E1 efecto de temperatura y fotoperlodo sobre la expresi6n de resistencia contra Phytophthora infestans fue investigada en cinco cultivares de papa con y sin.
A m e r J of Potato Res (2005) 82:139-146
139
E f f e c t o f T e m p e r a t u r e and P h o t o p e r i o d o n S y m p t o m s A s s o c i a t e d w i t h R e s i s t a n c e to Phytophthora infestans A f t e r L e a f P e n e t r a t i o n in S u s c e p t i b l e and R e s i s t a n t P o t a t o Cultivars O. A. R u b i o - C o v a r r u b i a s 1,~, D. S. D o u c h e s 3, R. H a m m e r s c h m i d t 2, A. d a R o c h a 2, a n d W. W. K i r k 2. IINIFAP,KM 4,5 Carretera, Toluca-Zitacuaro, Zinacantepec, Mexico 2Department of Plant Pathology, Michigan State University, East Lansing, M148824, USA 3Department of Crop and Soil Sciences, Michigan State University, East Lansing, MI 48824, USA *Corresponding author: Tel: (517) 353-4481; Fax: (517) 353-4940; Email: kirkw@msu.edu
ABSTRACT
however, t h e r e l a t i v e r e s i s t a n c e r a n k i n g a m o n g c u l t i v a r s was t h e s a m e in t h e f o u r e x p e r i m e n t s w i t h d i f f e r e n t
The effect of t e m p e r a t u r e and photoperiod on the
t e m p e r a t u r e s a n d p h o t o p e r i o d s . I t is a s s u m e d t h a t t h e
e x p r e s s i o n o f r e s i s t a n c e a g a i n s t Phytophthora infes-
r e s i s t a n c e in t h e M e x i c a n c u l t i v a r s m a y be c o n f e r r e d by
tans in five p o t a t o c u l t i v a r s w i t h and w i t h o u t r e s i s t a n c e
m i n o r r e s i s t a n c e g e n e s a n d by t h e r e s i d u a l e f f e c t o f
( R ) g e n e s was i n v e s t i g a t e d . F o u r e x p e r i m e n t s w e r e car-
d e f e a t e d R genes. T h e s e r e s u l t s e m p h a s i z e t h e difficulty
r i e d o u t u n d e r c o n t r o l l e d c o n d i t i o n s in g r o w t h cham-
in d i f f e r e n t i a t i n g b e t w e e n h o r i z o n t a l and v e r t i c a l resis-
bers. Two c u l t i v a r s (393295.236 and 391046.22) w i t h o u t
tance.
known R genes from the International Potato Center ( C I P ) in Lima, Peru, t w o M e x i c a n c u l t i v a r s w i t h m a j o r R
RESUMEN
g e n e s (Tollocan a n d M a l i n c h e ) , and a s u s c e p t i b l e cultiv a r ( A t l a n t i c ) w e r e u s e d in this study. P l a n t s w e r e
E1 e f e c t o de t e m p e r a t u r a y f o t o p e r l o d o s o b r e la
g r o w n f o r 32 days in g r o w t h c h a m b e r s a t t w o t e m p e r a -
e x p r e s i 6 n de r e s i s t e n c i a c o n t r a Phytophthora infestans
t u r e s (16 a n d 24 C) a n d t w o p h o t o p e r i o d s (12 a n d 16 h
fue i n v e s t i g a d a e n cinco c u l t i v a r e s de p a p a con y sin
day l e n g t h ) , t h e n i n o c u l a t e d w i t h a c o m p a t i b l e i s o l a t e o f
g e n e s de r e s i s t e n c i a (R). Se h i c i e r o n c u a t r o e x p e r i m e n -
P. infestans and i n c u b a t e d in a m i s t c h a m b e r a t 18 C.
t o s e n u n a c ~ n a r a de c r e c i m i e n t o b a j o c o n d i c i o n e s con-
T h e i n o c u l a t i o n efficiency, t h e p e r c e n t a g e o f l e s i o n s t h a t
troladas. Dos c u l t i v a r e s (393295.236 y 391046.22) de!
