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Plant Pathology (2000) 49, 389± 395
Characterization of new sources of resistance to Uromyces
viciae-fabae in a germplasm collection of Vicia faba
J. C. Sillero
a
, M. T. Moreno
a
and D. Rubiales
b
*²
a
CIFA, Dep. Mejora y Agronomõ
Âa, Apdo. 4042, 14080-Co
Ârdoba; and
b
CSIC, Instituto de Agricultura Sostenible, Apdo. 4084, 14080-
Co
Ârdoba, Spain
A collection of 648 accessions of Vicia faba was screened for resistance to faba bean rust (Uromyces viciae-fabae).
Two distinct types of resistance were identified, both resulting in reduced disease severity (DS) and area under the
disease progress curve (AUDPC), but differing in the expression of hypersensitivity. One should be regarded as
incomplete nonhypersensitive resistance and the other as incomplete resistance with late hypersensitivity. The
hypersensitive resistance, which has not been reported before, was not dependent on temperature or plant age. These
two types of resistance were characterized by three macroscopic components of resistance: increased latent period
(LP), decreased colony size (CS) and a relatively reduced infection frequency (IF), both on seedlings and on adult
plants. LP and CS were the components of nonhypersensitive resistance most highly correlated with DS and AUDPC
measured under field conditions. The presence of necrosis was an additional component in the hypersensitive
resistant response.
Keywords: colony size, faba-bean, hypersensitivity, latent period, necrosis, screening for rust resistance
Introduction
Faba bean rust (Uromyces viciae-fabae) is a disease
present in almost every area of the world where faba
beans (Vicia faba) are grown. It is a major disease in the
Middle East and North Africa. Normally, faba bean rust
epidemics begin late in the season, when pod filling has
started, so yield components are little affected by the
infection, and losses usually range from 5 to 20%.
However, when the infection starts early in the season
severe epidemics can occur and yield losses as high as
70% have been reported (Liang, 1986; Rashid &
Bernier, 1991).
Methods of rust control have been developed, such as
the use of fungicides (Yeoman et al., 1987; Marcellos
et al., 1995), but the use of genetic resistance is a more
desirable and efficient strategy. Several sources of
resistance to U. viciae-fabae have been reported.
However, most of the reports do not provide a detailed
description of the type of resistance. According to some
reports, the resistance in faba bean to U. viciae-fabae is
mainly quantitative in nature and is detected as a
reduction of disease severity in the field. Race-specific
resistance to rust has been reported in V. faba (Conner
& Bernier, 1982); this study was, however, performed
using a descriptive infection-type (IT) key based on
pustule size, rather than on the more generally adopted
presence or absence of necrosis. Rashid & Bernier
(1984) observed occasional chlorotic and necrotic zones
surrounding rust pustules but they found these to be
affected more by environmental conditions than by host
genotypes. They did not consider such aspects useful in
characterization of the infection type.
The purpose of this study was to identify sources of
resistance to U. viciae-fabae and to describe the
macroscopic components of resistance involved.
Materials and methods
Field experiments
A collection of 648 V. faba lines was studied for rust
resistance under field conditions at Co
Ârdoba, Spain, in
1995±96. The faba bean accessions, which originated
from around the world, belonged to the CIFA (Co
Âr-
doba, Spain) germplasm collections, or had been kindly
provided by ICARDA (Aleppo, Syria) or Prof. Bernier
(University of Manitoba, Canada). Each line was sown
in 1-m rows that were 0´7 m apart, with 10 plants per
row. The susceptible control cv. Troy was distributed
throughout the trial. Plants were inoculated three times
from mid-January, at two-week intervals, to ensure high
and uniform levels of rust infection. Inoculation
consisted of spraying with an aqueous suspension of
Q2000 BSPP 389
*To whom correspondence should be addressed.
²E-mail: ge2ruozd@uco.es
Accepted 5 January 2000.
