Trichoderma

Trichoderma harzianum has several sub-groups one of which, Th2, causes the vast majority of greenmould outbreaks in Ireland and the UK. Very rarely Trichoderma viride can cause greenmould by contaminating spawn. Even more rarely it may attack and kill mushrooms in later flushes. Low pH casing encourages Trichoderma viride.

Trichoderma can appear at the latter end of spawn run but is most likely to appear after the casing has been applied. It is normally only apparent as a white patch on the side of the bag, this rapidly changes to a green colour. When Trichoderma occurs in the middle of the bag its presence usually indicates that contamination has been present in the compost. If the growth is at the top of most bags, this indicates contamination at breaking or from the casing agent, if using chopped compost. The mould is very attractive to red pepper mites which multiply rapidly on it and eventually come to the surface to form clusters on the casing or on the mushrooms making them unpickable. The mites may prevent the green colour (sporulation) from occurring.

The original source of Trichoderma harzianum has not been identified although Trichoderma viride has been found in soil, casing, straw, and even water. Recent research indicates that some of these spores may survive peak heating, so some Trichoderma sp.may always be present in the compost. However, Th2 is rarely found in the compost ingredients. During outbreaks the source of contamination by Th2 have been found to be as follows:

The compost production personnel or machinery, the mechanical material handlers and pallets, airborne dust from contaminated compost, spores transferred from the outside to the inside of bags by 'levelling' personnel, dust from ventilation ducts and associated radiators, mice moving between bags, flies moving from crop to crop, mites dropping from clothing, 'levelling' personnel hands or tools.

Trichoderma spores are so small that they cannot be seen using a microscope and are very easily spread as indicated above.

If there is or has been a Trichoderma infection on the farm it is important to try and prevent further contamination. The following recommendations should therefore be carried out by growers who use the bag growing system.

Do not handle a delivery of bags unless wearing clothing which has just been washed or heated in a tumble drier and is thus free of Trichoderma spores and pepper mites.

Bags should not be allowed to heat up as temperatures of 27°C and above favour Trichoderma growth. They should be arranged in a mite and fly free house immediately after delivery.The smaller bags should be placed in the middle of the house with taller ones near the walls or doors as these areas tend to be cooler.

Trichoderma usually attacks spawn first and excess spawn favours growth.

Level the bags through the plastic, or use plastic disposable gloves which should be changed every 50 bags. Alternatively a tamping instrument can be used to level the bags. Again disinfect this instrument regularly. Bag tops should be left loosely folded over.

During the first week maintain the compost temperature at 21-22.5°C. Sterilise the thermometer in disinfectant if moving it from bag to bag.

During the second week compost temperatures should be raised to the optimum temperature of 25°C. Bags should be open enough to let the compost 'breathe' and exchange gases.

It should be unloaded onto a clean area that has just been sterilised with 0.4% Environ or Prophyl. Casing should not be left near contaminated bags or where they have been set. The casing trolley and implements should also be sterilised.

Compost should not be mixed through the casing as there may be Trichoderma present that is not yet visible.

Do not spill compost outside the house or dump it nearby. Do not create compost dust when emptying. This will contaminate the ventilation system and water butt.

Use traps and bait to control vermin.

Attention to detail is important. Always clean door handles, don't store spawn in the mushroom fridge, don't leave compost sitting outside overnight. Pickers can contaminate the rest room and canteen areas with spores.

The watering rose head should be cleaned with fungicide prior to each watering and never be allowed to touch bags or the floor.

Bags which are visibly infected with Trichoderma or mites should be sprayed on the surface with a sterilant such as Sudol or Jeyes fluid, then sealed and disposed of at a distant location. Care should be taken not to contaminate clothing or vehicles with spores or mites.

Careful application of these measures will have the benefit of reducing the incidence of most of the following pathogens: