PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
EXAMPLES OF BACTERIAL DISEASES
Crown gall
This disease is serious on nursery stock but occurs
sporadically. The same piece of ground may yield
badly infected plants one year and completely
healthy plants the following year.
Scientific name
Common soilborne bacteria (Agrobacterium spp.,
tumor state). Not all strains can infect all hosts.
Agrobacterium sp. is considered to be a natural
genetic engineer. The gall-inducing genes
causing crown gall can be removed, but the
infective ability retained to transfer genes, so that
DNA is inserted into another plant cell.
Host range
Wide host range, mainly Rosaceous plants but
other plants as well. Economic damage is largely
confined to Prunus spp., rose and beetroot.
Ornamentals, eg chrysanthemum, Prunus spp.,
rose, also dahlia, geranium.
Fruit & nuts, eg pome and stone fruits (especially
peach), bush fruits, grapevine, rhubarb, walnut.
Vegetables, eg beetroot.
Agrobacterium rhizogenes and A. tumefaciens (less
frequently) affect Rosaceae plants such as stone
fruit and roses. A. vitis infects grapevines and lives
systemically in the vascular tissue of the host.
Symptoms
Below ground/crowns. Galls ranging in
size from a pea to the size of a football develop at
ground level or on the roots (Fig. 154). Galls often
arise from root lenticels and are irregular in shape
and their texture depends on the host species. Galls
are the result of bacteria multiplying inside the
host, producing hormones which stimulate the host
to increase cell division and cell size resulting in
the formation of galls. The vascular system is
damaged, plants grow poorly and wilt readily.
Galls may also develop on side roots but they
probably do not do much damage. Other strains
cause excessive root proliferation.
Above ground. Aerial galls have been recorded
more than 100cm from the ground on grapevines
and on the branches of trees overseas. There is
some evidence that in these hosts, the bacteria can
move through the vascular system and that gall
formation may be associated with frost damage.
General. The effect of the disease is variable.
Infected plants may lack vigour, become stunted
and produce few flowers, however. Young plants
which are infected when planting out or which
become infected soon afterwards usually make
unthrifty growth and may eventually die. Older
plants which become infected may remain
vigorous for many years. It is not uncommon to
find established vigorous plants with large galls on
roots and crowns showing no apparent reduction in
vigour or other effects. Crown gall usually only
affects the vigour of older plants in the field if they
suffer moisture stress. Affected plants may die.
Grapevine. A. vitis unlike the gall-forming
species, lives systemically in the vascular tissue of
its host. Galls may develop where frost damage to
canes and trunks has occurred or at the bases of
cuttings used for propagation, or to major graft
wounds in warmer areas.
Diagnostics. Do not confuse crown gall with:
Natural adventitious ‘burr’ knots on Prunus sp.
which occur at the base of the trunk in some
species (page 397, Fig. 243).
With natural lignotubers on eucalypts.
Beneficial mycorrhizal roots on trees, eg alder.
With forked roots in crops such as carrots which
may be due to over-fertilization, unsuitable soil
structure, transplanting seedlings, etc.
With root knot nematode infections.
With clubroot disease in brassicas.
Expert diagnosis is necessary. As it is
difficult to isolate bacteria from galls, DNA
fingerprinting identifies strains on grapes.
Remember galls are often only seen when roots
or plants are dug up.
Fig. 154. Crown gall (Agrobacterium sp.).
Left: A large gall on rhubarb. Right: Galls on loganberry
canes. PhotoCIT, Canberra (P.W.Unger).
304 Bacterial diseases