Integrated disease and pest control in Irish mushroom tunnels

Soil

Nearly all of the common fungal diseases of mushrooms originate in the soil. Thus soil is the enemy of the mushroom grower and all possible sources of contamination must be eliminated. Soil can get into the mushroom production system in many ways and the most common include:

• Wind blown dust
• Soil on the wheels of vehicles or on shoes
• Soil in water running off soil banks on to the yard
• Soil contamination of the casing as it is being prepared
• Soil contamination of the ground limestone or sugar beet waste lime

Water

The use of dirty muddy water to wet the casing can lead to disease outbreak. Trials at Kinsealy have shown that bubble spores (Verticillium and Mycogone) can readily survive in water for many months.

Peat

Peat in a bog is relatively sterile except for the top layer. If casing is a source of disease it will almost certainly be from contamination after harvest of the peat. Sugar beet waste lime (SBL) coming from the sugar extraction process in the factory is very clean, as it has been subjected to a high temperature. However if it is subsequently stored in a dirty area it can become contaminated. In plating tests at Kinsealy we have isolated Verticillium from contaminated SBL. This can also occurwithground limestone if there is soil contamination.

The appearance, in some crops, of considerable wet bubble before and during the first flush points to contamination of the casing.

Compost

It is extremely unlikely that properly made compost is a source of diseases, such as bubble or cobweb, as the compost goes through a pasteurisation phase in its preparation. This entails holding the compost temperature at 56 to 58^oC for some hours, which will kill all the fungi causing these diseases.

There is still some uncertainty and debate as to whether the green mould fungus Trichoderma can survive the composting process. Very good hygiene procedures in the compost yards have succeeded in reducing that disease to a very low level.

Spawn

Mushroom spawn is made in factories where surgical levels of hygiene are used so that it is almost impossible for spawn to be a source of disease. Composters putting the spawn into the compost have to very careful to avoid contamination.

Survival of disease fungi

Trials at Teagasc, Kinsealy have shown that the bubble causing fungi can survive more than 6 months in water. Most of them produce survival spores, which are very durable. Swabbing tests have shown that they can also survive on tunnel walls and other structures.

Avoid dust

Dust blown by air currents or risen by brushing dry concrete is a serious disease source. Doors should be kept closed as much as possible.

Use air filters

One way in which disease organisms and flies enter mushroom tunnels is through the air intakes. The synthetic filters presently in use are not designed to catch very small particles such as fungal spores. A spore grade filter is installed in a specially constructed extension to the air intake vent.

Ensure personnel are free of contamination

Any person involved in emptying bags or who are in any way in contact with mushroom disease must shower and change clothing before coming in contact with a new crop - filling - casing - watering etc.

Minimise compost transfer at lining out

It may be necessary to transfer compost between bags at lining out in order to create a level cropping surface. This should be done carefully and minimised, as it is a definite way of spreading disease from bag to bag, particularly green mould.

Protect casing

Peat from a bog is relatively sterile with no disease but it is easy to contaminate it after harvest. The main thing is to avoid any possible contamination with soil. Make sure that the limestone used in casing is clean.

When casing is delivered to a mushroom unit it must be protected from contamination. The area on which it is dumped must be scrupulously clean. Remember that power hosing alone will not sterilise an area. If the casing is not being used immediately it should be covered with a sheet of plastic, particularly if it is a windy day.

Add fungicides to casing

Sporgon is the best fungicide approved for disease control. It may be applied in one single application after casing or can be split into two or three. The former is probably the best procedure to follow. Other approved fungicides include Bravo and the carbendazim types such as Bavistin. Disease-causing fungi can easily adapt to become resistant to a fungicide. To avoid this it is suggested that fungicides with different active ingredients should be alternated occasionally. Thus for example Bravo should be substituted the odd time for Sporgon. Underdosing with pesticides can lead to the development of strains of disease fungi or insects which are tolerant to the pesticide.

A list of approved chemicals for use in mushrooms can be obtained from the Department of Agriculture

Use foot dips

A foot dip containing an approved general-purpose disinfectant can be placed inside the door of the tunnel. Please remember that it will be no use if people keep stepping over it.

Do not brush a dry floor

A floor should be damp before it is brushed, to avoid producing a cloud of dust in the air, which may contain disease spores.

