The function of casing is to induce fruiting, support mushroom growth, and provide a source of water to the mushroom that compensates for its water lost through evaporation and transpiration. Sometimes, there has been an overemphasis on the influence of the moisture layer in the casing on mushroom yield and quality. A large variation in the compost substrate moisture may have more influence on mushroom yield and fresh mushroom quality. It is not to say that casing moisture does not influence yield or quality, but it may take a larger disparity in moisture to markedly influence the end product.

The mushroom relies on water potential gradients to move water through the mycelium. Water flows from regions of higher water potential to lower water potential, which helps maintain turgor pressure necessary for mycelial tip extension and growth. This same gradient also affects nutrient transport: nutrients, ions, and metabolites often move along with water via bulk flow or are actively transported across membranes, but their distribution is shaped by the underlying water potential differences within the mycelial network and between the fungus and the compost.

In a mycelial network, water potential gradients help the translocation of nutrients—moving them from the compost, the resource-rich region, to actively growing hyphal tips or fruiting bodies. Under dry conditions, the external water potential becomes too low, water movement into the hyphae slows, reducing nutrient uptake and impairing growth. Conversely, favorable water potential promotes efficient uptake and redistribution of resources throughout the mycelium.

Think of thicker rhizomorphs in the casing as the big pipes that move water and nutrients from the compost to the developing mushrooms. To keep mushrooms healthy and productive, this pipe system needs to stay in good working order all through the crop. Just like it’s easier to move water through a fire hose than a garden hose, well-developed rhizomorphs make water delivery more efficient. Keeping the casing wet or moist is what allows these larger “pipes” to form and continue feeding the mushrooms. If the casing dries out during production, most strains will give lower yields and poorer quality mushrooms. In the end, this whole water network relies on the compost as the main source of water for the crop.

Water is constantly moving during the cropping cycle. Mushrooms take up water into their cells, while water is also lost through evaporation and transpiration. Growers replace this loss mainly by watering the casing layer. However, we still know little about exactly how mushroom mycelium absorbs and transports water, or how water moves through rhizomorphs into the mushroom itself.

Some research suggests that water uptake depends on differences in water potential between the mycelium and the compost solution (Kalberer, 1987). As cropping progresses, mushrooms use water from both the compost and casing, while evaporation, transpiration, and respiration continually reduce the water content of the crop. This loss lowers the water potential inside mushroom cells. Because water moves from higher to lower potential, this gradient may allow mushrooms to absorb water with little energy cost. Another idea is that nutrient absorption helps drive water uptake (Holtz, 1971; 1979; Schroeder & Schisler, 1981; Kalberer, 1987). When nutrients are actively absorbed by the mycelium, the osmotic potential inside the cells decreases. Water then follows passively, moving in response to the concentration of nutrients. The developing mushrooms produce the sugar mannitol and therefore have a much higher concentration than the vegetative mycelium (Holtz, 1976). It was suggested that this different concentration of mannitol creates the osmotic and water potential gradient responsible for “pumping” nutrients and water from the compost mycelium through the casing rhizomorphs and into the developing mushrooms.

In summary, mushroom fruit body development depends on a balance of water movement between the compost, casing, and the developing fruit mushrooms. While casing moisture is important for yield and quality, compost moisture may play an even greater role. Water and nutrients move through the mycelium by water potential gradients, flowing from wetter regions in the compost toward drier hyphal tips and mushrooms. Rhizomorphs act like large pipes, efficiently transporting water and nutrients when casing moisture is maintained; if the casing dries, yields and quality decline. Water uptake is largely passive, driven by gradients in water and osmotic potential, which are influenced by nutrient absorption and the accumulation of compounds like mannitol in mushrooms. Together, these processes form a dynamic water transport system that sustains mushroom development throughout cropping.


REFERENCES
Holtz, R.B. 1971. Qualitative and quantitative analysis of free neutral carbohydrates in mushroom tissue by gas-liquid chromatography and mass spectrometry. J. Agr. Food Chem. 19 (6):1272-1273.
Holtz, R.B. and Smith, D.E. 1979. Lipid metabolism of mushroom mycelia. Mushroom Sci. 10 (Part 1):437-444.
Kalberer, P.P., 1987. Water potentials of casing and substrate and osmotic potentials of fruit bodies of Agaricus bisporus. Sci. Hort. 32:175-182.
Schroeder, G.M. and Schisler, L.C. 1981. Influence of compost and casing moisture on size, yield, and dry weight of mushrooms. Mushroom Sci. 11:495-509.

