PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Fig. 149. Symptoms of bacterial diseases (examples only)
Blight. A disease which produces
a general and rapid killing of
leaves, flowers and stems; may
be caused by bacteria, eg
bacterial blight of peas. Also
caused by insects, fungal
diseases, and many nonparasitic
problems, eg frost.
Canker. A dead or discoloured
area/spot on a stem, branch, or
twig of a plant, eg bacterial
canker of stone fruit. Also caused
by fungal diseases.
Defoliation. Leaves fall off
prematurely, eg bacterial canker
of stone fruit. Also caused by
many fungal diseases,
twospotted mites, senescence.
Dieback. Progressive death of
shoots, branches, and roots
generally starting at the tip, eg
bacterial canker of stone fruit.
Also caused by fungi, eg
Phytophthora, borers, drought,
etc.
Galls. Bacteria stimulate
plant cells to multiply and
enlarge abnormally causing
lumps to appear on plant
parts, eg crown gall. Also
caused by nitrogen-fixing
bacteria (see below), fungal
diseases, eg gall rust on
wattles, insects, eg gall
wasps.
Gumming/gummosis.
Production of gum by, or in
plant tissue, eg bacterial
canker of stone fruit
(gummosis). Also caused by
fungal diseases, eg shot
hole, injury, eg apricots.
Leaf spots. A self-limiting
lesion on a leaf, eg bacterial
leaf spot of mulberry (see
below). Also caused by fungi
and other agents.
Scab. A roughened cracked
diseased area on the surface of
plant tissues, eg bacterial scab of
gladiolus (below). Also caused by
fungal diseases, eg apple scab,
non-parasitic problems, eg oedema.
Soft rot. The material holding plant
cells together is destroyed by the
disease organism so that plant cells
collapse causing tubers and bulbs to
rot. May have an unpleasant smell,
eg bacterial soft rot of potatoes. May
also be caused by some fungal
diseases.
Wilts. Disease organisms multiply in
and block water-conducting cells
causing wilting of plant parts above
the blockage, eg bacterial wilt of
tomato; bacterial cells spread
quickly and may end up in fruits
and seeds. If these are used to
produce a new crop, the bacteria
will quickly produce diseased
seedlings which will probably die.
Also caused by fungal diseases, eg
Fusarium wilt of tomatoes.
Fig. 150. Bacterial leaf and corm scab of gladiolus
(Pseudomonas gladioli pv. gladioli). Scab lesions occur on corms,
leaf bases may rot. PhotoNSW Dept. of Industry and Investment.
Fig. 151. Nitrogen-fixing nodules
on legumes, eg clovers. On legumes do
not confuse with root knot nematode
galls (Meloidogyne spp.).
Fig. 152. Mulberry leaves. Left: Bacterial leaf spots (bacterial blight) of mulberry
(Pseudomonas syringae subsp. mori) with small, black, angular spots. PhotoCIT, Canberra (P.W.Unger).
Right: Fungal leaf spots of mulberry with larger round spots with dark margins and light-centres.
PhotoNSW Dept. of Industry and Investment.
296 Bacterial diseases