Agriculture Reference
In-Depth Information
petition for nutrients, antibiosis, degradation of pathogen cell walls, alteration
of plant surface wettability, interference with pathogenicity enzymes or by
triggering systemic resistance in the plant (see 'Bacterial Diseases', above),
these can have an inhibitory effect on pathogenic fungi (Elad, 1996). These
effects can be exploited for the biological control of leaf diseases. This has been
given impetus by the widespread pressure, backed by legislation in some
countries, to reduce pesticide - including fungicide - inputs in crop production.
The market for organic produce, the slow and expensive development of new
fungicides and the need for alternative strategies to fungicides to minimize the
risk of pathogens developing resistance gives further motivation to this work.
Various fungal antagonists of leaf pathogens have been tested to control
allium leaf diseases. Carisse
et al.
(2006) reported that treatment of onion plots
infected with
B. squamosa
with sprays of a spore suspension of the antagonist
fungus
Microsphaeropsis ochracea
reduced the number of
B. squamosa
conidia
produced by necrotic leaves by 82%. Furthermore, the disease control achieved
by sprays of spore suspensions of
M. ochracea
every 7-10 days was as good as
that achieved by spraying the protective fungicide mancozeb with the same
frequency. The antagonist also reduced the number of conidia produced by
B.
squamosa
sclerotia by 75%. Sprays of
M. ochracea
spores on to onion debris after
harvest could lower the number of conidia from sclerotia, thus lessening the
primary inoculum for succeeding onion crops. In earlier trials, sprays of
conidia of the antagonist fungus
Gliocladium roseum
(=
Clonostachys rosea
) were
about half as effective as the protectant fungicide chlorothalonil in reducing
leaf spots caused by
B. squamosa
(James and Sutton, 1996).
Gliocladium roseum
is a soil-dwelling antagonist and does not function as well as fungal antagonists
derived from necrotic leaves in the conditions of alternating wet and dry
periods typical of the leaf surface environment (Kohl
et al.
, 1995; see also
antagonists and biological control of Neck rot, below).
Notwithstanding the use of good cultural and crop hygiene practices,
outbreaks of leaf diseases are typical in allium-producing regions, and routine
fungicide sprays are essential for control. For example, in eastern England a
disease complex of downy mildew,
B. squamosa
leaf blight and
Cladosporium
leaf
blotch has to be controlled. Infection by the latter two diseases can make a crop
more susceptible to attack by downy mildew. Herbicide treatments that can de-
wax and scorch leaves can also leave them more susceptible to disease. Any
conditions that reduce leaf waxiness and cause mechanical damage to leaves
are thought to increase their disease susceptibility. Leaf disease is particularly
damaging to yield and storability if it occurs when onions are starting to bulb,
and growers spray intensively with fungicides at this stage. Disease-free foliage
is important at the late stage of bulbing if maleic hydrazide applications are to
be effective for prolonging storage life (see Chapter 7).
Fungicides divide into protectants and systemics. Protectants coat the leaf
surface and must be present on the leaf prior to spore germination and infection
to have any benefit, whereas systemic fungicides have the potential to eradicate