The function of casing is to induce fruiting, support mushroom growth, and provide a source of water to the mushroom that compensates for its water lost through evaporation and transpiration. Sometimes, there has been an overemphasis on the influence of the moisture layer in the casing on mushroom yield and quality. A large variation in the compost substrate moisture may have more influence on mushroom yield and fresh mushroom quality. It is not to say that casing moisture does not influence yield or quality, but it may take a larger disparity in moisture to markedly influence the end product.

The mushroom relies on water potential gradients to move water through the mycelium. Water flows from regions of higher water potential to lower water potential, which helps maintain turgor pressure necessary for mycelial tip extension and growth. This same gradient also affects nutrient transport: nutrients, ions, and metabolites often move along with water via bulk flow or are actively transported across membranes, but their distribution is shaped by the underlying water potential differences within the mycelial network and between the fungus and the compost.

In a mycelial network, water potential gradients help the translocation of nutrients—moving them from the compost, the resource-rich region, to actively growing hyphal tips or fruiting bodies. Under dry conditions, the external water potential becomes too low, water movement into the hyphae slows, reducing nutrient uptake and impairing growth. Conversely, favorable water potential promotes efficient uptake and redistribution of resources throughout the mycelium.

Think of thicker rhizomorphs in the casing as the big pipes that move water and nutrients from the compost to the developing mushrooms. To keep mushrooms healthy and productive, this pipe system needs to stay in good working order all through the crop. Just like it’s easier to move water through a fire hose than a garden hose, well-developed rhizomorphs make water delivery more efficient. Keeping the casing wet or moist is what allows these larger “pipes” to form and continue feeding the mushrooms. If the casing dries out during production, most strains will give lower yields and poorer quality mushrooms. In the end, this whole water network relies on the compost as the main source of water for the crop.

Water is constantly moving during the cropping cycle. Mushrooms take up water into their cells, while water is also lost through evaporation and transpiration. Growers replace this loss mainly by watering the casing layer. However, we still know little about exactly how mushroom mycelium absorbs and transports water, or how water moves through rhizomorphs into the mushroom itself.

Some research suggests that water uptake depends on differences in water potential between the mycelium and the compost solution (Kalberer, 1987). As cropping progresses, mushrooms use water from both the compost and casing, while evaporation, transpiration, and respiration continually reduce the water content of the crop. This loss lowers the water potential inside mushroom cells. Because water moves from higher to lower potential, this gradient may allow mushrooms to absorb water with little energy cost. Another idea is that nutrient absorption helps drive water uptake (Holtz, 1971; 1979; Schroeder & Schisler, 1981; Kalberer, 1987). When nutrients are actively absorbed by the mycelium, the osmotic potential inside the cells decreases. Water then follows passively, moving in response to the concentration of nutrients. The developing mushrooms produce the sugar mannitol and therefore have a much higher concentration than the vegetative mycelium (Holtz, 1976). It was suggested that this different concentration of mannitol creates the osmotic and water potential gradient responsible for “pumping” nutrients and water from the compost mycelium through the casing rhizomorphs and into the developing mushrooms.

In summary, mushroom fruit body development depends on a balance of water movement between the compost, casing, and the developing fruit mushrooms. While casing moisture is important for yield and quality, compost moisture may play an even greater role. Water and nutrients move through the mycelium by water potential gradients, flowing from wetter regions in the compost toward drier hyphal tips and mushrooms. Rhizomorphs act like large pipes, efficiently transporting water and nutrients when casing moisture is maintained; if the casing dries, yields and quality decline. Water uptake is largely passive, driven by gradients in water and osmotic potential, which are influenced by nutrient absorption and the accumulation of compounds like mannitol in mushrooms. Together, these processes form a dynamic water transport system that sustains mushroom development throughout cropping.


