What are common contaminants of the mushroom culture?
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Most specialty mushrooms are grown on sterilized substrates, and once a contaminant gets a foothold, it flourishes in the absence of competition from other contaminants. In nature, complex interactions among hundreds of other fungi, bacteria, nematodes, etc. maintain an ecological equilibrium. In a sterilized medium, the grower provides ideal conditions for the contaminant to prosper. In sawdust bags, contamination usually involves another fungus living off the waste products or on the remains of the cultivated fungus, or sometimes on the living mycelium or fruiting body of the cultivated fungus. The only competition for these contaminants is the cultivated fungus itself.
Wet Spot, Sour Rot - Bacillus sp
In grain spawn jars, one commonly encounters Bacillus, which sometimes survives the sterilization process as heat resistant endospores. A dull gray to mucus-like brownish slime characterized by a strong but foul odor variously described as smelling like rotting apples, dirty socks or burnt bacon. Bacillus makes uncolonized grain appear excessively wet, hence the name "Wet Spot". Pallid to whitish ridges along the margins of individual grain kernels characterize this contaminant. Bacillus primarily reproduces through simple cell division. In times of adverse environmental conditions, especially heat, a single hardened spore forms within each parent cell body - bacterial endospores, which can survive high temperatures for prolonged time.
Bacterial Blotch - Pseudomonas tolaasii (P. fluorescens)
Yellow to brown lesions form on mushrooms. Typically, spotting occurs at or near the edge of mushroom caps. Blotch occurs when mushrooms remain wet for a period of 4 to 6 hours or longer after water has been applied. The bacterium is spread in air-borne soil particles. Controls include lowering humidity and watering with a 150 ppm chlorine solution (calcium hypochlorite products are used since sodium hypochlorite products may burn caps). If the mushroom stays wet, however, chlorine has little effect since the bacterial population reproduces at a rate that neutralizes the effect of the oxidizing agent. Shiitake caps are affected by a bacterial disease caused by Pseudomonas gladioli (Burkholderia gladioli). Sanitation is a critical component of control measures.
Cobweb mold or Dactylium Mildew - (Hypomyces sp.)
A cottony mycelium grows over casing. When it contacts a mushroom, the mycelium soon envelopes the mushroom with a soft mildewy mycelium and causes a soft rot. It is also a parasite of wild mushrooms.
Green Mold - Trichoderma harzianum, T. viride, T. koningii
Green mold caused by Trichoderma harzianum is characterized by an aggressive, white mycelium that grows over the casing and onto mushrooms, causing a soft decay. Masses of spores that eventually form are emerald green. Heavily infested patches of compost are barren. This is currently the most important disease in the U.S. Agaricus industry. Many farms spread salt on the compost in affected areas when green mold is first recognized. Strict sanitation is essential. Shelving, trays, walls, floors, etc. may be surface disinfected as a matter of routine, but it is done with a sense of urgency following an outbreak of a disease. Many commercial products are available for cleaning surfaces. The base ingredients in these materials include chlorine, iodine, phenol, or quaternary ammonium, among others. Surface disinfectants are used farm-wide, from equipment sanitation to room washdowns to foot-dip solutions to picking basket pre-wash. Other Green Molds may be better defined as indicators since they don't seem to be as aggressive as T. harzianum. These species of Trichoderma also sporulate on the casing surface and may sporulate on infected mushrooms. These fungi indicate that carbohydrates are available, possibly due to inadequate nitrogen supplementation during Phase I or undercomposting. T. viride reportedly produce toxins that dissolve mushroom cells walls. A wet compost low in ammonia prior to pasteurization, flies, poor sanitation, anaerobiosis, and other factors influence green mold. These fungi are common in sawdust and commonly occur in the production of specialty mushrooms.
Cinnamon Brown Mold - Chromelosporium fulva (Peziza ostrachoderma)
The color of this mold ranges from yellow gold to golden brown to cinnamon brown. It grows rapidly in circular patches. It is very common in soil, and flourishes on damp wood. Areas in compost overheated during spawn run may be colonized. Improperly conditioned compost will also support growth, but it is most commonly known as a re-colonizer of overly pasteurized casing, possibly living on dead microorganisms. It often occurs on sterilized soil. Sexual fruiting bodies may appear several weeks after the first appearance of the mold. Spores are airborne.
Lipstick Mold - Sporendonema purpurescens (Geotrichum candidium)
This fungus colonizes compost or casing. As spores mature, the color of the mold changes from white to pink, to cherry red, and finally to dull orange. It is slow growing. Spores spread in air, during watering, and on pickers. The lipstick mold utilizes certain fats in the compost. It is an uncommon problem. Control is centered around sanitation.
