Coping with contamination


The impact of contamination depends on the stage of the culture. While in the initial phases of spore germination a contamination can be fatal to the planned culture, in later phases of fruiting, a contamination can mean just a cap or a bag to remove.

Substrate sterilization and air filtration are the main way to create an environment rich for the mycelium but free from competitors. Sterilization can be more or less effective in creating appropriate initial conditions, and air filtration in maintaining them. But these processes are never perfect, and in practice the competitor free environment has a limited duration. In this duration, the mycelium must find appropriate conditions (e.g. air,moist, light, air) to fully colonize the substrate, therefore colonization speed has a major impact on possible contamination. Inoculation rate (inoculant per inoculated substrate) also an impact on the fullness of the colonization after that limited time, as the more substrate is available, more time is required for the mycelium to achieve full colonization.

In practice, the mycelium growth stops when reaching nutrient restriction, when no more available substrate or available substrate is contaminated or colonized by another mushroom, or available substrate is not in appropriate conditions (e.g. moist, hardness, etc). If the conditions are appropriate, primordia fruiting phase is triggered and the fruits can pin and grow. But often contaminations make the conditions of fruiting inappropriate. The substrate may be too limited and create aborts. Contaminated substrate may also attract pests (e.g. mites, fly) bringing other contaminants.

Bacteria

Bacteria is very fast reproducing organism. Bacteria is fundamentally different from fungi, yiest and mushrooms, although they all live in similar conditions. Under microscopy, bacteria is much smaller, e.g. few micron, than a fungi spore, e.g. few dozen microns (like an orange and pips). Contaminations with bacteria visually differ from fungi, as bacteria is more "embedded" in the substrate. The substrate is likely to change color and substance (e.g. water release). Mycelium growth seems to stop without apparent reason.
Bacteria can be dealt with using pasteurization and sterilization for bulk substrate, and addition of antibiotics in agar culture.

Mold - Fungi

Regular monitoring insures detection early enough.
Control of mold include :
    • Salt : when mold is first recognized, many farms place a thick salt paste over the green spot and contain spores with a damp paper or/and a glass to hold air.
    • Baking soda
    • Peroxyde
    • Alcohol
    • Cinnamon
    • Electrolysis
    • Space sanitation : strict sanitation is essential. Space sanitation, shelving, trays, walls, floors, etc. may be surface disinfested as a matter of routine, but it is also done with a sense of urgency following an outbreak of a disease.
    • Heavily infested substrates are removed.

  • Aspergillus

  • Penicillum

  • Green Mold - Trichoderma

  • Most common fungi, that may easily be confused with Aspergillus and Penicillium. Trichoderma (e.g. harzianum) is characterized by an aggressive white mycelium that grows over the substrate, and possibly over mushrooms causing a soft decay. Masses of spores that eventually form are emerald green.This is currently the most important disease in Agaricus industry. Some Trichoderma molds may be better defined as indicators since they don’t seem to be as aggressive as T. harzianum. These species also develop and sporulate on substrate, casing surface and may sporulate on infected mushrooms. These fungi indicate that carbohydrates are available, possibly due to too high nitrogen composition, e.g. unbalanced supplementation 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. Trichoderma is often mistaken for Penicillium or Aspergillus molds(and vice versa), being that all three look very similar and are not easy to tell appart without the use of a microsope, especially before sporulation.
  • Conditions that favor this contaminant :
    • Excessive high or prolonged humidity
    • Stagnant air
    • High Carbone dioxide levels


Pests


  • Fungus gnats - Sciaridae

  • external image 19666.jpgMost common cultivator pest, that can ruin a fruiting chamber. 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.
  • They are attracted to the mushroom crop and their larvae feed directly on mycelium, swarm over the mushroom, and tunnel into the developing or developed mushroom. Tissues that have been physically damaged by flies often become colonized by bacteria which cause soft rot, thereby accentuating the problem.
  • Control method include :
    • Best method is to monitor carefully and spot the black headed larvae early and to get rid of all infected substrate. Any adult is the sign of many larvae behind with many potential flies.
    • Reduction of available food and moist environment for the flies in the farm and around. Fly larvae has a preference for fresh fruits, while the adults eat mycelium and moulds. The larvae need nitrogen rich nutriments, making the use of semi-selective substrate effective against both molds, bacterias in general and fungus gnat adults. external image 19668.jpg
    • Strict sanitation and general farm hygiene. For example, the grow room must be air tight. Fresh air that is used is filtered. Even a small crack will serve as an entry for the flies.
    • Most farms use sticky tape or some other method that allows monitoring of populations.
    • A biocontrol using nematodes and Hypoaspis mites offers effective control when populations of flies are low. These bio-agents are sold unsterile, and their application may introduce indesirable contamination (e.g. Trichoderma).
external image 19667.jpg
  • In addition to the damage which fly larvae cause by eating mushroom mycelium or killing pins, the adults also carry diseases such as Verticillium, Mycogone and Cobweb.


References