Overlay is a layer of dense mycelium on the surface of casing that does not produce pins. It is a subject I have addressed on several occasions. Recently, it appears to be on the increase. As it causes considerable frustration and loss I make no excuse for yet again discussing potential causes.
Carl Bozicek in The Mushroom People, February 2002 emphasises the point that an anaerobic layer in the casing is a potential cause of overlay. I am sure he is correct in this observation, thus my title. However, this is only part of the story, an open, high structure casing, particularly if lightly watered can also become overlayed.
When the contents of a casing bucket lands heavily, it can cause a dent in the compost, particularly if the latter was not sufficiently firmed at levelling. This impact can knock the structure and air out of the casing itself. Such compaction slows movement of mycelium into the casing from below in a central area where there is also an increased depth, in the dent.
These factors delay mycelium reaching the surface. A grower waiting for such areas to show visible mycelium, prior to airing is delaying the break although in other areas mycelium has reached the surface and has started to grow horizontally and begun to matt. Add to this the invisibly fine mycelium from the casing inoculum (CI). It can spread widely, in a horizontal layer, on or near the casing surface.
However, It is only when this connects with cords of mycelium bringing nutrients up from the compost below that it rapidly thickens up. If it is dense enough, it becomes a visible, white, unproductive, patch of overlay.
Some casings have more structure than others, That is, they tend to break up into particles with cracks between them that allow greater mycelium colonisation. For example, if the ‘lumps’ on the ‘bed’ are smaller than 2cm in diameter this is a casing with a tendency to become over colonised and thus overlayed.
Less CI, more ‘compaction’ in a controlled manner to create larger lumps and more frequent, but lighter watering are advisable.
A list of circumstances that singly or in combination may lead to overlay follows:—
1. Too much casing inoculum (CI), for the particular type of casing. A box of CI for a 1000 bag house may work fine with a wet, or ‘heavier’, relatively amorphous casing. This is because much of the CI is expected to become waterlogged and die. In casing with a lot of structure much less CI is needed because less dies.
2. Uneven distribution of CI. CI can break up into small or large particles. It is difficult to distribute evenly. By chance, some bags will have several times as much Cl as others. Some may have none.
3. Uneven depth of casing. Mycelium reaching the surface starts to grow horizontally. If the growing tips do not receive a cooling shock, or other pinning stimulus, within a relatively short period of time, they fuse to form a non pinning network or matt of mycelium (overlay).
4. Uneven original moisture content or uneven watering of the casing. Both of these lead to unevenness in mycelium growth so that some bags are far in advance of others.
5. Uneven house temperatures, warmer areas, sometimes middle bags are warmer or they may be sheltered, as under racks. Furthermore, CO2 may ‘pool’ in such areas further allowing greater run-on.
6. Onset of warmer weather. This may exacerbate risk of overlay for several reasons. Compost becomes drier, i.e. with more nutrients per bag. There is thus more potential for heat production. Consequently a belated heat surge during case run can combine with warmer house temperatures and with greater CO2 production from the compost.
Overlay advisory cases usually seem to involve a succession of houses. Consequently losses may be substantial, all the more if it is potentially highly productive composts that are involved. In other words, a poor yield from an overlayed crop, might otherwise have been a bumper yield.
Only enough mycelium is needed in the casing to form sufficient pins plus their compost connections. Excess mycelium in the casing wastes compost resource that could otherwise have produced mushrooms.
It also hampers water uptake, thus starving later flushes of sufficient moisture.
Action Points, if overlay appears on more than a few bags 1. Blocks are slightly less prone to overlay than bags because they have a slightly greater area relative to compost weight.
2. Successively reduce CI until it is the minimum necessary. Accept that some clumping and a slower case-run are preferable to a substantial proportion of bags being unproductive due to overlay.
3. Make sure your casing supplier understands the vital importance of an even distribution of CI throughout the casing. Don’t apply CI or CACing to the casing surface.
4. Normally, avoid CACing using spawn-run compost, due to the danger from Trichoderma. However, if it is used, straw pieces should be short, evenly distributed, buried in the casing, i.e. not mainly on the surface and in minimal amounts per bag.
5. Instead of using CI or CACing deep ruffle on day four or five to evenly distribute particles of spawn-run casing from below.
6. In general, don’t make big changes to a successful growing regime. Ideally, changes should be minimal, single and ‘proved’ before experimenting with a further change. If two or more changes are made together, one may be beneficial the other detrimental, or they may interact.
7. Water casing more lightly and more often, aiming to avoid water logged casing below and it relatively dry above. Water as evenly as possible.
8. Casing depth should be as even as possible and not more than 5cm deep.
9. If some bags become advanced, being ready to break several days early, water these frequently, but lightly, to try and hold mycelium growth back.
10. If some overlay occurs, a shortage of pins on such bags may make switching to large breakfast flats advisable, to make the best use of compost.
11. Some growers employ a rather unconventional system. However, this appears to give an even cover of good quality mushrooms, with high yields.
No Cacing is used, but on day nine of case-run the house is rapidly and thoroughly flushed of CO2. It is then quickly and evenly deep ruffled, i.e. right down to the compost. Following this, the casing surface is lightly flattened with an aluminium disc attached to a pole. Then the house is sealed to retain CO2 and the air and compost temperature are brought up to 26°C for two days before airing/breaking gently over at least four days.
12. If it takes a thump to dislodge the casing from the bucket, this may be due to an airlock. Bore several small holes in the bottom of the bucket.