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Damping-off is deadly

11 August 2023
neglected, dying seedling in pile of dirt
Damping-off can occur at any stage of seed-raising.

You don’t want damping-off fungal diseases getting a foot in the door.
Here’s how organic gardeners keep them out.
WORDS Diana Noonan

Where do damping-off diseases come from?
Damping-off fungal diseases can be present in the soil, or in the seeds we are sowing. As soon as seeds absorb moisture from the soil, their seed coats soften and become less resistant to any disease that is present. Fungal disease of this nature explains why our seeds sometimes rot in the ground before they even germinate, or why young shoots die before they push through the soil.
Perhaps even more disappointingly, seemingly healthy seedlings can succumb to fungal disease once they are a few centimetres high. We sometimes blame their wilting on having left them in the sun or the shade for too long, or for having sown seed into soil that is too high in nitrogen. But it is much more likely that the young plants have been infected by damping-off. If we inspect seedlings that have damped-off, we will notice that their root structure is poorly developed, and dark or bleached-out.
Stop it before it starts
Once damping-off takes hold, it’s difficult to slow it down. Occasionally, tough seedlings (usually brassicas such as broccoli, cabbage and kale) will recover, but they usually go on to produce poorly after transplanting. The organic gardener’s plan must be to prevent damping-off from the get-go, and this can be managed in several ways.

Seed-raising mix
Organic growers debate whether to sow seed into commercial, organic seed-raising mix (which is likely to be disease-free) or into homemade compost. Debate is further divided as to whether the homemade compost should be the result of hot composting (which is still unlikely to reach the temperatures required by commercial providers to kill off disease) or cold composting (which is likely to be rich in the sort of micro-organisms that some growers believe may be able to out-compete fungal diseases). Novice organic gardeners may be wise to opt for a commercial, organic seed-raising mix until they are experienced enough to make their own decision.

There’s little point in using a healthy seed-raising mix if seed trays and tools are contaminated with fungal spores. Choose an organic sterilising solution such as Biogrow’s Organic Wood Liquid Vinegar to clean your equipment.

Sow seed sparingly so that seedlings don’t become crowded, reducing air flow. Cover seeds (if required) with no more mix than is recommended so that shoots reach the light as soon as possible.

Sowing into mix that is below 20°C can contribute to damping-off. Once seeds have germinated, soil temperatures above 25°C can also be a catalyst for damping-off. Purchase a soil thermometer and, if necessary, a germination pad, and check your soil temperature regularly.

Pricking out
Transplant (prick out) tiny, newly germinated seedlings once they have two true leaves (leaves like those on the adult plant). As you prick out, hold the seedling by its leaves, not its roots (which can introduce disease). Plant the seedling into an individual container of mix, or into a ‘communal’ container at the distance recommended on the seed packet. Don’t plant any deeper than the seedling was in its original container.

Light levels contribute to seedling health in general, and disease takes advantage of the weakest plants. If your seedlings are ‘leggy’ (all stem, few leaves and more space between leaves than expected), insufficient light may be the problem. If your seedlings’ leaves are looking yellow or brown, they may be being exposed to too much light. Don’t be afraid to experiment by moving your seedlings around.

Overwatering contributes significantly to damping-off. Water seedlings only when a moisture test indicates it is necessary, and always avoid wetting leaves. (As a rough moisture test, push a wooden chopstick 2–3cm into the growing mix. If it comes out clean – that is, with no mix adhering to it – it’s time to water.)
Almost out of the woods…
Once your seedlings have mature leaves and are looking healthy (a sign that they have a strong root system), damping-off is much less likely to occur. However, it can still be a problem if seedlings are transplanted into a garden with cold, wet, compacted soil. Don’t be in a rush to transplant and, if necessary, repot seedlings into larger containers to allow for root growth until outdoor conditions improve.
Thinking ahead
The organic gardener is always thinking ahead in order to stem the cycle of disease. Some fungal diseases can’t survive the chill of winter and are reintroduced to the garden each year when spores are carried onto plants by the wind. However, other fungal diseases have adapted to survive in cold weather by hunkering down over the colder months in thick, hard, pellet-like structures. These structures form on plants and their fruit over the active growing months. As infected plant material drops to the ground, the fungal structures fall with it and rest in the soil, becoming active again when warmer weather arrives.
The organic gardener must train themselves to be observant and to recognise the first signs of fungal disease in their plants. When it’s found, infected plant material must be removed and binned before the protective fungal structures form. This is one of easiest ways to prevent the cycle of disease continuing.

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