لخّصلي

Downy mildew caused by the oomycete Hyaloperonos-
pora parasitica (syn.The objective of this study was to complete the
assessment of the downy mildew resistance sources
selected in previous work in order to determine if
they could provide potentially durable control of
downy mildew for UK vegetable production.The development and deployment of cultivars with sin-
gle major disease resistance genes has frequently failed toprovide durable disease control due to the quick evolu-
tion of pathotypes virulent on resistant cultivars, so in
cases such as downy mildew (Bremia lactucae) of lettuce,
blackleg (Leptosphaeria maculans) of oilseed rape,
northern leaf blight (Setosphaeria turcica) of maize, leaf
and stripe rust (Puccinia triticina and P. striiformis) of
wheat and potato late blight (Phytophthora infestans)
there has been a continuous effort to find sources of
broad-spectrum resistance and to devise strategies to
deploy the resistant cultivars (Crute & Norwood, 1981;
Sivasithamparam et al., 2005; Stuthman et al., 2007;
Zhang et al., 2009).Several sources of seedling resis-
tance to H. parasitica in B. oleracea have been reported
previously, and heritability of the resistance has been
characterized in many cases including: single dominant
genes in broccoli and cauliflower (Natti et al., 1967; Jen-
sen et al., 1999a; Farnham et al., 2002), a single recessive
gene in cauliflower (Hoser-Krauze et al., 1984), recessive
gene(s) in kale, Savoy cabbage and Brussels sprouts
(Carlsson et al., 2004) and multiple (additive) genes in
cauliflower, broccoli and cabbage (Moss et al., 1988;
Hoser-Krauze et al., 1995; Jensen et al., 1999b).The
same approach was subsequently applied to B. oleracea
in a coordinated effort to identify sources of resistance to
black rot (Xanthomonas campestris pv. campestris),
downy mildew (H. parasitica), white blister rust (Albugo
candida) and cabbage aphid (Brevicoryne brassicae) in a
core diversity collection of more than 400 B. oleracea
accessions (Leckie et al., 1996; Ellis et al., 1998; Taylor
et al., 2002).In the current study, a systematic approach pioneered
more than four decades ago at the National Vegetable
Research Station (NVRS), Wellesbourne, UK, for predic-
tive breeding of durable resistance to downy mildew in
lettuce, and subsequently applied to other major vegeta-
ble diseases (Crute, 1992; Taylor et al., 1996, 2002) has
been followed.Repeated application of
fungicides is generally used for control, but the effective-
ness of this approach has been limited because there is a
restricted range of active ingredients effective against
oomycete pathogens, and frequent prophylactic applica-
tion of fungicides has led to the selection and widespread
distribution of fungicide-insensitive variants of the path-
ogen (Brophy & Laing, 1992).To
achieve this, the resistance of selected lines was
assessed against a spectrum of UK isolates of H. par-
asitica, the inheritance of resistance in some of the
resistant lines was studied and breeding for the
development of broccoli and cauliflower lines pos-
sessing the resistance identified in selected lines was
initiatedInbreeding and selection of true-breeding
resistant lines may often be required at this stage, because
plant genetic resources (e.g. older open pollinated culti-
vars and land races or wild crop relatives) can comprise
heterogeneous mixtures of resistant and susceptible geno-
types. Peronospora parasitica) is an eco-
nomically important foliar disease of Brassica oleracea
crops (cabbage, broccoli, cauliflower, kale and Brussels
sprouts).Downy mildew can affect plants at all growth
stages, but the most damaging effects are generally
restricted to young seedlings, causing heavy losses in
plant nurseries, and to particular organs such as cauli-
flower curds (Channon, 1981).The
NVRS approach was used successfully to identify race
non-specific resistance to lettuce downy mildew and bac-
terial diseases of common bean and pea (Crute & Nor-
wood, 1981; Schmit et al., 1993; Taylor et al., 1996).Genetically uniform resistant lines can then be
screened against a more extensive sample of pathogen
isolates, collected widely from vegetable production
regions, to identify examples of broad-spectrum disease
resistance against the entire pathogen collection. A broadly virulent isolate (one
which could cause typical disease symptoms in the initial
subset of cultivars) was then chosen to screen a much lar-
ger host diversity collection to identify new sources of dis-
ease resistance.The approach began in each case with an
initial characterization of differential reactions amongst
standard vegetable cultivars for resistance to a small sam-
ple of pathogen isolates.Downy mildew resistant cultivars would offer a prac-
tical, environmentally acceptable alternative method to
control downy mildew.The
breadth of resistance provided against a collection of
H. parasitica isolates was not investigated in these previ-
ous reports, so the potential durability of each resistance
is unknown.The disease is distributed worldwide wherever
brassica crops are grown and is favoured by cool humid
weather that is frequent in spring or autumn (Channon,
1981).