The summer, such as it was, is turning to autumn with a vengeance. Soon warm weather will be a distant memory. However, despite June being the only true summer month, as far as I was concerned, I did come across several severe high temperature growing problems this year. It only goes to show that it doesn’t take days and days of consecutive sunshine to result in damaging yield losses. All that is necessary is the following combination — a house that contains more compost than the cooling system can cope with, an active compost, and a few days of constant elevated temperatures, with no possibility of night time tooling. It has been awhile since I wrote an article for The Mushroom People so this would seem to be an obvious start.

Some of the signs of over-heating are as follows:

1. Very white mycelium.

2. Excessive condensation on the plastic.

3. The centre of the bag or block burned out, grey, dry and powdery.

4. Better mushroom growth along the outsides edges of the bags or beds.

5. Low yields.

The optimum temperature for mushroom mycelium growth is around 24°C. For every degree higher than this, the rate of growth will decrease at a progressively steeper rate. However, if a high temperature puts the mycelium under stress this will produce more and more oxalate crystals around each strand and make these appear thicker, whiter and thus much more easily visible. If a grower tells me: “It was a great spawn run, the bad are as white as snow,” I am immediately put on guard that the mycelium has been heat stressed.

Another ‘great’ sign that I am wary of is the ‘whisky’ in the bag, drops of amber ‘sweat’ running down the inside of the plastic. Both the whiteness and the ‘whisky’ may indicate a potentially good yield because the compost was ‘active’. In other words it contained plenty of substance (dry matter) i.e. contents other than water, and it also had ample nitrogen. However, as I have written on many previous occasions, keeping the temperature down below say 26°C as an absolute maximum will give even better yields!
What I have seen this year is not just high spawn-run temperatures also very high case-run and cropping temperatures. The casing will always be a few degrees lower than that of the compost. This is due to evaporation from it thus removing heat, but if the compost temperatures become excessive, mycelium will not run into the casing at all. Similarly, if compost temperatures rise above 24°C cropping will stop. In both cases there is also the factor of duration of time. A high temperature (of say 34ºC) briefly (a lay or two) during case-run appears not to be too damaging. However, if it is for a week it could be the cause of a big or even a total loss of yield.
As regards what can he done. A list of possible action points is as follows.

1. Find out by trial and error, or from past records how much weight of compost a cooling system can cope with. For example, assuming there is a balance between heat production by the compost and heat removal by the cooling system then it only takes a small lack of capacity, i.e. a little too much compost, to tip the scales. There is also the point that as temperatures go up so does mycelium activity and this further adds to heat production, and it may not necessarily be just mushroom mycelium growth heat, as there are always other fungi in compost known as thermophiles. These are waiting dormant until the temperature is high enough for them to start their extremely vigorous and very heat producing growth.

2. If the system used is growing on bags then space these out to enable air to flow around each one to carry excess heat away.

3. If growing on blocks and these are placed on the floor some growers water the floor to cool them. However, there may be breathing perforations near the bottom of the blocks that can thus take in water so that the compost becomes saturated and/or diseased.

4. Contact your supplier and check that the type of compost being currently produced is suitable for summer growing. It should be well broken down, in other words not bulky for weight and not looking ‘strawy’.

5. Spraying the blocks themselves could make Trichoderma (green mould) worse if the blocks have not been handled when wearing absolutely clean clothing.

6. As fuel costs for cooling can cost several pence per pound of mushrooms it may be best to reduce compost tonnage in summer.

7. Most important of all, keep an eye on compost temperatures as it arrives and when it is in the various stages in the houses. Also make sure the house it is going into is cool, especially after a recent cooking out.

8. A twenty-one day spawn run may be best to take the excess energy out of the compost. Nevertheless make quite sure the start of case run is always below 24°C.