Trichoderma, as most of us know it (top left), and magnified many times.

Uncommon Trichoderma problems

There are at least 7 species of Trichoderma that are found in compost or on casing. This can sometimes cause confusion until the particular species has been identified because they have widely different importance/virulence. Furthermore, among the 7, only T. viride is easily identified, simply by using a microscope, the other species require culturing and/or molecular methods. However, in my experience, over 95% of serious problems are caused by T. harzianum 2 (Th2) or in North America by Th4, (the latter can be ignored, unless it is accidentally introduced here and becomes established).

T. viride (T.v) occasionally grows strongly on casing that has insufficient ground limestone in it. More uncommonly a virulent T.v strain attacks mushrooms causing them to rot. Equally uncommonly a packet of spawn gets torn and T.v becomes established in it. This leads to many small colonies in the compost, each one with a spawn grain at its centre.

T. pseudokoningii is very prevalent in the compost yard, but is only seldom found in compost. It tends to form small dark green colonies early in spawn run, but these are soon overgrown by Agaricus, with no noticeable reduction in yield. It typically has elongated spores but unfortunately with some strains these may be similar to those of Th2, short oval.

In the past, when the phase II stage of compost making was less well controlled, Th3 (now known as T. atroviride) and also common in the compost yard, occasionally caused serious problems. It is now rarely found forming large green patches in the compost, but it is one of several species sometimes isolated from casing, particularly in later mushroom flushes.

Major Trichoderma Problems

1. CACing and C.I.

Th2 is the main problem causer. Greatest loss occurs when one or more bags of badly contaminated spawn-run compost are used for CACing, possibly wiping out an entire house. Most affected bags then show a band of green sporulation just below the casing. This green mass of spores spreads down the bag after the dense white mycelium that precedes it, sporulates. Of course, the magnitude of the problem depends on the proportion of CACing bags affected and how they were mixed.

The machinery used for chopping or distributing the CACing can also cause problems, check for crevices in which Th2 could survive and form spores. Casing inoculum is much safer than CACing but it also has to be stored and handled in a rigorously hygienic way.

2. Contamination at levelling

Contamination of compost at levelling may cause problems that are also initially noticed in the upper part of the bag. Incidentally, this is the worse place for Th2 to grow, it creates a barrier between ‘healthy’ spawn-run compost and the casing surface. Th2 forms its largest colonies when contamination occurs at the start of spawn-run. At high levels of CO2, Th2 can grow as white mycelium for at least 10 days before ‘suddenly’ producing its green spores. The worst case (tested experimentally) follows use of heavily spore contaminated hands for pressing down the compost, soon after filling. This brings Th2 spores in contact with one or more spawn grains and these give Th2 a ’flying start’. For example if a single grain, dusted with Th2 spores is experimentally placed at the side of a bag it can produce a colony occupying a quarter of the bag by the week after casing. However, colony size also depends on spawn-run temperature. The nearer it is to 30°C, the larger the colony. As I have mentioned, several times in previous articles, Agaricus can grow just as fast at 19°C as it can at 30°C, whereas Th2 can hardly grow and compete at all at 19°C. Remember hands can become contaminated from a door handle, a dirty towel, or even from the sides of the bag.

Dust

Another source of contamination at levelling is airborne dust particles. This is probably the reason why there are more Trichoderma cases in the autumn, when dry windy conditions are most likely to occur. External sources of contaminated dust are, farm roads, aprons and house surrounds. Internally, the ventilation system is most suspect, particularly if bags have been emptied into a trailer inside the house.