Detect and remove disease early

Apart from very severe cases, there is usually only a small amount of disease in the first or second flush. Failure to detect and control this low level of infection will result in a disease epidemic in the third flush. It will not be possible to harvest a fourth flush. If a few disease spots can be detected early and treated with salt (directions below) or an appropriate chemical then further disease spread can be considerably curtailed. Disease inspection should always be done before watering or picking.

If there is a large amount of disease in the third flush then it should be killed off and disposed of. To do otherwise makes it almost impossible to devise a sensible disease control programme.

Best practice following early detection

If a tiny amount of a disease such as cobweb or bubble is seen early in the first flush then it should immediately be covered with salt to prevent further spread. Best practice is to cover it with a damp tissue before salting so that the salt application does not send up a cloud of spores which are then carried in air currents and spread disease.

A diseased mushroom should never be picked off with the bare hand as this will result in very efficient spreading of disease spores.

A bag, which is very bad with disease, should be closed off.

Best procedure in picking off a diseased mushroom is to drape a damp cloth over it and the carefully place the mushroom in a container of liquid. Another method is to put a plastic bag inside out over the hand. The bag is then closed over the diseased mushroom. The area on the bed, from which a diseased mushroom is removed, should then be treated with salt as above.

Remove stumps promptly

Old stumps should be bagged and removed quickly, as they are a definite source of disease spread. So also are mushrooms, which fall between bags.

Terminate diseased crops early

It is tempting for a grower to hold on to a crop for a fourth flush regardless of the disease level. The general finding is that this does not pay and results in a disease epidemic in the unit.

Emptying tunnels

Where there is a significant amount of disease in a unit the process of emptying a tunnel, after cropping, presents the single greatest risk of disease recycling. If bags are emptied, either in or outside the tunnel, the air can become laden with disease spores, which can then enter other crops. This is the time when many growers create a major hygiene problem in the unit. As there is no steam, for cook out, in most Irish mushroom units at this time growers must rely on chemical disinfection only. If the following recommended procedures for emptying and cleaning are followed carefully disease levels will almost certainly drop to a tolerable level.

Killing off the bag surface

If there is some disease present, the normal procedure is to apply a strong disinfectant to the surface of the bag to kill off surface spores and mycelium. A good drenching spray is required. It must be remembered that this will only kill to a small depth and that there will be an enormous amount of disease inoculum underneath in the casing and compost. Emptying the bag in the unit will therefore release a large amount of disease spores into the air thus undoing the benefit of the disinfectant. Another option is to use steam. This is discussed in the next chapter.

Ventilate

The tunnel must be cleared of disinfectant fumes before anyone enters to load bags. Failure to do this presents a serious health hazard.

Load bags without disturbance

Ideally bags should be loaded on to a trailer without disturbance of the compost or casing. Emptying bags at this stage or outside in the yard creates a very serious disease hazard, which is difficult to overcome. If it is essential to take the bags off the compost at this stage then serious consideration should be given to using a steam cook out system.

Close air intakes

When any tunnel is being emptied the air intakes and the doors on the rest of the tunnels in the unit should be firmly closed. This will reduce the risk of spores entering clean tunnels.

Take loads downwind or cover during transport

Trailer loads of used bags of compost should be taken downwind if at all possible, but this may not be practical. If so the load should be covered with a tarpaulin or sheet of plastic.

Driving upwind with an uncovered load of bags with disease has been shown to in a large shower of spores being blown back over the entire unit.

Cleaning out a tunnel

When the bags are removed the remaining dirt on the floor must be brushed out. Dampen the floor before brushing to ensure that a cloud of dust and spores are not sent into the air. Power hosing - while good for cleaning concrete does not kill spores and may spread them.

Disinfecting tunnels

The floors and walls of the cleaned out empty tunnel should then be sprayed with one of the approved disinfectants such as Formalin or one of the phenolics. This will kill any spores or mycelium remaining.

If there has been any significant amount of disease in the tunnel then fumigation with formaldehyde is advisable. This will kill any spores in the ventilation duct or radiator or on the fan blades. The air temperature should be raised to over 15^oC and the relative humidity above 50 % with the fan on full recirculation to ensure efficient fumigation.

