Agriculture Reference
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to powdery mildew is called adult plant resistance (APR), which retards infection,
growth and reproduction of the pathogen in adult plants but not in seedlings. It is
also called “slow mildewing” and “partial resistance”. APR in wheat cultivar Knox
and its derivatives remained effective against powdery mildew infection during the
20 years in which these cultivars were grown commercially (Shaner
1973
). Another
cultivar, Massey, which is a derivative of Knox62, was developed and released in
1981 (Starling et al.
1984
) and still has effective powdery mildew resistance in adult
plants. Common sources of
Pm
genes are different species within the primary, sec-
ondary and tertiary gene pools. Many of the resistance genes were introduced from
ancestral and other wild species related to common wheat such as
Triticum mono-
coccum,
close relative of the A genome progenitor
Triticum uratu
, the B genome
progenitor
Aegilops speltoides
, and the D genome progenitor
Ae. Tauschii
(Hsam
and Zeller
2002
). They reported a total of 22 resistance alleles at ten loci in com-
mon wheat indicating that
Pm
genes may still be found in cultivated wheat. Earlier
studies by Mains (
1933
) identified that the wild wheat relatives
T. monococcum
(AA genomes),
T. dicoccum
(AABB), and
T. timopheevi
(AAGG) are the sources
of resistance genes to powdery mildew as early as 1933. Screening of old wheat
cultivars, landraces and related species for resistance to powdery mildew started
in the 1930's (Hsam and Zeller
2002
).
Pm
genes were identified in many different,
widely distributed wheat cultivars and landraces.
Pm5a
and
Pm5b
, followed by
Pm2, Pm6,
and
Pm8
are the most common in Europe, Asia and Mediterranean cul-
tivars.
Pm3a
is commonly found in wheat cultivars grown in diverse geographical
locations including the Balkans, Japan, China and the US.
Pm3c
was identified in
Germany, while
Pm3d
was found in several European countries and China.
Pm4a
has been used in commercial wheat cultivars in Germany and China. A number
of commercially grown cultivars have been found to have
Pm
gene combinations
(Heun and Fischbeck
1987
). The best known cultivars are Normandie with
Pm1,
Pm2,
and
Pm9,
Maris Huntsman with
Pm2
and
Pm6
, Kronjuvel with
Pm4b
and
Pm8
, and 623/65 with
Pm4b
and
Pm8
(Liu et al.
1999
). Gene transfer from species
within the primary gene pool of
Triticum
with homologous chromosomes to wheat
can be done directly by hybridization, recombination and backcrossing.
Ae. tauschii
has proved to be an important source of resistance against pow-
dery mildew. Earlier Gill et al. (
1986
) screened 60
Ae. tauschii
accessions to four
different
Bgt
isolates and identified 11 highly resistant and 20 moderately resis-
tant accessions. Hsam and Zeller (
2002
) transferred two resistance genes,
Pm2
and
Pm19
, from
Ae. tauschii
to common wheat. Two germplasm lines, NC96B-
GTD3 and NC97BGTD7, were developed which carries resistance genes
Pm34
and
Pm35
against PM (Murphy et al.
1998
; Mirinda et al.
2006
,
2007
). Similarly,
Pm25
has been transferred from diploid
T. monococcum
to common wheat germ-
plasm NC96BGTA5 (Shi et al.
1998
; Murphy et al.
1998
). In tetraploid wild emmer
wheat (
T. dicoccoides
), seven PM resistance genes viz.
Pm16
,
Pm26
,
Pm3, Pm31,
Pm36, Pm41
and
pm42
have been identified and transferred to wheat (Rong et al.
2000
; Liu et al.
2002
; Hsam and Zeller
2002
; Hua et al.
2009
). Recently, Rafiq et al.
(
2012
) identified several A- (
T. monococcum
;
T. urartu
) and S-genome (
Ae. speltoi-
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