Noll, 1968) or other artificial media were used instead, the natural biological decomposition of added chemicals might not occur, with the result that some of them might control scab in the glasshouse but not in the field.

Table 1 shows comparative figures for yield and scab from glasshouse and field. The close numerical correspondence between the scab indices, particularly in the

first glasshouse test and the 1968 field trial, was fortuitous. The dry June weather in both 1968 and 1969 ensured that, in both years, field tubers were at least as severely scabbed as glasshouse tubers; in wetter years, with less scab in the field, the corre- spondence between glasshouse and field would not have been so close.

However, the figures show that the glasshouse tests predicted reasonably well the relative effects of the chemicals in the field in dry years, when scab incidence was severe. Every significant increase or decrease in scab or, more surprisingly, yield in the glasshouse (P ~ 0.05) was matched by a corresponding significant effect in the field. (These effects included two which are perhaps not really relevant in a comparison of two methods for measuring scab control : quintozene increased yield at the high rates used in the first glasshouse test and the 1968 field trial, possibly by controlling Rhizoetonia solani as well as scab; and thiram gave an increase in scab in the second glasshouse test and the 1969 field trial, possibly by controlling organisms antagonistic to S. scabies).

Potato Res. 13 (1970) 243

A. H . M C I N T O S H

Table 1. Compar i son of glasshouse test and field trials of scab-control chemicals.

Glasshouse test z No I ; t reatments at 50 ppm of a.i.3 ; variety Maris Pipet"

Field trial 2 1968; treatments at 150 lb a.i./acre (168 kg/ha)'*; variety Maris Piper

Treatment s yield (g/pot) 6 scab index r yield (tons/acre) s+ scab index 7

Untreated 9 32 25 12.6 26 Quintozene 36* 2**** 14.2" 3**** Captan 30 19 12.8 23 ' M 245T ++ 29 19 10.4'** 19" Drazoxolon 28* 17 11.2' 13"***

Glasshouse test j No. 2; treatments at 35 ppm of a.i. ; variety Majestic

Field trial z 1969; treatments at 70 lb a . i / acre (78 kg/ha); variety Maris Pipet"

Treatment s yield (g/pot) 6 scab index 7 yield (tons/acre) s+ scab index ~

Untreated 9 44 24 18.7 31 Quintozene 40 9*** 19.3 19"** Folpet 45 19 19.9 34 Thiram 39 35** 19.8 39* Zineb 46 25 19.4 34

Glashattstest - Test en serre 2 Feldversuch - Essai en champ

3 Behandhmgen mit 50 ppm akt . St. - Traitements gt 50 ppm de m. act.

* Behamthtngen mit 150 Ib akt. St. (168 kg/ha) - Traitements h 150 Ib m.a./acre ( 168 kg/ha)

+ ] ton/acre = 2510 kg/ha

s V e l f a h r e n - Traitement 6 Ertrag (g/Topf) - Production (g. /pot) 7 Schol f index - hu lex de gale 8 Ertrag (Tomten/acre) - Production (tonnes/ acre) 9 Unbehandelt - Non traitd

+ + D o w Chemical Company ' s code number for 0,0-diethyl phthal imidophosphonothioate - Code- nttmnler der Dow Chemical Company f i ir O,O-di~thyl phtkalimidosphonothioate - Numdro de code

de la Compagnie Dow Chhnique poltr O,O-diethyl phthalimidophosphonothioate * Difference from untreated significant at P = 0.05 - Unterschied zu unbehandelt signifikant bei

P = 0,05 - Diffdrence significative pal" rapport all non traitdh P = 0,05 ** idem at P = 0,02

*** idem at P = 0.01 **** idem at P ~ 0.001

Tabelle 1. Vergleich von Glashaus- und Feldversuchen mit chemischen Pr~iparaten fiJr die Schorf- bek/impfung. Tableau 1. Compara ison des tests en serre et des essais en champ sur produits chimiques contre la gale.