390 J. C. Sillero et al:
rust urediospores from a bulk population collected at
Co
Ârdoba. The urediospores were suspended in tap
water (1±2 £10
5
spores mL
21
), to which Tween-20
(0´03%, v : v) was added. Plants were inoculated after
sunset to benefit from the darkness and high relative
humidity at night. To maintain high relative humidity
during spring, plots were irrigated for 10 min, 4 times a
day, with microsprinklers. At maturity, the final disease
severity (DS) and infection type (IT) were assessed. The
IT scale of Stakman et al. (1962) was used, where
IT 0 no symptoms, IT; necrotic flecks, IT 1
minute pustules barely sporulating, IT 2 necrotic
halo surrounding small pustules, IT 3 chlorotic
halo, and IT 4 well-formed pustules with no asso-
ciated chlorosis or necrosis.
Twenty lines with low DS or low IT values were
selected for further field studies during the 1996± 97 and
1998±99 seasons. Lines VF-176 and VF-280 were
included as susceptible controls. Some of the faba
bean accessions were pure lines, others were faba bean
landraces. Lines were sown in double 1-m rows
(separated by 0´7 m), with five plants per row. A
randomized block design with three replications was
used. Inoculation was performed as described above.
When rust development started, disease severity was
assessed at two-week intervals by a visual estimation of
the leaf area covered with rust pustules. These data were
used to calculate the area under the disease progress
curve (AUDPC), using the formula:
AUDPC X
k
i1
1
2Si1Si11ti112ti
where S
i
is the rust severity at assessment date i,t
i
is the
number of days after the first observation on assessment
date iand kis the number of successive observations.
The means of the observed AUDPC values were
converted into relative values and expressed as a
percentage of the susceptible line VF-280.
The epidemic growth rate (r) was obtained by linear
regression of the transformed DS values against time:
transDSlogit DS110
ln DS/100 2DS110
Ten was added in this formula to obtain positive
numbers. The regression coefficients were considered
to be analogous to the apparent growth rate described
by Zadoks (1963).
For lines that segregated for hypersensitive resistance,
DS of resistant and susceptible plants within the line
were evaluated separately. Seeds were collected from the
hypersensitive resistant plants after selfing, to be sown
the following season.
In 1997±98, the development of the lines was
measured as the number of days from sowing to
flowering and to pod-setting.
Growth chamber experiments
Lines that displayed resistance in the field were studied
in a growth chamber experiment to determine the
components of their resistance. Seven of the lines with
the lowest DS and AUDPC values in the field were
selected. Four of them had low IT (,3) and three had
high IT ($3). The susceptible line VF-176 was included
as a control.
Three consecutive experiments were carried out, each
with 12 plants per genotype, using both seedling and
adult plant tests. In the seedling tests, two plants per line
were sown in plant boxes of 35 £35 cm in two parallel
rows. Each replicate consisted of three plant boxes, each
box containing two plants of each line. In the adult
plant tests, plants were sown in 2-L plastic pots, with
two plants per pot.
Seedlings were inoculated when the second leaf had
completely expanded, and adult plants when 10±12
leaves had expanded. Inoculation was carried out by
dusting the plants with rust urediospores (2 mg spores
plant
21
on seedlings, 5 mg spores plant
21
on adult
plants) diluted in pure talc (1 : 10) using a spore
settling tower. Plants were then incubated for 24 h in
an incubation chamber at 208C in complete darkness at
100% relative humidity, and subsequently maintained
in a growth chamber at 208C with a 14-h photoperiod
(8000 lux).
