Potato Res. 21 (1978) 9-14

Pathogenicity of Rhizoctonia solani Kfihn to potato leaflets

DOROTHY SPENCER and R. A. FOX

Scottish Horticultural Research Institute. Invergowrie, Dundee. DD2 5DA Scotland

Accepted for publication : 13 September 1977

ZusammeJ!/~tssung, R~;sum(; p. 13

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Summary

The extent of invasion of potato leaflets by Rhizoctonia solani Kfihn was related to the age of the plants sampled, their growing conditions and the cultivar being tested, but not to the sources of the fungal isolates. No correlation was found between behaviour of isolates on leaflets and their characteristics in vitro.

Introduction

Rhizoctonia solani Ktihn usually infects the underground parts of the potato plants where the results of pathogenicity tests are often variable and may require large numbers of replicates to establish treatment or host pathogen interaction effects. Hadley & Harvais (1964) found, in general, that isolates of R. solani from potato caused most infection on detached potato leaflets while those from other hosts caused little or none. They thought that their leaflet test might form a basis for assessing the pathogenicity of isolates from potato. This paper describes experiments devised to examine their suggestion and at tempts to relate the results to characteristics of the isolates used.

Materials and methods

Isolates and their culture

Isolates of R. sohmi were obtained from sclerotia on potato tubers sampled from stocks of over forty cultivars grown in a wide range of soil types in Scotland. Three isolates were obtained from lesions on potato sprouts and two from wheat stems. All isolates were grown on potato dextrose agar (PDA) in 90 mm petri dishes until the colony almost covered the plate. Agar discs (5 ram) were then cut from just behind the advancing edge of the colony and used to inoculate plates of tap water agar (TWA).

D O R O T H Y S P E N C E R A N D R O N A L D A. F O X

Leaflet source The cultivar Majestic was used unless otherwise stated. Leaves were removed from field or greenhouse-grown plants about 250 mm below the terminal bud and the two penultimate leaflets detached.

Pathogenicity tests Isolates were grown on TWA plates for three days at 22~ Leaflets were soaked in tap water for six hours, rinsed in sterile water and placed singly with the lower epidermis on a culture of an isolate, which was then incubated for a further seven days on the laboratory bench at about 16~ in diffuse daylight. The virulence of each isolate was expressed as the percentage of leaflet tissue discoloured (invaded), as measured with a planimeter. Replicate plates generally gave comparable results. Check leaflets on uninoculated plates were usually undamaged although, rarely, colonies of Phytophthora infestans (Mont.) de Bary or Boto,tis cinerea Pers. ex Fr. developed on them.

Characteristics o / isolates Each isolate was characterised by the kind (brown to jet black), quantity (nil to many), and distribution (concentrated, concentric or scattered) of sclerotia produced in vitro, and the cultivar from which it had been obtained. The growth rate of the twelve isolates most frequently used was assessed on PDA at 10, 15 and 22~ by measuring the increase in two diameters at right angles, on five replicate plates, 48 and 72 hours after inoculation.

A nalysis Experiments were laid out as replicated randomized blocks. Data in the tables are shown as percentage values which were transformed before analysis ; thus the standard error values used are not relevant to the tables. Significance levels are for P < 0.05.

Results

Leq/Tet source Preliminary tests indicated that three isolates from sclerotia on tubers, No 42, 57 and 63, were respectively weakly, moderately, and highly pathogenic to leaflets from greenhouse-grown plants. They were tested on leaflets at a similar stage of develop- ment but from plants grown in two different environments, the greenhouse and the field (Table 1 ). All isolates differed significantly from each other in their reaction to each type of leaflet but only one, No 63, behaved similarly in both tests. Since the results could apparently be affected by the environment in which the leaflets were grown, further experiments were limited to leaflets from field-grown plants, chosen as being more relevant to a pathogenicity test likely to be of practical value.

1 0 Potato Res. 21 (1978)

PATHOGENICITY OF RHIZOCTONIA SOLANI TO POTATO LEAFLETS

Table 1. Percentage of potato leaflet tissue from two sources invaded by three isolates of R. solani.

Isolatet Source 2

greenhouse 3 field 4

63 100.0 100.0 57 77.9 41.6 42 34.0 84.5

XLwlate Souche," 2Herktmlt OrLgine: 3Glashaus - Serre." "~Feld- Champ

Tabelle 1. Prozent Kartoffelblattgewebe yon zwei Herktinften, befallen dutch drei Isolate von R. solani. Tableau 1. Taux d'envahissement par 3 souches de R. salani des tissus de folioles de pomme de terre provenant.de 2 origines diff6rentes.

Table 2. Percentage of potato leaflet tissue from plants of three ages invaded by three isolates of R. solani.