244 Potato Res. 13 (1970)

T E S T I N G C H E M I C A L S FOP. C O N T R O L OF P O T A T O C O M M O N SCAB

The two methods disagreed in that some effects were found in the field, but not in the glasshouse. There may be several reasons for this. One is that the standard errors of the scab indices were always smaller in the field than in the glasshouse, so that significant differences were more easily found in the field (compare the indices for 'M2452" in the first glasshouse test and the 1968 field trial). The standard errors in the glasshouse could no doubt be decreased by increasing the replication, but it is un- likely that the activity of a chemical like quintozene would be missed in a test with twenty pots per treatment.

Lack of agreement between the methods may also be related to the difference between the more or less constant conditions in the glasshouse and the varying condit ions in the field. Thus, drazoxolon slightly damaged the plants, giving signi- ficant decreases in yield in both glasshouse and field. One effect of this may have been to delay the time of tuber formation, and if this happened to coincide with a change from dry to wet soil in the field, any direct control of scab by drazoxolon would have been exaggerated. The apparent control of scab by drazoxolon in the 1968 field trial may thus have been an artifact (cf. kapwood and Dyson, 1966, who found a similar effect with nitrogen). The failure of the glasshouse method to show a decrease in yield by 'M 2452", corresponding to that in the 1968 field trial, is un- explained.

The glasshouse method is reasonably quick (and could be made quicker by harvest- ing the tubers a little sooner); it seems to be reliable and fi'ee from an artifact which may occur in the field ; and it can be used throughout most of the year. Although it was designed for testing scab-control chemicals, it could as easily be used for measuring varietal resistance to scab.

Acknowledgments

[ thank Mrs G. M. Parkes, Miss A. k. Gathergood and Mrs I. Covell for their help; Dr D. H. Lapwood for much advice; and Plant Protection Ltd and the Dow Chemical C o m p a n y (U.K.) Ltd for providing materials.

Zusammenfassung

Eine methode zur Prii[ung yon chemischen Kartoffel-SchotJbek6mpJimgsmitteht hn

GewOchshaus

Fiir die lnfektion von Knollen durch Flach- schorf (Streptomyces scabies) sind nachstehende Bedingungert notwendig: die Stolonen einer an- f/illigen Sorte sollten, um kleine Knollen zu bilden, in warmem, trockenem, schorfbefallenem Boden mit pH .':- 5,5 anschwellen.

Die Anforderungen an eine Glashausmethode zur Priifung von chemischen Pr~iparaten fi.ir die SchorfbekSmpfung unter den obenerw/ihnten

Potato Res. 13 (1970)

Bedingungen bestehell in folgendem: Tont6pfe wurden mit drei gleich hohen Schichten Erde gefi.illt. Die unterste Schicht bestand arts ge- w6hnlichem Kompost fi.ir Topfversuche. AIs mittlere und obere Schicht. die durch die Ein- lage eines "Terylene-Netzes" voneinander ge- trennt wurden, verwendete maa einheitliche, nicht sterilisierte, mit Schorf infizierte Acker- erde, der die zu prtifenden Chemikalien in der

245

notwendigen Menge beigefiigt wurden (in der Regel 50 ppm). Ein einzelner, gut bewurzelter Trieb (Sorte Majestic oder Marls Piper) wurde in die oberste Schicht eines jeden Topfes ein- gepflanzt (Abb. 1). Die T6pfe (20 pro Verfah- ren) wurden wahllos in einem Glashaus bei 20~ (Tag) und 15~ (Nacht) aufgestellt. Sie wurden die ersten 10 Tage reichlich begossen, aber nachher bekamen sie nur noch 50 ml pro Topf und Tag in den Unterteller. Nach weiteren 10 Tagen wurde dreimal pro Woche das Wasser durch 50 ml Hoagland-L6sung ersetzt.

Die Wurzeln durchdrangen das Netz rasch, um die feuchte Nahrungsmischtmg am Boden zu erreichen, aber die Stolonen taten dies nicht; die Knollen wurden im trockenen Boden nahe der Oberfl/iche gebildet, und in tmbehandeltem Boden waren sie fast immer stark mit Schorf be- fallen. Bei der Ernte (8-10 Wochen nach dem

A. H. MC I N T O S H

Eintopfen) wurden die Knollen gewogen und auf SchorfbefalI beurteilt.