The components of resistance measured in this
experiment were: infection type (IT), latent period
(LP), infection frequency (IF) and colony size (CS). IT
was estimated according to the 0±4 scale of Stakman
et al. (1962; see above for classes). LP, the period of time
between inoculation and sporulation of 50% of the
pustules, was determined by counting daily the number
of uredosori visible in a 1´5-cm
2
marked area on the
leaves, using a pocket lens (magnification £7), until the
number of uredosori no longer increased. The time at
which 50% of the final number of uredosori had
appeared was estimated by interpolation. IF, the number
of pustules per unit area, was calculated from the same
area in which LP was estimated. CS was measured using
a Leica DM-LB epifluorescence microscope (magnifica-
tion £100, excitation filter 385±425 nm) with a
micrometer. Eight days after inoculation, samples of
leaf (2±3 cm
2
) were cut and stained with Uvitex 2B
according to Rohringer et al. (1977). The length (L) and
width (W) of 25 randomly chosen colonies per leaf were
measured. CS was calculated using the formula:
CS pLW/4. For statistical analysis, when the compo-
nents of resistance were expressed as percentages, data
were transformed with the formula: arcsin (p(1/x)); CS
data were transformed to square roots (Niks, 1986) to
obtain a linear rather than a quadratic value for colony
area. The means of the observed values of LP and IF
were converted into relative values per experiment
Q2000 BSPP Plant Pathology (2000) 49, 389± 395
391Resistance to Uromyces in Vicia faba
(seedlings or adult plants) by expressing them as
percentages of the susceptible control, VF-176
(100%).
To estimate the influence of temperature on the
occurrence of hypersensitive reactions, 30 seedlings of
each of the seven hypersensitive lines were sown in 2 L
plastic plant boxes. Plants were inoculated and incu-
bated overnight in darkness at 208C, as described above.
Following inoculation, 10 plants were maintained at
108C, 10 plants at 208C and 10 at 258C. Infection type
was observed 20 days after inoculation, when sporula-
tion occurred profusely in the susceptible plants and
hypersensitivity in resistant plants was clear. The same
plants were also studied as adults (10- to 12-leaf stage),
following the same procedure.
Results
Field experiments
A wide range of disease susceptibility was found in the
germplasm collection. High susceptibility to rust was
very common, but good levels of resistance were also
found. Six out of the 648 lines studied showed low
infection levels, with less than 20% of the leaf area
covered with pustules. Fifty lines showed moderate
infection levels, with DS values between 20 and 40%.
However, most of the accessions (91%) displayed DS
values higher than 40%, with some more than 70%.
High IT values, indicating a nonhypersensitive reac-
tion, were observed on almost all the lines. However,
seven lines with low disease severity displayed segrega-
tion for hypersensitivity. These plants produced small
pustules surrounded by necrotic areas. This resistance
response was first noticed late in the growing season,
suggesting that it could be a result of an adult plant
resistance or to a resistance induced by high tempera-
tures. Data presented in Table 1 show that six of the
seven lines displayed a hypersensitive response (IT ,3)
at all three temperatures tested, demonstrating that
their hypersensitivity did not depend on temperature.
However, hypersensitivity on the line V-313 was
expressed only at 258C and that on V-300 was more
frequent at higher temperatures. Plants that displayed
hypersensitivity as seedlings also did so as adult plants
and, in some genotypes, the hypersensitive response was
stronger in adult plants. Seedlings that displayed IT 2
often showed IT ; as adult plants. Hypersensitive
resistance was confirmed in the field during the seasons
1996±97, 1997± 98 and 1998± 99 (Table 2). Hypersen-
sitivity could already be observed at the seedling stage,
when temperatures were moderate. Accessions in which
hypersensitivity was found also contained susceptible
plants. The field plots were covered with a nylon mesh
to exclude insects and to avoid cross-pollination, so
plants with low IT were selfed and their seeds sown the
next season. During the three seasons the plants
showing hypersensitivity (low IT) also had the lowest
DS and AUDPC values, generally showing DS values
lower than 20% and AUDPC values between 10 and
30% of the susceptible control.
Another 12 lines showed rather low DS and AUDPC
values but with a high IT. In addition, those segregating
plants from the lines displaying hypersensitivity that
showed high IT also displayed low DS and AUDPC
values, so they should also be considered slow-rusting
accessions. DS and AUDPC were not correlated with
speed of maturity (days from sowing to flowering or
pod-setting).
The percentage of plants showing hypersensitivity in
1998±99 (Table 2), after repeated annual selection and
selfing, suggests either a 3 : 1 segregation ratio or
complete homozygosity, which would imply a single
dominant gene encoding for this resistance.
