Organization of Genes Conferring Resistance to Anthracnose in Common Bean - Translational Genomics for Crop Breeding: Biotic Stress - page 154

Variables such as temperature, relative humid-
ity, time, and physiological state of the plant
or pathogen can modify the outcome of the P.
vulgaris-C. lindemuthianum interaction (Tu and
Aylesworth 1980). Use of standardized methods
to test response to pathogen is relevant to com-
pare results among different studies. Different
methods were used to investigate the resistance
of bean genotypes in controlled conditions. Dri-
jfhout and Davis (1989) dip germinated bean
seeds in suspensions of conidia prior to plant-
ing in moist river sand. Champion et al. (1973)
and Menezes and Dianese (1988) immersed
seedlings in a conidia suspension and wrapped
the stems of inoculated seedling in wet paper. Tu
and Aylesworth (1980) applied a suspension of
conidia with a paintbrush on primary leaves and
maintained inoculated seedlings in high relative
humidity using a transparent plastic bag. Cur-
rently, the most widespread inoculation method
is based on spraying a suspension of 10 6 coni-
dia ml − 1 onto seedling plants. Plants are main-
tained at moderate temperature (20-24 ◦ C) under
high relative humidity (

or 1 to 2). Currently, a 1-9 scale based on the type
of lesion and proportion of seedling with disease
symptoms is commonly used to describe dis-
ease reaction on seedlings (Balardin et al. 1997).
Plants with no visible symptoms or with few very
small lesions mostly on primary leaf veins are
recorded as resistant (values from 1 to 3). Plants
with numerous small or enlarged lesions, with
sunken cankers on leaves and seedling stem, or
death are recorded as susceptible (values from
7 to 9). Plants with small lesions on leaves and
seedling stem are recorded as intermediate reac-
tion and classified as susceptible or moderate
resistance. Use of scales including wide range of
reactions helps identify genes conferring moder-
ate resistance that appears to behave as a quan-
titative trait. The most common method is to
classify reaction types into two broad categories,
which implies qualitative resistance that further
assists in race characterization.

Pathogenic Variability

80%). Symptoms are
observed 8-10 days after inoculation and they
are reliable and precise and resemble reaction
in the field. Reaction of the same check geno-
types to different inoculation methods should be
considered before comparing results of different
studies using different types of resistance tests.

> Pathogenic variation can be described as the dif-

ferential response of a plant genotype to different
isolates of a pathogen. Barrus (1911) reported
the first evidence of pathogenic variation in the
interaction of P. vulgaris- C. lindemuthianum.
Using bean cvs. Detroit, Imperial, Blue Pod But-
ter, Navy Pea, and China Red Eye, Barrus (1911)
found variation in the response of two local
isolates of C. lindemuthianum . Later, the same
author (Barrus 1918) tested 70 bean genotypes
against ten isolates of C. lindemuthianum and
classified the ten isolates into two pathogenic
groups: strains alpha and beta based on reaction
profiles. In 1923, Burkholder described a third
strain, gamma. Since then, many studies have
reported pathogenic variants or races of C. lin-
demuthianum and at least 14 races have been
described prior to 1988; alpha, beta, gamma,
delta, Mex I, II, and III, epsilon, alpha mutant
later designated as lambda, Brazil I and II, ebnet
later designate as kappa, alpha Brazil, and C236
(reviewed by Drijfhout and Davis 1989). Unfor-
tunately, many of these studies used different

ReactionScore

In order to describe the response of bean geno-
types to C. lindemuthianum , three main types of
qualitative scales have been reported in the lit-
erature. Several studies classified the reaction of
bean genotypes as resistant or susceptible (Bar-
rus 1911; Cardenas et al. 1964; Kruger et al.
1977). Other classical studies used scales with
five levels of response (Yerkes and Ortiz 1956;
Bannerot 1965; Leakey and Simbwa-Bunnya
1972; Muhalet et al. 1981; Drijfhout and Davis
1989; Menezes and Dianese 1988) where the
resistant reaction is considered as plants with no
disease symptoms or a few small lesions (0 to 1