Agriculture Reference
In-Depth Information
been cleared, or on tracks and waste land, are potential sources of disease per-
sistence and should be destroyed, as should any alternative hosts like sus-
ceptible weeds. After harvest, crop remains normally break down much faster
and have less time and opportunity to form spores if they are buried than if they
are left on the soil surface.
For example,
Stemphylium vesicarium
can cause severe leaf blight of garlic;
it can colonize garlic debris and develop into the sexual stage,
Pleospora allii
.
This stage gives rise to black structures called pseudothecia, which ultimately
give rise to ascospores that infect garlic crops in the spring. Studies in the south
of Spain showed that garlic leaf debris left on the soil surface from October to
December produced
c
. 6.3 pseudothecia/mm
2
of dead leaf area, whereas leaf
debris buried 10 cm in the soil produced only about 2/mm
2
. Moreover,
pseudothecia formed on buried leaves degenerated faster than those on the
surface (Prados-Ligero
et al.
, 1998). The same study showed that inflorescence
stalk residues were more resistant to degradation upon burying than leaf
debris and gave rise to 6.5-9.5 perithecia/mm
2
, whether buried or not. The
crop hygiene recommendations from this study were that it was best to gather
and destroy waste seedstalks and then to plough in and bury leaf residues, in
order to minimize the disease inoculum in the spring.
Crop rotation is highly desirable from a disease prevention viewpoint, so
that susceptible crops do not overlap in time nearby to each other and are
distanced both from each other and from the potentially diseased residues of
previous crops. However, although desirable, the separation of susceptible crops
in both space and time using rotation is not always economic, particularly in
areas of intensive production. To reduce humidity in the leaf canopy and
minimize the duration of periods of leaf wetness, thereby shortening the periods
suitable for sporulation and infection, it is desirable to have widely spaced plants
and a low LAI. A wide inter-row spacing with rows oriented in the direction of
the prevailing wind also helps achieve this objective and is important for downy
mildew control in onion seed crops. Excessive nitrogen fertilizer should be
avoided, as this promotes a lush leaf canopy within which high-humidity
microclimates are likely. However, low LAI reduces yield potential and delays
bulb crop maturity (see Chapter 4), and green salad onions need to be grown at
a high plant density to be economical. So again, practical and economic
considerations may override what is optimal to minimize the risk of leaf disease.
For some leaf diseases, cultivars with disease resistance have been
identified or are being developed (see Table 5.5). Most promising is the intro-
duction of resistance from wild species, e.g. the introduction of downy mildew
resistance from
Allium roylei
to onion. There is also potential to use
A. roylei
as
a 'bridge species', to introduce the
B. squamosa
resistance of Japanese bunching
onion
A. fistulosum
into the common onion,
A. cepa
(see Chapter 3, 'Breeding
for Disease and Pest Resistance').
The surface of leaves supports a community of 'phyllosphere' micro-
organisms (Lindlow and Brandl, 2003). In various ways, including com-
