S U S C E P T I B I L I T Y O F P O T A T O V A R I E T I E S T O S K I N S P O T ( O O S P O R A P U S T U L A N S ) I N R E L A T I O N T O T H E

S T R U C T U R E O F T H E S K I N A N D E Y E

G. A. N A G D Y 1 and A. E. w. BOYD

Edinburgh School of Agricul ture , Wcst Mains Road, Edinburgh 9, Scotland

ZIIS~IIIIIIN'I{fEIA'XIIII~. R(IXIIIll(;. l). 212

S U M M A R Y

Tubers of 30 commerc ia l pota to varieties, g rown in two different soils, were inoculated with Oo.v~ora

pu.s'Hdans and incubated in two different condi t ions in two successive years. Varietal differences in skin

and eye susceptibi l i ty were in general agreement with those pre~iously recorded in field exper iments .

There was no consis tent varietal dilTerence in s y m p t o m expression but in some cases a superlicial or

slightly depressed skin necrosis developed in addi t ion to the normal pustules.

Over all the varieties, there w a s a significant corre la t ion between surface and eye infection. Skin infec-

tion was little influenced by the t empera tures of incubat ion which flucluated over the range 33-50 F

(0 ,6 - I0 C). Eye infection tended to be lower where the m a x i m u m tempera ture was higher. Thickness

of the tuber per iderm and of the suberised layer, and the content o f c r u d c fibres in the per iderm were

all negatively correlated with susceptibil i ty to skin infection.

No obvious relation was found between the s t ructure of the eyes and susceptibil i ty to eye infection.

The two types of soil used did not influence any of the per iderm charac ters observed or skin or eye

susceptibil i ty to skin spot. In the 9 varieties tested, i m m a t u r e and lll,;tttlI'C tubers showed no signilicant

difference in mean skin susceptibi l i ty or in mean per iderm thickness.

1. INTRODU('-IION

Potato varieties differ markedly in susceptibility to skin spot. Previous work with British varieties ( A n o i l . , 1932) indicated that Ker f s Pink, Arrcm Chi<f and Arran Bamwr were frequently badly affected without much damage to the eyes, and no infec- tion was observed in Golden 14/omh, r. On the other hand the varieties All_r, Mqjestic and King Edward &equently showed both skin and severe eye infection whicla led to considerable blanking after planting. BOYD (1957) found that in field trials of 24 com- mercial varieties, l<err's Pink, Arran Bamler, Majestic, King Edward and Ulster Chi~J- lain were highest in order of susceptibility and no infection was observed in Home Guard and Golden Wonder. In conti n uation of these studies over a 3-year period, BOYD and LENNARD (1961) noted that of 19 varieties which could be compared directly with the previous work only three, MqjesHc, Gladstone and Home Guard, occupied some- what different positions in the two sets of data. Go~den I,l'onder, Dtmhar Rorer ',tnd Dlmhar StamNrd were most highly resistant and Craigs Alliam'e, Kerfs PiHk, Craigs

Present address : Seed Test ing Dcparmlen t , Ministry o f Agricul ture , O r m a n , Giza, U .A.R.

Accepted for publ icat ion 28th M a y , 1965.

2OO Era'. Potato J,, Vol. ,'r (1965) No. 4 (Dec.)

SUSCEPTIBILITY OF P ( ) I A T O VARIEIIFS TO SKIN SPOT (OOSPORA PUSTULANS)...

Royal, Sharpe's Express and King, Edward were most highly susceptible to both skin infection, and, particularly King Edward, to eye infection. Since Golden Wonder was so consistently highly resistant it was suggested (BoYD and LENNARD, 196[) that differ- ences in varietal susceptibility might in certain cases be attr ibuted to differences in skin characters.

Previous work has relied on natural contaminat ion of tubers in the field. The present work describes artificial inoculation of tubers grown under two different soil condi- tions and subsequently incubated under different storage conditions.

2. EXPEI~,IMENTAL METIIODS

Thirty commercial varieties were grown in the seasons 1960 and 1961 in plots of me- dium loam (Busfi) and light sandy soil (Scoughall). The seed tubers were fl-ee from skin spot by visual examination and had been surl'ace-sterilised in an organo-mercury solution prior to planting. The crops were harvested by hand in mid-October and from each variety 20 equally-sized tubers were selected, thoroughly washed and surface- sterilised in a 21}{; formalin solution. Al'ter drying they were immersed for 1 rain. in a spore and mycelial suspension of Oosp~Jrapustu/anscontaining 1,7 - 10 ~and I,I 10 ~spores per ml in 1960 and 196I respectively, and stored in sterilised cardboard boxes, 10 tubers per box. A high humidity was maintained in the boxes by. means of damp cot ton wool while the boxes themselves had been previously clipped in water. The boxes were then incubated for about 5 months until the following spring under different temperature conditions as follows:

1960 61 1961 -62

M in. Max . M i n. Max .

A 35 F(I,7 C) 50 F(10 (') 33 F(0,6 CI 48 F(9 (') B 36 F(2,2 C) 44 F(6,6 C) C 40 F 14,4 C)

In B, the temperature varied about a mean throughout the storage period whereas in A, which more closely simulated natural variations, the higher temperatures occurred d uri ng the early period of storage.

Assessment of skin-spot infection on the skin was made in March using the scale described by BOYD (1957) and the mean surface-infection index (S.I.1.) obtained. Eye inf'ection was assessed by noting the presence of pustules associated with buds on some or all eyes of tubers in each surface-infection category. To simplify these results for comparat ive purposes it was assumed that tubers with some eyes infected had 50 <:,,

50 (2A ! S) of the eyes attacked. Thus the eye-infection index E.I.I. N where A :

number of tubers with all eyes infected, S - number of tubers with some eyes infected and N total number of tubers in the sample.

From 4 tubers of each variety grown at each site in 1961, 8 cubes of about 1 cm

Eur. Potato J., Vo/. ,~' (1965) No. 4 ( Dec. J 201

o. A. NA(;,)V AND A. E. W. l~o','l)

square and 4 r a m in depth were cut a round typical eyes on the tuber sides, and 8 simi- lar cubes were taken from the same tubers from similar areas with no eyes. Sections of skin and eyes were made, stained and examined. Measurements were made of the thickness o f the periderm and the size o f cells and the thickness of the suberin layer, ten readings being made in each case. Typical eye structure was described for each variety and these were divided into two groups according to whether the buds were open or closed. Photomicrogl aphs were made of periderm and eye sections of all varieties from each site.