did n o t g r o w b e y o n d t h e i n o c u l a t i o n spot, t h e s p o r a n g i a
C e n t r o I n t e r n a c i o n a l de la P a p a ( C I P ) Lima, Perd, dos
density, and t h e AUDPC w e r e r e c o r d e d . T h e p e r c e n t a g e
c u l t i v a r e s m e x i c a n o s con g e n e s m a y o r e s ( R ) (Tollocan y
o f a r r e s t e d l e s i o n s d e c r e a s e d w i t h t e m p e r a t u r e in t h e
M a l i n c h e ) y u n c u l t i v a r s u s c e p t i b l e ( A t l a n t i c ) se u s a r o n
t w o m o s t r e s i s t a n t c u l t i v a r s (393295.236 a n d M a l i n c h e ) ,
en e s t e estudio. Las p l a n t a s se h i c i e r o n c r e c e r p o r 32
a n d t h e AUDPC was l o w e r a t 16 t h a n a t 24 C in f o u r o f
dias e n c~maras de c r e c i m i e n t o a d o s t e m p e r a t u r a s (16
t h e five cultivars. T h e i n o c u l a t i o n efficiency a n d t h e spo-
y 24 C) y dos f o t o p e r i o d o s (12 y 16 h o r a s de l o n g i t u d del
r a n g i a d e n s i t y w e r e n o t a f f e c t e d by change in t e m p e r a -
dia); f u e r o n i n o c u l a d a s con un a i s l a m i e n t o c o m p a t i b l e
t u r e . S p o r a n g i a d e n s i t y i n c r e a s e d a t 16 h p h o t o p e r i o d ;
de P. infestans e incubadas en u n a c~mara de crec-
however, t h e final i n f e c t e d l e a f a r e a was n o t a f f e c t e d .
i m i e n t o a 18 C p r o v i s t a de vapor, y se r e g i s t r 6 la eft-
Our results demonstrate that the expression of horizon-
ciencia de inoculaci6n, p o r c e n t a j e de l e s i o n e s que no
t a l a n d v e r t i c a l r e s i s t a n c e w a s a f f e c t e d by t e m p e r a t u r e ;
c r e c i e r o n m&s all~ del p u n t o de inoculaci6n, densidad de e s p o r a n g i o s y AUDPC. E1 p o r c e n t a j e de l e s i o n e s que no c r e c i e r o n a u m e n t d con la t e m p e r a t u r a e n dos de los cul-
Accepted for publication 21 September 2004. ADDITIONALKEY WORDS: late blight, resistance mechanism, hypersensitive response
t i v a r e s m~s r e s i s t e n t e s (393295.236 y M a l i n c h e ) y el AUDPC rue m~s b a j o a 16 q u e a 24 C e n c u a t r o de los cinco c u l ~ v a r e s . La e f i c i e n c i a de l a i n o c u l a c i 6 n y la den-
140
AMERICAN JOURNAL OF POTATO RESEARCH
sidad de e s p o r a n g i o s no s e a f e c t 5 por c a m b i o s de tem-
Vol. 82
or decrease in luminance. In wheat, increasing temperatures
peratura. La d e n s i d a d de esporangios a u m e n t 6 a 16
inhibited the expression of resistance genes against Puccinia
horas de fotoperlodo; sin embargo, el ~rea f i n a l de hoja
graminis f.sp. tritici Eriks. and E. Henn (Harder et al. 1979).
i n f e c t a d a no se afect6. N u e s t r o s r e s u l t a d o s d e m u e s t r a n
These studies suggested a possible influence of temperature
que la t e m p e r a t u r a a f e c t 6 la e x p r e s i 6 n de las resisten-
and fight on the expression of resistance against pathogens.
cias vertical y horizontal; sin embargo, la c a t e g o r i z a c i 6 n
The relationship between host resistance and environ-
de la r e s i s t e n c i a r e l a t i v a e n t r e cultivares f u e la m i s m a
mental factors has relevance for the prediction of the behavior
e n los cuatro e x p e r i m e n t o s con t e m p e r a t u r a y fotop-
of potato cultivars in sites with different climatic conditions
e r i o d o s diferentes. S e a s u m e clue la r e s i s t e n c i a e n culti-
and also the effect of world climatic changes, mainly tempera-
v a r e s m e x i c a n o s p u e d e ser conferida por g e n e s m e n o r e s
ture, on the resistance of cultivars to P. infestans. This issue is
y por el e f e c t o r e s i d u a l de los g e n e s R v e n c i d o s . E s t o s
also related to the stability of the different kind of resistances
resultados
to late blight. Since the initial concepts of Van der Plank (1963)
e n f a t i z a n la d i f i c u l t a d de d i f e r e n c i a r la
r e s i s t e n c i a h o r i z o n t a l de la vertical.
concerning horizontal and vertical resistance, there has been consistent interest on the differentiation of both types of resis-
INTRODUCTION
tance. However, the distinction between the two types of resistance is still not clear. Recent evidence has shown that the
The effect of environmental factors on the development
hypersensitive reaction, which has been regarded as exclusive
of Phytophthora infestans (Mont.) de Bary on potato plants as
to plants with R genes, may occur in host plants with horizon-
well as the interaction between potato cultivar, environment,
tal as well as vertical resistance (Vleeshowers et al. 2000).
