Lars Lund

08 Sep 2020 14:34

Greenhouse plants also get sick

How to keep diseases away

The summer started already in April, which has boosted the greenhouse plants. This also means that diseases which would have appeared later in the season are here now. Some greenhouses are filled with lice and mildew, while others are filled with yellow and scorched leaves.

You can’t always be completely sure what causes the problems. In some cases, it can be caused by multiple things, but with a magnifying glass and a precise description of how the plant is nursed it can be figured out.


Plants suffer from diseases because:


Plants get too much or too little nitrogen.

Make sure to dose according to the directions. If you make your own fertilizer, the colour of the leaves will indicate how they are doing. Green leaves are a sign of health. If they turn brighter than normal, they might need more fertilizer. Too much nitrogen (N) is not good, especially not for greenhouse plants. In the greenhouse phosphor (P) and potassium (K) are more important. Vine needs very little fertilizer, but gladly compost.


Plants have stress

Kids that are born prematurely can get stress, and elders too. Plants get stress for a number of reasons, but typically because they are transplanted too early. If the soil temperature is below 14 degrees plants need phosphor as it can only be absorbed in temperatures above 14 degrees.

Cucumbers that are transplanted before the 1st of June (maybe on the 20th of May) risk getting a poor root system which weakens the plant. The consequences of transplanting too early will show latter, typically by a bad fruit development or sick leaves.


Excessive humidity

A greenhouse is a very closed environment and many diseases are carried by drops of water or need humidity to develop. Mildew, paradoxically, develops when the soil is too dry, but it spreads when the humidity is high. Generally, all fungi diseases develop in humid environments. That is why it is important that plants are not placed in a constantly humid environment.

Outside plants dry out quickly and get fewer fungi diseases, especially when the wind is blowing. You can irrigate with a sprinkler that showers your plants. You should irrigate close to the soil (if cucumbers, then irrigate away from the stem) and air out the greenhouse morning and evening. If your plants are often infested with diseases, open up all windows and doors. When a plant is growing, it does not need as much heat as you think.

The soil is too poor

Soil, coconut and sphagnum are the typical methods of cultivation, but the size of coconut is micro as the product is difficult to obtain. If you grow in beds you should as a minimum apply one new layer of compost every year. The most important matter in the soil is the millions of beneficial microorganisms and fungi.

They need to be fed with compost to give back important minerals to the soil and plants. By planting directly in the ground, the plants will develop deeper roots and thereby a better root system. One of the important and beneficial fungi is the Mycorrhiza fungus, which among other things increases the plants’ ability to absorb phosphor. If your plants lack phosphor (indicated by scorched leaves) you can add Mycorrhiza.

90% grow in sphagnum. Some (approximately 10%) grow in growbags on the ground because they don’t want to change the soil or add a layer of compost. Most people grow in sphagnum because growbags combined with capillary boxes are the best solution for the busy one. Some say that when growing in capillary boxes you are more exposed to fungi diseases like blossom end rot on tomatoes. Given that 80% grow in capillary boxes, it must be true that most diseases are found there. The most common reason is that people fertilize too much. When it is hot plants need more water to cool down, so if you have fertilized a bit too much, the plants easily get fertilized too much. That is why it is important to skip fertilizing if there is a heatwave on its way, or just fertilize less. Also, remember that tomatoes easily get diseases if irrigated only with rainwater. Irrigate every other time with tap water. Sphagnum is a good product, not in an organic way, but because it absorbs a lot of water. Choose the most expensive, which is more porous and better quality, if you plant in capillary boxes. Look for the volume in the bag. The more the better. Mushroom compost is a composite product of horse manure and sphagnum. Those bags often have a good volume.


Products that help prevent fungi diseases

Skimmed milk can be sprinkled on the leaves to cover them with a layer of calcium.

An extract of horsetail or garlic protects and, in some cases, kills fungi.    

Om Lars Lund

Lars Lund
Danish horticulturist and journalist
Lars Lund has for many years engaged in the garden and greenhouse. Lars has published many books about greenhouses, and he has participated in many Danish horticultural TV shows. He is a walking garden encyclopaedia, and he has answers for most basic cultivation questions – also the more ambitious ones.  

Get to know Lars Lund