Ergebnisse mit sieben chemischen Pr~iparaten wurden mit solchen aus Feldversuchen in den Jahren 1968 und 1969 mit den gleichen Pr/ipara- ten verglichen, als die trockenen Sommer Ge- w~ihr boten, dass die Knollen auf dem Feld ungef/ihr ebenso stark schorfig waren wie jene im Glashaus. Die Ergebnisse beider Methoden stimmten bei der Schorfkontrolle gut iJberein und, was erstaunlicher war, auch in Bezug auf Einfluss der Chemikalien auf den Ertrag (Ta- belle 1). Jede signifikante Zu- oder Abnahme im Schorfbefall oder im Ertrag im Glashaus (P < 0,05) stimmte i.iberein mit einem entsprechenden signifikanten Einfluss auf dem Feld; dagegen traten nicht alle im Feldversuch festgestellten Schwankungen auch im Glashaus auf.

R~sum~

M~thode de serre pour tester les produits chimiques pour la lutte contre la gale commune de la p o m m e de terre

Les conditions n6cessaires pour l'infection des tubercules par la gale commune (Streptomyces scabies) sont le renflement des stolons d'une Vari6t6 susceptible pour devenir de petits tuber- cules, un sol chaud et sec infect6 de gale d 'un pH > 5,5.

Les principes d'une m6thode de serre pour tester les produits chimiques de lutte contre la gale, principes qui r6alisent les conditions ci-dessus, sont les suivants. Des pots d'argile sont remplis de trois couches de sol d'6gale 6paisseur. La couche inf6rieure consiste en compost de rempo- tage. Les couches moyenne et sup6rieure, qui sont s6par6es l 'une de I'autre par un filet circu- laire de 'Terylene', sont consitu6es de sol prove- nant d 'un champ uniform6ment infest6 et non st6rilis6, auquel les produits chimiques ~. tester sont ajout6s suivant les n6cessit6s (habituelle- ment / t 50 ppm). Un simple germe bien enracin6 (var. Majestic ou Maris Piper) est plant6 dans la couche sup6rieure de chaque pot (Fig. 1). Les pots (20 par traitement) sont plac6s au hasard dans une serre/t 20~ (jour) et 15 ~ (nuit). Les pots sont arros6s abondamment pendant les 10 premiers jours, mais, par apr,~s, ils reqoivent seulement 50 ml par jour, et par pot, dans la

soucoupe. Apr+s les I0 jours suivants, on sub- t i tue/t I'eau 50 ml de la solution de Hoagland et cela, trois lois par semaine.

Les racines traversent rapidement le filet pour atteindre le m~lange nutritif humide de la base, mais les stolons ne le traversent pas; les tuber- cules sont form6s dans le sol sec pr+s de la sur- face et, dans le sol non trait6, sont presque tou- jours s6v6rement infect6s de gale. A la r6colte (8-10 semaines apr6s la raise en pot) les tuber- cules sont pes6s et l'infection de gale appr~ci~e.

On a compar6 les r6sultats obtenus avec sept produits chimiques avec les r6sultats des m6mes produits obtenus dans les champs en 1968 et 1969, ann6es or) les conditions de s6cheresse faisaient que les tubercules des champs 6taient approximativement aussi s6v6rement atteints de gale que ceux des serres. Les r6sultats des deux m6thodes s 'accordent bien pour le con- tr61e de la gale et, ce qui est plus surprenant, pour les effects des produits chimiques sur la production (Tableau 1). Chaque augmentation ou diminution significative de gate ou de pro- duction en serre (P < 0,05) est 6gale h u n effet significatif correspondant darts le champ, quoique I'oppos6 ne spit pas toujours vrai.

246 Potato Res. 13 (1970)

TESTING CHEMICALS FOR CONTROL OF POTATO COMMON SCAB

References

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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.

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Vruggink, H. & Maat, D. Z., 1968. Serological recognition of Streptomyces species causing scab on potato tubers. Neth. J. PI. Path. 74: 35-43.

Potato Res. 13 (1970) 247