and pathogen have been studied extensively (Flores-Guti6rrez
Based on links between R genes and QTLs for quantitative
and Cadena-Hinojosa 1996; Grtmwald et al. 2002). However,
resistance, it has been stated that there is no difference
the systematic effect of individual environmental factors on
between genes controlling quantitative or qualitative resis-
the resistance of potato plants to late blight has received lim-
tance to potato late blight (Gebhardt and Valkonen 2001).
ited attention. Parker et al. (1992) demonstrated that the culti-
Their studies focused on comparing the expression of both
var Alpha, which is regarded as moderately susceptible to late
types of resistance under different temperatures, but interac-
blight, exhibited higher resistance in northern (New York,
tions with photoperiod were not included.
USA, Lat ca. 44 45) than in southern latitudes (Toluca, Mexico,
The report addresses the influence of temperature and
Lat 19). This work was done in the field and differences
photoperiod on the resistance to penetration and growth of P.
between sites may have been due to the presence of local
infestans in leaves of potato cultivars with different types and
races of P. infestans in Toluca, which could have influenced
levels of resistance. The objectives of this study were to eluci-
disease severity. Different climatic conditions in both sites
date the effects of temperature and photoperiod on disease
could have also influenced the development and growth of the
expression after inoculation of P. infestans onto foliar tissue
pathogen. However, observations made during field evalua-
of potato cultivars with and without R genes and diffenng in
tions (Rubio et al. 1998-2001, unpublished data) of Mexican
susceptibility to P. infestans that could influence the subse-
cultivars in different sites in Mexico have indicated that resis-
quent epidemic. The factors measured included expression of
tance may be influenced mainly by temperature. This hypoth-
development and growth of late blight lesions on leaves, the
esis required testing under controlled conditions to avoid
percentage of arrested lesions on leaves and sporulation
interference of the influence of climatic factors onP. infestans
potential.
growth and also to avoid interference due to the presence of different races of the pathogen.
MATERIALS AND METHODS
The influence of environmental factors on the expression of resistance against pathogens has been studied in other
Two potato cultivars (Malinche and Tollocan) with major
crops. Jenns and Leonard (1985) found that resistance to
R genes, two cultivars (393295.236 and 391046.22) without
Bipolaris maydis (Nisikado and Miyake) Shoemaker in dif-
known R genes and one susceptible cultivar (Atlantic; R1)
ferent lines of maize diminished with increase in temperature
were selected for this study. Malinche and Tollocan are Mexi-
RUBIO-COVARRUBIAS et al.: RESISTANCE TO P. INFESTANS
2005
141
can cultivars bred by INIFAP (National Institute for Forestry,
the isolate, it was continuously propagated on potato leaves of
Agriculture and Livestock Research). These cultivars possess
the cultivar CIP2. For each experiment, fresh sporangia were
moderate and high resistance against potato late blight,
harvested from two leaves (8 days after inoculation) and sus-
respectively. Their resistance is based on a combination of
pended in a glass beaker with 50 mL of sterile distilled water
unknown major and minor genes derived from Solanum
and incubated at 4 C. After 2 h incubation, the concentration
demissum. The cultivars 391046.22 (CIP1) and 393295.236
of zoospores was adjusted to 3x104 mL-1 for inoculation.
(CIP2) were bred in the CIP (International Potato Center) and
The primary and first pair of leaflets of the fourth leaf
form part of a group of cultivars identified by CIP as popula-
from each plant were detached. The three leaflets were placed
tion B3 whose sources of horizontal resistance are genetic
in a Petri dish with water-saturated filter paper and inoculated
materials without R genes derived from Solanum demissum
by pipetting a 10 pL droplet of inoculum suspension (3x104 of
(Landeo 1997). The cultivars CIP1 and CIP2 have been tested
zoospores mL 1) on the abaxial side of each leaflet.
in Toluca, Mexico, where they have shown medium and high
The Petri dishes with the inoculated leaflets were incu-
resistance against late blight respectively (C. Diaz and 0.
bated at 18 C in a dew chamber with a photoperiod of 12 h light
Rubio 2000, unpublished data). Both cultivars are part of the
(RH > 95%). Forty-eight hours after inoculation, leaflets were
group of materials known as population B derived from par-
used to assess their susceptibility to potato late blight by mea-
ents that did not show the hypersensitivity reaction after inoc-
suring the expansion of the lesions daily for 7 to 8 days after
ulation with race 0 of P. infestans, indicating absence of R
inoculation. The largest length (L) and width (W) of each
genes (J.A. Landeo, pers comm).
