I got an email from Susannah who had picked up on my garden theme in my last post on this blog. Susannah’s got a problem with damping off in seedlings. She has just lost three young cucumbers and asked if I had any advice. Damping off is a fungal infection of plants, usually seedlings but can affect more mature plants also. I too had recently had a problem after repotting a supermarket bought basil plant. Within a couple of days in its new pot, almost the entire surface of the compost around the base of the basil was white with fungal mycelium. After a few more days most of the stems of the basil were covered in fungus and turning black…

Readers of my blog will know that fungi features regularly in my posts. There’s a very good reason for this of course. Fungi need a good supply of moisture for growth. They get this moisture from the vegetable or animal thing they are growing on, or from the atmosphere. Any organic matter that is damp is good potential fungal growing ground. To extract moisture from the atmosphere, generally, the relative humidity must be above 70%.

So, the quick, but maybe not so simple to put into effect, answer for Susannah’s damping off problem is not to have the surface around the plants damp and keep the surrounding humidity below 70%.

Not wanting to dump my new purchase of a basil plant I had a close look at what was going on. Most but not all of the basil stems were infected. In amongst the white mass of mycelium were a few stems that had not succumbed to the fungal onslaught. For a second time I repotted the basil. Carefully, I scrapped off as much of the surface compost and white mycelium that I could, cut out the dead and dying black stems, and removed a lot of the outside roots and potted up the good stems with their root ball into fresh compost.

This time I made sure the potted basil was watered from the bottom by placing it in a container with water up to about half way on the pot. That way the fresh compost gets sufficient water up to the root ball, but importantly the top surface of the compost is kept dry. After leaving the plant for a while, I lifted the pot out of the water container to check the weight by the feel of it. Happy that there was enough water taken up I set up the basil in a container indoors on a window shelf, only watering from the bottom when, by the feel of the weight, I think some water was needed.

Here’s the result:

Basil recovered from damping off

Basil growing well after rescuing from fungal infection


You may be able to see a little bit of the fungal legacy on the surface, but I have got it under control by limiting the amount of moisture getting to the surface. Although the basil is sitting on a bathroom window shelf and the humidity will peak upwards at times, it is otherwise well enough ventilated. The surrounding humidity will not be high enough for long enough to allow the fungi to really get going again. Also the sun coming through the window, not only provides light for the basil, but the temperature effect will also drive the relative humidity down in the room. It is only in an enclosed space, where water is around, that a rise in temperature pushes moisture levels up. For example, greenhouses, cloches, plant incubators, etc. that are not well ventilated.

To find out more about damping off, such as the causes and the species of fungi involved, here’s a link to a page on the The Royal Horticultural Society website.

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Useful background and various topics on moisture and humidity are discussed in my eBook “A Wet Look At Climate Change”.

Welcome to my world of moisture