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