In addit ion, chemical analyses of the skin of 10 tubers of each wtriety was made to determine the content of crude libres. The skin for analysis was obta ined most easily by first heating the tubers to 100 :'C lor 1 hour and the method used was that described by MAHIN and CARP, (1923). Two determinat ions were made for each wu'iety.

3. RESULTS

3.1. V a r i e t a l d i f f e r e n c e s in s u r f a c e n f e c t i o n

Tile results in TAFn_E 1 show the mean surface-infection indices in 1960 and 1961 o1" 30 varieties f rom two sources stored under different condi t ions after inoculation. Tile 1960 results show a significantly higher degree of infection than those in 1961, but whether this is due to a somewhat higher iitocululn concentra t ion or to some other factor is not clear.

Al though it is unlikely that the results of" susceptibility tests can ever be completely uniform ['rom year to year, the general pattern of ranking is reasonably consistent and the order is in fairly close agreement with those given by BOYD (1957) and Bovo and LENNARD (1961) from field experiments. Thus the varieties Calriona, Record, Duke q/ York, Arran Peak, Doon Star, Dunl)ar Rorer and Goldc'#z 14'o#zder, which were shown to be resistant in the previous work, are a m o n g the first 15 varieties in order of resis- tance in TABLE 1. ,dl'lall Pilol, Dr. klchllosll and Up- to -Dale differ in this respect. Of the varieties which had previously given anomalous results, klqjeslic was found to be of modera te susceptibility and Home Guard highly resistant.

It is clear that, in general, the differences in varietal susceptibility to skin infection obtained in field experiments have been confirmed by inoculation tests under labora- tory conditions.

3.2. V a r i e t a l d i f f e r e n c e s in eye i n f e c t i o n

TABLE 1 also presents the eye-infection assessments of the 30 varieties, In this case the mean index for infection in 1960 was signiticantly lox~er than that for 1961. This is the reverse of the surface-infection result and the reason for finis is not known. Some in- consistent variations between individual treatments occurred and for this reason deti- nite conclusions cannot be drawn f'rom the results of only one or two tests. However, if the mean eye-infection indices of the 8 tests of the 20 c o m m o n varieties used in this in- vestigation and in that of BOYD and LENNAm) (1961) are compared , only 5 are not consistent in order of susceptibility. Sharpe's E.vpress and Epicure are much less sus-

202 E,,'. Potato J., liol. 8 (1965) No. 4 (Dec.)

SUSC'|:~PTIBILITY ()F P()TAI() VARIt-IIES TO SKIN SPOT (OOSPORA PUSTULANS),..

TABI.I i I. Skin spot surface- and eye-infection indices of 30 potato varieties; mean of 4 inoculat ions in each year

Variety Surface-infection i n d e x t E y e - i n f e c t i o n i n d e x ~ Sorle l/ari~;t~; 1960 1961 I 9 6 0 - I961 1960 1961 1960 �9 1961

Gohlen Wonder 2,9 5,0 4,0 16,3 48,8 32,5 Dunbar Rover 7,6 5,4 6,5 14,5 51,3 32,9 Great Scot 11,0 4,2 7,6 30,0 51,3 40,6 Bintje I 1,3 4,7 8,1 23,2 40,0 31,6 Home Guard 14,3 5,2 9,8 20,3 37,5 28,9 ,4rran Consul 17,7 3,7 10,7 25,6 23,8 24,7 Epicure 18, I 5,7 I 1,9 19,3 16,3 17,8 Ulster Beacon 17,7 8,4 13, l 26,2 4 I, l 33,6 Catriona 26,7 6,9 16.8 32,5 32,5 32,5 Record 27,9 7, l 17,5 40,0 53,8 46,9 Chancellor 30,9 4,8 17,9 37,5 33,8 35,6 .4rr~:n Peak 28,9 7,5 18,2 46,8 53,8 50,3 British Queelt 28,7 I 0,0 19,4 43,8 39,5 41,6 DooH Star 32,0 8,7 20,4 32,5 43,8 38, I Duke t~/" ) "ork 32,4 10, 3 21,4 46,6 55,0 50,8 ~la/es/ic 28,0 15,3 21,7 35,9 36,3 36, I Up-to- Date 36, I 7,2 21,7 59,0 50,0 54,5 Dr. Mch~tosh 33.1 1 t ,8 22.5 50.0 40.0 45.0 Ulster Torch 36.5 12,3 24,4 36,3 52.5 44.4 Arran Pilot 41.0 8.7 24.9 37.8 28,8 33.3 Redskin 41.3 15. I 28,2 47.3 56.3 51,8 Crat',es D~[vance 49.0 8,2 28,6 50,0 41,3 45 6 Sharp's Express 50, I 8,4 29,3 37,5 45,0 41,3 Van,guard 40,6 19,2 29,9 45. I 71,8 58,5 Ulster Supreme 56,5 7,9 32,2 57,0 28,8 42,9 .4rrall l/ikiHg 50.4 15, I 32,8 51,3 57,5 54,4 Ker f s Pi~tk 51,5 15, I 33,3 80,3 70,0 75,1 Crai~s Royal 48,2 29,9 39,1 47,4 66,3 56,8 King Edward 72,8 17,6 45,2 78,8 55,0 66,9 .41"t'all Ballllt'l" 74,6 22,7 48,7 60.8 51,3 56,0

A lean" 33,9 10,4 40,9 45,7

' htdc.v l?ir Oher/hh'heninl~4, tio/t imtice d'inl~'ction de ~mtht'e. Index/?it .qugenin/~,l, tion indite d'inJi'cHon d'oei/.

' %l i l l e~ l l lo.VC'lt l le.

T,xm ELl 1. lmlex./)ir OhetJliichen- und ,4ugepdt(/~,ktionen mit Tiip/~,l/lecki~keit hei 30 Karto[]blsorten: Mitre/yon vier hu)ku/ationen pro Jahr

T",I'ILliAU I. lndice.v de I'b(/Oction d'oo.sporiose de /a smJace et de I'oeil the: 30 i'aridtds de Pomme de terre." iiio.v[,lllleS do qlla[l'r illOCll/aliolls' c]laqllr allll~;e

c e p t i b l e a n d A r r a n P e a k , D u k e ~!/' Y o r k a n d U p - t o - D a t e m o r e s u s c e p t i b l e in the p r e s e n t

w o r k . T h e r e is f u r t h e r c l ea r e v i d e n c e th: t t o f all the c o n m a o n c o m m e r c i a l va r i e t i e s

Kerr ' s P i n k a n d Kil tg E d w a r d are m o s t s u s c e p t i b l e to eye in fec t ion . T h e resu l t s a l so

i n d i c a t e a g a i n t h a t M a j e s t i c , a l t h o u g h s o m e t i m e s d e s c r i b e d as b e i n g l iable to b l i n d n e s s

Fur. Potato J., 1"ol. ,%' I 1965) No. 4 ( Dec. ) 203

G. A. N A G D Y A N D A. E. W . BO",'I)

because of skin-spot infection, exhibits a very moderate eye susceptibility in relation to other varieties.