lesion were measured and used to calculate the lesion area
Plant Growth Conditions
under disease progress curve (AUDPC [Madden and Hughes
(A); (A = 0.25nLW) and the lesion growth rate (LGR). The area In vitro plantlets of each cultivar were grown for 3 wk in
1995]) was calculated with the expansion of the area of the
sterile plastic magentas, then transferred into pots of peat
lesions over time (7-8 days after inoculation). The inoculations
moss and placed in temperature-controlled environment
that did not result in lesions and the lesions that did not
chambers, 1.8 m ~ volume (Environmental Growth Chambers,
expand beyond the inoculum droplet area were recorded as
Chagrin Falls Ohio, USA). Ten plants (one per pot) of each cul-
arrested lesions and were not included in the estimation of the
tivar were grown during 32 days in independent growth cham-
LGR and AUDPC but were recorded as useful components of
bers
the late blight resistance response.
under the
four
combinations of two
constant
temperatures (16 and 24 C) and two photoperiods (PPD, 12
Sporangial density was quantified 8 to 9 days after inocu-
and 16 h day length). The relative humidity was programmed
lation. Six leaf tissue discs, 12.6 mm in diameter, were
in the climatic chambers at 80%, and the illunfinationwas pro-
removed with a cork borer from the infected tissue of two
vided by 500 W Philips-HPIT lamps. The fourth fully expanded
inoculated leaflets from each Petri dish. The sporangia were
compound leaves, counted from the apex of the main stem
washed from both sides of the discs by stirring them in 5 mL
from each plant, were detached 32 days after plants were incu-
of chilled (4 C) water, and the sporangia] density was deter-
bated at the temperature and photoperiod treatments.
mined with a hemacytometer. The average measurements of the lesions on the two inoculated leaflets in each Petri dish
Phytophthora infestans I s o l a t e a n d
were used for statistical analysis as a single observation. The
Inoculation
10 combinations of five cultivars and two temperatures (16
The P. infestans isolate CO-42 (mating type A2;
and 24 C) constituted a single experiment with a factorial
mefenoxam resistant; mt haplotype 1A; GPI 100/100; PEP
design, which was repeated twice at a photoperiod of 12 h and
100/100) collected in Mexico and kindly provided by K. Deahl,
another two times at a photoperiod of 16 h. The effect of pho-
USDA, was used in this study. The compatibility of this isolate
toperiod could not be statistically tested due to possible dif-
with all five potato cultivars was verified by inoculating
ferences in the experimental conditions among the four
detached leaflets of each cultivar prior to imposition of the
experiments. Four variables (percent of positive inoculations,
incubation treatments. In order to preserve the virulence of
percent of arrested lesions, AUDPC, and sporangia density)
142
A M E R I C A N J O U R N A L O F POTATO R E S E A R C H
Vol. 82
w e r e analyzed in e a c h o f t h e four e x p e r i m e n t s i n d e p e n d e n t l y
m e n t s 1, 2, a n d 3 the c o m p a r i s o n b e t w e e n cultivars indicated
as a factorial design with cultivar and t e m p e r a t u r e as the main
that t h e cultivar Malinche had the l o w e s t p e r c e n t a g e of posi-
effects (Proc GLM - SAS/Stat, SAS Institute, Cary, NC, USA).
tive inoculations.
Arrested Lesions
RESULTS
In the inoculated sites on t h e leaves, s o m e lesions did n o t
Infection Efficiency
e x p a n d b e y o n d the inoculum d r o p l e t a r e a during the 7 to 8
This p a r a m e t e r w a s m e a s u r e d as the p e r c e n t a g e o f the
days o f i n c u b a t i o n (Table 2). T h e s e a r r e s t e d lesions w e r e only
inoculated sites that d e v e l o p e d lesions on the leaves. Testing
in t h e cultivars CIP2 and Malinche, a n d it w a s evident t h a t
the effect o f t e m p e r a t u r e by cultivar interaction r e s u l t e d in
t h e s e a r r e s t e d lesions w e r e p r e s e n t in higher p e r c e n t a g e in
only t w o significant c o m p a r i s o n s , f r o m a total o f 20, in exper-
plants g r o w n at 16 t h a n at 24 C. The c o m p a r i s o n b e t w e e n cul-
i m e n t s 2 a n d 3 (Table 1). The average effect o f t e m p e r a t u r e
tivars s h o w s a higher p e r c e n t a g e in Malinche t h a n in CIP2 in
w a s not significant in any o f the four experiments. In experi-
three o f the four experiments.
