Gadenin日本語Get the appDamping off is the one real disease
Radish shoots have no pests worth discussing indoors and one disease that matters. Damping off is caused by soil-borne fungi and water molds that are already present in most potting mixes at harmless levels and become destructive when the conditions swing their way: a saturated medium, a dense mat, warmth above 72F (22C) and no air movement.
It shows as a water-soaked, pinched, brown or translucent band right at the base of the stems, and it moves fast. A patch the size of a coin at breakfast can be the size of your hand by evening, and the shoots above it fall over as a sheet because the stem has been girdled rather than eaten.
There is no treatment once it is running in a tray this dense. Nothing you can apply reaches the base of a crowded mat, and nothing you would want to apply belongs on a crop you eat in three days. If more than a third of the tray is affected, tip the whole thing out. If it is a single patch, cut that patch out at the base, get a fan on the tray, and cut the rest a day early.
| Symptom | Likely cause | What to do |
|---|---|---|
| Pinched brown stems at the base, mat collapsing | Damping off in a wet, warm, still tray | Cut out the patch, add airflow, harvest early |
| Gray cobweb strands across the surface | Mold on ungerminated or broken seed | Pick out the seed, mist less, keep airflow up |
| Sour or yeasty smell, no visible damage yet | Standing water under the medium | Pour off, do not water for a day, add a fan |
| Fine white halo around every root | Root hairs, entirely normal | Nothing, mist to confirm and carry on |
| Yellow shoots long after uncovering | Not enough light, or a light leak in blackout | Move under a lamp for 12 to 16 hours a day |
| Tall thin shoots falling flat | Blackout ran too long, or sown too thin | Cut early, shorten blackout on the next tray |
Root hairs are not mold, and this is worth ten seconds
Radish produces a dense white fringe of root hairs a day or two after germination, and it looks alarmingly like fungal growth to anyone seeing it for the first time. More perfectly good trays are thrown out over this than are lost to actual disease.
The test is water. Mist the suspect area and root hairs flatten instantly against the root and effectively disappear, reappearing as the surface dries. Mold does not respond to misting at all.
The distribution is the other tell. Root hairs appear on every seedling at once, evenly across the tray, in a neat collar right at the root. Mold starts as one patch, usually centered on a seed that failed to germinate, and spreads outward in strands that bridge between stems rather than clinging to each root.
Smell the tray as well. A healthy tray at any stage smells faintly of fresh radish and damp earth. A tray that smells sour, yeasty or of drains has a problem at the base whether or not you can see it yet, and that smell always arrives a day before the collapse.
Prevention is four habits, not a product
Nothing you spray will save a dense tray of shoots, so the whole of disease control here is in how the tray is set up and run. Four habits between them remove nearly all of the risk.
Growers who lose trays regularly are almost always doing two of these four things wrong at once, usually overhead watering in a warm room with no fan. Fix those two and the problem tends to disappear entirely rather than improve.
- Weigh the seed instead of guessing An overloaded mat holds water against its own stems and cannot dry. About an ounce (28 g) per 10 by 20 in (25 by 50 cm) tray is the ceiling, not a starting point to build on.
- Water from below after uncovering Wick from a solid tray beneath, or pour down one corner, and tip off whatever is not absorbed within fifteen minutes. Water sitting in the crown of a crowded mat is the whole problem.
- Keep air moving A clip fan on low pointed at a wall, or an open shelf and an open window twice a day. Never a humidity dome after the blackout ends.
- Start clean every time Fresh medium for every tray, and trays washed in hot soapy water and rinsed between crops. Reused medium carries the previous tray's root mat and roughly doubles the failure rate.
Problems that look like disease and are not
Seed coats stuck on the tips of the seed leaves are cosmetic. They are common when the surface dried out during the blackout, and they mostly drop off once the leaves open. Mist lightly the day before cutting and pick out the stragglers as you harvest.
Purple or reddish tinting in the stems is normal in some seed lots and is more pronounced when the tray is cool or the light is strong. It has no effect on flavor or keeping.
Uneven height across a tray is a sowing or a light problem, not an illness. A tilted or dished surface, a light leak into one end of the blackout cover, or a cold draft on one side will all give you a mat you cannot cut level. Fix it at the next sowing rather than trying to correct the tray you have.
Log the failures, briefly
When a tray goes wrong, write down the room temperature, whether a fan was running, how you watered and what day the trouble appeared. It takes one line, and it is the only way to separate a bad seed lot from a bad shelf.
The pattern almost everyone finds in their own notes is a temperature threshold: below some point in the low seventies Fahrenheit their trays are reliable, and above it they are not. Knowing your own number is worth more than any general advice, because it tells you exactly when to move the trays and when to sow thinner.