3.3. R e l a t i o n b e t w e e n s u r f a c e a n d eye i n f e c t i o n

Although the differences of mean surface and eye infections between the two seasons 1960 and 1961 cannot be explained, comparison was made within each variety. The relation which is apparent between these two factors (FIG. 1) was shown to be highly significant by calculation of the correlation coefficient. Only the varieties Great Scot, Up-to-Date, Arran Pilot, Craigs Defiance, Sharpe's Express and Ulster Supreme are markedly out of position.

80 .g ' - 70 r

0 ....=

e,.a 60

4 0

. J

10

S u r f a c e - i n f e c t i o n i n d e x =

r : 0.745; P 0,01 ' hzde.v fiir ,4u,~eni#d~'ktion - i/Mice d ' in fec t ion d'oeil . " huh' .v lTir O b e r l l d c h e M n f e k t i o n �9 imlice d'inl~,ction ~h, .surl~we.

FIG. I

P o t a t o s k i n s p o t : R e l a t i o n b e t w e e n s u r f a c e

a n d e y e i n f e c t i o n o f 30 v a r i e t i e s

AB~. I Tiip.l~,l/teckigkeit an Karto/lk'ht: Beziehang zwischen Ober)qgichen- uad ,4uge#Upzfektio#t hei 30 Soften

Fie; . 1 Oosporiose de la Pomme de terre." Relation entre I'intection tic, sm;l}we et I'iJ{l'ection de I'oeil chez 33 l'ari~st{;s

These results were obtained under conditions which were as far as possible uniform. Under natural field conditions, stocks of a variety may sometimes be found on which infection is considerably higher on the surface or on the eyes. The reason for this is not clear but may be associated with environmental conditions during or after infection.

3.4. Tile e f fec t o f i n c u b a t i o n c o n d i t i o n s oil s u r f a c e and eye i n f e c t i o n

The three different incubation conditions used in this investigation have been described previously and TABLe 2 shows the infection differences which occurred. Tubers t'rom both sources in 1960 stored at B showed a higher surface and eye infection than those stored at A, although only with eye infection on tubers from one source was this sig- nificant. The minimum temperatures in each case were practically the same but the maximum was considerably lower at B. The development of the disease is favoured by low temperatures and it is likely that this is the reason for the trend towards higher in- fection under incubation B.

204 Eur . P o t a t o J . , Val. 8 ( 1 9 6 5 ) No . 4 (Dec ' . )

SUSCEPTIBILITY OF POTATO VARIETIES TO SKIN SP()F (OOSPORA PUSTULANS)...

TABLE 2. Effect of difl'erent incubation conditions after inoculation on the mean surface (S.I.I.) ttlld eye (E.I.I.) skin-spot infection indices of 30 potato varieties grown on two types of soil

Infection index Year Soil Incubat ion ( F)' Significance (t)

ln, lbktionsindex Jahr Boden B A C Sig#Ufikanz ( t ) lndice d'htfection ,4mtde Sol 36~t4 1960:35-50 __ De~rddr162

- - 1961:33 48 40

S.I.I. 1960 Scoughall 33,9 29,3 1,034 N.S. ~ Bush 37,0 35,4

1961 Scoughall 9,4 9,8 0,990 N.S.-' Bush - 12,7 9,4

E.I.I. 1960 Scoughall 51,1 35,2 3 ,918"** Bush 42,6 34,7 - 1,872 N.S.-'

196[ Scoughall - 46,5 46,3 0,055N.S. ~ Bush 37,0 53, I 7,007 ***

I TI'IIIpCI'(I[III" ll'tl,tl#'('lltl d c r hl/<llhflliOILs2(# ( 1 7 ) [el#lp(;l'll[llr~: t l( , l ldtl l l / /~1 p(;l'iofh- d'ill<'ul)aliOll ( [") Ix F (x 32) 5 9 CI.

: Nicht si.~,nitil<ant non.vivni/icative.

TAI~I~LLt! 2. l+Trkm~g i'erschiedener Bedingungen wOhreml der Inkuhations-eit ate/die mittleren Imh,.w' ./Tir OhetJtficheniql~'ktion ( S.I.I.) tom ,4ugeni#(l~,ktion ( E.l . l .) nach der h{fektion #nil dem Erreger der TiipJOffteck~4,keit (3(7 Sorten yon zwei verschiedenen Bodentypen:

T,'~I~LI-AU 2. E/let de di/J~;rentes conditions d'incuhation sur les indices mo.vens d'infectioo de sur[~we ( S.l .I .) et d'oeil (E. l . l . ) aprOs inoculatio#t avec oosporiose the: 30 varidtds de Pomme de terre poussdes darts d~'tt.v types de sol

In 1961 the controlled temperature of 40 F (4,4 'C)at C was almost midway in the temperature range at A. While this made no difference to infection in tubers from Scoughall. those from Bush developed a slightly lower surface infection and signifi- cantly higher eye infection at 40'F than under the fluctuating conditions.

While temperature differences in these experiments had no significant effect on sur- [1tee infection, the effect was significant in two of the fourcomparisons of eye iil{'ection. Where this did occur, lower infection was apparently associated more with a wider temperature range where the higher temperatures tended to occur during the early period of storage. More work however is necessary to determine more precisely the effects of storage temperatures particularly those of a fluctuating nature.

3.5. R e l a t i o n b e t w e e n sur face i n f e c t i o n of d i f f erent var ie t i e s and skin c h a r a c t e r s

Since obvious differences in susceptibility to surlhce infection exist between different varieties of potato, measurements were made of periderm characters of 30 varieties grown at two centres. These included the total thickness, the average immber o[" rows of cells, the thickness ol'the suberin layer and the percentage of crude fibres. These last consist ot" a mixture of cellulose, hemicellulose and lignin of the cell walls and w a s

Eur. Potato J., Vol. 8 (1965) No. 4 (Dec.) 205

G. A. NAGDY AND A. E. W. BOYD

shown by HAWKINS and HARVEY (1919) tO be related to resistance to mechanical puncture of the skin and to penetration by Pvthitml deharyamml.

These measurements were made on samples of potatoes grown at Bush and Scough- all in 1960. In TABLE 5 they are compared with the average surface-infection index for eacll variety.