Ventilate before entry

All fumes must be cleared from the tunnel before anyone enters, to avoid a serious health hazard. The controls should be located outside so that ventilation can take place without having to enter a tunnel full of fumes.

Disinfect equipment

Any piece of equipment, which has been used in a tunnel with disease, must be thoroughly disinfected before the new crop is started. This includes hoses, roses, trays, brushes, knives etc.

Ensure clean personnel

Anyone involved in cleaning out a tunnel with disease will be covered with disease spores, including clothes, hair, hands, shoes etc. Such persons must shower and have a complete change of clothes before coming in contact with other crops.

Thermal death points

This is the temperature at which spores or mycelium of pathogens are killed when held at a given temperature for a specified time. There are various reports in the literature of researchers measuring thermal death points. Some of these are set out below.

A. van Zaayen and A. A. Rutjens, 1981. Mush Sc. X1. 393-402

Wuest and P.J. Moore, 1970. Phytopathology 60: 1274-1275

Mycogone, Trichoderma viride, Verticillium and Agaricus were eliminated when infected soil was treated at 54.4 ^oC for 30 minutes.

"Cookout" in practice

Our trials have shown that cooking out tunnels, with bags of compost on the floor, is more difficult than with trays or shelves. It is almost impossible to get to the kill temperature required in the compost in the bag particularly that near the floor. This may not be so important for bubble and cobweb, which are on the casing surface, as for green mould, which is deeper seated.

In our studies we found that if the air temperature in the tunnel is raised to 65⁰C for about 5-6 hours this is sufficient to kill bubble and cobweb diseases on the top 5cm of the casing.

Cookout is more expensive than disinfectant but is worth it particularly for a grower with a chronic disease problem.

Sciarids

These are the main mushroom pests. Larvae kill pinheads and pea-sized mushrooms, and tunnel in stalks and caps. Adults are the main culprits in disease spread. They are small black flies longer than broad and have long antennae. They are attracted to compost for egg laying immediately after cool-down and throughout the life of the crop. Larvae have a distinct black head and their gut contents can be seen through their body wall.

A generation takes about 5 weeks to turn over so that eggs laid early in the spawn run give rise to adults at about the first flush. It is the offspring of these that do most damage. Sciarids are mainly inhabitants of the casing, where they do most damage.

The key to good sciarid control is exclusion during spawn run by having well sealed growing tunnels.

Phorids

Adults phorids are much smaller and shorter than sciarids and tend to fly less. They are actively attracted (often from considerable distances) to compost or casing in which spawn is running. The life cycle takes 2 weeks at 25^oC and 5 weeks at 18^oC. Consequently eggs layed early in the spawn run give rise to adults soon after casing and these reproduce again to give very high populations of adults 4-5 weeks into cropping.

The adult flies are not as significant in disease spread as sciarids but are a great nuisance to pickers. Larvae are pointed at one end and blunt at the other. They do not have a black head. They feed on mushroom mycelium. The key to efficient phorid control is exclusion during spawn run.

Cecids

Cecids are unusual in that mature larvae regularly split to give many daughter larvae thereby dispensing with the adult stage in the interests of rapid population increase. This means that they have the potential to reach huge numbers in a short time and the consequence of this is large-scale contamination of mushrooms, bags etc and easy spread between crops. Larvae also carry bacteria that cause browning of the stalks of mushrooms. Adults are rarely seen. Larvae live mostly in compost and suck the contents from mushroom mycelium.

Tarsonemid mites

These invisible microscopic mites can occur in vast numbers in crops and their feeding causes reddish-brown discoloration of the base of the stem. Badly anchored and off-colour mushrooms are also symptoms of their feeding on mycelium. The key to their control is good between-crop hygiene.

Pepper mites

These minute brownish mites sometimes swarm in huge numbers on the casing surface and mushrooms. They do not feed on mushroom mycelium but on weed moulds particularly green mould, their occurrence being a symptom of its presence. The huge numbers disappear after a few days and there is no yield loss.

Predatory mites

Robber mites (Parasitus sp) can occur in large numbers. They are predators of the other small organisms present e.g. fly larvae, mites, eelworms. They can reach numbers that annoy pickers. Long legged mites (Linopodes sp) behave similarly. These are beneficial mites as they are natural enemies of mushroom pests. Parasitus has been used experimentally at Kinsealy to control sciarids.

