Meeting the Challenge of Yellow Rust in Cereal Crops - ICARDA

For each resistant genotype (Table 1) we determined, under artificial
inoculation, the Area under disease progress curve (AUDPC) using the
average coefficient of infection (ACI). The Coefficient of Infection (CI)
combines disease severity and host reaction in the form:
Coefficient of Infection (CI) = Severity × Value of host reaction
where: Severity values vary from 0 to 100
and:
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Values of host reaction are attributed as follows: 0.0 = Immune
reaction; 0.2 = Resistant (R) reaction; 0.4 = Moderately
Resistant (MR) reaction; 0.6 = Moderately Susceptible (MS)
reaction; 0.8 = Susceptible (S) reaction; and 1.0 = Very
Susceptible (VS) reaction.
AUDPC combines the Average CI and the duration of the observation in
weeks. Infection type was recorded weekly. First measurements were taken 7
days after first appearance of symptoms (21 March 2005) on the universal
susceptible check cv. Morocco. The disease recording dates were: 21 March =
appearance of symptoms on universal susceptible check, then 28 March =
Week 0, 16 May = Week 7, and 23 May = Week 8.
A modified Cobb scale was used that considers the actual percentage (0–
100%) occupied by rust pustules and the rust severities (R, MR, MS, S and VS)
of the modified Cobb scale (Peterson, Campbell and Hannah, 1948).
Results and discussion
Adult plant resistance (APR) could offer a possible means for control of yellow
rust disease in wheat. Yellow rust suffers most damage at the adult growth
stage. Rust spores blown by wind from within an area or the region can
rapidly infect susceptible plants and produce more inoculum for subsequent
infection, particularly in the facultative winter wheat area, where the
vegetative period of wheat extends for up to six to seven months. Effective
adult plant resistance would reduce the inoculum intensity and hence reduce
spread of yellow rust spores, in addition to the direct protection it provide for
the wheat cultivars. In order to determine effectiveness of APR, known
resistance genes were assessed singly and in combination. Sixteen yellow rust
resistance genes from the Avocet isogenic lines, and three gene combinations
among differential cultivars were evaluated under artificial inoculation at the
Tel Hadya station of ICARDA, and assessed at different location in Central,
West Asia and Nile Valley regions (including Ethiopia). Commercial cultivars
that are commonly grown in Central and West Asia and 200 advanced
breeding lines were also assessed. The ACI combined repeated reading during
the same period (week 0 to week 6) and those realized at different locations
(Week 7). The disease recording extended over seven weeks. In that short time
(23 March to 18 May 2005) it was possible to discriminate the different levels of