What to Do When Shiitake Mushroom Logs Are Burned by Heat and Rot? Cause Analysis and Preventive Measures

From late June to early August 2003, Lishui City, Zhejiang Province experienced a historically rare continuous high-temperature and low-rainfall weather lasting more than 40d, with temperatures exceeding 40°C for 14d, among which the urban area of Lishui reached 43.2°C, breaking the historical extreme and ranking first in the country. This brought serious impacts on people's production and daily life, especially dealing a devastating blow to shiitake mushroom production. The mycelium of shiitake mushroom logs, harmed by high temperatures, exhibited phenomena such as black liquid oozing, rotting and log disintegration. Of the city's 350 million bags of flower shiitake mushroom logs, 150 million bags rotted due to high-temperature mycelium burning, accounting for 43% of the total flower shiitake mushrooms, and the remaining over 200 million bags were also affected to varying degrees, causing enormous economic losses. More than 40,000 mushroom farmer households in the city had all their logs rot, losing their entire investment.

Faced with this unexpected natural disaster, the Lishui City Agriculture Bureau promptly organized technical personnel from relevant departments to conduct on-site investigations and guidance, and to carry out production self-rescue. From the investigation of severely affected areas in various counties (cities), the sustained high-temperature weather was the main cause of this shiitake mushroom log rot. However, analysis of the degree and distribution of log rot shows that it was also closely related to the varieties selected, geographical location, log stacking methods, and environmental conditions, with certain patterns to follow. Many human factors were also key factors exacerbating mycelium burning and log rot, and these factors could have been completely avoided. Therefore, a summary analysis is provided below to guide future production.

1 Cause Analysis of Shiitake Mushroom Log Burning and Rotting

1.1 The impact of rare sustained high-temperature weather was the main cause of shiitake mushroom log burning and rotting. Shiitake mushroom mycelium is very sensitive to high temperatures, stopping growth above 35°C, with a limit temperature of 38°C. Mycelium dies when exceeding the limit temperature for more than 4h. At that time, temperatures exceeded 40°C for 14d. If the summering site for logs had poor ventilation, it would inevitably cause mycelium death and log rot.

1.2 The relatively uniform variety selection and violation of the altitude requirements for the natural distribution of varieties were indirect causes accelerating log rot. For example, the 135 variety is a low-temperature-loving, long-mycelium-age variety with poor stress resistance and intolerance to high temperatures, generally requiring an altitude above 300m for safe summering. However, in Jingning County, the 135 variety was almost exclusively used from altitudes of 100-1000m, and the cultivation method was mainly spring-planted flower shiitake mushrooms. Mushroom farmers in Liandu District also chose 135 as the dominant variety in areas above 100m altitude, without adapting varieties and cultivation modes to local conditions, resulting in total loss. In Songyang County at the same altitude, using the more heat-tolerant varieties 9015 and 939 greatly reduced the log rot rate to only 40%.

1.3 Mushroom farmers' weak awareness of safe summering for logs exacerbated the degree of log burning. In previous years, farmers in high-altitude areas stacked logs on rooftops, second floors, or hung them under beams and eaves for summering. As a result, during that year's high-temperature season, the upper floors were the hottest, and nearly all logs were burned, while those placed on the ground fared better. In low-altitude villages, only a few layers of logs near the ground survived. Some farmers, although aware that rooftop temperatures were high, held onto the hope that it would rain and cool down, thus missing the best opportunity for rescue. Logs stacked on upper floors or in single-layer, poorly ventilated brick-concrete houses rotted very severely. The main reason was that farmers blindly expanded production scale, exceeding the capacity of the mycelium incubation site, leading to overly dense arrangement and poor ventilation. In addition, many farmers underestimated the high temperatures, neglected the importance of spreading stacks for ventilation and cooling, and failed to implement flexible management measures in a timely manner according to weather changes, resulting in greater irreparable losses.