TABLE 1--Percentage of inoculations of P h y t o p h t h o r a i n f e s t a n s resulting in lesions on leaves of f i v e potato cultivars grown under two photoperiods (12 and 16 h) and at two temperatures (16 and 24 C) inoculated with P. i n f e s t a n s in f o u r independent experiments.
Cultivar
CIP1 CIP2 Malinch~ ToUocan Atiantic MeanT ~
Percentage of inoculations with P. infestans resulting in lesions on the leaves 12 h photoperiod-16 h photoperiod Experiment 1 Experiment 2 Experiment 3 Experiment 4 Temperature (C) Temperature (C) Temperature (C) Temperature (C) 16 24 Mean 16 24 Mean 16 24 Mean 16 24 Mean 95 85 70 95 100 89
100 100 65 100 100 93
98a a 93a 68b 98a 100a
75 75 60 95 100 81
100*b 95 65 100 85 89
88a 85a 63b 98a 93a
100 90 100" 93 96 96
96 100 76 96 100 94
98a 95ab 88b 95ab 98a
100 96 96 100 100 98
96 100 96 100 100 98
98a 98a 97a 100a 100a
aMeans of cultivar in each experiment: values followed by the same letter are not statistically different at P < 0.05%, t test. ~rhe comparison of the means of each cultivar at the two temperatures is by t test (P < 0.05%) and the * indicates the highest value. cThe main effect of temperature in each experiment was tested by ANOVA (P < 0.05%) and the * indicates the highest value.
TABLE 2--Percentage of arrested lesions on leaves of f i v e potato cultivars grown under two photoperiods (12 and 16 h) and at two temperatures (16 and 24 C) inoculated with P h y t o p h t h o r a i n f e s t a n s in f o u r independent experiments.
Cultivar
Percentage of arrested lesions on leaves after inoculation with P. infestans 12 h photoperiod .16 h photoperio4 Experiment 1 Experiment 2 Experiment 3 Experiment 4 Temperature (C) Temperature (C) Temperature (C) Temperature (C) 16 24 Mean 16 24 Mean 16 24 Mean 16 24 Mean
CIP1 CIP2 Malinch~ Tollocan Atlantic Mean T~
0 0 25*b 0 0 5
0 0 10 0 0 2
0ba 0b 18a 0b 0b
0 5 20* 0 0 5*
0 0 0 0 0 0
0b 3b 10a 0b 0b
0 13" 7 0 0 4*
0 0 0 0 0 0
0b 7a 3ab 0b 0b
0 13" 17 0 0 6
0 0 17 0 0 3
aMeans of cultivar in each experiment: Values followed by the same letter are not statistically different at P < 0.05%, t test. LThe comparison of the means of each cultivar at the two temperatures is by t test (P < 0.05%) and the * indicates the highest value. ~I'he main effect of temperature in each experiment was tested by ANOVA (P < 0.05%) and the * indicates the highest value.
0b 7b 17~ 0b 0b
RUBIO-COVARRUBIAS et al.: RESISTANCE TO P. INFESTANS
2005
143
Tollocan, and Atlantic increased when grown at 24 C (Table 3).
A UDPC Three days after inoculation, the lesions in all five culti-
However, AUDPC in CIP1 decreased when grown at 24 C
vars were apparent and expanded very slowly during the fol-
(Table 3). CIP1 did not adapt well at 24 C, and the leaves gen-
lowing two days. Then after the fifth day, the lesions abruptly
erally looked wilted even though well watered. The interaction
increased their expansion rate (Figure 1). The same trend was
between cultivar and temperature, in three of the four experi-
observed in each of the four experiments and only the disease
ments, indicated that the effect of temperaRtre was significant
progress curves from experiment 1 are shown. The average
and the AUDPC was higher at 24 C than at 16 C. The comparison between cultivars indicated that Mal-
AUDPC of each experiment is presented in Table 3. The interaction between cultivar and temperature was significant in all
inche and CIP2 had lower AUDPC and Atlantic had the highest
experiments (data not shown) and AUDPC in CIP2, Malinche,
AUDPC (Table 3). Even though the effect of photoperiod could not be tested statistically, there seemed to be no effect of the photoperiod on the AUDPC of
1000 A
7
_.
CIP124°C CIP116°C
~) /,~i //y
Atlantic 24°C
800
g
Atlantic16°C Malinche24°C
(~) /~
the different cultivars (Table 3).