3.5.1. Peridernt thickness

Considerable differences in periderm thickness exist between varieties from each cen- tre (TABLE 3) and in both it is clear that Golden Womh, r possesses a skin very much thicker than that of any other variety. A highly significant negative correlation was shown to exist between periderm thickness and surface infection (FK;. 2). It is not possible to suggest why the four varieties Ulster Beacon, British Queen, Ulster Torch and Ulster Suprenle were out of position.

=.... 16o!

15(

.(J 14( ,J=

,'- 13[ .g

r 12( Ca.

11(

9(

8(

7( 10 20 30 40 50

S u r f a c e - i n f e c t i o n i n d e x e

r 0 ,831; P - 0 . 0 1 ' Peridermdicke (~) - ~;paisseur du pdriderme (1*). = hldex fiir OberflgicheninJ'ektion - indite d'infectioit de sur/}lce.

10(

Fic;. 2 P o t a t o sk in s p o t : R e l a t i o n b e t w e e n s u r f a c e -

i n f e c t i o n i n d e x a n d p e r i d e r m t h i c k n e s s o f

30 v m i e t i e s

AIIB. 2 Tiipfe!fleckigkeit an KartofJ~'/n: Beciehmlg zwischen lndex J~ir ObeiJtiicheMnfektioH trod Pet'idermdicke hei 30 Sorten

FIG. 2 Oosporiose de la Pomnte dc terre, relatiou entre I'hldice d'infection de smJace et I'dpais- seur du pdriderme chez 30 varidtds

3.5.2. Periderm ('ells

As might be expected, tile number of rows of cells in tile periderm (TABLE 3) appears to follow the order of periderm thickness of the varieties in most cases. The walls of the cells themselves were described arbitrarily as thick or thin, and a tendency was noted for thin cell walls to be associated with a thinner periderm.

3.5.3. Suherin-layer thickness

TABLE 3 shows the average thickness of the suberin layer of the pedderm or the 30 va- rieties and tiffs also appears to follow the order of thickness of the respective periderm.

206 E,,.. Potato J., Vol. 8 (1965) No. 4 (Dec).

SUSCEPI 1BILIq Y ()I: P{)IAT{} VARIEI IES T(} SKIN SPO~[ (OOSPORA PUSTULANS). . .

TABLE 3. Relation between mean skin spot surface-infection index of 30 commercia l po ta to varieties grown at Scoughall and Bush in 1960 and thickness o f pcr idcrm and suberised layer and pcrccntagc of crude libres

GoldeH l,Vomh, r Dtmhar Rover Great Scot Bin@, Home Guard Arran Consul Epicure Ulster Beacon Catriona Record Chancellor .-Irran Peak British Queen Doon Star Duke o f )'ork Mq/estic Up-to-Date Dr. Melntosh ULster Torch Arran Pilot Redskilt Cra&,s Defiance Sharpe's E.vpres.s' Vanguard ULvter Supreme .4rral l P7kiHg Kerr 's Pink Craig,~ Royal King Edward .x|/'rall BaHIIeI"

Mean:'

Periderm pdriderme

Thickncss (Iz) t Av. no. of ce[{ rows ~

B. So. av. B. So. av.

164 154 159 12 12 143 135 140 9 II t38 124 31 t0 9 128 130 29 9 9 128 145 36 10 II 144 131 37 9 9 127 115 21 9 8 119 119 19 8 8 123 126 24 7 8 113 117 15 8 7 119 117 18 8 7 96 116 106 7 7

135 130 133 9 9 104 110 107 7 7 125 113 119 9 8 I10 101 106 8 7 116 1t9 1~7 7 8 111 123 117 7 8 90 105 97 7 7

116 99 108 8 8 112 95 104 7 8 102 92 97 7 7 106 94 100 7 7 94 99 97 6 6

112 I10 I11 8 7 100 96 98 7 7 94 96 95 6 7

102 96 99 7 6 99 83 91 6 6

108 96 102 9 8

116 I13 7,9 7,9

Thickness (p.) Crude fibres (00)' S.I.I. o f suber in laycr:'

B. Sc. av. B. Sc. av.

12 I0 12 II 32,3 31,5 31,9 4,0 10 10 10 10 30,9 30,6 30,8 6,5 9,5 9 10 9,5 25,5 25.8 25,7 7,6 9 l0 10 10 28,2 26,7 27,5 8,1

10,5 I1 10 10,5 30,5 29,7 30,1 9,8 9 13 9 ] I 33,9 30,9 32,4 10,7

8,5 I0 9 9,5 24,1 24,5 24,3 II ,9 8 II 10 10,5 25,8 24,2 25,0 13,1

7,5 10 9 9,5 23,4 24,8 24,1 16,8 7,5 10 10 10 27,8 26,0 26,9 17,5 7,5 8 10 9 23.0 21,8 22,4 17,9 7 9 9 9 24,8 23,2 24,0 18,2 9 10 10 10 24,2 25,7 25,0 19,4 7 9 9 9 24,4 25,1 24,8 20,4

8,5 9 8 8,5 27,3 25,1 26.2 21,4 7,5 9 9 9 28,0 28,9 28,4 21,7 7,5 l0 10 10 26,7 25,8 26,2 21,7 7,5 9 9 9 24,2 23,5 23.8 22,5 7 9 10 9,5 27,5 25,9 26,7 24,4 8 9 I0 9,5 26,6 25,6 26,1 24,9

7,5 1 I 8 9,5 26,6 24,9 25,8 28,2 7 8 9 8,5 25,3 25,0 25,1 28,6 7 9 9 9 22,2 23,1 22,6 29,3 6 9 10 9,5 21,6 23,4 22,5 29,9

7,5 9 10 9,5 21,0 21,3 21,1 32,2 7 9 9 9 22,3 23,9 23,1 32,8

6,5 8 9 8,5 25,6 24,6 25,1 33,3 6,5 8 9 8,5 21,8 22,2 22,0 39,1 6 9 8 8,5 24,3 22,9 23,6 45,2

8,5 8 8 8 23,0 22,0 22,5 48,7

9,4 9,4 25,8 25,3 25,5

B Bush; Sc. Scoughall. DicLe (!z) -- dpaix.seur (~x).

'- 4]ill/{'l'e ,.Inzahl Zelheihen mmlhre moyen de rtm~,des tit, celluh, s. ' Dicke I!~) der Km'kschicht c;pais,~'em" de la couche tie lic;~'e (!~).

Rohl~tser,uehalt ( ",.I - fibres hrules (",;). " Mittel- h'lO'.l'ell/le.

