PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
NUTRITION
AND
PARASITISM
HOW BACTERIA
INFECT HOST
PLANTS
Bacteria cannot
physically penetrate
protective barriers of
plants
Most bacteria parasitic on plants develop on host plants as parasites on the plant
surface, especially on buds as epiphytes and partly in plant debris or in the soil as
saprophytes (Agrios 2005). Under suitable conditions, they can become parasitic. Most
plant pathogens are facultative saprophytes and can be grown artificially on nutrient
media (page 324).
NATURAL OPENINGS
There are always some bacteria on plant surfaces. Some never harm the plant; others
which cause disease can infect the plant through natural openings (leaf stomates, leaf
scars, lenticels, small pores at the margins of leaves (hydathodes), or through relatively
fresh wounds (hail, pruning wounds, etc).
Bacteria that cause
leaf spots randomly
on the leaf surface are
likely to have invaded
leaves through
stomates
WOUNDS
Damaging the intact surface of a plant can facilitate the entry of bacteria.
DISTRIBUTION
WITHIN A PLANT
Some bacteria destroy the material holding plant cells together, plant cells
collapse causing sunken areas on stems, tubers or bulbs, eg soft rots which may be
accompanied by unpleasant smells, eg soft rot of potato.
Some remain mostly on the surface of plant tissue, eg galls, and tend to
decrease in numbers as they invade the gall tissue, eg crown gall bacteria.
Bacterial cells may invade water and food-conducting tubes of plants:
– Spreading quickly to fruits and seeds. If these are used to produce a new crop,
bacteria will quickly produce diseased seedlings which may die.
– Bacteria also multiply in, and block, water-conducting cells causing wilting of
plants parts above the blockage (page 296, Fig. 149).
DISEASE
CYCLE
Many bacteria parasitic on plants develop partly on the host as parasites and partly in
the soil, or on plant debris in the soil, as saprophytes.
HOST ONLY
These diseases produce their populations on the host plant. If the bacteria do reach the
soil, eg via fallen leaves or fruit, their populations rapidly decline and they do not play
a part in the spread of the disease, eg bacterial blight of walnut, bacterial canker of
stone fruit. These bacteria have developed sustained plant to plant infection cycles.
HOST, HOST DEBRIS AND SOIL
These diseases build up large populations on the host. If the bacteria reach the soil,
through decaying parts of the plant, they can remain there for many years, populations
only gradually declining over years. Susceptible plants in contaminated soil will soon
become infected, eg crown gall.
HOST, HOST DEBRIS, BUT MAINLY SOIL.
These diseases produce most of their populations by growing on plant matter in the
soil. They only attack plants incidentally. Plants are not essential for their continued
existence, eg bacterial soft rot of iris or potato.
Bacterial diseases 299