Rice blast

small spots on the leaves that grow into spindle- or diamond-shaped ('eye-shaped') lesions, pointed at both ends — from about the size of a rice grain up to your thumbnail. The centre is dead grey or ash-white, ringed by a brown to reddish-brown edge. When the weather stays wet the spots multiply and join up until whole leaves and patches of the field look scorched, as if burned.

as the crop heads out, look just below the grain head — the short 'neck' turns brown-black and rotten, choking off the head. The head then turns pale grey-white, stands up empty, and the grains stay blank and light (chaffy); rotten necks often snap and hang over. The stem joints (nodes) can also turn black and rot right through.

with brown spot, which makes small, even, round brown dots scattered like sesame seeds (no pointed ends), or with bacterial leaf blight, whose yellow-white streaks run in from the leaf tip and edges in long wavy lines. Blast's spots are diamond-shaped, with a grey dead centre, out on the flat of the leaf — that shape is the giveaway.

Walk a big 'W' across the field once or twice a week so you check all four corners and the middle — not just the edge by the path. Make about 10 stops, and look hardest in the lush, dark-green, crowded patches and low wet spots, where the fungus loves the extra nitrogen and moisture.

Blast can strike at every stage, so scout from seedling and tillering (when the plant throws out its side-shoots), right through to heading and grain-fill. Look hardest after cool, cloudy, humid spells with heavy dew or drizzle (around 20–28°C, air almost saturated at 90%+) — those long wet nights are when spores infect. Check the leaves early in the morning, while the dew still shows up the spots.

On the young crop, turn the leaves over and hunt for fresh, spreading diamond spots with grey centres (a still-growing spot has a soft grey middle; an old, stopped spot is dry and fully brown). At heading — as the grain heads push out — switch to the necks: check the short stalk just below each head for the first brown-black staining, especially if your field has ever had blast before.

— blast explodes fast, so don't wait for more. And if your field has any past history of blast, treat the neck at heading as a must-protect even before you see any rot — once the neck rots, it is too late to save that head.

the strongest single defence is to plant a blast-resistant rice variety suited to your area (ask your extension officer or seed dealer which ones hold up locally). Because the fungus can slowly break a variety's resistance (learn to attack it), rotate or mix the varieties you grow rather than planting the same one every season.

go easy on nitrogen fertiliser — never dump it all at once or late, because soft, lush, over-fed leaves are the fungus's favourite. Split the nitrogen into small doses, and if your soil is poor add potassium and silicon to toughen the leaves. Keep the paddy evenly flooded (steady water; don't let it dry out and stress the crop — upland and un-flooded rice suffers most), and don't sow too thickly, so the canopy can dry out between dews.

always plant clean, healthy, seed-treated seed — the fungus rides on infected grain. After harvest, plough in, remove or burn the old rice straw, stubble and volunteer plants where the fungus survives the dry season, so it can't jump into your next crop.

as soon as fresh spots appear, stop adding nitrogen and help the leaves dry faster (thin out any over-dense patches and cut the weedy grasses on the bunds that trap damp air and can also host blast). Pull out and remove the worst hotspot plants. Getting on it early, while it is still only a few leaf spots, is what keeps it from ever reaching the neck.

if a resistant variety plus the steps above aren't holding it, use tricyclazole, a systemic fungicide (systemic means it moves inside the plant and protects new growth, unlike a contact spray that only coats the surface). Timing is everything: spray a young crop while the leaf spots are actively spreading, and — to save the grain — spray at late booting to early heading, just as the heads are pushing out and BEFORE any neck rot shows. Spray safety: cover your skin, eyes, nose and mouth, spray in the cool of the day with the wind at your back, keep it off food, drinking water, children, animals and bees, wash yourself and the sprayer afterwards, and respect the pre-harvest interval (the set number of days you must wait between the last spray and harvest so no chemical is left on the grain). To stop the fungus becoming immune (resistant), do not spray tricyclazole every time — rotate to a different fungicide group between sprays.

if the spots keep multiplying after you spray and the wet weather holds, the strain may be resisting that fungicide — switch to a different chemical group and make sure the neck is protected at heading. When it's too late — the heads have already gone white and the grain is blank — no spray can refill them, so cut your losses: harvest whatever filled, and for next season fix the causes — switch to a resistant variety, cut back the nitrogen, plant clean seed, and destroy this crop's straw and stubble so blast can't carry over.