TAFIILLI~ 3. ZusammenhaHg zwisehen mitt/erem Imlex J)ir Oheu'tiicheninfektion mit Tiip/bffh, ek~kei t hei 30 Handelssorten ~#r Herkiinfte Scoughall utul Bush im Jahre 1960 trod der Periderm- dicke, der l~orkschieht sowie dem RohJbser~ehah

TABLEAU 3. Relation entre, d'ltne part, I'indiee mo)'ett d'ittfectiott d'oos77oriose de sttrJ~tce the-- 30 varid- tds comtnereiales de Potnnw de terre t't Scottghall et t) Bush en 1960 et, d'autre part, Ik;pais- settr cht pdriderme, de la touche de li@e et le pourcentage de fibres hrtttes

Ettr. Potato J., I/ol. A' (1965) No. 4 (Dee.) 207

G . A. N A G D Y A.NI) A. I{. W . l t (~YD

There is also a highly signil icant negative cor re la t ion between surl i lce-infect ion index and suberin thickness (r - - - 0 , 7 2 2 ( N 3 0 ) : P 0,0I) .

3.5.4. Crude fihres

Tile varietal differences in crude-f ibre content (TABLE 3) are ilOt o1" a very high order and 15 of the 30 variet ies are separa ted by less than 4~ fibre content . Ditrerences tend to be clearer a m o n g tile variet ies with highest and lowest susceptibi l i t ies . Crude fibres are highest in 5 of the first 6 most suscept ible variet ies and are lowest in 7 of the 8 least susceptible. This re la t ionship was conf i rmed cor re la t ion between surface- infect ion index (N : : 30): P 0,01).

3.6. R e l a t i o n s h i p b e t w e e n e y e i n f c c t i c h a r a c t e r i s t i c s

by showing a highly signil icant negative and crklde-fibre content (r 0,669

oil o f d i f l ' e r e n t v a r i e t i c s a n d eye

Tile eye of a po t a to conta ins at least 3 buds pro tec ted by more or less consp icuous scales. They may vary in depth in relat ion to tile surface of tile tuber and such featLircs are var ietal character is t ics . Sections of tile eyes of 30 variet ies grown at tile two centres were examined and descr ibed accord ing to tile depth of the eye and to the shape of the bud. These were open if tile scales were not closed over tile p r imary bud and closed i[" tile scales su r rounded the bud. Both these charac ters were found to be col ls lant for each variety in ahnos t every case at both growth centres.

No ou t s t and ing differences in eye s t ructure could be detected between variet ies wlnich might have a bear ing oil suscept ibi l i ty to skin spot. A l though ra ther more var ie t ies in the h igh-suscept ib i l i ty g roup and fewer variet ies in the low-suscept ib i l i ty g roup possess- ed closed buds, this tendency was much too indefinite to d raw any conclus ions . Fur ther , no differences were observed in suber i sa t ion of scales or buds of selected suscept ible and resis tant varieties.

Most of the variet ies examined have slnallox~ or mode ra t e ly shal low eyes, in l,ict, in only three, namely Epicure, Arran Consul and Ker f s Pink, \~ere tile eyes classified as deep or mode ra t e ly deep. Of these, the first two were least suscept ible and the last \vas most suscept ible to eye infection. It is clear therefore that this is not dependen t oil depth of the eye.

Nei ther the ma tu r i ty nor the earl iness of sprout growth appea r s to bear ;.lily re la t ion to l iabi l i ty to eye infection.

3.7. T h e e f f e c t o f d i f f e r e n t s o i l t y p e s oll s u r f a c e a n d e y e s u s c e p t i b i i t y

The possibi l i ty that suscept ib i l i ty of tubers may be influenced by the type o[" soil in which they were grown was examined by c o m p a r i n g both surface- and eye-infect ion in- dices of the 30 variet ies grown in two seasons in soils of two different types. At Bush it was a loam and at Scougfiall it was a lmos t ent irely sea sand. TABLE 4 shows that differ- ences in the mean surface- and eye-infect ion indices were not signific-int except in one case where eye infection was higher in tubers from Scoughal l under one method of in- cubat ion . There is no exp lana t ion for this result which seems to be out o[ 'keeping with the others.

2 0 8 E#o'. P o t a t o J. , I/ol. ,~' ( 1 9 6 5 ) No . 4 ( Dec . J

SUSCEPTIBILITY OF POTATO VARIETIES TO SKIN SPOT (OOSP()RA PUSTULANS)...

TABLE 4. Effect of different soil types on the skin characters and the mean skin-spot surface-IS,l.l.I and eye-(E.l,I.) infection indices of 30 potato varieties incubated under different conditions after inoculation

S.I.[. 1960 Incubation A ,, 1960 B ,, 1961 A

1961 (7 E.'i.I. 1960 A

1960 B ,, 1961 A

1961 C Periderm thickness (f;_)~ Rows of periderm cells ~ Suberin-layer thickness (:z):' Crude fibres ( ~, )l

Soil types Bodenarte , Significance (t) types de sol S igni f ika ,z ( t

Degrd de sigm~Cicatiolt ~ t J Scoughall Bush

29,3 35,4 1,653 NS 33,cj 37,0 0,926 NS

9,4 12,7 1,435 NS 9,8 9,4 0,027 NS

35,2 34,7 0,117 NS 5 I, I 42,6 1,940 NS 46,5 37,0 2,699 * 46,3 53, I 1,940 NS 113 II6 1.645 NS 7,9 7,9 9,4 9,4

25,3 25,8

' Per idermd iuke ( [ a . ) - ~;pai.vse.r t l . i;Uriderme (t.tl. .4nzahl Per iderm-Zu lh ' e ihen . o m h r e de ra~r~,t;e.v de celluh'.; m;ridermiqm'~.

' Dicl, e der Kurl,.vchiullt {uI (;pais.vetlr de la cliouche de/i~;r ({o.). R~dllT~.~'ervehalt ( " , , I - f ihres hrute.~ ( '>.l.

TABELLE 4. 14/l'rl<ung i 'ersehiede,er Bodenarte , au f die Sehule#teiyenschaJlen uml die mitlle,'en hMexe ./7it" Oherflfiehe,- r ) umi ,-luge,i#{/kklio, r E.I.I. ~ m # TiipJ~'lfteeki,~l, eit ,ach der h!fek- lion wgi/*re,d der Ullle#" i'ersc/u'edene#t Bedin,gun,gen ,ge/~altene~l lH/~ut~atio,.~.Te# f 3() Sot'ren ~

TAULE-',U 4. E~tEts ~& d~PJi;re,ts o'pes de.sol sur les caraeMres de la oeau et.vur les imliees mo.ve,s d'i~{l{'e- lion d'oosporio.se de sin:face I S. l , l . ~ el d'oeil ( E.I.I. I ehe-- 30 vari&& de Pomme de lerre incuhdes .wins dl[ff~;#'elltes coHditio#ts apr~;.v illo('ulglliO#!

