The aphid arrives on a schedule
Mustard aphid builds up in the cooler part of the season and attacks the flowering and pod-forming stage. Heavy infestation covers the shoots, drains the plant, and leaves sticky honeydew and sooty mould behind.
The single most effective control is sowing on time. Early-sown mustard flowers and sets pods before the aphid population peaks; late-sown mustard flowers straight into it. In most districts the difference between a well-timed and a late crop is larger than the difference between spraying and not.
When a spray is justified
Scout the terminal shoots. The usual threshold is when a significant proportion of plants carry aphid colonies on the top portion.
- Spray the top of the canopy, where the aphids are.
- Use a selective product where possible — mustard is a heavily bee-visited crop at flowering and broad-spectrum insecticides at that moment do real damage to pollination as well as to natural enemies.
- Spray in the late evening, when bee activity has stopped. This matters more in mustard than in almost any other field crop.
- Remove and destroy the first heavily infested terminal shoots, which are often clustered.
Alternaria blight and white rust
Both are common. Alternaria shows as dark concentric spots on leaves and pods; white rust as white pustules on the underside with distorted, swollen inflorescences.
Manage them with a tolerant variety, adequate spacing for airflow, and a protectant application when conditions turn humid, applied before the disease reaches the pods.
Sulphur is not optional
Mustard is a heavy sulphur user. Sulphur drives glucosinolate and oil content — the two things that decide what the crop is worth. Deficiency shows as pale, cupped younger leaves and poor pod filling.
Apply it at sowing, in a form that will be available in a rabi season: elemental sulphur needs warmth and time to oxidise, so on a short cool season a sulphate form may serve better.
Two smaller things
Thinning to the recommended spacing. Dense mustard lodges and gets more disease.
Irrigation at flowering and pod formation, if available. These are the two stages where water pays.
Late-evening spraying at flowering is a beekeeping courtesy and a yield decision. Poor pollination costs pods.