Cavity spot

you only see it when you pull a root, because all the trouble is below ground (nothing shows on the leaves). Look on the skin of the carrot or parsnip for small, sunken oval pits, from the size of a pinhead up to about 1 cm — the width of your smallest fingernail — usually running crossways around the root.

the pits are grey-to-tan and dished in like a thumbnail pressed into the skin, often with a darkened or slightly cracked rim. Early ones are small and shallow; later they grow bigger and join up into rough patches.

once the skin over a pit breaks, soft wet rots move in, turning that part of the root slimy and smelly — now the whole root is a reject, not just a downgrade.

scab makes rough, raised, corky brown patches you can feel standing proud of the surface. Cavity spot is the opposite — smooth-edged and sunken INTO the root.

carrot fly leaves rusty, winding tunnels and you may find a small pale maggot; cavity spot has no tunnels and no grub, just the surface pits. Cracks from stones show clean, pale torn flesh — not a dark, sunken oval rim.

begin lifting sample roots from mid-season, once the roots are sizing up (about finger-thick), and keep checking right through to harvest (late season). That whole stretch is when the fungus is pitting the swelling root.

the fungus loves wet, cool soil (about 15–25°C) and low, poorly drained patches. After a long wet spell, lift some extra roots — that is when fresh pits appear.

cross the field in a "W" so you sample all four corners, the middle, and — most important — the low, wet, heavy-soil patches where it always starts. Don't just check the dry ground by the path.

(roughly 20–30 roots in all). Rub the soil off each one and look all the way around it — pits often hide on the underside — from the shoulder down to the tip.

find sunken pits on even 1 in 10 lifted roots and you must treat the whole field as infected. There is no rescue spray — from here the decision is when to harvest and how to grade, not what to spray (see Control).

if pits are still small and few, plan to lift the crop soon, before they enlarge and wet rots set in. If many roots are pitted, plan to grade hard and sell for processing, not the fresh market.

Choose well-drained fields and avoid low, wet, heavy or compacted ground. Where water sits, break up hard pans and improve drainage before you sow. Cavity spot needs wet soil; take the water away and you take away the disease.

the fungus builds up in soil under carrots and parsnips, so rotate away from them (and other susceptible crops) for several years before coming back. And don't grow over-long crops — the longer a root sits in wet soil, the more pits form, so harvest on time.

avoid over-irrigation and waterlogging, and don't over-apply nitrogen — soft, lush roots in soggy soil are exactly what the fungus wants.

where you can get them, plant less-susceptible carrot varieties. Remember, "less susceptible" means they pit less — not that they can't get it, so you still need the drainage and the rotation.

Where it is registered and allowed in your area, an oomycete fungicide (for example metalaxyl) can be applied preventively — preventive means it must go on before any pits show. It works only about half the time and only alongside good drainage and rotation; once you see pits, no spray will fix them.

cover your skin, eyes, nose and mouth, spray in the cool of the day with the wind at your back, keep it off food, water sources, children and bees, and wash well afterwards. Respect the pre-harvest interval — the number of days the label says you must wait between spraying and lifting the crop. And rotate to a different chemical group rather than using the same one every time, so the fungus never gets used to it.

there is no rescue spray, so lift the crop early before the pits enlarge and wet rots set in, grade out the pitted roots, and sell the sound ones for processing rather than the fresh market. Then fix the real cause for next season — better drainage, a longer rotation, and a less-susceptible variety in a different, drier field.