PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Bacterial leaf spots
Scientific name
Almost all bacterial spots and blights of leaves,
stems and fruits are caused by bacteria in the
genera Pseudomonas and Xanthomonas (see also
page 297).
Host range
Wide range of plants, eg
Ornamentals, eg begonia, carnation,
chrysanthemum, ferns, geraniums, gladiolus,
hibiscus, poinsettia, Prunus, statice, stock, zinnia.
Fruit & nuts, eg mulberry, strawberry, walnut.
Vegetables, eg.cucurbits, lettuce.
Field crops, eg lucerne. Although some species
of bacterial leaf spots can infect several species of
plants, specific strains may be restricted to one
host, or group of related hosts, eg one species may
attacks lettuce another geranium and so on.
Symptoms
Spots (and blights) are the most common type of
bacterial disease. Symptoms vary depending on
the host and the specific disease organism.
Leaves.
Leaf spots usually start as small lesions 1 mm
in diameter on the leaf surfaces. The brownish
spots enlarge to 2-10 mm across, have irregular
borders and translucent or yellow halos.
– Shape is usually affected by the leaf veins which
may make the spots more angular.
– On some hosts the center of the leaf spot falls out
giving the leaf a shot hole appearance.
– Fungal structures are absent.
Blight infections. Spots can begin on leaf
edges leaf dieback or the spots can join together
killing the leaf.
– Tissue is water soaked and slimy when newly
rotted but dries out with age becoming brown
and papery.
Systemic infection. Bacteria may extend
along leaf veins and establish in the vascular
system which is blocked inhibiting the flow of
water and nutrients to the foliage.
– Young foliage or whole plants may wilt and die.
– Older leaves may turn yellow and eventually die.
– Discolored vascular bundles are visible when
stems and petioles are cut.
Fruit. Leaf spot bacteria may also attack fruit, eg
bacterial leaf spot of cucurbits
Stems. Black section may develop on stems.
Disease cycle
The disease cycle varies with the particular leaf
spot disease.
‘Overwintering’
Bacteria exist at very low levels:
In or on plant parts and seed.
Infected crop debris in soil.
On contaminated tools, containers, or in the soil.
Spread
Water splashed from infected to healthy plants.
Recycled and untreated irrigation water, surface
water. Wind blown rain, direct contact with host,
insects such as flies, bees and ants, handling of
plants, tools.
Infected propagation material, eg cuttings, seed.
Contaminated tools during pruning and
cultivation.
Clothes when brushed against diseased foliage,
especially during wet weather.
Infected soil in pots, on vehicles and footwear.
Diagnostics.
Do not confuse with fungal leaf spots, chemical
toxicities, environmental problems, etc.
Bacterial leaf spots are generally angular with a
yellow halo. Fungal leaf spots are generally
round with fungal structures present (often can
only be seen with a hand lens or microscope).
Sometimes bacteria invade already damaged
plant tissue, ie they are secondary infections.
Can test for bacterial ooze (page 295).
Need an expert to confirm diagnosis (page 295).
Fig. 158. Bacterial leaf and stem rot of pelargonium
(Xanthomonas campestris pv. pelargonii). Upper: Leaf spots
on ivy-leafed geranium appear sunken and water-soaked.
Lower left: Stem rot, the brown withering and rotting
progresses from the stem tips downwards. PhotoNSW Dept. of
Industry and Investment. Lower right: Black sections on stems.
310 Bacterial diseases