Alternaria Blight of Mustard Control

Symptoms

Dark brown to black round spots with clear concentric rings, like a target, appear on leaves, often surrounded by a yellow halo. Spots enlarge and join so leaves dry up and fall. On the stem and flower stalks dark elongated lesions form, and on pods dark sunken spots appear that shrivel the seeds inside. Warm humid and cloudy weather causes rapid spread.

Damage

Alternaria blight is one of the most widespread diseases of rapeseed and mustard in India and average yield losses of 30 to 57 percent have been reported, with damage to both leaves and pods. Pod infection reduces seed number, seed size and oil content, so it directly hits quality as well as quantity.

Organic & cultural control

Use clean disease-free seed and remove and destroy infected crop residue after harvest, since the fungus survives on debris and seed. Spray neem or other plant extracts that have shown some effect against the fungus. Maintain proper plant spacing for airflow and avoid excess irrigation and nitrogen that keep the canopy damp. Follow crop rotation with non-crucifer crops to lower soil and residue inoculum.

Chemical control

Spray a recommended fungicide at first appearance of spots and repeat as needed. Mancozeb 0.2 percent is the common protective spray, and a programme of mancozeb at about 45 days after sowing followed by a systemic such as hexaconazole or difenoconazole later has given good control. Treat seed with a recommended fungicide before sowing. Always follow the product label for dose and pre-harvest interval and consult your KVK or an agronomist before use.

Always follow the product label, the recommended pre-harvest interval, and your local KVK or agronomist. Pesticide approvals change, so verify before use.

Prevention

Use healthy certified seed from a disease-free crop and treat it before sowing. Sow at the recommended time and spacing to avoid a dense damp canopy. Follow crop rotation with non-crucifer crops for at least 2 to 3 years. Remove and destroy infected stubble and weed crucifers. Avoid late evening irrigation and excess nitrogen, and scout the crop from the leafy stage so you can start fungicide at the first spots.