Components of resistance
In the growth chamber experiment, the susceptible
control VF-176 had a fully compatible reaction type
(IT 4) (Table 3). The IT of the remaining lines was
either low or high, both in seedlings and in adult plants,
in agreement with field observations. However, in some
lines hypersensitivity was stronger in adult plants than
in seedlings, as necrotic flecks (IT ;) were observed on
some adult plants that had displayed IT 2 at the
seedling stage. As in the field, there was some
segregation for IT reaction in hypersensitive lines.
Plants from those lines that showed IT 4 were
discarded.
Latent period was longer on adult plants than on
seedlings (Table 3). The longest LP values were mea-
sured on the hypersensitive lines, both at the seedling
and adult plant stages. Only one of the nonhypersensi-
tive slow-rusting lines, BPL-261, showed longer LP than
the susceptible control at both plant development
stages. The longest LP was found on the lines with
lowest DS, AUDPC and rin the field. Infection
frequency was lower on adult plants than on seedlings,
even though the spore concentration at inoculation was
higher in the adult plant experiment. On all resistant
lines the IF tended to be lower than on the control, but
was significantly lower only on four of the lines (VF-16,
Q2000 BSPP Plant Pathology (2000) 49, 389± 395
Table 1 Percentage of plants (seedlings and adults
a
) showing a
hypersensitive response (IT
b
,3) at three different temperatures
Lines 108C208C258C
V-300 12´5 22´2 70´0
V-313 0´0 0´0 55´6
V-1271 44´4 30´0 42´9
V-1272 36´4 40´0 55´6
V-1273 40´0 30´0 50´0
V-1335 10´0 10´0 16´7
2N34 44´4 60´0 60´0
a
The same plants were studied in at the seedling and adult plant stage,
and gave a similar reaction type at both stages.
b
IT, infection type; 0± 4 scale according to Stakman et al. (1962).
392 J. C. Sillero et al:
Q2000 BSPP Plant Pathology (2000) 49, 389± 395
Table 3 Macroscopical components of resistance of selected faba bean lines resistant to rust, U. viciae-fabae, under controlled conditions
and in the field (season 1995± 96)
a
Seedling Adult plant Field data (1995± 96)
b
Line
Infection
type
c
Latent
period
d
Infection
frequency
d
Colony
size
e
Infection
type
c
Latent
period
d
Infection
frequency
d
Colony
size
e
Infection
type
c
DS AUDPC
d
r
V-1273 2± 21136´0 a 70´0 ab 44´3 c ; 22 133´6 c 77´3 ab 6´1 h 2 12 d 16 d 0´075 cd
2N-34 ; 221130´4 ab 53´4 b 32´0 cd ; 22 121´6 d 81´6 ab 8´4 f 2 13 d 20 cd 0´070 d
V-300 21127´3 ab 74´0 ab 36´7 cd 2 149´3 a 68´9 bc 8´7 e 2 110 d 14 d 0´068 d
V-1271 2± 21124´3 b 58´7 b 28´2 d ; 22 136´1 b 29´1 c 6´3 g 2 10 d 15 d 0´072 d
BPL-261 4 111´9 c 60´7 ab 75´3 b 4 118´8 e 50´5 ab 23´9 d 4 25 c 24 bc 0´073 d
VF-16 4 104´9 cd 52´1 b 164´7 a 4 98´1 g 70´3 ab 92´5 a 4 33 b 47 b 0´086 b
VF-40 4 101´0 d 77´6 ab 142´1 a 4 98´0 h 78´8 ab 48´2 c 4 35 b 44 b 0´081 bc
VF-176 4 100 d 100 a 152´2 a 4 100 f 100 a 74´9 b 4 67 a 100 a 0´095 a
a
Data with the same letter, per column, are not significantly different (Duncan, P,0´05).
b
Field parameters measured: DS, disease severity; AUDPC, area under the disease progress curve; r, epidemic growth rate. When the line
segregated for hypersensitivity, the susceptible plants were discarded and only those plants showing hypersensitivity were evaluated.
c
Infection type according to 0± 4 scale of Stakman et al. (1962).
