PLANT PROTECTION 4

PLANT PROTECTION 4 – How to Diagnose Plant Problems
VIRUS &
VIRUS-LIKE
DISEASES
Hydrangea mosaic
BACTERIAL
DISEASES
Bacterial gall of
oleander
Viruses.
1. Most can only multiply in living
cells. Viruses make plant cells
produce more virus particles. Some
phytoplasmas can be cultured.
2. Are infectious and can spread from
one plant to another.
3. Can only be seen with the aid of an
electron microscope.
4. Vary in structure and size.
8 Background briefing – Causes of plant problems
Viroids, Viruses Phytoplasmas, larger
(x 100,000) (x 10,000) than viruses but
smaller than bacteria
Signs
None. Viruses are too small to be seen with the naked eye or by dissecting or
compound microscopes.
Symptoms
Symptoms are distinctive on some hosts and so disease can be quickly identified. In
many cases, though, this is not possible.
Colour changes, eg flower breaking, greening of flowers, yellowing of foliage (line
patterns, mosaics, mottling, ringspots).
Distortion of leaves or flowers.
Viruses often weaken rather than kill plants, causing affected plants to be stunted.
Some virus diseases produce no symptoms, eg carnation latent virus.
Camellia yellow mottle
Bacteria.
1. Are small single-celled organisms which
can only be seen under high magnification
(x 1,000). The ones which attack plants
are mostly short, rod-shaped, with one or
more flagella which enable them to move
through a film of water.
2. Have a cell wall surrounding the cytoplasm
but do not have the nucleus found in higher
plants.
3. Have no chlorophyll and cannot
manufacture their own food. They obtain
it from external sources.
Tulip flower breaking
Apple russet ring
Signs
None. Individual bacteria are too small to be seen with the naked eye. When bacteria
are observed on the surface of diseased tissue with a compound microscope, they may
be the actual cause of disease or growing on dead tissue killed by some other agents.
Bacterial ooze may be observed.
Symptoms
Blights, cankers.
Galls, gumming.
Leaf spots, often angular or irregular in shape.
Rots, often soft and wet. Infected areas look water soaked, have a slimy texture and
often smell. If the disease persists the plant tissue may totally disintegrate.
Wilting followed by death of the plant.
Leaf yellowing is often associated with bacterial diseases.
Crown gall Bacterial blight (leaf spots) – walnut
Bacterial canker
(gumming) – stone fruit