Isolate t Sampling date-'

21 July 25 August 6 September

63 100.0 100.0 100.0 57 41.6 100.0 92.3 42 84.5 77.4 19.4

I L~'olat Saache, 2 Datum der Musterentnahme Date ~Ft;chantilhmage

Tabelle 2. Prozent Kartoffelblattgewebe yon Pflanzen dreier Alterstufen, befallen durch drei Isolate von R. solani. Tableau 2. Taux d'envahissement par 3 souches de R. salani des tissus de folioles de pomme de terre provenant de plantes prelevees il 3 dates diff~rentes.

Age of plant The susceptibility of leaflets was examined on three occasions in the growing season, using the same three isolates. The results, given in Table 2, show significant interac- tions, No 63 consistently giving 100",, infection wheras No 42 gave decreasing, and No 57 increasing leaflet infection as the plants aged.

In the following year nine isolates from sclerotia and three from sprouts were tested at weekly intervals from the beginning of July to the end of August. An isolate from a sprout, No 279, invaded 100"., of the tissue at every sampling date. Among the other eleven isolates, the percentage of tissue invaded in late July usually increased, but not significantly, and there was no clear pattern of relative pathogenicity over the whole period. The behaviour of isolates from sprouts and those from tubers did not differ significantly in any test.

Potato Res. 21 (1978) 1 1

DOROTHY SPENCER aND RONALO A. FOX

Table 3. Percentage of potato leaflet tissue of five cultivars invaded by three isolates of R. solani.

Cultivar ~ Isolate 2

63 57 42 mcan

Record 100.0 80.6 89.7 90.1 Pentland Envoy 78.9 90.6 86.5 85.3 King Edward t 00.0 48. I 65.5 71.2 Majestic 1000 88.3 22.9 64.5 Golden Wonder 64.6 23.0 48.4 45.3

Mean 3 88.7 66.1 62.6

1Sorte - l,'ariOtO : 2 Isohtt Souche ." 3 Mit te l - M o y e m w

Tabelle 3. Prozent Kartoffelblattgewebe von ffinf Soften. befallen durch drei Isolate von R. solani. Tableau 3. Taux d'envahissement par 3 souches de R. sohmi des tissus de folioles de pomme de terre provenant de 5 vari6tds,

Cl t ] t i var SOIIFCe

Five potato cultivars were tested for their reaction to the three isolates used in the preliminary test, Nos 42. 57 and 63. The cultivar means shown in Table 3 differed significantly; Record and Pentland Envoy were the most and Golden Wonder the least susceptible. All interactions between cultivars and isolates differed significantly except for Majestic, Record, and King Edward in their response to isolate No 63 and Pentland Envoy and Majestic in their response to isolate No 57.

I s o l a t e s o u r c e

On one sampling date in mid-August, 89 isolates were tested, 86 from sclerotia on tubers, 1 from a potato sprout, and 2 from wheat stems. Leaflets showed a continuous range of infection from mild to very severe..Isolates could be assigned to 12 over- lapping sets by Duncan's Multiple Range Test (Duncan, 1955) but there was no evidence of distinct groups of isolates differing significantly from all other groups.

C o r r e & t i o n o [ r e s u l t s w i t h c h a r a c t e r i s t i c s o / i s o l a t e s

Two isolates had contrasting features; No 42, from a sclerotium, grew slowly at all temperatures but invaded leaflets well whereas No 57, also from a sclerotium, grew quickly but had low to moderate infectivity. The remaining ten isolates, which included three from sprout tissues, showed neither contrasts nor equalities in growth and pathogenicity at any of the three temperatures.

No correlations were found between isolate behaviour on leaflets and the kind, quantity, and distribution of sclerotia produced in vitro, and the cultivar from which they had been obtained.

12 Potato Res. 21 (1978)

PATHOGENICITY OF RHIZOCTONIA SOLANI TO POTATO LEAFLETS

Discussion

The extent of leaflet invas ion was affected by both the growing condi t ions and the age of the plant and whereas the former are amenab le to control in, say, a glasshouse the latter is no t ; moreover , there was evident difficulty in assigning an absolute age to any leaflet. The var ia t ions between cult ivars were not entirely unexpected as the leaflets clearly differed, at least morphological ly . The extent to which leaflets were

invaded was not de te rmined by the source of an isolate, those from sclerotia not differing markedly from those from pota to sprouts and both groups, in turn, varying little from cereal isolates.

Thus the leaflet test, a l though simple and quick, does not reveal differences in pathogenici ty a m o n g isolates of R. solani from different pota to tissues nor between isolates from potatoes and those from cereals.

Several corre la t ions were a t tempted between in vitro characterist ics of isolates and their behav iour on leaflets. Only two isolates showed a growth rate which had a positive, but inverse, re la t ionship to their pathogenici ty , as has been reported pre- viously by Akai et al. (1960) for R. solani isolates on cucumber seedlings. No correla- t ions were found between pathogenici ty and any other simple characteristic, thus conf i rming work of m a n y other authors such as Shephard & W o o d (1963).

Acknowledgment

The work reported here was suppor ted by a grant from the Potato Marke t ing Board.