1.4 From a technical perspective, the following situations also exacerbated the occurrence of mycelium burning: (1) Logs produced too late in the season with poor mycelium running and colouring had severe burning. (2) Some farmers, in pursuit of first-flush bag-opened mushroom yield, deliberately made logs with higher water content during production, causing decreased mycelium vitality and more severe high-temperature burning. (3) During the high-temperature season, some farmers were still piercing holes for aeration and turning and moving logs, which inevitably raised the temperature of the mycelium incubation site and exacerbated burning. (4) Many farmers placed logs in outdoor sheds for summering and covered the sheds with shade nets and various shading materials, but did not remove the plastic film covering the shed (saying it was to prevent rain). As a result, hot air could not dissipate or circulate out of the shed, causing stuffy heat inside and burning the logs. Although some people continuously sprayed water on the shed to cool it down, the result was still in vain.

2 Prevention of Shiitake Mushroom Log Burning and Rotting

Although natural disasters are irresistible and the losses and lessons are severe, as long as we fully raise awareness of prevention from an ideological perspective and adopt a rigorous scientific attitude in managing shiitake mushroom log summering during high-temperature periods, it is possible to reduce the losses that may be suffered.

2.1 Scientifically and reasonably arrange production. Operations must strictly follow shiitake mushroom technical regulations, and production should be arranged according to local conditions without violating the biological characteristics of shiitake mushrooms. Particular attention should be paid to the production season, variety selection, and water content. Reasonably adjust the planting structure of shiitake mushrooms, change traditional concepts, and seek efficiency from science and technology.

2.2 Advocate outdoor shade sheds for mycelium running and summering of flower shiitake mushrooms. Outdoor shade sheds, due to good ventilation and rapid cooling, can effectively reduce the occurrence of mycelium burning, and are a relatively good summering method. The top and surroundings of the shade shed should be shaded, with covers such as shade nets or reflective film on the outside, but plastic film must not be used on the shed top, to avoid stuffy heat inside the shed burning the logs. Around the shed, trellises can be set up to grow climbing crops such as pumpkins and loofahs to improve shading. The shade shed should be thickened and heightened, with the shade net at least 1m above the shed (the shed top is divided into two layers: the upper layer is a shading layer of weeds, and the lower layer is a shade net). The surroundings can be enclosed with materials favourable for ventilation such as bamboo branches and tree branches to improve ventilation. In case of continuous high-temperature weather, interconnected drainage ditches can be dug around the inside of the shed and along both sides of the walkways inside the shed, and "running horse water" can be introduced for cooling. For old mushroom sheds, a comprehensive cleaning and disinfection/insect elimination should be carried out before entering the shed.

2.3 Strengthen the management of logs summering indoors. For indoor summering, it is advisable to run mycelium in ground-floor rooms with good ventilation and cool summer temperatures, and ventilation passages should be left when arranging logs. Shading facilities around the incubation room should be increased, such as building shade shelters and hanging shade nets. During high-temperature periods, attention should be paid to spreading stacks, handling gently to avoid vibrating the logs, and this should be arranged during the low-temperature periods of morning and evening. The requirement for spreading stacks is to change from the original four-horizontal and four-vertical stacking to triangular or well-shaped stacking, with a stack height of 3-5 layers. Near noon, doors and windows can be closed, and in the evening, ventilation should be used to exchange air and dissipate heat. In short, use all available conditions for ventilation and cooling.

During high-temperature periods, measures such as piercing holes for aeration and turning stacks are prohibited for logs, to prevent enhanced mycelium respiration from raising the stack temperature. Hole-piercing for aeration should be carried out before high temperatures when the weather is cool, and should be done in batches within the same room. After piercing holes, the temperature rises quickly, and ventilation and heat dissipation should be strengthened within 2-3d to prevent logs from dying from stuffy heat due to high temperatures.

2.4 Properly handle contaminated logs. For early-stage localised log rot, the rotten mycelium blocks can be dug out down to healthy mycelium, then coated with quicklime solution or a slightly higher-concentration fungicide to prevent and control further spread of the pathogen. For logs that have been completely stuffily burned, the bags can be broken open and the material sun-dried for reuse. They can be used to produce shaggy ink cap mushrooms, Agaricus blazei, and king oyster mushrooms. In plain areas, shiitake mushroom logs can still be produced before the end of September, but short-mycelium-age varieties such as L66 and L33 should be selected. When using contaminated material, quicklime water should be used to adjust the pH value, and the sterilisation time should be extended. It is best to add new material, which is beneficial for mycelium running.