MANY growers still suffer substantial losses due to overlay. Overlay is strong, horizontal surface mycelium that seldom pins. A number of potential causes are listed. Most have featured in experiments or discussions with ten growers who have had a recurring problem. All growers who have made sufficient changes to their airing program and casing management and possibly spawn-run time have not had overlay again. They may however have gained the relatively minor problems of a little pre-pinning and some breaking deep (mainly in the first flush).
The principle reason for most cases of overlay appears to be that many growers do not realise that supplemented compost or bunker or other 'strawy' high structure compost,although having the potential for yields well over 500Ib/t, may stimulate greater heat and CO2 production. Consequently, unless casing is correctly managed it tends to become overloaded with mycelium, particularly at the surface.
A bad combination appears to be 'active' compost, insufficient spawn run time (14 days), deep, wet casing that is not opened up by early deep ruffling, plus abundant CACing. Especially if this CACing is in the form of strawy, unchopped compost.This tends to end up near the surface, adding to the load of horizontal mycelium. Dry casing can lead to over-colonisation right through it.
Overlay in concrete houses is uncommon but has been deliberately created simply by placing a sheet of newspaper over each experimental bag after last watering and leaving the paper in place until the beginning of airing. It is assumed the paper either traps CO2 and this is not dispersed rapidly enough to allow pinning. Or it stops some vital stimulus that occurs prior to airing, possibly evaporation chilling of the surface. This fits in nicely with the observation that growers with concrete 'houses' or those with plastic tunnels who deliberately leak in a little fresh air, only get overlay rarely.
1.Too high a temperature at airing pinning at least in part of the house. Bags temperature should be brought down gradually to 21-21.5°C. This has to be reduced below 23°C before pinning gets well underway. Do not rely on just one or two readings. A house should have at least six thermometers. Readings should be from all over the house, particularly from inner rows, especially from below racks and from the warmer, fan-end of the 'house'. The tip of the thermometer should go to the bag centre. This is much hotter than the outside. Allow space between bags for cooling especially in the summer. Problems occur if insufficient time is allowed for spawn run. Nineteen and preferably 21 days are needed, otherwise bag temperatures may exceed 30 °C during case-run particularly with tall bags.
If bags appear to run out of 'power' in the third flush this is not the result of too long a spawn-run. It is due to insufficient nutrients in the compost (check the analysis), or to bags overheating. Aim for 25°C max. during both spawn and case-run, peaks of 28°C are acceptable but exceeding 30°C will probably result in a poor yield. If bag temperature is above 28°C at the start of airing it is very difficult to get it down sufficiently to start pinning and simultaneously create suitable humidity and air temperature conditions for pinning.
2.Too much CO2 at airing at least in part of the house may prevent pinning. A reason for high CO2 may be a blocked exit port or a partially blocked radiator grill. The fins can be bent over by power hosing them. It has also been reported that houses that back onto others are prone to overlay, their air exit ports are either less effective, or less cool air leaks in.
*Beware! Your CO2 meter can give a lower reading than it should. The small lock-washer that retains the water drain device at the bottom of the plastic cup is prone to rust. It then does not hold tightly enough to compress the rubber ring seal, resulting in an air leak. When the pump draws air from the mushroom house it also draws in outside air via the leak thus diluting the CO2 so the reading is low.
A large amount of CO2 is produced in each bag (1.5-3 litres per m2 of bed = to 5-6 bags). This must get away so that there isn't a strong upsurge during airing. Beware wet casing it may delay CO2 release. If a substantial amount comes up at airing the concentration in the pinning layer may be much higher than that read by the meter. A heavy casing with large surface lumps may have hollows in which CO2 tends to 'pool'. This gas is heavier than air. It is safest to bleed in some fresh air during case-run so CO2 remains just above 3000 ppm, not vastly higher. It is also beneficial to have re-circulating air moving rapidly enough to disperse 'pooled' CO2 and possibly to slightly 'chill' the surface pinning layer. If during case run air temperature is set to allow for a variation of up to 2°C this allows an expansion and contraction of gas in the bags, helping them to 'breath'.
Holes in the bag are probably also beneficial, although if less CO2 comes up through the casing from below, mushrooms may tend to break deep. The CO2 intake should be tied to a peg stuck into the casing so that the meter accurately reflects the concentration near the pinning layer, also move the intake about the house. *Beware! Bunker or other 'strong' or supplemented compost may produce more CO2 than 'traditional' compost. If a house is broken using humidity alone there may be more CO2 in the house than expected from previous experience.
3. A delay in airing or in achieving airing conditions may lead to overlay
Delay in airing that leads to overlay is sometimes accidental, but may be deliberate to make picking more convenient, or to allow parts of the house to catch up. Overlay was seen in a house were the usual ruffling day was delayed. Depending on strain, pinning does not get well underway till mean bag temperature is below 23°C and CO2 below 1300 ppm. If either parameter is much higher this may delay pinning and allow mycelium to 'run on'.