d
Components of resistance expressed as values relative to the susceptible control, VF-176 (100%). The actual values for VF-176 are: latent
period (seedling), 176 h; infection frequency (seedling), 70 pustules cm
22
; latent period (adult plant), 224 h; infection frequencey (adult plant), 44
pustules cm
±2
; AUDPC, 2485 units.
e
Colony size, £10
22
mm
2
´
Table 2 Response to rust, U. viciae-fabae, of selected faba bean lines in the field. Growing seasons 1996 ±97 to 1998 ±99
Growing season 1996± 97 Growing season 1997± 98 Growing season 1998± 99
Line
a
DS
b
AUDPC
c
%Hpl
d
DS
b
AUDPC
c
%Hpl
d
DS
b
AUDPC
c
%H pl
d
2N34-n46´7 56´48 30 54´29 30 44´8
2N34-R 13´3 19´93 36´4 17´5 21´61 71´4 11´7 12´5 75
2N52-n60 73´3 ± ± ± ±
2N52-R 11´7 17´13 25 13´33 11´28 100 8´7 5´8 100
V-300-n40 52´13 40 59´27 30 55´2
V-300-R 10 13´85 36´8 10 14´08 30 6´7 8´1 73´3
V-313-n40 37´25 20 32´09 30 37´4
V-313-R 20 22´05 28´6 10 29´89 11´1 6´7 13´2 68´8
V-1271-n53´3 55´2 36´67 55´19 ± ±
V-1271-R 13´3 14´45 45´5 11´67 14´37 69´2 5 6´9 100
V-1272-n46´7 55´15 30 36´05 ± ±
V-1272-R 10 14´27 57´1 10 12´71 85´7 6´7 8´9 100
V-1273-n33´3 41´91 30 27´78 28´3 44´1
V-1273-R 11´7 11´95 60 10 16´04 72´2 5´7 7´6 75
V-1335-n56´7 62´92 43´33 52´69 46´7 63´0
V-1335-R 30 40´1 10 ± ± 0 ± ± 0
2N29 40 60´52 50 68´25 20 38´0
BPL-261 46´7 46´6 25 24´23 25 35´3
ILB 938 40 39´9 nd nd 28´3 32´6
Primus 10 21´29 30 49´06 23´3 34´6
VF-16 46´7 52´5 40 43´68 38´3 40´9
VF-36 40 43´21 40 38´67 46´7 64´1
VF-40 46´7 50´52 30 42´3 26´7 35´5
VF-47 36´7 41´93 30 39´85 28´3 37´6
VF-50 33´3 41´87 40 51 20 31´3
VF-59 50 57´54 43´33 54´46 40 49´8
VF-131 46´7 59´71 46´67 62´06 30 41´8
VF-159 50 47´82 43´33 52´69 30 40´3
VF-176 66´7 84´13 66´67 93´27 60 91´4
VF-280 66´7 100 70 100 65 100
a
As segregation for IT was found in some lines, plants with low and high IT were scored separately; -nfollowing the name of a line indicates those
plants of the line which showed high IT; -R following the name of a line indicates those plants of the line which showed hypersensitivity.
b
DS: final disease severity.
c
AUDPC: area under the disease progress curve, expressed as percentage of the most susceptible line value, VF-280 (AUDPC 3618 in 1996±
97; 2961 in 1997± 98; 2740 in 1998±99).
d
% H pl: percentage of plants within a line that showed a hypersensitive response. Only those plants with this response were selfed and sown the
following season. This procedure was repeated annually.
393Resistance to Uromyces in Vicia faba
2N-34 and V-1271 as seedlings and V-1271 and V-300
as adult plants).
Colonies measured at 8 days after inoculation were
larger on seedlings than on adult plants. In the
susceptible control, the CS on seedlings was almost
double that on adult plants. The hypersensitive lines
showed the smallest colonies at both plant maturity
stages. Two of the nonhypersensitive slow-rusting lines
showed smaller colonies than the control (line BPL-261
on both seedlings and adult plants, and line VF-40 only
on adult plants).
In the field studies, all the lines showed lower DS and
AUDPC values than the susceptible control (Table 3).
