PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
SYMPTOMS
Symptoms are not
related to virus
concentration in the
host but depend on:
Strain of virus
Environment, climate
Duration of infection
Type, variety, age,
physiology, stage of
development of host
Presence of other
viruses and disease
organisms
. GENERAL SYMPTOMS.
The interference in photosynthesis, respiration and other cell processes results in
a range of symptoms including those described below. Often they cannot be
easily observed or quantified. Sometimes a virus disease does not show
symptoms in an infected host, eg tobacco mosaic virus infection in African
violet and is called a latent virus; apple stem pitting virus, apple chlorotic leaf
spot virus and apple stem grooving virus are common, economically important,
and symptomless in commercial pear and apple cultivars. Other viruses only
produce symptoms under certain conditions of light and temperature and are
called masked viruses. Although virus may infect all parts of the plant,
symptoms generally are most obvious in young foliage. Leaf symptoms can
easily be confused with other plants problems (Table 54 below).
STUNTING
YIELD
PLANT LIFE
May be so slight that it is often unnoticeable, especially in
the initial stages, or so severe that the disease is called
‘stunt’, affected plants being unproductive.
Yield may be slightly or severely reduced.
Life is usually shortened, probably only important for
perennial plants. Seedlings can be killed.
. SPECIFIC SYMPTOMS.
Some symptoms are
easier to see and
identify than others,
some are more subtle
Some viruses, eg
cucumber mosaic virus
(CMV) tend to produce
the same type of
symptoms on all plants
they infect regardless
of the host species
Others such as tomato
spotted wilt virus
(TSWV) may produce
different symptoms on
different hosts
Ringspots on watermelon
PhotoCIT, Canberra (P.W.Unger).
LEAVES
FLOWERS
FRUIT
STEMS
OTHERS
COLOUR CHANGES, eg yellowing and reddening
Bronzing, eg tomato spotted wilt virus
Line patterns, eg plum line pattern virus
Mosaics, eg poinsettia mosaic virus
Mottling, eg camellia yellow mottle virus
Ringspots, eg tomato spotted wilt virus
Streaks, eg garlic yellow streak virus
Veinbanding, eg strawberry veinbanding virus
Veinclearing, eg malva veinclearing virus
MALFORMATIONS, eg potato leafroll virus (rolled leaves)
WILTING, eg tomato spotted wilt virus
Breaking, eg tulip breaking virus (stripes, intensifies colour)
Greening, eg tomato big bud phytoplasma (greening)
Malformation, eg iris severe mosaic virus
Malformation, eg stony pit virus of pear
Ringspots, eg tomato spotted wilt virus
Russet. eg russet ring virus of apple
Malformation, eg apple flat limb virus
Streaking, eg tomato spotted wilt virus
Death of plants (not common)
Table 54. Comparison of virus diseases with other plant problems (typical generalizations).
LEAVES,
DISTRIBUTION
VIRUS
DISEASES
NUTRIENT & TOXICITY
DEFICIENCIES
HERBICIDE
INJURY
GENETIC
ABNORMALITIES
Distribution of
affected leaves
on plant
Distribution of
symptoms on
leaves
Other
features
Distribution of
affected plants
in the field
Usually uneven, patchy,
often only a few leaves
show symptoms on the
plant, often seen on new
growth in spring
Usually uneven pattern
on leaf
Some plants are
susceptible to specific
viruses
Symptoms on a few
randomly scattered
patches of plants which
may gradually spread
Usually either all over plant,
only on young leaves, or only
on older leaves
Usually even, bilateral
symmetry, often specific
pattern
Some plants are susceptible
to specific deficiencies
Sudden appearance of
symptoms on all plants in a
crop, or evenly in an area
within the crop
Often on new growth,
may be on windward
side of plant or crop
May be even, a
specific pattern
Leaves may be
distorted. History of
chemical applications
Sudden appearance of
symptoms on all plants
Usually all over plant,
or on one branch or
shoot (a "sport")
Often even over leaf,
tendency to bilateral
symmetry
Pattern on leaf or fruit
Rare, 1-2 in a
population of plants
Virus and virus-like diseases 275