Agriculture Reference
In-Depth Information
General characteristics of bacterial diseases
Symptoms and infection
Pathogenic bacteria cause infected tissue to become macerated and water-
congested through the destruction of plant cell walls by their pectolytic
enzymes. Extracellular polysaccharides embed the bacteria and are thought to
induce water-congested intracellular spaces within which the bacteria can
spread. In onion bulbs, bacterial infection often spreads within, rather than
between, fleshy scales.
Moisture is necessary for initial infection, and ingress is often via wounds.
Bacterial diseases of onion are favoured by warm temperatures, frequently
with optima around 30-32°C. Wet, windy weather followed by warm
temperatures is conducive to disease. Wind will cause leaf damage, rain-splash
facilitates the spread and entry of bacteria and warmth accelerates subsequent
rapid disease development. Potential causes of damage include leaves snapping
under their own weight and by wind movement, abrasion by hail and wind-
blown soil particles, cultivation operations, trimming to remove foliage from
onion necks at harvest or before transplanting seedlings and damage by pests.
Both soil particles and cultivation equipment can be contaminated by
pathogenic bacteria. Equipment can brush past and pick up the bacteria from
weeds or other crops on which they may exist as asymptomatic surface
colonizers (epiphytes). Some of the pathogenic bacteria have been shown to
survive within the digestive tracts of pests - for example,
Erwinia carotovora
spp.
carotovora
within larvae and adults of the onion fly,
Delia antiqua
and
Pantoea
ananatis
can survive within, and infect onions via, the tobacco thrips,
Frankliniella fusca
(Gitaitis
et al.
, 2003). When onion leaves fold under their
own weight the inner side of the fold can retain moisture and present a
vulnerable point of ingress for leaf blight bacteria (Gitaitis
et al.
, 1997). In wet
weather water can accumulate where young leaves emerge from the top of the
pseudostem, and this region has been shown to be very susceptible to infection
by
Burkholderia cepacia
. Onion bulbs are susceptible to infection around harvest
time when bulb rot bacteria can invade leaf tissue and enter the top of the bulbs
via the neck if leaves and cut necks are wet (Wright and Triggs, 2005).
Diagnosis
The identification of the primary pathogen causing a bacterial rot is not always
straightforward, since rotting tissue can be invaded by numerous secondary
opportunistic microorganisms when it starts to decay. The aggressive pathogen
must be distinguished from these and identified. In novel situations, the suspected
pathogen isolate must be shown to induce disease in healthy plants when they
are inoculated with it. The identification of bacterial pathogens is technically
demanding and frequently involves 'polyphasic characterization', where a range
of different biochemical, DNA-based and sometimes immunological methods are
combined to establish identity (e.g. Gent
et al.
, 2004a; Roumagnac
et al.
, 2004a).
Sometimes, rapid and sure diagnostics can be designed, increasingly using PCR-
