Stop making these 6 mistakes and get rid of mould on your plants

Every grower fears mould. It is difficult to defend against, and once plants are infected, you will never get rid of the disease completely. The good news is that you can prevent mould from spreading very effectively. All you need to do is avoid a few common mistakes. What are they? You will find out in the following paragraphs.

There are many types of mould that attack plants. Today, however, we will focus only on the two most dangerous – powdery mildew and grey mould, which causes flower rot. These appear on plants most frequently, both when growing indoors and outdoors, as well as in greenhouses. Moulds are fungi and spread through microscopic spores, which can be found practically everywhere. Mould thrives particularly in high humidity and poor air circulation, and under suitable conditions it can begin to colonise plants.

Powdery mildew is caused by parasitic fungi from the Erysiphaceae family and manifests itself as a characteristic white coating on the leaves that resembles flour. Powdery mildew usually first appears on young leaves, where circular clumps of white mould develop. As the infection spreads, the patches grow and the mould attacks other parts of the plants.

Grey mould is caused by the parasitic fungus Botrytis cinerea, which causes rot in the above-ground parts of plants. Botrytis most commonly attacks ripening flowers, which usually begin to rot from the centre, meaning that the infection can be difficult to notice in time.

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Mistake 1 – High air humidity

A fundamentally important factor supporting the spread of mould is high air humidity. Botrytis thrives best at humidity levels above 65 % and powdery mildew requires air humidity of 70 to 90 %. When growing indoors, it is therefore almost always necessary to use dehumidifiers, which keep air humidity within reasonable limits. Low air humidity is particularly important during flowering, but mould can also attack plants during the vegetative stage. When growing outdoors or in greenhouses, you cannot influence the weather, but it is always better to plant your plants in a sunny, well-ventilated location rather than in a shady, damp spot.

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Mistake 2 – Insufficient air circulation

Growers often underestimate how important fresh air and the right microclimate are for plants. Mould thrives in poorly ventilated spaces or wherever the air is “stagnant”. When growing indoors, insufficient air circulation is most commonly caused by a weak extract fan and an insufficient number or incorrect positioning of circulation fans. In greenhouses, ventilate thoroughly and maintain a constant gentle draught.

You may also be interested in: How and where to position fans correctly in a growing room

Mistake 3 – Plants that are too dense

With large plants or when growing in a confined space, it can easily happen that air cannot flow freely  through a dense canopy. This creates an ideal microclimate for mould to develop. Therefore, care for your plants throughout the entire growing cycle, regularly remove unnecessary leaves and keep the canopy airy.

You may also be interested in: Defoliation (leaf removal) during flowering

Mistake 4 – Untidy surroundings

“Cleanliness is half of good health” also applies to growing. Botrytis spores and those of other moulds often first attack or survive on dead leaves and decomposing organic material. You should therefore regularly remove yellow or damaged leaves from your plants and keep the area around your garden tidy. Especially when growing indoors, never leave fallen leaves or debris lying on the growing medium.

Mistake 5 – Condensation

Very few growers realise how great an impact the difference in temperature between day and night can have. Rapid temperature changes cause droplets to condense on the leaves, creating suitable conditions for mould spores. Morning dew is one example, but a similar phenomenon also occurs when growing indoors. Especially after the lights are switched off, the temperature in the growing space drops very quickly and water may condense on the leaves, the walls of the grow tent and other surfaces. If you cannot prevent condensation, make sure that the plants dry as quickly as possible, for example by increasing extraction or ventilating the greenhouse.

Mistake 6 – Inconsistent inspection

If at all possible, inspect your plants every day to check for signs of mould infection. Do not only pay attention to clearly visible parts, but also focus on places where moisture accumulates. When growing outdoors, be vigilant during periods of rain and significant cooling, particularly in the second half of summer and in autumn. Botrytis often attacks the largest ripening flowers and progresses from the centre. Sometimes you can recognise an infected flower by the noticeably dried flower bracts on an otherwise green plant.

And what should you do if you discover during an inspection that mould has attacked your garden? In the case of powdery mildew, much depends on the severity of the infection and the age of the plants. If the plants are only mildly infected and there is not much time left until harvest, it may still be possible to save part of the yield. Botrytis is often discovered during harvesting. In such a case, discard the infected parts. Consuming mould can have serious health consequences!

You can find more detailed instructions on protecting plants from mould, as well as further tips and advice for growers, on the higarden blog.