Zusammenfassung

Pathogenitdt yon Rhi=octonia solani Ki'thn at t / Kart~fft, lhldtt('hen

Die An~illigkeit yon gepfltickten FiederbRittern der Kartoffel fiir Infektionen durch Rhizoctonia solaui wurde in Versuchen dazu beniitzt, Unter- schiede in der Pathogenit/it zwischen lsolaten festzustellen, die entweder yon Sklerotien von Kartoffelknollen aus dem Feldanbau. von Lii- sionen auf Kartoffelkeimen oder yon Getreide- halmen gewonnen wurden. In diesem Artikel ist der Zusammenhang zwischen dem Blattbefall und der Herkunft und den Eigenschaften der Isolate beschrieben.

Bl~itter der Testpflanze wurden bis ca. 250 mm unter der Endknospe entfernt. Die beiden vor- letzten Fiederbl,ittchen wurden gepfl0ckt, wiih- rend sechs Stunden in Leitungswasser einge- taucht und dann mit sterilem Wasser abgesp/.ilt. Die Blfittchen wurden dann einzeln mit ihrer Unterseite auf eine Kultur des Versuchsiso[ats,

das auf Wasser-Agar in 90 mm Petrischalen gedieh, gelegt. Die Virulenz jedes Isolates wurde sieben Tage nach der lnkubation bei ca. 16~ unter diffusem Tageslicht in Prozent des ver- 9,irbten Gewebes gemessen, Das Ausmass des Befalls der Blfitter schwankte je nach Sorte (Tabelle 3), Wachstumsbedingungen (Tabelle 1) und Alter der Pflanze bei der Musterentnahme (Tabelle 2). aber nicht nach der Herkunft der Isolate.

Zwischen dem Verhalten der Isolate auf den Bl.~ittern und ihrer Wachstumsrate in der Kultur. der Art (braun bis peckschwarz), der H~iufigkeit (null bis viel) und der Verteilung (geballt. konzentrisch oder zerstreut) der in vitro oder aufden Sorten, yon denen die Isolate gewonnen wurden, erzeugten Sklerotien wurde keine Kor- relation gefunden.

Potato Res. 21 [ 1978) 13

D O R O T H Y S P E N C E R A N D R O N A L I ) A. FOX

R6sum6

PathogdnicitO de Rhizoctonia solani Kiihn surJblioles de pomme de terre

La sensibilit~ des folioles de pomme de terre ",i une inoculation par Rhizoctonia solani a 6t6 utilis6e dans le but de d6terminer des diff6rences de pathog6nicit6 entre des souches obtenues /t partir de scl6rotes pr61ev6es sur tubercules, de I+sions sur germes, ou de tiges de bl&

Cette publication concerne la relation existant entre le taux d' infestation des folioles, l 'origine et les caract~ristiques des souches.

Les feuilles sont pr6lev6es sur des plantes test environ 250 mm en dessous du bourgeon termi- nal, Les deux avant derni~res folioles ont +t6 d6tach6es, tremp6es dans l 'eau du robinet durant 6 heures et ensuite rinc6es dans de l 'eau st6rile.

Les folioles sont plac6es s+par6ment avec leur 6piderme inf6rieur sur une culture de la souche test poussant en boite de p6tri de 90 mm sur milieu eau du robinet-g61ose,

La virulence de chaque souche est exprim6e en pourcentage de tissus ddcolor6s, mesur6 apr+s 7 jours d ' incubat ion fi 16~ en lumi6re du jour diffuse.

L'extension de la maladie sur les folioles varie en fonction de la varlet6 (tableau 3); des con- ditions de croissance (,tableau 1); de l'/ige de la plante et de l 'echantil lon (tableau 2), mais non pas en fonction de Forigine des souches.

Aucune corr61ation n 'a 6t6 trouv6e que ce soit entre le comportement des souches sur les folioles et leur taux de croissance en culture; la nature (brun fi noir de jais), la quantit6 (zero ',i beaucoup) et la distribution (concentr6, con- centrique ou disperse) des scl6rotes produites in vitro; ou la vari+t~ fi partir de laquelle les souches ont 6t6 obtenues.

References

Akai, S., H. Ogura & T. Sato, 1960. Studies on Pellicullaria./Hamentosa (Pat.) Rogers. I. On the relation between the pathogenicity and some characters on culture media. Am~. phytopath. Soc., Japan 25: 125-130.

Duncan, D. B., 1955. Multiple range and multiple F tests. Biometrics 11 : 1-42. Hadley, G. & G. Harvais, 1964. A potato leaf test for pathogenicity of Rhizoctonia solani. Eur.

Potato J. 7: 72-73. Shephard, M. C. & R. K, S. Wood, 1963. The effect of environment, and nutri t ion of pathogen and

host, in the damping off of seedlings by Rhizoctonia solani. Ann. appl. Biol. 51 : 389-402.

14 Potato Res. 21 (1978.)