Sporangia Density There was no consistency in the effect of the temperature on the production of sporan-
600
gia (Table 4). Testing the effect of the temperature by cultivar interaction resulted in only
400
three significant comparisons, from a total of 20, favoring the sporangia density under 16 C
200
and three significant comparisons with higher
o
grown at 24 C. There was no significant effect
production of sporangia in plants that were
E E
lOOO
in the comparison of the average effect of the
C
ri q ~i
Tollocan24°C 800 -/;x- T°ll°can16°C
D
_.
CIP224°C CIP216°C
(,~) ~
temperature in any of the four experiments. The variation in sporangia density depended
)
mainly on the cultivar. Malinche, Tollocan, and CIP2 had the lowest density of sporangia per
600
)
unit area of lesion. These cultivars also had the lowest AUDPC values and the correlation
400
between sporangia density and AUDPC was significant (P < 0.01) in the four experiments
200
(data not shown). Even though the effect of photoperiod can not be tested statistically, it was evident that all cultivars had higher spo-
0
rangia density at 24 h than at 12 h of photope-
3
4
5
6
7
3
4
5
6
7
Days after inoculation F I G U R E 1 A-D. Effect o f t h e p r e - i n n o c u l a t o i n t e m p e r a t u r e ( 1 5 or 2 4 C) o n t h e e x p a n s i o n o f l e a f l e s i o n s o f Phytophthora infestans. The graphs a r e from e x p e r i m e n t 1 u n d e r a p h o t o p e r i o d o f 12 h. T h e v e r t i c a l bars i n d i c a t e t h e s t a n d a r d e r r o r o f t h e m e a n . T h e r e s p o n s e o f t h e cultivar Atlantic is i n c l u d e d o n e a c h o f t h e four graphs as a r e f e r e n c e . A: Malinch6; B, CIP1; C, Tollocan; D, CIP2.
riod (Table 4).
DISCUSSION This study dealt with changes in resistance to Phytophthora infestans in potato cultivars with and without R genes, induced by changes in temperature and photoperiod dur-
144
A M E R I C A N J O U R N A L O F POTATO R E S E A R C H
Vol. 82
TABLE 3---Average A UDPC of lesions on leaves inoculated with P h y t o p h t h o r a i n f e s t a n s of five potato cultivars grown under two photoperiods (12 and 16 h) and at two temperatures (16 and 24 C) in four independent experiments.
Cultivar CIP1 CIP2 Malinchd Tollocan Atlantic Mean Td
AUDPC of leaves inoculated with Phytophthora infestans -12 h photoperio4 16 h photoperiodExperiment I a Experiment 2 Experiment 3 Experiment 4 Temperature (C) Temperature (C) Temperature (C) Temperature (C) 16 24 Mean 16 24 Mean 16 24 Mean 16 24 Mean 660 300 169 611 698 488
517 644*¢ 550* 732 993* 687*
589bc b 472cd 359d 672b 845a
1041 570 359 1001 1341" 862
819 808 858* 1195 763 889
930a 689b 608b 1098a 1052a
582 321 258 425 965 510
739 872* 244 485 1130 694*
660b 597b 251d 455c 1047a
1117" 199 296 567 1184 673
723 673* 375 876* 1272 784*
920b 436d 336d 721c 1228a
aThe AUDPC in the expe~ments 1, 3, and 4 were calculated over a period of 7 days after inoculation and in experiment 2 over 8 days. bMeans of cultivar in each experiment: values followed by the same letter are not statistically different at P < 0.05%, t test. cThe comparison of the means of each cultivar at the two temperatures is by t test (P < 0.05%) and the * indicates the highest value. dThe main effect of temperature in each experiment was tested by ANOVA (P < 0.05%) and the * indicates the highest value.
TABLE 4--Average
sporangia density (number of sporangia m m -2) from lesions on leaves inoculated with P h y t o p h t h o r a of five potato cultivars grown under two photoperiods (12 and 16 h) and at two temperatures (16 and 24 C) in f o u r independent experiments.
