Sesame

Aphids are a major pest, causing leaves to curl and attacked capsules become unattractive to customers.

Aphids feed by sucking plant sap. Small aphid populations may be relatively harmless, but heavily infested plants usually have wrinkled leaves, stunted growth and deformed capsules. Plants, in particular young plants, may dry out and die under heavy aphid attack. Heavy attack on older plants may cause crop loss by decreasing flower and seed production. Damage may also reduce seed viability.

Water soaked, small and irregular spots are formed on the leaves which later increase in number and turn brown. Severely infected leaves defoliate. Later, the spots are formed on the twigs which bear poor capsules. Spots appear from 4 to 6 leaf stage of the crop and continue till maturity. Seed treatment with hot water at 52°C for 10 minutes is recommended.

Light brown angular spots with dark purple margin appear in the leaf veins. Defoliation and death of plant may occur in severe leave and stem infection. Sunken and shiny spots appear on the capsules. Early capsule infection renders them black and seedless. The pathogen is seed borne. High temperature, rainfall and persistent humidity favour the disease.

Cutworms are caterpillars of Agrotis moth. The adult moth is grey to brown with a wingspan of about 4 cm and have lighter coloured hind wings. Whitish yellow eggs are laid at night on leaves. The eggs turn darker as hatching approaches. Young larvae may feed on leaves and cause tiny holes but they drop to the ground after a few days.

Mature larvae are about 4 cm long. They are easy to recognise by their smooth skin, greasy grey / orblack colour and C-shaped posture when disturbed. Cutworms emerge at night causing serious damage by cutting young plants at the base of the stem. Cutworm infestation is often associated with fields that are weedy, have high amounts of organic residue or are very wet due to poor drainage or heavily irrigated.

It is a fungal disease. Symptoms include partial or total wilting of plants at flowering and podding, a purple band of stems extending from the base upwards, browning of the stem tissue in the purple band area, and browning or blackening of internal tissue when the main stem or primary branches are split. Infected young plants may not show the purple band symptom but have conspicuous internal browning and blackening. Affected fields show patches of dead plants. The fungus survives on infected crop debris in the soil for about three years. Infection occurs from seeds or soil. Non-opening varieties are not as susceptible.

The pathogen attacks all parts of the plant at all stages. Small, dark brown water soaked, round to irregular lesions, with concentric rings, 1-8 mm in diameter appear on the leaves and under excessive atmospheric and soil humidity the spot increases in size and number. The lesions may also appear on the midrib and veins of the leaves. Milder attacks cause only defoliation, in severe cases the plant may die. The pathogen is seed-borne. Temperature of 20-30⁰C and humid conditions favour the disease.

The disease can infect all aerial parts: leaves, flowers and pods. Characteristic of the disease is white greyish powdery fungal growth on affected plant parts. Small pale yellow chlorotic spots develop on the upper surface of leaves and the corresponding lower surface develops white greyish powdery fungal growth. With time the powdery growth covers the entire upper and lower leaf area. Severe infection causes heavy leaf drop. The fungus develops at temperatures ranging from 20 to 35°C, but 25°C is the optimum. The fungus survives on perennial pigeon peas and volunteer plants, and on the ratoon growth of the harvested plants.

The adult is a 5 mm long red-bodied midge (mosquito-like fly). Female midges lay eggs along the veins of terminal leaves. The larvae are typical maggots; they are whitish to orange in colour, legless and with body tapering exteriorly and grow up to 3 to 4 mm in length. Maggots feed inside the floral buds and young capsules leading to formation of galls of up to 6 mm in diameter. They pupate inside the galls. Attacked flower buds wither and drop, or become twisted and stunted and do not develop in to flower/capsules. The simsim gall midge is usually a minor pest, but occasionally high infestations occur resulting in considerable crop losses. Generally plants with green capsules appear to be more susceptible to attack than plants with black capsules.

Eggs of red spider mites are round, white or pink and of size about 0.1 mm. Eggs are usually laid on the under-surface of leaves. They hatch into six-legged larvae, light green in colour, which become reddish coloured adults. The adults are about 0.25 -0.6 mm long. They have eight legs, and produce a fine silk webbing that protects them predators and pesticide sprays. Infested leaves are spotted, yellowish or silvery as a result of feeding by red spider mites. Infested capsules exhibit white speckling. The red spider mites are dispersed by wind and human activity on clothes while working in infested fields. Their development is favoured by warm weather and insufficient rains.

Initial symptom on stems and branches are spindle-shaped spots with light grey centres surrounded by brown margins. The centres of the spots have scattered dots (pycnidial bodies - fungal spores). The spots may join up and cause the branches or whole plants to dry up and die.

The fungus mainly attacks secondary finer roots. These roots have dark, blackened streaks underneath their barks with dots (pycnidial bodies - fungal spores).

Diseased plants suddenly wilt. When diseased plants are uprooted their roots are rotten and shredded. Disease development is favoured by hot dry weather (30 degC). Crops are more susceptible to the disease in the reproductive than in the vegetative stage.

Whiteflies are extremely polyphagous. They are a vector of many virus diseases. Whitefly feeding produces a honeydew, which is conducive to the development of sooty mould and also attracts ants. Whitefly incidence is favoured by dry conditions .