PLANT PROTECTION 1 – Pests, Diseases and Weeds

1
PLANT PROTECTION 1 – Pests, Diseases and Weeds
CLASSIFICATION
IDENTIFICATION
Symptoms
Bacteria are
identified by what
they do rather than
what they look like,
(Brown & Ogle 1997)
ELISA Testing
Service
DNA
SYMPTOMS
Many bacterial
diseases produce
more than 1 symptom,
eg crown gall may
cause dieback,
galls and wilting
Symptoms caused
by bacterial diseases
may be confused with
those caused by
fungal and
non-parasitic
diseases and
other causes
BACTERIA ARE CLASSIFIED BY VARIOUS FEATURES including colony
characteristics, pigments, stain reactions and morphology, eg shape of cells, motility,
flagellation and a range of chemical tests (Fahy and Persley 1983, Agrios 2005).
Bacteria causing diseases of plants include:
Agrobacterium Erwinia Ralstonia Streptomyces
Clavibacter Pseudomonas Xanthomonas Xylella
Phytoplasmas are classified with bacteria but in this book are studied with virus
diseases because of similarities in symptoms, methods of spread, etc.
Some organisms with mycelium-like forms, eg Streptomyces scabies (common scab
of potatoes), are classified with bacteria rather than with fungi.
SYMPTOMS EXHIBITED BY THE HOST PLANT
For those without access to specialized facilities this is often the main
method of identification but considerable expertise is needed.
Other disease organisms, environmental extremes and chemical toxicities may
cause similar symptoms. Bacterial leaf spots may be confused with fungal leaf spots,
bacterial wilts with fungal wilts, senescence, other agents.
Secondary bacterial infections may be associated with above conditions.
Bacterial ooze may be observed using a high-powered compound microscope when
suspect stems or leaf lesions are cut transversely with a razor blade and placed on a
microscope slide in water. Similarly when kept in a moist chamber, creamy bacterial
ooze may ooze from the vascular system, eg bacterial wilt of tomato. However, this
does not identify the bacterial species.
DETECTION AND IDENTIFICATION BY EXPERTS
Experts test for the presence of bacteria in seeds, food supplies and in parent stock and
certification schemes, eg strawberry, cut flowers, potato, grape.
Microscopic morphology is of little value in identifying bacteria.
Pure bacterial cultures can be isolated on selective media and identified.
Continuous culture-indexing includes regular checks of plant material for bacterial
infection over a 2-year period. Pathogenicity tests can be carried out.
Biochemical tests and molecular techniques are precise, species and
subspecies can be identified. Some test kits have been developed.
– ELISA tests are relatively low cost, give a quick specific answer (a color change
indicates a positive test result) but are not as sensitive as some other methods. An
ELISA test is available for bacterial leaf & stem rot of pelargonium
(X. campestris pv. pelargonii) and is useful when scouting in IDM programs.
– Other techniques include gram staining reaction, substances used by bacteria for
food, the fatty acid composition of cells. Serological tests which produce a colour
change can be used for quick and fairly accurate identification of bacteria.
Sensitive DNA tests, eg PCR (polymerase chain reaction) enable researchers to
distinguish one bacteriium from another by comparing segments of DNA.
For some bacterial organisms on some hosts there are specific tests.
DIRECT DAMAGE.
LEAVES Blights, eg bacterial blight (cotton, pea, stock, walnut, etc)
Defoliation, eg bacterial canker (stone fruit)
Galls, eg bacterial gall of oleander
Leaf spots, eg bacterial leaf spots (begonia, hibiscus),
bacterial canker (stone fruit), bacterial blight (mulberry)
BUDS, Blights, eg bacterial canker of stone fruit
FLOWERS
FRUIT Sunken black areas, eg bacterial blight (walnut)
Rots, eg bacterial soft rot (stored fruit and vegetables)
STEMS, Cankers, eg bacterial canker (stone fruit)
TRUNKS Dieback, eg bacterial canker (stone fruit), bacterial blight (walnut)
Gumming, eg bacterial canker (stone fruit)
Rots, eg bacterial leaf and stem rot (pelargonium)
Wilts, eg bacterial wilt (tomatoes, internal staining of vascular tissue)
CROWNS, Galls, eg crown gall
TUBERS, Rots, eg soft rots
ROOTS Scabs, eg leaf spot/corm scab (gladiolus), common scab (potato)
INDIRECT DAMAGE.
Secondary bacterial infections may be associated with injury caused by other
disease organisms, environmental effects, injuries and toxicities.
Nematode-bacterial disease complexes may occur (page 253).
Bacteria may clog screens in pumps and reticulation systems.
Bacterial diseases 295