DS and AUDPC were particularly low on the hypersen-
sitive lines, followed by BPL-261. The epidemic growth
rate (r) was significantly lower than the control value on
all the lines, especially on the hypersensitive lines and
the line BPL-261, followed by the other two slow-
rusting lines.
Pearson's linear correlations between the components
of resistance of the resistant lines (Table 4) showed that
LP was highly and negatively correlated with CS, both
on seedlings and on adult plants. There was also a good
correlation between the LP values measured on seed-
lings and on adult plants, as well as between the CS at
both plant development stages. Correlations between IF
and the other two components of resistance were not
significant.
For the resistant lines, the latent period on seedlings
and adult plants was negatively correlated with the
three epidemiological parameters measured in the field,
DS, AUDPC and r(Table 5). CS at both development
stages was correlated with DS and AUDPC, but only at
the seedling stage with r. IF was correlated with AUDPC
only when measured on seedlings. The epidemiological
parameters DS, AUDPC and rwere also compared using
Pearson's linear correlation coefficients (Table 5). The
correlation between DS and AUDPC was very high and
significant (P.0´001), but there was no correlation
between either of these field parameters and r.
Discussion
Two distinct types of resistance were identified in the
collection, both resulting in reduced DS and AUDPC,
but differing in the occurrence of hypersensitivity. The
resistance of one of the lines (BPL-261) should be
regarded as incomplete nonhypersensitive (partial resis-
tance sensu Parlevliet, 1979), and that of the others as
incomplete resistance with late hypersensitivity (V-300,
V-1271, V-1273 and 2N-34, as well as V-313, V-1272,
V-1335 and 2N-52). Such a hypersensitive response has
not been reported before in faba bean in field tests.
Rashid & Bernier (1984) observed occasional chlorotic
and necrotic zones surrounding rust pustules but they
considered that these were influenced more by environ-
mental conditions than by host genotypes. They found
various levels of resistance when some lines were
inoculated with certain races, but the main resistance
component they reported was reduced pustule size,
rather than the presence of necrosis. They also found
lines with absence of symptoms (IT 0), but no clear
hypersensitivity. Polignano et al. (1990) suggested
hypersensitivity in a greenhouse test on material of
Ethiopian origin, but did not detect hypersensitivity
under field conditions. As part of the present work,
their lines were studied in growth chambers using a
Spanish and an Italian isolate, and in the field with only
the Spanish isolate (data not presented), and no
hypersensitivity was found.
Q2000 BSPP Plant Pathology (2000) 49, 389± 395
Table 4 Pearson's linear correlation
coefficient between latent period (LP),
infection frequency (IF) and colony size (CS)
in seedlings and adult plants, under growth
chamber conditions
Seedling stage Adult plant stage
LP IF CS LP IF
Seedling
LP
IF 20´40 ns
CS 20´92** 0´37 ns
Adult plant
LP 0´86** 20´21 ns 20´91**
IF 20´27 ns 0´59 ns 0´48 ns 0´46 ns
CS 20´85** 0´27 ns 0´96*** 20´86** 0´43 ns
**Significant at P,0´01; ***significant at P,0´001; ns, not significant.
Table 5 Pearson's linear correlation coefficient between latent period (LP), infection frequency (IF) and colony size (CS) measured on
seedlings and adult plants in a growth chamber, and between disease severity (DS), area under the disease progress curve (AUDPC) and
epidemic growth rate (r) measured in the field
Seedling stage Adult plant stage Field parameters
LP IF CS LP IF CS DS AUDPC
DS 20´84** 0´69 ns 0´84** 20´79* 0´59 ns 0´81*
AUDPC 20´76* 0´72* 0´78* 20´72* 0´63 ns 0´77* 0´98***
r20´71* 0´21 ns 0´79* 20´89** 0´50 ns 0´69 ns 0´60 ns 0´55 ns
*Significant at P,0´05; **significant at P,0´01; ***significant at P,0´001; ns, not significant.
394 J. C. Sillero et al:
The hypersensitive resistance against faba bean rust
found in the present work does not depend on
environmental conditions, as had been suggested in
previous reports (Rashid & Bernier, 1984). Clear
hypersensitivity was detected both under field and
growth chamber conditions at different temperatures.
