COMPOST The dryer
weather in The Netherlands in 2025 seems to have been a common,
decisive factor.
surrounding area. The most dangerous situation is a mushroom farm next door, while the least dangerous is a composting yard literally in the middle of nowhere. Spawning in a poorly disinfected hall at a yard that produces phase III compost carries a higher risk than spawning outdoors at a facility that only produces spawnable compost. In a closed hall on a yard that produces phase III, the infection pressure will increase every week. However, spawning compost outdoors benefits from a continuous supply of clean fresh air which continually removes any infection pressure in the compost. Composting yards next to a mushroom farm must be much more alert. As an example: most compost yards use F9 spore filters installed in the climate units of the tunnels and in the overpressure system of the spawning hall. These F9 filters trap>99.9% of Trichoderma spores (2.5-3.5 microns). That is a high capture efficiency (photo 8). But this can be interpreted in another way: 1 in 1000 spores will pass through the filter and it takes just 80 spores per kg of compost to trigger an infection. If the air in the surrounding area is potentially heavily contaminated (neighbouring mushroom farm), I recommend using an F10 filter. This filter class captures 99.99% of the spores and is ten times more effective than the F9. A Trichoderma contamination originating from phase I does not overly concern me, but I am extremely concerned about infections from phase III or the mushroom farm itself. While there is potential chance of Trichoderma infections entering the farm on phase I compost, under normal circumstances the concentration will be very low. However infections that originate from phase III or on the farm have already multiplied significantly and are therefore far more dangerous.
Problem analysis in the Netherlands This article was prompted by the specific problem that occurred in the Netherlands this year I mentioned at the beginning. Numerous compos- ting yards experienced issues with Trichoderma simultaneously and these problems subse- quently vanished at the same time. I visited several of these yards, inspected and assessed the hygiene myself and concluded that it was acceptable. There are always minor areas of improvement, but certainly not serious enough to explain this Trichoderma outbreak. So there had to be another common factor causing these problems. This is not a scientific study; I have simply attempted to analyse the problem. As all these composters work with practically identical raw materials (>95% horse manure) and the same type of buildings, and there are no major differences in the working methods such as processing cycles and recipes, I looked for a common factor all of these composters faced. The fact that the outbreak started in spring and ended in autumn suggests a seasonal factor. And I do not believe the problem is the hygiene standards. My hypothesis pinpoints the weather as the common, decisive factor. The Netherlands had an exceptionally warm and dry spring in 2025, and a summer and an autumn with significantly lower average humidity. This has a number of consequences in our sector: • horse manure was supplied much drier (photo 9). 1300 litres of water per 1 ton of horse manure must be added which is not easy and increases the risk of dry, hard patches forming,
8) These filters no longer capture 99.9% of spores.
9) Patches of green mould already growing on dry horse manure.
22 MUSHROOM BUSINESS
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