Bacterial blotch (Pseudomonas tolaasi)

The main source of bacterial blotch in the mushroom farm is the casing soil. The disease is favoured by dampness [high humidity (over RH 85%), high temperatures (over 20degC ) and poor ventilation in the growing houses]. It can be spread by the workers' hands, irrigation water and on the inventory. Mushroom flies and mites also spread the disease. The first disease symptoms appear in areas of caps that have been moist for a longest time and areas where mushroom caps touch.

Symptoms include spots that darken becoming chocolate-brown in colour and slimy. It can also cause distortion and splitting of the stipes. When the infection is severe, spots spread throughout the whole mushroom surface. Seriously diseased mushrooms can become deformed and the caps can decay giving a foul (unpleasant) odour. Young pins affected by the disease become brown and do not further develop.

Die-back disease (Virus)

It causes spots in the casing soil where no mycelial growth occurs. Around these spots, mushrooms of low quality appear with long stems and dirty caps. Sometimes the only indication of a virus infection is low yield. In severe cases, a few deformed mushrooms are produced. The disease can be introduced to the farm by infected spawn. It is spread by spores and mycelium from infected mushrooms. Mushrooms affected with the virus open fast, releasing infected spores. Sometimes, mushrooms that were formed inside the casing layer come out already open. Spores from infected mushrooms are easily carried by wind, insets, on implement, clothes and hands of personnel.

Green mould (Trichoderma aggressivum var. aggressivum)

This fungus is soil-borne. There are many species of the fungus but Trichoderma aggressivum var. aggressivum (Trichoderma harzianum biotype 4 (Th4)) is a major problem particularly to button mushrooms. It gains entry to growing rooms primarily through contaminated personnel and equipment. Other sources include poorly composted substrate or carryover in rooms that were not sufficiently steamed off. Once introduced, it rapidly spreads into large disease areas from infection source. Infection of casing soil can result in serious losses. Green mold is characterized by dense white mycelial growth followed by extensive green patches on casing/compost or mushroom caps. The spores of the fungus are sticky and can easily be spread by flies, mites and workers to previously uninfected area. Red pepper mites are often associated with the disease.

Mites

Mites belong to the spider family. They are eight-legged compared to insects which are six-legged. They are very small and are usually not noticed until large numbers are present. Many different kinds of mites are encountered during mushroom cultivation process. Some mites feed on fungi and bacteria that exist in great amounts in decaying organic materials used as a substrate in mushroom production. Other members of mites are predators that mainly feed on nematodes and smaller mites. Only a few of them are considered as mushroom pests. These include mushroom mites or tarsonemid mites (Tarsonemus myceliophagus) and red pepper mites (Pygmephorus sellnicki). The mushroom mites are so small, that they are not seen with a naked eye. They are shiny, light brown in colour feeding on the mushroom mycelium and mycelium of many other competing fungi present in the substrate. They gnaw the bases of mushroom stipes (stalks of mushroom) which in result become rounded and obtain a reddish-brown shade. The amount of damage they bring to the growing crop will depend on the time and degree of infestation. If a large population of mites enters the batch at spawning, the damage can be significant.

Red pepper mites got their names from their reddish-brown colour and are often found moving on the mushroom caps or the casing surface. These mites are not regarded as primary pests, their presence is usually an indicator that Trichoderma (green mould) is present in the compost. These mites feed on various weed moulds but not mushrooms, thus their presence indicates that the compost is unsatisfactory. They are yellowish-brown in colour, 0.25 mm in length and have a flattened appearance. They spread spores of Trichoderma from bag to bag. They feed on competing fungi and that is why their presence indicates poorly prepared compost and possible presence of Trichoderma. They reproduce very fast one female can lay up to 160 eggs within 5 days. They cause browning of mushroom heads and promote the spreading of green molds not only in the infected room, but also in the whole farm. When their population is high, they cause discomfort and even allergy to mushroom pickers at work.

Mushroom flies

Sciarid flies (Lycoriella spp.) and phorid flies (Megaselia spp.) can be a major problem in mushroom production. Adult flies enter mushroom houses through openings and cracks. They are attracted by the smell of substrate (compost) and growing mushrooms. They lay eggs in the substrate, casing layer and mushrooms. Each female can lay up to 170 eggs. At a temperature of 24.5degC development from egg to adult takes about 21 days. Emerging larvae (maggots) feed on the substrate, mushroom mycelium and developing mushrooms. Damaged mycelium results in the formation of pins brown in colour and having a leathery surface. Developing pins and young mushrooms may not survive after larval attack.

Infested young mushrooms become hollow and shrink, and may eventually die. Larvae may also tunnel ripe mushrooms forming many passage ways and holes, which makes the mushrooms unsuitable for human consumption. The affected substrate areas turn into swampy masses with a foul odour in which mushroom mycelium will not be able to grow. Adult flies are known to spread mushroom diseases and mites. They are also a nuisance to workers.

Mycophytic nematodes (Aphelenchoides composticola and Ditylenchus myceliophagus)

These nematodes (eelworms) are common inhabitants of most agricultural soils. They feed on mushroom mycelium. They gain access to growing rooms in substrate and casing soil. They are spread by insects, personnel and implements. Affected areas become unpleasantly wet and soft with a foul odour because of anaerobic (not needing oxygen to live) bacteria activities. Nematodes affect yield and quality. The degree of crop losses depends on the time and level of the initial infection. Infection at spawning time can have an effect on mycelial growth and thus making cultivation unprofitable. Infection occurring later causes only minor crop losses and goes unnoticed by the mushroom grower. The presence of nematodes on mushroom beds indicates bad cultivation conditions, that is, poor substrate and casing soil preparation and non-observance of sanitation and hygiene.

Wet bubble (Mycogone perniciosa)

This disease like dry bubble is soil-borne. Infested soil may be primary source of infection. Infection usually occurs at casing. The disease is spread within a house mainly by water splash, on clothing, equipment, flies or mites. Cross-contamination between houses can also take place. Young pin heads infected by Mycogone grow into shapeless lumps cauliflower-like which have a velvety appearance initially and eventually break down producing small amber droplets of liquid on the surface. It can also appear as gray fuzzy growth on gills (a series of radially arranged (from the center) flat surfaces located on the underside of the cap on which spores are formed).