On all resistant lines, a reduced DS and AUDPC in the
field, and an increased latent period, a decreased colony
size and a relatively decreased infection frequency of the
rust under controlled conditions, were observed, both in
seedlings and in adult plants. The two distinct types of
resistance found in the material used here differ in the
presence of necrosis in the hypersensitive lines, while
partial resistance is characterized by a susceptible
infection type.
The epidemiological parameters DS and AUDPC
were highly correlated in the present work. This
correlation has also been found by other authors with
faba bean rust (Rashid & Bernier, 1986b) as well as
wheat leaf rust (Broers, 1989) and wheat yellow rust
(Broers et al., 1996). In the current study, DS and
AUDPC were not correlated with r. However, such a
correlation has been observed in wheat rust (Broers,
1989; Broers et al., 1996). The present findings agree
with those of Rashid & Bernier (1986b) in indicating
that DS and AUDPC are more informative than rwhen
assessing the level of resistance of faba bean. The
components of resistance measured under controlled
conditions are better correlated with DS and AUDPC
than with the apparent infection rate, r. In addition, ris
a regression coefficient with a large error variance in
field evaluations, as reported by Broers et al. (1996) in
the yellow rust/wheat pathosystem. A greater number of
observations would be needed to obtain a more accurate
estimation of r.
LP and CS were found to be the most reliable
components of quantitative resistance, being most
closely correlated with DS and AUDPC. Bhalla &
Bernier (1984) found IF to be the most important
component of slow-rusting resistance to U. viciae-fabae.
However, in the present study IF was poorly correlated
with other components, even when special care was
taken to apply inoculum uniformly. Low IF seems to be
a component of resistance only in some lines. In the faba
bean collection used, only one line, BPL-261, has low
DS and AUDPC values as a result of a low IF, in
addition to a long LP. The quantitative resistance found
in the field in lines VF-16 and VF-40 could not be
explained sufficiently either by a reduction in IF or by a
prolonged LP.
The colonies on seedlings were larger than on adult
plants in all the genotypes studied. LP and CS were
negatively correlated because CS was measured at a
fixed time (8 days after inoculation) and both para-
meters depend on colony growth rate, so the pustules
appearing late resulted from smaller colonies. It has yet
to be established whether colony growth is permanently
retarded in the lines with the two postulated types of
resistance. In wheat leaf rust, colony growth in partially
resistant genotypes was continuously retarded, com-
pared with the susceptible control (Jacobs & Buurlage,
1990).
Niks (1986) found that the prolonged LP and reduced
IF and CS of Puccinia hordei on partially resistant
barley resulted from a continuous failure in haustorium
formation. Infection units that failed to form any
haustoria were responsible for a reduced IF, and the
infection units that formed primary haustoria but few
subsequent haustoria explained the reduced colony
growth rate, and hence resulted in a reduced CS and
prolonged LP. The present findings of a lack of
correlation between IF and the other two components
suggest that reduced infectibility arising from early
abortion of colonies is not as important as in the barley/
P. hordei system. Studies of the histology of the
infection process will help to identify the mechanisms
involved in both partial and hypersensitive resistance.
The material in which a hypersensitive response was
found originated from various regions of the world. Five
of the lines (V-300, V-313, V-1271, V-1272 and V-1273)
came from Central Europe, one (2N34) from East
Africa, one (V-1335) from Spain and one (2N52) was of
unknown origin. The segregation found in selfed
hypersensitive plants suggest that this type of resistance
to U. viciae-fabae in faba bean was controlled by a
single dominant gene. This is in accordance with the
results of Conner & Bernier (1982) and Rashid &
Bernier (1986a), although they described resistance
based on pustule size rather than necrosis. Further
studies are needed to clarify the genetic basis of this
hypersensitivity and of the partial resistance, and to
determine their race specificity.
Acknowledgements
The authors acknowledge the projects SC. 97±005-C2±
1 and 1FD97± 0393 for financial support.
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