Diplodia root and stalk rot

whole plants wilt and die too early, weeks before the rest of the field is ready. The leaves fade to a dull grey-green, then to a bleached, straw colour — the plant looks 'frosted' or scorched from the ground up while nearby healthy plants are still green.

The lowest one or two internodes (the first 'knuckles' just above the soil) go soft, brown and spongy. Squeeze that section between your thumb and finger: a healthy stalk stays firm and round, but a rotted one crushes flat and hollow like a dry drinking straw. These plants lean over and snap at the base after wind — this falling-over is called lodging.

with your knife. Inside, the white pith is shredded and dry, gone tan-to-brown, and scattered over the rind and pith you'll see tiny black specks like grains of pepper or pinheads — these are pycnidia, the fungus's tiny seed-bodies (that is the sure sign it is Diplodia).

look for a white, cottony mould growing over the husk and creeping between the kernels. Affected cobs feel light and dry, the kernels are glued together, the husks bleach pale but stay clamped shut and upright, and tiny black specks appear on the husk. A cob may rot right through from the base.

Charcoal rot also shreds the pith but leaves a grey-black, sooty dust inside (like charcoal), not brown rot with pepper-specks. Gibberella/Fusarium rots turn the inside pink or reddish, not brown. Ordinary drying at maturity leaves the base firm — if the base is soft enough to crush, it is rot, not ripening. Drought scorches from the tips and the base stays hard; Diplodia softens and rots the base and roots.

start once the crop has flowered (silking / tasseling) and keep checking right through grain fill and as the crop nears maturity — this is the mid-to-late season window when the rot shows itself. Look hardest in fields that had a dry spell around flowering followed by wet weather during grain fill, and in fields grown on maize after maize with old stalks lying on top of the soil.

don't just check the edge by the path. Walk a big 'W' shape across the whole field so you touch all four corners and the middle. Make about 10 stops along the 'W', and at each stop check 10 plants (100 plants in all).

push the top of each plant about one hand's width (30 cm) to the side. A sound plant springs back; a rotting plant leans and stays leaning, or its base crumples. For any plant that fails, squeeze the lowest internode — if it crushes flat, that plant is rotting inside. Also glance at the cobs for white mould and at the stalk base for browning and black pepper-specks.

count how many of the 100 plants fail the push test. If more than 1 in every 10 plants (about 10 in 100) crush or lean, the field is rotting from the base and will lodge — mark it to harvest that field first, before the wind flattens it (see control). Even a few soft plants early on is your warning to plan the break-the-cycle steps for next season.

Once the fungus is inside the roots and stalk, fungicides cannot reach it and there is no rescue spray for this rot. Anyone selling you a spray to 'cure' collapsing stalks is selling you nothing — put that money into rotation, clean residue and better seed instead. (This dossier lists only cultural methods and a tolerant variety — there is no chemical control for Diplodia stalk and root rot.)

If your push test shows many plants soft and crushing, those plants will not recover — the job now is to rescue the grain before it hits the ground. Harvest that field early, ahead of your other fields, before wind lodges the plants (cobs on wet soil rot and are lost completely). Dry the grain well and throw out any white-mouldy cobs: heavily moulded Diplodia grain can carry toxins, so keep badly mouldy cobs out of your family's food and don't feed them to animals. Then, before next season, break the cycle: rotate away from maize, bury the residue, and come back with a stronger-standing variety.