Mites

The preferred food source for red pepper mites (RPM) is Th2 mycelium. If they consume most of this, relatively few green spores will form. However RPM laden bags can be counted as ‘Trichoderma bags’. RPM can be experimentally shown to be usually contaminated with Th2 spores and also capable of transferring such contamination to compost. Once in they may even seek out spawn grains. At one of the stages of their life cycle, RPM swarm and migrate to the highest point available. If a bag is against the sidewall of a house the RPM will migrate up it. RPM are also commonly seen clustered on top of mushrooms and can thus get transferred to the picker’s clothing, to the packing shed and its fridge. RPM are not long lived, but are highly mobile and when dead form contaminated, easily airborne, dust particles. A Th2 house plus mites is much worse than one with Th2 alone. It either implies a double contamination or that the mites have been instrumental in the Th2 outbreak.

Some researchers claim to have occasionally detected living mites in fresh Phase II compost, taken from inside the peak heat tunnel. I have always found this hard to believe. They are sensitive to heat and are in any case short lived. It is also the case that if compost is experimentally inoculated with Th2 spores alone, it is unusual for such bags to develop mite colonies. This indicates the compost most likely did not contain living RPM.

The Wisdom of Solomon!

Determining where an outbreak of Trichoderma has originated is fraught with difficulty. On the one hand there is an abundance of Trichoderma species and mites in the compost yard. On the other hand Th2 is rare among these and it is easily shown experimentally that neither Th2 nor mites can survive in containers that are put through a phase II cycle.

In the bagging shed there is constant ‘flow’ of compost, however, it is all freshly peak heated and thus most unlikely to contain Th2. A danger would be if the bagging ‘shed’ were adjacent to mushroom production, as this would allow the possibility of airborne contamination of the new compost during mushroom house emptying. This possibility is avoided by specialist compost production only.

The compost maker has most to lose in a serious Trichoderma outbreak. Consequently, the bagging shed and its machinery are thoroughly washed and disinfected every week, particularly the spawn hopper and the bagging machine. None of the personnel is permitted to also work in mushroom production, for example on a home farm. The sensitivity of spawn to contamination is well known. Particular care is taken in its storage and handling.

Growers sometimes make an equally strong case. However, even if there is only one mishandled Th2 affected bag, this comprises a huge potential for contamination. Furthermore the process of mushroom production is almost identical to that for deliberately producing Th2! The only difference being, Th2 is favoured by a high ‘spawn run’ temperature (30°C), whereas Agaricus is disadvantaged. A consequence is the high likelihood of there being hugely greater levels of potential Th2 inoculum on mushroom growing units than are now found in or around bagging sheds.

There is also the observation that some growers who are noted for their obsessive attention to farm hygiene detail, appear to only rarely get a serious Trichoderma problem.

In the days before compost producers knew the vital importance of the hygiene measures outlined for them above, or where there is still mushroom production nearby, growers had/have justification for blaming their supplier. However it is now, in my opinion, more likely for most contamination to originate by circulation on farm. If I am correct, it is in the grower’s long­term best interest to accept this opinion in order to take avoiding measures.

Control suggestions

The greater the Trichoderma problem on farm, the more drastic the hygiene needed to break the in-house cycle or cross contamination between neighbouring houses.

Thinking all the time about how this could occur and paying attention to prevention detail is essential. There are so many potential contamination routes. For example, picking up a single bag on which there are mites and later handling some ‘new’ blocks could result in a ‘disaster’ house.

Use of formalin as a disinfectant appears to stimulate, rather than suppress Th2.

Levelling bags late, i.e. a few days before casing and/or from the outside, to avoid touching compost, minimises the risk of Th2 developing. However if the compost is going to be levelled by hand or redistributed between bags, remember the door handle the wash hand towel and the outsides of the bags may all have accumulated Th2 spores.

No animals should have access to ‘houses’, this includes dogs or cats and in particular mice.

There should be no ‘fly problem’ and houses should have fly-proof filters. Spawn run temperatures should be around 23-25 rather than 26-30+ °C. House doors should be open for minimum time, particularly if it is windy. The ideal compost removal method would be cookout, then taking bags/blocks in a trailer to a remote area to empty them, using non-farm personnel! Other options have specific risks.