Pink Mold, Red Bread Mold - Neurospora
Commonly to occasionally seen on agar and grain. Neurospora is fast growing, sometimes taking only 24 four hours to totally colonize a media filled Petri dish. It is ubiquitous in nature, occurring on dung, in soils and on decaying plant matter. Since this fungus grows through cotton stoppers or filter discs, a single contaminated jar, though sealed, can spread spores to adjacent spawn jars within the laboratory. This condition is more likely if the filter discs or cotton plugs are the least bit damp; or if the external humidity is high. Furthermore, Neurospora spores germinate more readily at elevated temperatures. The pink mold seen in mushroom culture is most frequently Neurospora sitophila, a pernicious contaminant that is difficult to eliminate. All infected cultures should be removed as soon as possible from the laboratory and destroyed. A thorough cleaning of the laboratory is absolutely necessary. If contamination persists, remove all spawn and start anew.
Sepedonium Yellow Mold - Sepedonium spp.
This white, sparse mold grows in the compost during spawn run. With age, it turns dull yellow to tan. Spores are airborne. Thick-walled spores may survive peak heat. The mold colonizes compost considered ideal for spawn growth.
Black Whisker Mold - Doratomyces spp.
This fungus produces black powdery spores that appear as smoke when disturbed. This mold indicates the presence of certain carbohydrates in the compost at spawning time. It also indicates that the straw has been incompletely caramelized or underheated in Phase I (therefore, carbohydrates are in a form easily utilized). The proportion of carbohydrates, particularly cellulose, may be too high. The black whisker mold is also present in compost that overheated during spawn run. Simple carbohydrates are utilized by this fungus but it can also utilize lignin. Doratomyces, Aspergillus, and Penicillium produce copious numbers of spores and may cause respiratory problems (nasal and throat irritation, chest congestion, breathing difficulty, etc.).
Abundant blue-green spores are produced on the surface of the substrate. Similar to Aspergillus. Favorable conditions parallel those for the black whisker mold. Penicillium spp. utilize simple carbohydrates, as well as cellulose, starch, fat, and lignin. These fungi are very common on specialty mushrooms and are one of the chief concerns in agar and grain culture. Spores are airborne and ubiquitous.
Very common in agar and grain culture, and in compost making. Found on most any organic substrate, Aspergillus prefers a near neutral to slightly basic pH. Well used wooden trays and shelves for holding compost are frequent habitats for this contaminant in the growing house. Species range in color from yellow to green to black. Most frequently, Aspergillus species are greenish and similar to Penicillium.
Inky Cap - Coprinus spp.
These are evidence of free ammonia in the compost. Ammonia seems to be a nitrogen source. Their delicate gray caps autodigest quickly. Inky caps are indicators of nitrogen over-supplementation or a poorly managed Phase II compost. If there is too much residual ammonia, Phase II thermophilic microflora may be unable to convert all the ammonia into microbial protein. In addition, areas in the compost that did not remain within the range of 115 to 135 F from 72 to 96 hours after pasteurization may contain residual ammonia. This fungus is strongly cellulolytic.
Oedocephalum (Brown) Mold - Oedocephalum spp.
This mold makes light gray growth on compost surface, later becoming brown as the spores mature. It forms erect spore-bearing structures with spherical clusters of large spores at its top end. This mold indicates that ammonia and amines were not completely eliminated during Phase II (which might be the case when carbon sources are limiting and nitrogen can't all be converted into microbial protein). Its ecology is similar to Coprinus, and often occurs with it.
Olive Green Mold - Chaetomium spp.
Fruiting structures of this mold look like olive-green cockleburs - 1/16 inch in diameter - that develop on compost. Although its heat tolerant spores survive 140 F for 6 hours, the mold appears only in compost improperly managed during Phase II, especially where Phase II ventilation is inadequate. Lack of oxygen when compost temperatures are greater than 142 F permits formation of compounds produced in anaerobic conditions. These compounds are toxic to spawn growth but are utilized by the olive green mold. It is highly cellulolytic.
Pin Molds - Rhizopus spp.
A very fast growing fungus. Once it sporulates, it forms many tall aerial hyphae adorned with black-headed pins. It grows on readily available carbohydrates. Along with Aspergillus and Penicillium, species of this genus are the primary contaminants of grain spawn. It is also very common on straw.
Plaster Molds and Flour Molds - Papulaspora byssina, Thielavia thermophila, Botryotrichum piluiliferum, Trichothecium spp., and others
These molds develop when nitrogen sources (ammonical compounds and amines) from Phase I are not completely utilized by the microbes during Phase II and when the nitrogen is not converted into microbial protein. They are often seen in raw compost. Aerial hyphae aggregate on compost surface, resembling plaster of paris. White plaster mold (Botryotrichum piluiliferum) forms dense white colonies. T. thermophila is thermophilic (unique among the indicator molds), and may grow rapidly during the last days of Phase II. It indicates hot spots during spawn run, inhibiting spawn growth (resulting in black areas). Brown plaster mold (Papulaspora byssina) forms dense brown colonies on compost.
