Causes of soft rot and nest rot in Gastrodia elata and prevention methods

Soft rot and nest rot in Gastrodia elata is one of the reasons for low and unstable yields in Gastrodia elata cultivation. In the small tubers, white tubers and arrow tubers affected by soft rot, the tuber skin turns yellow and withered, the central tissue decays, and a foul-smelling white milky fluid oozes from the cut surface of the tuber. In the rotting nests of Gastrodia elata and inside the tubers, the rhizomorphs of Armillaria mellea are luxuriant and dense. Gastrodia elata depends on Armillaria mellea for growth, so why does soft rot and nest rot occur, and why is the area beneath the tuber skin filled with Armillaria mellea mycelium? To explore this question, we should start with the nutritional ecology of Gastrodia elata.

Gastrodia elata has no roots and no green leaves, its autotrophic organs are degenerated, and it relies on Armillaria mellea to provide nutrition, living in a special symbiotic relationship with Armillaria mellea. The establishment of this relationship begins when Gastrodia elata is treated as a host by Armillaria mellea, when the mycelium comes into contact with the tuber of Gastrodia elata.

The mycelium of Armillaria mellea generally extends along the surface of the Gastrodia elata tuber. The outer sheath of the rhizomorph is initially brownish-red with a white tip, gradually becoming dark brown, with white mycelium inside the sheath. Armillaria mellea has strong adaptability to the environment and can grow and develop at soil temperatures of 6-32°C, but is most suitable at soil temperatures of 15-25°C and soil humidity of 50%-70%. While developing, Armillaria mellea not only obtains nutrition from the host, but also continuously extends in search of new hosts in its surroundings. After the mycelium comes into contact with the tuber, it quickly spreads along the tuber epidermis and produces root-hair-like mycelium, penetrating the epidermis of the Gastrodia elata tuber and invading the bacteriolytic layer (located in the inner layer beneath the tuber epidermis of Gastrodia elata, appearing under the microscope as columnar cells arranged in neat rows), which is dissolved to continuously provide nutrition for the growth of Gastrodia elata. Therefore, under environmental conditions favorable for the growth of Gastrodia elata, the more Armillaria mellea mycelium that invades, the faster the tuber grows, until the inner layer of the Gastrodia elata becomes hard and the young sprout of the terminal scape is formed. However, because Gastrodia elata has weaker adaptability to the environment than Armillaria mellea during growth, it begins to grow at soil temperatures above 10°C; it grows quickly at 15-25°C and humidity of 40%-50%; when the soil temperature reaches 28°C and humidity is above 60%, tuber growth is inhibited, and humidity exceeding 70% will lead to necrosis and nest rot of Gastrodia elata.

Soft rot and nest rot in Gastrodia elata is a physiological disease, and the main cause of the disease is adverse environmental conditions.

① Improper selection of soil quality. Soft rot and nest rot are mostly seen in gravelly land and dead yellow mud land with poor soil moisture retention and air permeability, where Armillaria mellea grows and develops poorly, and Gastrodia elata wilts and rots to death because it cannot obtain nutrition from Armillaria mellea.

② High temperature and high humidity environment. Under such climatic conditions, the growth of miscellaneous fungi is favored, and grayish-white miscellaneous fungal mycelium quickly appears on the Armillaria mellea material, gradually spreading into the humus branches in the nest. The characteristic of this pathogenic fungus is that yellow-white, rapeseed-like sclerotia form among the hyphae, and once Gastrodia elata is infected by the pathogenic fungus, it rots from a localized area to the whole body.

③ Excessive rain and poor drainage. Under such harsh conditions, the growth of Gastrodia elata is inhibited, and its bacteriolytic function is significantly weakened. However, because Armillaria mellea has strong stress resistance, it can still grow normally, and Armillaria mellea breaks through the bacteriolytic layer of Gastrodia elata and rapidly reproduces within the tuber. Its characteristic is that beneath the skin of the rotting tuber, it is filled with Armillaria mellea rhizomorphs that have "eaten" the Gastrodia elata, and the Armillaria mellea rhizomorphs on the fungal material in the rotting nest are dense. This nest rot phenomenon mostly occurs in the rainy seasons of July, August and September.

④ Mistakes in Gastrodia elata cultivation techniques. This situation is mostly seen when there is insufficient fungal material, the sowing rate is too large, and there is no oak leaf or other humus filling in the nest. Despite climatic conditions suitable for the development of Gastrodia elata, because it cannot obtain nutrition from Armillaria mellea, the Gastrodia elata becomes thin and elongated, gradually leading to soft rot.

So how can the occurrence of soft rot and nest rot in Gastrodia elata be avoided

① Choose sandy loam soil with good granular structure, strong water permeability and rich humus content, and require forest land with a 15°-25° slope, soil pH of 5.8-6, and humidity of 40%. In high mountain areas, choose sunny slopes; in medium and shallow mountain areas, choose semi-sunny slopes; in low mountain and hilly areas, choose cultivation under the forest.

② The cultivation season is generally arranged mainly in winter (October-November). Usually, old fungal material plus new fungal material is used to supplement the nutrition of the Armillaria mellea fungal material and control excessive growth of Armillaria mellea. During cultivation, open the fungal bed cultivated in March-April of that year, take out 5 well-developed fungal materials and add 4 new materials to plant another nest, and add 4 new materials to the original fungal bed to plant one nest (that is, the "take five, add four method"). Generally, 500-750 grams of seed is sown per 10 fungal materials, the Gastrodia elata seed is placed close between the fungal materials, more fungal branches are added between the new materials, and in addition to a layer of oak leaves spread at the bottom of the nest, each layer planted is filled with an appropriate amount of oak leaf humus soil, and after planting, cover with leaves and earth 10-15 cm.

③ Gastrodia elata management is simple: no weeding, no fertilizing, mainly preventing drought, preventing waterlogging, preventing cold and preserving soil moisture. In low mountain areas, to prevent drought, a few corn plants can be planted beside the nest as drought indicator plants, and when the corn leaves curl, spray an appropriate amount of water to prevent drought. In rainy seasons, open ditches on the upper slope of the cultivation area for drainage. In high mountain areas, add leaves or straw curtains in the cold winter season to prevent freezing. In addition, prevent trampling by people and livestock and contamination by pesticides.