Agriculture Reference
In-Depth Information
Many bacterial pathogens can persist on the surface of other crops and on
weeds. For example,
Xanthomonas axonopodis
pv.
allii
can survive epiphytically
on dry bean crops in Colorado, USA and on a number of weeds common there.
Survival on such vegetation does not normally persist for more than 1 year after
a diseased onion crop has been grown (Gent
et al.
, 2005). In Georgia, USA,
Pantoea ananatis
and
Pseudomonas viridiflava
persist on a number of weeds of
onion fields.
Pseudomonas viridiflava
was found on the cutleaf evening primrose
several kilometres from onion crops and several years after onions had been
grown nearby. An association was noticed between the herbicidal control of this
weed in onion fields and the extent of crop infection (Gitaitis
et al.
, 1998).
Pseudomonas viridiflava
was not found in soil or water in Georgia. On the
other hand,
Burkholderia cepacia
can survive in soil for long periods, although
its frequency increases with onion cropping (Yohalem and Lorbeer, 1997).
Irrigation water that has drained off infected crops is another potential source
of inoculum, as was shown for
X. axonopodis
pv.
allii
(Gent
et al.
, 2005).
A number of bacterial pathogens have been shown to be seed transmitted,
as indicated in Table 5.5. The genetic similarity of most strains of
Pseudomonas
syringae
pv.
porri
causing leek leaf blight around the world points to its
transmission on seed from Europe, from where most leek seed is supplied (Noble
et al.
, 2006). Roumagnac
et al.
(2000) detected
X. axonopodis
pv.
allii
on a
number of onion seed-lots. A study of disease development in fields sown with
0.04% infected seed showed a pattern of spread around random diseased plant
foci, probably derived from infected seed, when warm weather followed wet,
windy weather (Roumagnac
et al.
, 2004b). Infected onion sets can also
originate disease. In the UK, onion bulb crops grown from sets were found to be
infected with
Burkholderia gladioli
pv.
alliicola
, whereas crops grown from seed
were not (Davies and Taylor, 1994).
The spread of bacterial disease depends on weather conditions, wetness
being necessary for infection, with temperatures around 30°C being generally
optimal for disease development. Schwartz
et al.
(2003) derived regression
models relating the development of leaf blights to weather conditions in
Colorado. For both
Pantoea ananatis
and
X. axonopodis
pv.
allii
blights, disease
appearance was associated with high July temperatures and, for the latter
disease, there was a strong association of severity with high temperatures in
July and high rainfall in July and August.
Changes in agronomic practices often give rise to new disease problems. For
example, bacterial rots of stored onions became prominent in the UK when
forced ventilation drying using air around 30°C was applied to field-topped
bulbs (Davies and Taylor, 1994). High temperatures were introduced to develop
brown-coloured bulb skins (see Chapter 7), an attractive feature for purchasers.
Control measures
As always in plant pathology, rational control measures require knowledge of
what is causing the disease and information on the life cycle of the pathogen,
