PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
EXAMPLES OF VIRUS & VIRUS-LIKE DISEASES
Tomato spotted wilt
Cause
Tomato spotted wilt virus (TSWV), spotted or
bronze wilt. TSWV occurs wherever its vectors
occur. Losses can be serious and are likely to
increase because the exotic Western flower thrips
(WFT) is a very efficient vector of TSWV, has a
very wide host range, readily develops resistance
to insecticides and can reach very high numbers on
host plants. There are different strains of the virus.
Host range
Has the widest host range of any plant virus. It
can attack over 900 species of plant, including:
Vegetables, eg tomato, potato, capsicum,
lettuce, celery, eggplant, spinach.
Ornamentals, eg aster, chrysanthemum, dahlia,
Iceland poppy, nasturtium, petunia, zinnia.
Fruit & nuts, eg peanut.
Field crops, eg cowpea, lupin, tobacco.
Weeds, eg dandelion, lamb's tongue, nightshade.
Symptoms
TSWV may be symptomless on some plants.
On other hosts a variety of symptoms may be
produced which are dependent on plant species,
cultivar, growing environment and virus strain.
Symptoms start to show 14-21 days after infection
and may occur on leaves, stems and fruit.
Plants may be distorted, stunted and show reduced
vigour. In some cases leaves and/or whole plants
may die.
Vegetables.
Tomato. Small areas of bronzing develop on the
upper side of young leaves in the terminal
growth and spread over the whole leaf (bronze
wilt). Older leaves have bronze spots, rings or
crescents up to 3 mm long between the veins.
These spots may extend and join up. Affected
leaves may wither and die and tissues blacken and
shrivel until the shoots look as if they have been
scorched by flame. Leaf stalks and stems may
develop dark streaks. Young vigorous plants
may be killed in a few days but in older plants
disease may take several weeks to develop. Fruit
on more mature plants may show irregular or
circular blotches as they ripen (Fig. 141).
Symptoms are usually obvious. Taste is not
affected. Young fruits shrivel and fall.
Broad bean. Tips of main shoots blacken and
may die, dark streaks may develop on stems and
black sunken lesions on pods.
Capsicum. Leaves show yellowish parallel lines
or concentric rings, the fruit is marked with yellow
rings and blotches up to 10 mm across which may
not show up until the fruit ripens. Rings and
blotches may darken.
Ornamentals.
Chrysanthemum leaves are marked with
irregular wavy lines, one inside the other. Leaves
in very susceptible varieties go brown and die.
Dahlia. Leaves develop yellow spots or rings.
Later concentric yellow or brown rings or wavy
lines appear. Symptoms are clearest on the first
formed foliage, especially in early-planted dahlias.
As the plant grows, new leaves formed during
summer may only show slight mottling or no
symptoms at all. Young stems may have brown to
purplish streaks (Fig.143).
Arum lily. Leaves develop yellow spots or streaks
parallel to the veins (Fig. 144). The stunted and
yellow appearance is distinctive.
Nasturtium. Leaves develop straw-colored spots,
become cupped, distorted, enlarged (Fig. 145).
Diagnostics. Diagnosis of TSWV can be quite
difficult. Symptoms are similar to, and can be
confused with, nutritional disorders, pesticide
injury, genetic patterns, etc, depending on the host
(page 275, Table 54).
Leaf symptoms usually occur initially on a few
scattered patches of plants which gradually spread
as thrips transmit TSWV to healthy adjacent plants.
Knowledge of typical leaf symptoms on a specific
host is required. Considerable experience is needed
for a confident diagnosis.
Fruit symptoms are usually easier to recognize.
Plant tests. Confirm diagnosis with an on-site
test, or send a plant sample to a diagnostic service
(page xiv). Nepo viruses and their diagnosis
www.daff.gov.au/ba/publications/nepoviruses
‘Overwintering’
In infected weeds, diseased stock plants, other
host plants, eg volunteer crop plants, cuttings.
TSWV is not seedborne, except for broad bean.
PP1-4 TSW final
Fig. 141. Tomato spotted wilt. Left: Circular
blotching of tomato fruit. Right: Ringspots and other
dark streaks on capsicum. PhotoCIT, Canberra (P.W.Unger).
286 Virus and virus-like diseases