PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
OVERWINTERING,
OVERSUMMERING
BACTERIAL DISEASES MAY ‘OVERWINTER’ IN SEVERAL WAYS
Crown gall may overwinter on the host, in soil or in or on seed.
HOST
Parasitic bacteria which attack trees or shrubs overwinter in bacterial lesions, galls or
cankers on the host plant, eg bacterial gall of oleander. They may also exist on the
surface of a plant or plant organ without causing infection.
SOIL
Some parasitic bacteria can accumulate and survive in the soil on or in debris from
infected plants, seeds or insects. Remaining alive for varying periods of time they are
then able to infect a future susceptible crop, eg crown gall. Some like those causing
bacterial soft rot can live in the soil indefinitely while others will decrease in numbers
unless plants they are able to attack are grown.
SEED
Many bacterial diseases are seedborne, which means that the bacteria are present either
in, on or in association with the seed. Plants produced from such infected seed will
automatically produce infected plants, eg bacterial blight (black rot) of Brassicas (stock,
cabbage, cauliflower), bacterial blight and halo blight of beans.
SPREAD
WATER, WATER SPLASH
Flagella enable bacteria to move only very short distances on their own.
Rain and irrigation water can wash bacteria from one part of a plant to another, from
plant to plant and from soil to the lower leaves of plants.
Wind may assist spread of rain and irrigation water.
Drainage water or any other running water, in or on soil, can wash bacteria downhill
to where susceptible plants are growing.
Pruning or other activities within a wet crop can assist spread.
WIND, AIR CURRENTS
Airborne dispersal of bacteria may occur in tomato and pepper transplant fields.
SEED
Many bacterial diseases are seedborne, eg bacterial wilt of tomato. Bacteria can
therefore be spread by any of the agents which aid seed dispersal.
Bacterial cells which get into the water and food-conducting tubes of plants spread
quickly to fruits and seeds. If these are used to produce a new crop, the bacteria will
quickly produce diseased seedlings which will probably die.
VEGETATIVE PROPAGATION MATERIAL
Bacterial diseases can be transferred to new plants on or in buds, cuttings, cormlets and
similar vegetative propagation material, eg bacterial canker of stone fruit is spread in
infected budwood.
INSECTS
MITES
INSECTS
Insects and mites do not commonly spread bacterial diseases but they may:
Carry bacteria from one part of a plant to another or to other plants.
Inoculate plants with bacteria during feeding, eg the walnut blister mite is thought to
aid in the spread of bacterial blight of walnut.
HUMAN ACTIVITIES AND ANIMALS
Pruning activities. Bacteria can be carried on pruning tools and cause infection
through pruning cuts, especially during cool wet conditions, eg bacterial canker of
stone fruit.
Handling plants. Bacteria can be spread on hands, shoes etc.
Bird and other animals may carry bacteria on their bodies when moving among
plants.
Movement of soil. Bacteria may be spread in soil on machinery and vehicles, in
containers and in soil deliveries when these are moved from one place to another.
Movement of plant material. Bacterial diseases can be transferred in infected
plants, nursery stock, bulbs, seed, cuttings and other vegetative propagation
material.
300 Bacterial diseases