Identification and Control Methods of Southern Blight in Luo Han Guo (Monk Fruit)

Luo Han Guo, also known as false bitter gourd, Luo Han Biao, Jin Bu Huan, smooth-fruited Momordica, and naked tortoise gourd, is a fruit with both edible and medicinal value. It is highly nutritious, rich in vitamin C, fructose and other substances, and medicinally it can relieve cough and moisten the lungs. Luo Han Guo is grown relatively widely in southern China, and if not managed properly it is very prone to southern blight, so how should it be controlled?

I. What is southern blight in Luo Han Guo?

Southern blight, also known as sclerotial root rot or sclerotial seedling blight, mainly damages the roots, stems and vines of the plant. The cortex of the infected area slowly turns brown and becomes necrotic, and in relatively humid environments, white silky mycelium appears on the surface of the affected roots and stems or on the nearby soil surface. Within this mycelium, many small sclerotia that closely resemble rapeseed form. If the plant is attacked by this disease, the harvest is basically a total loss.

This pathogen is a habitual inhabitant of the plant root zone and can only survive on diseased plants. Its optimum growth temperature is around 30°C. It readily forms sclerotia, which have no dormancy period, so the pathogen has very strong viability and great resistance to adverse environments, and can survive in the soil for 5-6 years. The sclerotia overwinter on diseased plant residues. In the following spring, when soil temperature and moisture are suitable, the sclerotia germinate and produce mycelium. The disease is more likely to occur when temperatures are high, humidity is high, the soil is relatively clayey, and there is abundant rain with poor drainage.

II. How to control southern blight in Luo Han Guo?

1. According to local conditions, choose varieties with better disease resistance for planting.

2. Strengthen management, apply more decomposed organic fertilizer, irrigate reasonably, and keep the plants growing vigorously, which can improve the plant's resistance to this pathogen. Also prune diseased and disabled branches as well as relatively dense branches, so that ventilation and light are more even. In rainy seasons with plenty of rain, pay attention to drainage to avoid waterlogging, plow the soil and remove weeds, and maintain appropriate temperature and humidity.

3. The soil should be relatively fertile and loose, with good drainage. Try not to choose melon or vegetable plots or sandy soil. When preparing the land, an appropriate amount of lime can be added to adjust the soil pH, keeping the soil slightly acidic to neutral. Before planting, the seed stems can also be soaked in lime water or carbendazim to eliminate the pathogen.

4. For orchards where infection has already appeared, lime water can be used to disinfect the soil and prevent it from spreading. It is recommended to use organic fertilizer composted with straw decomposition inoculant to increase beneficial microbial flora.

5. In the early stage of the disease, the soil around the roots can be pulled away to expose the roots to sunlight for sterilization. Chemical agents such as shenaning, baikang, fenxiuning, and thiophanate-methyl can also be used to drench the roots. Deep plowing should be carried out every winter, and diseased plant residues can be buried deep in the soil to eliminate the source of infection.