Eelworms

Rhabditid nematodes or eelworms are bacterial feeders and their presence on the surface of compost or casing indicates some deficiency in the compost, which has resulted in bacterial decomposition. As they do not feed on mycelium, their presence does not necessarily result in yield loss.

Protecting compost in transit

Ensure that compost is protected from the moment of cool-down after peakheating and spawning. While much of this period is not under the control of the grower, it is important that compost is protected from sciarids while in transit to the mushroom farm. When it reaches the mushroom farm, the compost should be transferred to the growing tunnel as quickly as possible to avoid access to egg-laying flies.

Hygiene

Good hygiene in the tunnel is essential to avoid transfer of pests from one crop to another. This is particularly important with cecids and tarsonemid mites, as these will survive in tunnel structures and crop debris. Containers, utensils, even hoses must be kept clean and not shared between houses. Always work in newer crops before going into older ones, as pests are easily transferred between crops. Discard spent compost well away from the mushroom farm and preferably upwind from it.

Pest exclusion during cropping

Exclude pests from a new crop especially during spawn run and when spawn is running in the casing. This involves fitting an appropriate screen on air inlets and exits (at least 16 apertures per cm) and install well sealed doors. The door seal should be similar to that on a domestic refrigerator. Routinely inspect seals from inside a dark tunnel and carry out repairs if necessary. Caulking of poorly sealed houses between crops may be advisable. Very few growing tunnels are properly sealed. Perfect seals are of little value if the doors are needlessly left open.

It is advisable to put a stainless steel fly filter in front of a dust or spore filter to prolong the life of the spore filter. A typical specification for a suitable fly filter is:

• Stainless steel woven wire (or nylon mesh),
• Material AISI 316,
• Mesh No. 39,
• Wire diam 0.25 mm,
• Mesh opening 0.40 mm,
• Open area 38%.

Generally the area of the filter should be about twice the area of the air intake so that there is little resistance to air intake created.

The filter should be brushed down regularly to prevent blockage.

Rapid spawn run

Ensure a rapid spawn run so that the compost is quickly colonized by the mycelium, thereby taking up moisture and nutrients so that they are not available to undesirable organisms. Use spawned casing also for the same reason.

Pest monitoring

Pest populations can double in a day or decrease at a comparable rate, consequently, routine estimates of population size should be made. Monitoring adult fly numbers gives information on what pests are present, when they appear, and whether their numbers are increasing or decreasing. The necessary action taken is based on the information gathered.

Various devices have been used for monitoring adult fly numbers. Sticky fly paper hung beside a light bulb is an old method while more modern "Insectocuter " type devices commonly used in food shops are also useful. The Pennsylvania Mushroom Fly Monitor (see sketch ) is an easily made and cheap alternative.

An advantage of a standard trap is that fly numbers in different locations and on different mushroom farms can be compared.

After a period of use, it should be possible for a grower to recognise pest levels that will lead to problems later in the life of the crop. It is particularly important to monitor fly numbers during spawn run and take the necessary action if appreciable numbers are detected. As high numbers of flies can enter a tunnel during casing, it is important to check these and use an insecticidal smoke on the night after casing.

It is important to remember that flytraps are only useful for monitoring and have little influence on reducing fly numbers.

The Pennsylvania Mushroom Fly Monitor

Figure 1. Fly monitoring

Pest identification

One person should be given responsibility for pest control and this person should be trained in the recognition of pests and their symptoms. An essential requisite for this work is good lighting equipment, preferably mobile, and an appropriate magnifier such as a good-quality glass hand lens. Crops should be monitored by checking the mushrooms at random throughout the house, paying additional attention to possible "hot spots" such as near doors, along walls, and corners.

When to use pesticides

Establish the need to make a pesticide application before treating a crop and never apply pesticides routinely. It may be possible to establish that it is not necessary to make pesticide applications at certain times as a result of good monitoring. Always evaluate the effectiveness of a treatment for example by placing dishes containing water and a little detergent around the house to catch dead flies.

Crop termination

Terminate severely infested crops early thereby so that they do not act as a serious source of pest infestation in other crops. Before termination, take steps to curtail the movement of flies out of crops by giving a knock- down treatment at the end of the crop. Never delay casing or cropping as pests will continue to build up in the compost.