Seeing the waste involved in a series  of cases of overlay always depresses me. Compost, casing, time and money all lost just because too much mushroom mycelium has developed, particularly across the casing surface, thus preventing pinning.

A recap on an old story

I’ve written several articles in The Mushroom People on overlay so I thought I had covered most aspects! However, I came across a case recently that illustrated an additional reason why wet sticky casing could be more prone to this problem.

I’ve twice mentioned buckets landing heavily, creating a central dent, casing ending up twice the depth in the centre compared to the sides. Usually the centre then becomes somewhat anaerobic.

The result - lots of ‘black centres’ later on, or, if the grower delays airing, in the hope the centres will catch up, there is the risks of letting the surface mycelium at the edge of the bag run too far  horizontally, resulting in matting, then overlay.

What to do if casing is too wet?

If a bag grower receives peat that is rather wet and sticky, it may be necessary to leave it to dry somewhat for a day or two, on the bag, before levelling. This is a good idea.

However, if levelling is done in such a way that the centre is tumbled, compost straws that have stuck to the bottom of the casing are brought to the surface.

These straws can effectively double, even treble, the amount of CAC. If this happened with every bag, airing a couple of days ‘early’ might be all that was necessary to avoid overlay. However, what happens when only half the bags have added compost straws and these require earlier airing?

Getting the right amount of CAC

In my opinion it is very counter productive to high yields to use too much CACing (casing inoculum). A full box per house appears to be more than enough in most circumstances.

With some types of peat, particularly in warm summer conditions a full box per 1000 bag house may be too much.
In some types of peat, mushroom mycelium runs much faster. Such peat requires much less casing inoculum.

Consequently, if the casing is inadvertently, additionally CACed with both straw and mycelium brought up from below, one can only expect trouble.

At best there will be over-pinning, leading to poor  quality in the first and second flushes, with few small, slow growing mushrooms in the third.

At worst there will be overlay and virtually no mushrooms on a high proportion of bags.

Pattern, probability and chance

I am often asked why is there overlay on one bag and a heavy crop of mushrooms on the bag beside it? There are a variety of reasons.

If there is a completely random pattern of overlay in a house it could be poor distribution of CACing, some bags having double, some none. If overlay shows up on groups of bags, these may correspond to areas where case-run temperature was just a degree or two higher and/or the airing was a bit slower in taking effect (i.e. due to less air movement).

If overlay is in double lines of bags along a house, or in one half and not the other, this could be due to one worker doing something different from the other/s, going deeper could double the CAC by bringing up straws for example.

The faster the case run, the narrower the window for airing

When the amount of casing inoculum is more than adequate and the casing temperature is about 24C (maximising mycelium growth rate) case-run is reduced to about 7 days or even less. There is then little margin for error in timing the house’ airing. In other words it is a lot harder to stop suddenly if you’re travelling at 60 than at 30mph!

Speed of case run is not predictable

The fact that case run may usually take say 7 days, does not guarantee that now and then it will not take 5 days!

Mycelium runs quicker in some types of peat. It may just run over the surface of the casing particles without much penetration of them. It should be remembered that peat is formed from the remains of a wide variety of plants and may have a very wide age range.

Although it may all look the same, nevertheless it is rather variable. Similarly casing inoculum is sometimes particularly fresh and active.

Add to this the fact that case run temperatures could by chance be just a fraction nearer the optimum. Such a combination of factors could lead to a very fast case run. Then, half day in timing of airing could make all the difference between having a serious loss due to overlay and having no overlay.