infestans
Cultivar
Average density of sporangia (Number of sporangia mm-2) in leaves inoculated with P. infestans 12 h photoperiod-16 h photoperiod. Experiment 1 Experiment 2 Experiment 3 Experiment 4 Temperature (C) Temperature (C) Temperature (C) Temperature (C) 16 24 Mean 16 24 Mean 16 24 Mean 16 24 Mean
CIP1 CIP2 Malinch6 ToUocan Atlantic Mean Td
128 83 39 36 141 86
113 83 68*c 46 152 93
121bb 83c 54d 42d 147a
99 72 38 118 224* 110
142 151" 74 121 135 125
120b l12b 56c 120b 179a
215 180 106 77 259* 167
210 170 114 77 175 149
212a 175b 110c 77d 217a
293* 83 80 68 227 150
82 95 29 42 461" 142
188b 89c 54c 55c 344a
aAverage sporangia densities in experiments 1, 3, and 4 were measured 8 days after inoculation and in experiment 2 after 9 days. bMeans of cultivar in each experiment: values followed by the same letter are not statistically different at P < 0.05%, t test. cThe comparison of the means of each cultivar at the two temperatures is by t test (P < 0.05%) and the * indicates the highest value. dThe main effect of temperature in each experiment was tested by ANOVA (P < 0.05%) and the * indicates the highest value.
ing the pre-inoculation growing conditions of t h e plants. The
temperature. There is n o available i n f o r m a t i o n in the literature
results indicated that t e m p e r a t u r e h a d no effect o n t h e inocu-
a b o u t t h e pre-inoculation effect o f t e m p e r a t u r e o n each one o f
lation efficiency or s p o r a n g i a density; however, t h e p e r c e n t a g e
the r e s i s t a n c e c o m p o n e n t s of the p o t a t o late blight pathosys-
o f a r r e s t e d lesions i n c r e a s e d with t e m p e r a t u r e in t h e t w o m o s t
tem, a n d generally, the AUDPC h a s b e e n t a k e n as the final
resistant cultivars and t h e AUDPC w a s l o w e r at 16 t h a n at 24
result o f t h e e x p r e s s i o n o f resistance. The influence o f climatic
C in four of the five cultivars. These results suggest t h a t the
c o n d i t i o n s o n r e s i s t a n c e against
different c o m p o n e n t s o f r e s i s t a n c e may be differently a f f e c t e d
b e e n previously r e p o r t e d b y P a r k e r et al. (1992), w h o o b s e r v e d
by climatic conditions during the pre-inoculation period. Our
that the cultivar Alpha had higher AUDPC in Freeville, NY,
P. infestans in p o t a t o has
o b s e r v a t i o n s are similar t o J e n n s a n d Leonard (1985) in t h a t
USA, t h a n in Toluca, Mexico. In t h a t study, the average tem-
lesion length and sporulation i n c r e a s e d with increasing tem-
p e r a t u r e during the epidemic w a s l o w e r in Toluca (18-20 C)
withB, maydis, the
than in Freeo.11e (20-22 C). This m a y b e i n t e r p r e t e d as a nega-
infection efficiency w a s n o t affected by the pre-inoculation
tive effect o f t h e t e m p e r a t u r e o n t h e r e s i s t a n c e to P. infestans,
perature, and, as in c o r n plants inoculated
2005
RUBIO-COVARRUBIAS et al.: RESISTANCE TO P. INFESTANS
145
which is opposite to what was observed in our study. However,
lated the AUDPC with the expansion of the area of the lesions
it is important to point out the study of Parker et al. (1992) was
in a period of 7 to 8 days after inoculation; however, an AUDPC
carried out in the field and the influence of different races of P.
based on the percentage of foliar infection during the total life
infestans and of different climatic conditions on the growth of
of the plant may lead to different results because the length of
the pathogen could not be eliminated. The present study was
the vegetative phase of the plants may be changed by environ-
conducted under controlled conditions and results clearly
mental factors.
indicated a negative effect of higher temperature (24 C) on the
Our results obtained under controlled conditions in
total area of infected leaf tissue. Our results are in agreement
growth chambers, confirmed what was previously observed in
with the results of Jenns and Leonard (1985), who observed
field tests in Toluca, Mexico (C. Diaz and O. Rubio 2002,
that different maize lines diminished their resistance to B.
unpublished data) where Malinche and CIP2 had high resis-
polaris maydis with increasing temperature, and also with the
tance to late blight, CIP1 and Tollocan medium resistance, and
fmdings of Harder et al. (1979), who demonstrated that
Atlantic was susceptible. Comparing the five cultivars, it
increasing temperatures inhibited the expression of resistance
seems that regardless of their type and level of resistance, their
genes against P. graminis f.sp. tritici in wheat. The effect of photoperiod during plant growth on the per-
relative resistance (resistance ranking among the five cultivars) was not affected by changes in temperature and pho-
centage of successful inoculations, the percentage of arrested
toperiod; however, the AUDPC values were
lesions and the AUDPC did not show a clear trend. The pho-
differently by temperature. All the cultivars, except CIP1,
affected
toperiod only had an observable effect on the increment of
showed a general trend of lower AUDPC at the lower temper-
sporangia density under 16 h of day length, which did not
ature (16 C). The cultivar CIP1 showed no adaptation to the
affect the fmal infected leaf area. This is not in agreement with
high temperature (24 C), and the extreme heat stress may have
the postulations of Parker et al. (1992), who suggested that
induced some resistance to P. infestans in the wilted leaves.