REFERENCES
Holtz, R.B. 1971. Qualitative and quantitative analysis of free neutral carbohydrates in mushroom tissue by gas-liquid chromatography and mass spectrometry. J. Agr. Food Chem. 19 (6):1272-1273.
Holtz, R.B. and Smith, D.E. 1979. Lipid metabolism of mushroom mycelia. Mushroom Sci. 10 (Part 1):437-444.
Kalberer, P.P., 1987. Water potentials of casing and substrate and osmotic potentials of fruit bodies of Agaricus bisporus. Sci. Hort. 32:175-182.
Schroeder, G.M. and Schisler, L.C. 1981. Influence of compost and casing moisture on size, yield, and dry weight of mushrooms. Mushroom Sci. 11:495-509.

LaFrance virus disease

Of all the diseases confronting mushroom growers, none have been the subject of more confusion than viral diseases. Viral diseases can be confused with the effect of poor cultural practices or the bacterial disease mummy. Since no known commercial mushroom strain is resistant to viruses, growers must incorporate preventive measures into the IPM plan and rigorously carry out control measures.

The virus lives in mushroom spores and mycelium (spawn). Infected spores spread the disease to other new crops. Infected mycelium (spawn) may survive in the bed boards or quickly spread in bulk phase III facilities. Spores survive many years and can be released during farm renovations.

Symptoms (Figure 1-4):

• Portabellas don’t size up; lower yield, Fig 1.
• Bare areas with few pins and mushrooms, Fig 2.
• Premature opening of the veil (small caps)
• Mild yield loss

Severe infection:

• “Drumstick” (small caps, long stems) Fig 3.
• Weak growth in the casing that often disappears over time
• Die-back of mycelium in the compost, Fig 4.
• Stems discolor quickly when cut
• Significant yield loss

Control:

• Exclude, eradicate, or reduce inoculum.
• Mushroom spores at spawning
• Mushrooms should be harvested before they mature and the caps open, releasing spores.
• Infected mycelium at spawning or casing
• Isolate the crop.
• Regularly scheduled replacement of filters/filtration
• Air movement—high positive pressure in spawning and casing areas
• Practice postharvest steaming to eliminate pathogens and mushroom mycelium/spores between crops.
• Virus-infected mycelium in the wooden tray/bed boards
• Virus-infected mushrooms and spores left on the bed

figure 1

figure 2

figure 3

figure 4

Bacterial diseases

1)  Bacterial Blotch

Signs and Symptoms:

• Superficial discoloration which leads to lower quality in the marketplace
• First pale yellow, then darkens to golden yellow to brown color.
• Bacterial pathogen: Pseudomonas tolaasii, recently other species have been found to cause similar symptoms

2)  Mummy Disease

Signs and Symptoms:

• Stunted growth, swollen base
• Sometimes mushrooms develop curved stipe with translucent, longitudinal streaks on the side
• Tissue appearance: spongy, dry and leathery
• First break can be harvested; second break mushroom does not grow in the affected area
• Scientific name: Pseudomonas species

By David M. Beyer, Penn State University

Fungal Diseases

The life cycle for fungal pathogens like Dry Bubble, Trichoderma, and Cobweb is simple, Figure 1. Spores germinate into mycelium, which forms structures that produce spores. In a petri dish culture that may take less than a week; in compost or casing, it is probably pretty much the same. However, other factors like pH, moisture, and nutrient availability may influence this life cycle timing. Much of that, however, is unknown for these pathogens.

Looking at the disease cycle in mushroom cultivation, we know a relationship exists between spore load, time of infection, and symptoms or signs of disease development. Let’s look at the three most common fungal diseases and what we know about these relationships.

Dry Bubble, caused by Lecanicillium, or Verticillium has symptoms that develop based on spore load and timing of infection. Spores coming in contact with a fully colonized spawn run don’t germinate well and little disease will develop. It may be possible that spores landing on the substrate the day before or the day of casing could cause an early disease development. Spores in contact with the rhizomorphs in the casing will easily germinate. How fast they germinate, and the vegetative mycelium growth may be influenced by casing pH, moisture, relative humidity, and temperature.

It is unknown what the optimum conditions are but in general the warmer the conditions the faster the growth and the shorter time from spore to symptom development. In general, spore to symptom takes about seven to 14 days depending on the above factors. However, when Dry Bubble mycelium is in contact with mushroom pins,metabolites are produced that degrade mushroom tissue. This process seems quick, perhaps hours to a day or two.

Read the full factsheet here.

Written by: David M. Beyer