4. Too high a level of humidity
After the start of airing humidity should not exceed 95% and at pinning it should be between 80 and 95% according to strain recommendations. A popular hybrid white has a manufacturers specification that humidity should be 90- 92% at pinning. However ensure the monitor is accurate and readings represent what is happening at the casing surface and throughout the house, particularly if humidity and not CO2 is used as the sole measurement used for breaking. The wick of the wet and dry bulb thermometer must be clean and distilled water used in the 'well'.
If part of a house is too far advanced at airing this may become overlaid.
5. A list of causes for part of a house being more advanced.
1. A higher temperature in that part. This may be only 0.5°C, but over time can result in some bags becoming much more advanced. Extra radiation from the radiator unit usually means the fan end of the house is warmer during case run.
2. A variation in casing structure may allow mycelium to colonise some bags more rapidly than others.
3. A variation in type of casing may allow mycelium to reach the surface more rapidly in some bags.
4. A variation in method of ruffling may allow mycelium in some bags to grow faster. Different workers tend to ruffle to different depths and this affects speed of case-run.
5. A variation in the amount of CACing will advance some bags. Beware using more CACing than recommended, particularly compost CACing that has not been chopped. Levelling the casing it can bring long straws near the surface where they will lead to excess horizontal mycelium.
6. Beware the additive effect of CACing and ruffling, together they could swamp the casing with mycelium.
7. A variation in timing when casing a house may mean part of it is half a day ahead. The first part cased should not be in the warmer, fan-end of the house.
8. A variation in watering, or a delay, or less water applied, can all result in more mycelium colonising the casing.
9. Large houses are more at risk of overlay for several reasons. These include greater difficulty in getting an even strength of air circulation over the casing. There are more heating and cooling demands. Also more workers are needed, leading to more variation in casing management,or longer to complete tasks.
6. Genetic factors There may be strains of Agaricus that regularly produce a type of overlay. Such strains mayor may not be infected by a virus. Such strains are low yielding and are not used for commercial spawn.
7. Factors that might add to the severity of overlay (this is an unknown area). 1. Applying Sporgon at a concentration higher than that recommended, particularly in a minimum quantity of water and at the last watering before airing.
2. Using casing that has been accidentally sterilised or stored in anaerobic conditions.
Reducing the overlay risk
1. Breeze block houses are less liable to overlay. 'Bleeding in' fresh air and allowing a 2°C fluctuation in temperature, day to night, can make a plastic house more similar to a block house. The risk of getting some pre-pinning and/or some mushrooms breaking deep is more than compensated for by higher yields and less chance of overlay. Usually there is less overlay at the door end of a plastic house This is probably due to both less CO2 and more heat loss from the metal doors at night, resulting in larger temperature fluctuations.
2. Deep ruffling the casing on day 4-6 helps CO2 to escape. This also evens out colonisation so no CACing or CI spawn required. Alternatively some growers rapidly flush the house of CO2 on day nine before deep ruffling, followed by gently firming the surface and bringing the temperature and CO2 up for a day or two before gently breaking.
3. The greater the amount of 'power' in a compost the more likely it is to getoverlay. This is partly because at airing-pinning the grower is 'fighting' against over pinning. Bunker compost and phase three may be better in blocks or at least in smaller quantities per bag (16kg or 62 bags per ton).
4. Casing structure can be used to influence the number of pins. The larger the lumps that make up the surface of the casing the smaller the number of pins.However if the casing surface is very rough, a strong flow of air is required to make sure there are not pockets of high CO2 in the crevices where the pins should form. Some growers somewhat level and slightly compress the surface after ruffling to decrease the amount of breaking deep.
5. If a grower wishes to avoid ruffling, the casing should be put through a casing machine. This will aerate it and even out the amount of casing spawn in it. When CACing made from compost is added to the casing it is vital that it is evenly added and not overdone. A grower who used almost double the recommended amount of CACing had a very rapid case-run (5.5 days) but had severe overlay in a succession of houses (although this may have had other contributing causes).
6. A slow break is desirable to prevent overpinning.However a rapid spawn-run mitigates against this. Breaking slowly has the danger of allowing the mycelium to run-on due to the rapidity of spawn growth into the casing.Growth has a certain 'momentum'.
7. There may be a danger in applying Sporgon in the minimum quantity of water just before airing. A light watering after Sporgon application, to make sure the pinning layer does not have too high a salt-concentration is advisable.
8. Check for air leaks around the water drain valve on the CO2 meter by putting one or two drops of soapy water into the water trap and slightly blocking the air inlet tube. If air is entering at the valve bubbles will form around it.
9. A well known grower pointed out that if the surface of the casing layer is allowed to dry somewhat before pinning, breaking deep has minimal effect on quality as the peat on top of the mushrooms is relatively dry.
Acknowledgements I would like to thank the UFU for helping to support this work also the advisors at Reen compost and Sylvan Spawn for many helpful pieces of information and for taking me to see a wide variety of cases of overlay.
