The main diseases affecting tomatoes are Early blight, Late blight, Bacterial wilt and Fusarium wilt. The main pests of tomato are Mites, Whiteflies, Leafminers (Tuta absoluta) and Thrips.
1. Bacterial wilt (Ralstonia solanacearum)
The disease causes rapid wilting and death of the entire plant
without any yellowing or spotting of leaves. Total collapse of the plant
usually occurring when temperatures reach 32°C and above. Plant wilts while
still green. When the stem of a wilting plant is cut across, the pith has a
darkened water-soaked appearance, and on squeezing the cut stem, a white,
yellow or greyish, slimy exudate may appear. In later stages of the disease,
decay of the pith may cause extensive hollowing of the stem.
Photo. Tomato plant infested by bacterial wilt |
Management:
- If only a few wilted plants are found, immediately remove them from the field.
- Practice long-term crop rotation (3 – 4 seasons).
2. Fusarium wilt (Fusarium oxysporum sp. Lycopersici)
The lower leaves of the plant usually turn yellow and die. One or
more branches may exhibit such symptoms. Leaves on one side may be affected
while those on the other side are symptomless. Diseased leaves readily break
away from the stem. When affected stems just above ground level and petioles
are cut diagonally, a reddish-brown discolouration of the water conducting
tissues will be observed.
Photos. Symptoms of fusarium wilt on tomato |
Management:
- Use resistant tomato varieties.
- Use certified disease-free seeds.
- Do not locate seedbeds on land where Fusarium wilt is known to have occurred.
- Where soil is acidic, raise the pH by applying lime or farmyard manure.
- Avoid excessive nitrogen fertilisation and control root-knot nematodes.
3. Early
blight (Alternaria solani)
Leaf spots of early blight are circular, up to 12mm in diameter,
brown, and often show a circular pattern, which distinguishes this disease from
other leaf spots on tomato.
Photos. Symptoms of early blight on tomato |
- Use
resistant varieties.
- Use
certified disease-free seeds. If using own seeds, hot water treat the
seeds.
- Use
disease-free plants.
- Practice rotation with Non-solaneceous crops (e.g.
eggplant, peppers, potatoes etc.).
- Stake and
prune indeterminate varieties.
- If disease
is endemic, applied preventative sprays of copper compounds (e.g. copper
hydroxide).
4. Late blight (Phytophthora
infestans)
Symptoms of late blight are irregular, greenish-black, water soaked
patches, which appear on the leaves. The spots soon turn brown and many of the
affected leaves wither, yet frequently remain attached to the stem.
Photo. Late blight symptoms on tomato plant and fruits |
- Use
resistant varieties.
- Use
disease-free transplants.
- Stake and
prune indeterminate varieties.
- Practice
crop rotation.
5. Powdery mildew
Photo. Symptoms of powdery mildew on tomato |
Photo. Stages of blossom end rot |
Management:
- Avoid
water stress during early stages of fruit development.
- Apply lime
in calcium deficient soils.
- Apply
foliar sprays of 0.5% calcium chloride at fruit development stage.
7. Tomato viruses
Often spread in the plantation by insect vectors such as whitefly,
thrips and aphids. The most important symptom of viral infections is the light
(white or yellow) colour of the leaves, or a mosaic pattern of light and darker
shades of green on the leaves. In many cases, viral disease leads to dwarfed
growth, rosette formation or other strange stem and leaf deformations.
Viruses reported on tomato crop include:
- Tobacco
mosaic virus or tomato mosaic virus (TMV or ToMV)
- Cucumber
mosaic virus (CMV)
- Tobacco
etch virus (TEV)
- Potato
virus-Y (PVY)
- Potato
leafroll virus (PLRV)
- Tomato
spotted wilt virus (TSWV)
- Pepper
veinal mottle virus (PVMV)
- Chilli
veinal mottle virus (CVMV or Chivmv)
- Tomato
yellow leaf curl virus (TYLCV)
- Tomato
Big-Bud mycoplasma (TBB)
Common control measures include:
- Use
tolerant varieties and certified disease-free seeds
- Protect
seedlings with a net in the nursery.
- Control
the insect vector.
- Eliminate
weeds and ornamental plants that harbour the virus.
- Remove and
destroy infected individual plants.
- Avoid an
overlap of tomato crops
- Remove
crop refuse and roots from fields
- Workers
should not smoke or take snuff when working in tomato fields since the
virus can be transmitted from tobacco.
8. Major Tomato Pests
The main insects of tomato are Red spider mites and Russet mites, Fruitworms,
Whiteflies, Leafminers and Thrips.
8.1. Whiteflies
Photo. Whiteflies under leaf. Adult whiteflies are about 1mm long |
Management:
- Time
sowing and transplanting. If possible avoid the season when whiteflies are
more likely to occur
- Check the
crop regularly. Use yellow sticky traps to monitor the presence of
whitefly adults.
- Remove
weeds in advance of planting tomatoes, and keep tomato fields weed-free.
- If
necessary spray neem extracts. Neem-based insecticides are reported to
substantially reduce egg laying by B. tabaci, inhibit the growth and
development of nymphs, and significantly reduce the risk of TYLCV
transmission.
- Spraying
with soap and water solutions reportedly controls whiteflies.
- Plant
barrier crops.
8.2. Aphids (Aphis gossypii, Myzus persicae)
Photo. Green peach aphid (Myzus persicae) |
Management
- Conserve
natural enemies. Aphids are usually kept under control by a wide range of
natural enemies. In particular, avoid use of wide spectrum pesticides
since they kill natural enemies.
- Use
reflective mulch. Reflective aluminium mulches deter aphids from landing
on plants. The effect is lost once plants are large enough to cover the
mulch.
8.3. Tuta Absoluta
It’s a harmful mining moth with a strong preference for tomatoes and
other crops in solanacea family.
Photo. Tomato leafminer, the moth has barrowed into the tomato fruit |
Prefers leaves and stems. It also occurs underneath the crown of a
fruit and sometimes inside.
Severe infection leads to drying of the leaves and sometimes dying
of the whole crop.
Control:
Use of chemicals e.g. Emamectin benzoate and Abamectin alternatively for attacking both adult and larva.
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