photoperiod may be the most important environmental factor
The comparison of the tested potato cultivars provides
that influenced the susceptibility to P. infestans of the potato
information for a better understanding of horizontal and verti-
cultivar Alpha in Toluca, Mexico, and in Freeville, NY. The dis-
cal resistance. Horizontal or quantitative resistance to late
crepancy between results of these two studies may be
blight has been described as stable and durable (Parlevliet
explained by the different experimental conditions mentioned
1979; Van der Planck 1968). In the present study, it was demon-
above. The effect of photoperiod could not be statistically
strated that the resistances of cultivars with horizontal resis-
tested in this study and further experiments are needed to clar-
tance (CIP1 and CIP2) and those with a combInation of
ify this issue.
horizontal and vertical resistance (Malinche and Tollocan)
Similarly to results reported earlier (Krug 1965; Steward
were affected by temperature; however, their resistance rank-
et al. 1981), we observed that the lower temperature and the
ing was the same in the four experiments with different tem-
shorter photoperiod induced earliness in all the tested potato
peratures and photoperiods. Therefore, the tested cultivars
cultivars. Using molecular techniques, it has been shown that
with horizontal and vertical resistance were not stable in
resistance to late blight in potato is correlated with maturity
terms of their susceptibility to infection but were stable in
and vigor (Collins et al. 1999; Visker et al. 2003a). It was there-
terms of their relative resistance expression.
fore expected that a resistant cultivar may express decreased
It is well known that the durability of resistance based on
resistance as a consequence of its reduction in maturity under
major (R) genes can be shortened by continuous changes in
the influence of a lower temperature and shorter photoperiod.
the pathogen (Goodwin et al. 1995; Van der Planck 1968); how-
This effect was not observed in our study. A partial explana-
ever the combination of major and minor resistance genes has
tion is that changes of resistance with respect to leaf position,
been a successful strategy in many Mexican potato cultivars
plant age, and leaf age (Dorrance and Inglis 1997; Visker et al.
selected under natural infection conditions in the Toluca Val-
2003a), were avoided in our study by inoculating leaves from
ley, Mexico (Flores-Guti~rrez and Cadena-Hinojosa 1996;
the same position on the plant and before tuber growth
Grtinwald et al. 2002). The four resistant potato cultivars used
started. The methodology for measuring the infection severity
in this study were previously tested in the field in Toluca,
may also influence the interpretation of the results. We calcu-
where at least 11 known races of P. infestans are present, and
146
A M E R I C A N J O U R N A L O F POTATO R E S E A R C H
it is e x p e c t e d that t h e r e s i s t a n c e of the four cultivars m a y be durable. Resistance in the cultivars free o f R g e n e s (CIP1 and CIP2) is a s s u m e d to b e c o n f e r r e d by m i n o r g e n e s (Landeo et al. 2000). The o b s e r v e d r e s i s t a n c e in the Mexican cultivars (Malinche and Tollocan) after inoculation with a c o m p a t i b l e isolate o f P. infestans m a y b e c o n f e r r e d by m i n o r r e s i s t a n c e g e n e s and by the residual effect of d e f e a t e d R genes, w h i c h w a s previously d e m o n s t r a t e d in p o t a t o (Stewart et al. 2003) a n d o t h e r c r o p s (Durel et al. 2000; Li et al. 1999). In conclusion, our results indicate that r e s i s t a n c e against
P. infestans in potato cultivars with and without R g e n e s can b e affected by the t e m p e r a t u r e at w h i c h the plants have b e e n growing before inoculation. The general t e n d e n c y w a s to d e c r e a s e resistance w i t h i n c r e a s e in t e m p e r a t u r e f r o m 16 to 24 C; however, there may b e s o m e cultivars that r e s p o n d differently. The relative stability o f the resistance a m o n g t h e t e s t e d cultivars did n o t c h a n g e w i t h c h a n g e s in t e m p e r a t u r e a n d photoperiod. These results e m p h a s i z e the difficulty in differentiating horizontal and vertical r e s i s t a n c e and c o n f m n that a c o m b i n a t i o n o f b o t h kinds o f r e s i s t a n c e is the b e s t strategy to develop cultivars with stable a n d durable resistance.
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