TABLF 4 also shows that the di f ferent soils i nduced no d i f ferences in the m e a n thick-

ness o f ' t u b e r p e r i d e r m or o f its suber in layer, or in the c o n t e n t o f c rude fibres in the

pe r ide rm.

3.8. S u s c e p t i b i l i t y o f i m n l a t u r e t u b e r s

In 1961 tubers of '9 c o m m e r c i a l wu ie t i e s g r o w n at Bush were l i f ted in n f id -Sep tember ,

one naonth be fo re those used in the n o r m a l suscept ib i l i ty tests and whi le the h a u h n s o f

the p lan ts were still green. These tubers were i nocu la t ed as desc r ibed p rev ious ly and

incuba t ed at 4 0 F (4,4 :C). Pe r ide rm sec t ions were m a d e fronl s imi la r samples but un-

f o r t u n a t e l y none were ava i l ab l e f rom m a t u r e tubers . Af t e r i ncuba t ion , the sk in- infec-

t ion indices o1" these i m m a t u r e tube r s were c o m p a r e d with those of" m a t u r e tube r s oi"

the same variet ies . The pe r ide rm th icknesses o f the i m m a t u r e tubers were c o m p a r e d

with those o f m a t u r e tubers g r o w n at Bush in 1960. The inl 'ection d a t a in TAbLe 5 show that there are no o u t s t a n d i n g di f ferences be-

tween m a t u r e and i m m a t u r e tubers o f any of the var ie t ies except Kin<,,, E d w a r d where

those which were i m m a t u r e b e c a m e m o r e highly infected. W i t h o u t this var ie ty there is

Ett#.. Polato J., l"of ,~' (1965~ No. 4 fDee. ) 209

G. A. NAGDY AND A. E. W. BOYD

TAnLE 5. Surface-infection index and periderm thickness of mature and immature tubers of potato varieties

Surface-infection index Periderm thickness (l J-) Variety

Sorte 1961 1961 1961 1960 Varidtd immature mature immature mature

unrei f - 11011 mftr re i f - nlt'tr unret f - non iii1"11 re i f - mfo"

Arran Consul 7,0 4,8 141 144 Doon Star 7,4 8,0 100 104 Majest ic 7,7 6,7 123 I 10 Up-to-Date 8,0 7,4 118 116 Golden Wonder 9,7 5,4 167 164 Kerr's Pink 10,0 8,0 104 94 Redskin 12,0 17,2 99 112 Craigs Royal 15,6 12,0 87 102 King Edward 51,8 I 0,0 82 99

Mean t 14,4 8,9 I 13 I 16 Mean (without King Edward) ~ 9,7 8,8 117 118

Mit te l - moj'enne. '-' Mittel (ohne King Edward) - moyenne (sans King Edward). Fiir die attderen Einschriften siehe TABELLE 1 - pour ]es atttres inscriptions voh" TABLEAU I.

TABELLE 5. Index f i ir Oberfliicheninfektion und Peridermdicke bei reifen und unreifen Knollen yon 9

Kartoffelsorten TABLEAU 5. Indi te d'illfection de surface et dpaisseur du pdridernle de tubercules mr'o's et Ilotl mt'lrs chez

n e u f varidt(s de Pomme de terre

a significant correlation (r = 0,687; P = 0,05). Also the overall mean infection index without King Edward is very similar for both maturity groups and even with the inclu- sion of this variety, the mean difference is not statistically significant (t = 1,195).

There is also a significant correlation in the periderm thickness of mature and im- mature tubers even when King Edward is included (r = 0,921 ; P = 0 ,01)and the mean figures do not differ significantly (t = 0,744).

The infection index of the immature King Edward tubers thus appears to be aber- rant but no reason for this can be suggested.

4. DISCUSSION

In previous investigations, assessment of varietal differences to skin-spot infection was based on field experiments. A laboratory method developed from that used by ALLEN (1957) and BOYD and LENNARD (1961) was used to inoculate tubers of 30 commercial varieties which were then stored under different temperature conditions. The varietal order of skin and eye susceptibility which was obtained was in reasonable agreement with previous results (Anon. , 1932; BOYD, 1957" BOYD and LENNARD, 1961) and with field observations. A method of assessment of eye infection was devised and was found to be convenient in making varietal comparisons.

210 Eur. Potato J., Vol. 8 (1965) No. 4 (Dee.)

SUSCEPTIBILITY OF POTATO VARIETIES TO SKIN SPOT (OOSPORA PUSTULANS)...

In the Mtirmansk area of the U.S.S.R. four symptom types are recognised: black superficial stains, raised pustules, depressed irregular patches and deeply-depressed pits (KHARKOVA, 1961) of which some may be peculiar to certain varieties. It was suggested that different races of O. pustulans were involved. In the present investiga- tion no abnormal symptoms were noted as being associated with particular varieties. However, in many cases, after incubation, tile fungus developed as a white mycelial growth over the whole surface of the tuber, producing both normal pustules and symptoms which might be described as skin necrosis, consisting of either superficial stains or slightly-depressed patches. The pitted type of lesion noted by BOYD and LENNARD (1962) was assumed to be induced by very low-temperature conditions and it is possible that different symptom types may be induced by varying the temperature of storage.

BOYD and LENNARD ( 1961 ) stated that surface and eye infection of different varieties grown tinder field conditions were not necessarily related. In the present investigation, under standard conditions of inoculation, there was a highly significant correlation between skin and eye susceptibility. This does not mean that, in a given stock of pota- toes, severe surface infection will always be accompanied by severe eye infection. Envi- ronmental conditions of humidity and temperature may be such as to encourage one in relation to the other. Other factors within the tuber may also influence their subse- quent development: e.g. tubers of two varieties, King Edward and Arran Pilot, may show an equally severe eye infection but in the field the eventual loss in plant establish- ment may be quite different (BoYD, 1957). This is no doubt connected with vigour of sprout growth but further work is necessary on these lines.

In 1960, surface infection was higher and eye infection lower than in 1961. The reason for this is not known but this had little effect on the order of susceptibility.

In most of the series of experiments, the temperatures fluctuated during the incuba- tion periods, but, within the ranges used (max. 50':F = 10~ surface infection was little affected. Possibly differences were not wide enough to influence wound-periderm formation. On the other hand, eye infection was more sensitive to temperature change, and tended to be less under fluctuating conditions of normal storage where the maxi- mum temperature, usually at the beginning of the storage period, was higher than 40- 44~'F (4,4-6,6'-'C). This may be related to the activity of bud growth in certain varieties but more precise work is necessary on the effects of temperature of incubation. The results do confirm that skin spot is a disease associated with low-temperature storage conditions.

