Causes and Control Techniques of Rotting in Substitute-Material Shiitake Mushroom Logs

In recent years, varying degrees of log rotting (i.e., rotting and disintegration of logs during the spawn-running period) have occurred in some substitute-material shiitake mushroom production areas, causing waste of resources and economic losses for mushroom growers, and in some areas it has seriously threatened the development of the shiitake mushroom industry. For example, in Qingyuan County, the log rotting rates in 1999 and 2003 reached 17% and 31% respectively, with some individual growers exceeding 50%, resulting in severe losses. In response, we have conducted dedicated investigations and research since 1997, and have preliminarily identified the causes of log rotting and its control techniques.

1. Causes of log rotting

Log rotting mostly occurs in old mushroom-producing areas with many years of cultivation. The fundamental cause is invasion and infection by competitive contaminant microorganisms. All factors that trigger invasion and infection by contaminant microorganisms are specific causes leading to log rotting, which can be summarized in the following aspects:

1.1 Weakening of stress resistance in cultivated varieties

Weakening of stress resistance is a manifestation of variety degeneration, and this is the internal cause of log rotting. In old mushroom-producing areas, some dominant varieties have been cultivated for many years, leading to degeneration of varietal characteristics, manifested as decreased mycelial vitality, reduced adaptability to the environment, and declined ability to resist contaminant microorganism invasion, allowing competitive contaminant microorganisms to invade and cause rot. For example, the shiitake variety 241-4, due to many years of cultivation, has shown obvious degeneration in yield and stress resistance in recent years, and its log rotting has worsened year by year.

1.2 Mycelial damage and contaminant microorganism invasion

When the mycelium in the log is directly damaged, contaminant microorganisms infect through the wounds, causing log rotting. This is the external cause of rotting.

1.2.1 Harsh cultivation environment. Harsh environmental factors cause blocked mycelial growth, autolysis, weakened stress resistance, or aging and death, leading to log rotting. Factors such as high summer temperatures, insufficient ventilation, improper use of disinfectants, and deterioration of raw materials can all trigger rotting.

1.2.2 Damage from insect pests. Mushroom pests enter the log through inoculation holes and ventilation holes to grow and reproduce, directly gnawing on the mycelium. After the mycelium is injured, a localized environment of high humidity and high nutrients forms, and competitive contaminant microorganisms invade and infect, forming cross-infection of insects and microorganisms, and the log rots. This is why old mushroom-producing areas with high insect population density and severe pest damage also have severe log rotting.

1.2.3 Damage from piercing holes. During the cultivation process, when piercing holes for ventilation, the piercing tools (iron nails, bamboo skewers, etc.) directly pierce and injure the mycelium, contaminant microorganisms invade and infect, and subsequently the log rots.

1.2.4 Damage from pesticide phytotoxicity. Shiitake mycelium and fruiting bodies are very sensitive to most pesticides and are prone to pesticide damage. For example, dichlorvos and dimethoate have obvious toxic effects on mycelium, yet mushroom growers are more accustomed to using these conventional pesticides for pest control, resulting in killing the pests but rotting the logs.

1.3 Damage from pathogenic microorganisms. Persistent parasitic microorganisms or viruses directly invade or interfere, causing log rotting.

2. Control of log rotting

In view of the causes and triggering pathways of log rotting, the control of log rotting should focus on prevention as the primary approach and treatment as supplementary, and should be implemented throughout the entire cultivation management process.

2.1 Selection of superior varieties and purification and rejuvenation of degenerated varieties

Select suitable varieties according to local climatic characteristics. In low-altitude areas of Zhejiang Province below 350 m, varieties with strong stress resistance such as "Qingyuan 9015" and "Qingke 20" should be selected, while varieties with weak stress resistance such as "135" are not suitable for cultivation.

2.2 Standardized cultivation management

2.2.1 Renovate mushroom sheds and improve the environmental sanitation of the cultivation site. Increase the height of mushroom sheds from the original 2.5 m to 3.5 m, and increase the width from 3.5 m to more than 5.4 m, and consolidate the structure to thoroughly improve the spawn-running and fruiting environment. Waste logs left from the previous season should be cleaned up promptly, and old mushroom yards should be sprayed with high-efficiency, low-toxicity fungicides and insecticides to reduce sources of contaminant microorganism contamination and lower insect population density.

2.2.2 Strictly control the dosage of disinfectants. Currently, the active ingredients of disinfectant products on the market are all the same. The optimal dosage is 3-4 g per cubic meter, and must not exceed 6 g. Excessive use will injure and kill shiitake mycelium, causing log rotting.

2.2.3 Seal inoculation holes with wax or adhesive tape to prevent insects and microorganisms from invading the log through the inoculation holes.

2.2.4 Piercing holes for ventilation must be done in a standardized manner. The diameter of the iron nail should be less than 2 mm, the hole depth should be 1-1.5 cm, piercing should be done on sunny days, and holes should not be pierced at tumor-like protrusions.

2.2.5 Change indoor summering to outdoor mushroom shed summering. After the mycelium has fully colonized the bag, move the logs to outdoor mushroom shed shelves and place them in a single layer to improve ventilation and heat dissipation, avoiding damage to the logs from high summer temperatures.

2.2.6 Rational pest control and disease prevention. For example, select high-efficiency, side-effect-free, non-toxic, long-lasting mushroom-specific insecticides such as "Guchongjing" (Mushroom Pest Clean), spray once after the first hole-piercing and again two months later, which can effectively control pest damage and protect the logs. Insecticides such as dichlorvos that are not suitable for use on shiitake logs must not be used.

2.3 Treatment measures for rotted logs

When log rotting is discovered, the rotted logs should be promptly selected and stacked separately to prevent cross-infection between logs, and treatment should be carried out.

2.3.1 Treatment of diseased scars. Use a clean blade to cut off the rotted part, wipe and wash the cut edge of the diseased spot and the plastic film with the fungicide kemeling (carbendazim-based fungicide), then put the plastic film back and seal it tightly with adhesive tape to prevent water loss and re-invasion by insects and microorganisms. This operation is best completed under sterile conditions.

2.3.2 Re-making logs and re-running spawn. If log rotting begins in the early stage of spawn running, decisive action should be taken to re-make the logs, disinfect, and inoculate (i.e., redo).

3. Discussion

The mechanism of log rotting is a relatively complex topic. This paper is only a preliminary research discussion. Issues such as the types and characteristics of competitive contaminant microorganisms (or viruses) and the screening of specific effective therapeutic agents remain to be further explored.