La France Disease - an isometric virus
Symptoms of this disease include a degeneration of the mycelium, suppression of fruiting, and rapid dying of mushrooms. In time, the mushroom mycelium disappears. Infected mushrooms are off-white and drum-stick shaped. Other fruitbody symptoms include dwarfing, premature opening of veil, development of an elongated spindly stem with a small cap, formation of a thickened stem with a thin flat cap, and malformed or absent gills. Mushrooms should be picked before the veil opens since spores may carry virus particles (75% infection rate of spores from infected mushrooms). Pasteurization of wood after the compost has been removed is essential. Initial sources include infected spores or mycelium in or on wood, compost, casing, people, and equipment. Wood should be cleaned, disinfected, and steamed. The virus may enter a mushroom farm from neighboring farms and from wild mushrooms. Controls include pasteurization (145 F for 6 or more hours) of compost, spawn, equipment and empty rooms), cleaning HEPA filters, and general sanitation. In 1962, Hollings first identified viruses in mushrooms, also the first report that fungi had viruses. Another virus with a lipid membrane is found in all hybrid spawn; its effect is unknown. PCR is now used by makers of spawn for early detection of viruses. An outbreak of viruses on a farm can be devastating.
Mummy Disease - Pseudomonas aeruginosa
Symptoms of infected Button mushrooms include curved stems surrounded at the base by an overgrowth of mycelium. Internally, stems have water-soaked, longitudinal streaks. Caps are tilted and dwarf. Tissues become spongy and dry (mummified). Sanitation and reducing free water are control measures.
Wet Bubble - Mycogone perniciosa
Symptoms include malformed mushrooms with swollen stipes and reduced or deformed caps. Undifferentiated tissue becomes necrotic and a wet, soft rot with an offensive odor may follow. An amber liquid appears on infected mushrooms. Mushrooms become brown in color. Bubbles may be as large as a grapefruit. The fungus is spread via airborne dust and contaminated casing. It is also a parasite of wild mushrooms. Controls include sanitation and in some countries the use of the compound Sporogone, which is also very effective against Verticillium. Wet bubble is the most important Button mushroom disease in China.
Dry Bubble - Verticillium
This disease is caused by Verticillium, a species which produces sticky spores.
Verticillium sp. commonly occur in the soil which may be a primary source of the infection, however, it is more likely that they come with casing or by transfer of infection from house to house by pickers, flies or machinery. Spores can lie dormant until they come into contact with mushroom mycelium which stimulates them to grow. Because the spores are sticky the disease is spread on dust particles from the movement of earth or spent compost. This dust can contaminate fresh casing, or can enter through fans or doorways or be carried in by flies, pickers or even mites. The disease within the production house can be spread by water splash. The spores can also be spread on any other equipment used in an infected growing house. Appearance of the disease at pin head formation signifies infection at an early stage of development, probably at the time of casing. Development of the disease at later stages of the production cycle usually indicates infection has occurred from other cropping houses or from outside sources. Spread by water, flies and pickers can result in 30% of the crop being infected at the third flush and by the last flush virtually all of the crop. The highest standards of hygiene are essential for the control of Verticillium. Other methods of control are as follows:
Fungus gnats (Sciarids) - (Lycoriella spp.) and phorids (Megaselia spp.)
Adults are small (1/8 inch long), fragile grayish to black flies with long, slender legs and thread-like antennae. Their wings are clear or smoky-colored with no pattern and few distinct veins. Larvae are clear to creamy-white and can grow to about 1/4 inch long. They have shiny black head capsules.
Many mites are commonly found in straw and manure, most species are beneficial to mushroom growing as they feed on eelworms and other mites, although some can cause damage.
1. Tarsonemid mite
These mites are pale brown and are so minute that they are only visible with the aid of a microscope.
2. Tyroglyphid mites (Tyrophagus spp)
These mites can be identified as they are slow moving, translucent, with long hairs on their bodies.
3. Red Pepper Mites (Pygmephorous spp)
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.
Nematodes - Aphlelenchoides composticola and Ditylenchus myceliophagus
These nematodes are common inhabitants of most agricultural soils. Symptoms include a degeneration of mushroom mycelium and failure of mushrooms to form. Normally, an infestation is noticed at the time of third break. Mycelium in affected areas is completely destroyed and as the compost decomposes, it turns black and a medicinal odor is detectable. An effective Phase II is the primary control.
Several disorders have abiotic origins. Common ones include:
Browning - tyrosinase (phenolase) - Is the main enzyme responsible for browning in Agaricus. Calcium chloride in irrigation water decreases bruising by increasing the integrity of vacuole membranes (thus, tyrosinase is not released).
Flock, hardcap, and open veil - Physiologically induced malformation of cap and gill tissue. Cap opens prematurely. Causes include some diseases, petroleum based materials, and genetic abnormalities. Hollow core and brown pith - related to water stress, but exact factors unknown. Long stipes and small caps - insufficient light and/or fresh air.
Rosecomb - Condition where pink gill tissue, often with a porous appearance, develops on the surface of a mushroom cap. The cause has been attributed to contamination by petroleum based materials.
Scaling - The natural reaction of the mushroom cap to dry air.
Stroma - Dense mycelial growth without fruiting. Stroma occurs if spawn is mishandled or exposed to harmful petroleum-based fumes or chemicals. It also occurs in dry environments.
Weepers - Mushroom exudes water from cap. The cause is not known, but it is seen in low-moisture compost and high-moisture casing.