It would seem that varieties react differently to tuber infection and to stem-base in- fection since the order of skin and eye susceptibility shows little resemblance to that for infection of the stem bases given by SALT (1964). In this case, the brown cortical rotting of the underground parts of the comparable varieties was greatest in Majestic, King Edward, Dr. Mclntosh, Ulster Torch, and Redskin. Two of these were highly sus- ceptible and three moderately susceptible to tuber infection in the present investiga- tion. Ulster Beacon was rated moderate ['or stem-base infection but is of low tuber susceptibility and Ulster Supreme and Arran Viking, which showed little stem-base in-

Ettr. Potato J., Vol. 8 (1965) No. 4 (Dee'.) 211

G. A. NAGDY AND A. E. W. BOYD

fection, are in the high tuber-susceptibility group. The reason for this is not clear. The significant negative correlation between susceptibility to skin infection of dif-

ferent varieties and thickness of the periderm and its various components such as suber- in layer and crude-fibre content, supports the suggestion of BOYD and LENNARD (1961) and indicates that resistance to invasion is primarily mechanical. Where this mechanical barrier is ruptured (FucHs, 1954: BOYD and EENNARD, 1961) the fungus may penetrate without restriction but whether further varietal reactions come into play is not known.

MANTUROVA (11960) also noted that in the far north of the U.S.S.R. varietal selections with a thick, rough periderm were practically non-susceptible.

Although in varietal comparisons eye infection is strongly correlated with surface in- fection, the basis of resistance of the eyes requires fuller investigation. It could not be accounted for by any obvious difference in eye structure found in this work.

Tuber susceptibility to skin spot was not influenced by growth in soils of two quite different types nor did this in any way affect the periderm characters although this may be worthy of investigation on a wider range of soils. It is probable that soils of different types alter conditions for natural infection of the tubers as they do for infection of the stem bases (SALT, 1964), which was least in neutral peat (black fen) and alluvial soils, more in sandy light loam and greatest in clay soils. GOMOLYAKO (1959) showed that in the U.S.S.R. infection was heavier on tubers grown in iron podzol than in peat-bog soil. Since wet soil conditions at harvest (BOYD and LENNARD, 1962)encourage extensive skin spot, it is likely that less-affected crops of tubers will be produced in soils of a light and dry nature.

Tubers of 9 varieties lifted about one month before complete haulm decay, and to this extent inamature, were shown to have the same mean periderm thickness as those lifted in the previous year in a mature condition and, with the exception of King Edward (where the result cannot be explained), susceptibility to skin infection of im- mature and mature tubers was little different within each variety. Tiffs agrees with the findings of GREEVVS and MUSKVTT (1939) who found that early digging did not reduce infection. BoYo (1957) and ALLEN (1957) however found that early-lifted tubers show- ed a lower incidence of infection but it is possible in these cases that the tempera- ture of storage may have been higher in September than in October and so may have reduced the chances of natural infection.

ZUSAMMENFASSUNG

ANFALLIGKEIT VON KARTOFFELSORTEN GEGEN TiJPFELFLE('KIGKEIT (Oospo- rapustulans) IM ZUSAMMENHANG MIT DER STRUKTUR VON SCHALE UND AUGE

In zwei aufeinanderfolgenden Jahren wurden Knollen yon 30 im Handel erh~Itlichen Kartof- felsorten aus zwei verschiedenen Bodenarten ge- waschen, ihre Oberfl~iche sterilisiert und durch Eintauchen in eine Sporen- und Mycelium-Sus- pension mit Oospora pustulans sofort nach der

Ernte geimpft. W/ihrend der lnkubationszeit wurden sie unter zwei verschiedenen Bedingun- gen gehalten. Dazu wurden 20 Knollen jeder Sorte verwendet, und nach fiinfmonatiger Lage- rung wurden sie anhand des Indexes betreffend Oberfl/i.chen- und Augeninfektion auf Ttipfel-

212 Eur. Potato J., Vol. 8 (1965) No. 4 (Dec.)

SUSCEPTIBILITY OF POTATO VARIETIES TO SKIN SPOT (OOSPORA PUSTULANS)...

fleckigkeit un te r such t [TABELLE 1L Die Ober- flS.chcninfektion wurde yon BOYD (1957) be- schrieben, w/ihrend man den Index ftir dic Augen- infektion gem/iss fnlgender Formel berechnete: 50 (2A ~ S)

- - , wobei A Anzahl Knollen, bei N

denen allc Augen befallen sind, S Anzahl Knol len mit einigcn befallenen Augen und N Gesamtkno l l enzah l des Musters bedeutet . Die sor tenbedingten Unterschiede in der Anfiil- ligkeit von Schale und Augen s t immten im allge- meinen mit denjenigen der frtiheren Feldversu- che iiberein. Es gab keinen Libereinst immenden, sor tenbedingten Unterschied in den Krankhe i t s - merkmalen , aber in einigen Fiillen entwickelten sich ausser den no rma l en Pusteln oberflfichliche oder Ieicht e ingesunkene Nekrosen . Bei allen Sorten bes tand eine signifikante Korre- lation zwischen Oberflfichen- und Augeninfck- tion {ABB. ]). Unter nati.irlichen Bedingungen dagegen mOgen Lagerpar t ien einer Sorlc gefun- den wcrden, bei denen einer dieser Faktoren m/3glicherweise infolge unterschiedl icher U m - wel tsbedingungen wfihrend der Lagerung bedeu- tend wichtiger ist als der andere. Die Schalenin-

fektion wurde du tch die Tempera tu r wS.hrend der lnkubat ionszei t , die zwischen 33 (0,6 :C) und 50 F (10 C) schwankte , nu r wenig beeinflusst (IABELLE 2). Dic Augeninfek t ion schien st/irker beeinflusst zu werden, und sie war eher schw/i- cher, wenn die Max ima l t empcra tu r hOher war. Dic Sorten waren sehr unterschiedl ich in den Per iderm-Eigenschaf ten der Knollen, wic Gc- samtdicke, Dickc der verkorkten Schicht und Gc- gehalt an rohen Fasern, und diese waren alle negativ korreliert mit der Anffilligkeit gegen Schaleninfekt ion (ABB. 2, TABEI.LE 3). Kein offensichtl icher Z u s a m m e n h a n g wurde ge- f tmden zwischen A u g e n s t r u k t u r und Anf/illigkeit ftir Augeninfek t ionen . Dic beiden Bodenar ten , in denen die Knollen gewachscn waren - Lehm und Sand - beeinflussten keine der beobachte ten Per iderm-Eigenschaf ten oder die Anffilligkeit der Schale oder der Augen (T:,BELt_E 4). Bei 9 un te rsucb ten Sorten bestand kein signifi- kanter Unterschied in der durchschni t t l ichen SchalenanDilligkeit oder in der durchschni t t - lichen Peridermdicke zwischen reifen Knollen und solchen, die einen Mona t friiher geerntet wurden (T:,BELLE 51.

RF-SUME

S U S C E P T I B I L I T I ~ DES VARII~TF.S DE POMME DE T E R R E A L ' O O S P O R I O S E (Oospora

f)llSlll/aHs) EN F O N C T I O N DE LA S T R U C T U R E DE LA PEAU ET DE L 'OEIL

Les tubercules de 30 varietds commerc ia les de P o m m e de terre, ayant pouss6 dans deux sols dif- ferents, sont laves, Ia surface est sterilisde et ino- culee par immers ion dans une suspens ion de spo- res et de myce l ium de Oospora pustldalls, inlllle- d ia tement apres a r rachage et ce, pendan t deux annees successives. Les tt, bercules sont alors mis ",.i incuber dans deux condi t ions differentes. On utilise vingt tubercules de cbaquc varidtd pour determiner , apres cinq mois de conservat ion , les indices d ' infect ion de st, rfacc et d'oeil (TAI~LEAU 1 ). L ' indice d ' infcct ion de surface a 6t6 ddcrit par BOYD (1957) et I 'indice d ' infect ion d'oeil est de-

50 (2A + S) tern'fine selon la formule N , off A est Ic

n o m b r e de tubercules dont t o u s l e s yeux sont a t taques , S est Ie nombre de tuberct, les dont quelques yeux sont a t taques et N e s t le non'tbre total des tubercules de l 'echanti l lon. Les differences varietales dans la susceptibili t6 de la peau et de l'oeil sont gdneralement concordan-

Ear. Potato J., Vol. 8 (1965) No. 4 (Dec.)

tes avec celles p rdcedemment etablies dans les c h a m p s d'essais. II n 'y a aucune difference varie- tale consequen te dans I 'expression des sympte - rues reals, dan quelques cas, se developpent , en plus des pus tu les normales , des necroses super- licielles de la peau ou des necroses en 16gere de- pression. Chez toutes les varietes, la correla t ion est signi- ficative entre l ' infection de surface et I ' infection d'oeil (FIG. I). Cependant , en condi t ions na tu - relies, on peut t rouver des lots d ' une varidt6 off un de ces facteurs est cons iderab lement plus grand que l 'autre, peut-etre 'a cause des differences dans les condi t ions de milieu pendant la conservat ion. L ' infect ion de la peau est peu influencee par la tempera ture de Fincubat ion qui varie entre 33 et 50 Fahrenhei t (0,6 et 10 C) (TABLE,XU 2). L' infection d'ucil parai t e, tre plus sensible et tend ',). etre moindre avec un n tax imunl de t empera tu re plus eleve. Les caractt}res du per iderme du tubercule, tels que

213

8paisseur totale, ~paisseur de la couche de li6ge et la teneur en fibres brutes, varient largement avec les vari6t8s; ces caract+res sont tous en corr61a- tion n~gative avec la susceptibilit6 b. I ' infection de la peau ( F IG . 2, TABLEAU 3 ). On ne t rouve aucune cor re la t ion 8vidente entre la st ructure de l 'oei l et sa susceptibi l i t6 a l ' in fect ion. Les deux types de sol dans lesquels les tubercules

O. A. NAGDY AN[) A. E. W. BOYD

ont pOUSS~ - argi le et sable - n ' in f luencent aucun des caract~res de 1"Spiderme ou la susceptibi l i t~ de la peau et de I 'oei l (TABLEAU 4). Chez 9 vari~:t~:s test~es, il n 'y a aucunc di f f6rence signif icat ive dans la susceptibi l i t~ moyenne de la peau ni dans FSpaisseur moyenne du p~r iderme entre Its tubercules mOrs et ceux arrach~:s un mois plus lot (TABLEAU 5).

R E F E R E N C E S

ALLEN, J. D. (1957): The development of pota to skin spot disease. Ann. appl. Biol. 45,293-298. ANON. (1932): Skin spot and blindness in seed potatoes. Scot. J. Agric. 15,191-196. BOYD, A. E. W. (1957): Field experiments on potato skin spot disease caused by Oospora pustulans

OWEN & WAKE[:. Ann. appl. Biol. 45,284-292. - - - - and J. H. LENNARD ( 1961 ): Some effects o f pota to skin spot (Oospora pustulans) in Scotland.

Ear. Potato J. 4, 361-377. - - and - - ( 1962): Seasonal fluctuation in pota to skin spot. Plant Path. 11, 161-166. FUCHS, W. H. (1954): Einige Beobachtungen i]ber die Pickelbildung (TiApfelfleckigkeit) der Kartoffel.

Nachr.BI. dtsch. P[tschDienst, Stuttgart. 6, 75-76. GOMOLYAKO, L. G. (1959) : The effect o f oosporosis disease on the chemical composi t ion of the pota to

tuber. Biokhim. Plod. Ovoschch. 5, 159-164. GREEVES, T. N., and A. E. MUSKETT (1939): Skin spot (Oospora pustulans OWt~N & WAKEFIELD) of the

pota to and its control by tuber disinfection. Am1. appl. Biol. 26, 481-496. HAWKINS, L. A., and R. B. HARVEY ( 1919): Physiological study of the parasit ism of P.vthium deharya-

nitro HESS. on the pota to tuber. J. Agric. Res. 18, 275-297. KHARKOVA, A. P, ( 1961 ) : On the biology of the causal agent of oosporosis , Oospora pustulans OWEN

& WAKEr. Bot. Zh. S.S.S.R. 46, 399~07. MAHIN, E. G., and R. H. CARR (1923): Quanti tat ive Agricultural Analysis. McGraw-Hil l , New York

& London. MANTUROVA, I. M. (1960): Oosporosis of potato in the Far North. Zaschch. Rast. Mosk t'a., 5, 49-50.

(R.A.M. 39,492). NAGDY, G. A. (1962) : The mechanism of infection of pota toes by Oo.wora pustulans causing skin spot.

P h . D . Thesis, University of Edinburgh. SALT, G. A. (I 964): The incidence of Oospora pustulans on potato plants in different soils. Plant Path.

13, 155-158.

214 Eur. Potato J., Vol. 8 (1965) No. 4 (Dec.)