Main Root Diseases of Tea Plants and Their Control Methods
1. Tea seedling white silk blight
(1) Distribution and symptoms: Tea seedling white silk blight is a common root disease in nurseries. It has a wide distribution range and causes severe damage. Besides tea, it can also harm more than 200 kinds of plants including cucurbits, Solanaceae, hemp, tobacco, and peanut. It occurs at the root collar, where the diseased part initially shows brown spots, and a white cottony material grows on the surface; after expanding, it encircles the root collar, forming a white silky mycelial membrane that can spread toward the soil surface. Later, rapeseed-shaped sclerotia form on the diseased part, changing from white to yellow-brown and then to dark brown. Due to the pathogenic action of the fungus, the cortex of the diseased plant rots, and the transport of water and nutrients is blocked, causing the leaves to wither and drop, and finally the whole plant dies.
(2) Pathogen: It is a fungus of the genus Pellicularia in the Basidiomycotina. The mycelium is initially colorless, later slightly brown, dense, and forms sclerotia. The sclerotia are round, with a smooth, hard surface, and dark brown. Under hot and humid conditions, it produces reproductive bodies, namely basidia and basidiospores, but they are not common and play little role in disease transmission.
(3) Occurrence pattern: It mainly overwinters as sclerotia in the soil or attached to diseased tissue, and can survive for 5 to 6 years under dry conditions. At the turn of spring and summer the following year, when temperature and humidity are suitable, it germinates and produces mycelium, which spreads along soil crevices or is transmitted by rainwater, irrigation water, farm tools, etc., infecting the root collar of seedlings and causing damage. High temperature and high humidity favor disease development, with the most severe occurrence from June to August. The disease is also severe in sticky soil, soil with excessive acidity, low-lying terrain, poor-growing tea seedlings, and where the previous crop was a susceptible host.
(4) Control methods: Choose virgin land or land with non-susceptible crops as the nursery. Pay attention to drainage in tea gardens, improve the soil, promote robust seedling growth, and enhance disease resistance. In susceptible nurseries, diseased seedlings should be removed promptly and the soil disinfected. For chemicals, use 50% carbendazim 500-fold solution, 0.5% copper sulfate solution, or 70% thiophanate-methyl 500-fold solution. For transplanted tea seedlings, roots can also be soaked and disinfected in 20% lime water.
2. Tea root canker disease (compared with tea seedling damping-off)
(1) Distribution and symptoms: It mainly harms tea seedlings and occurs severely in some tea areas, causing tea seedlings to die. It is common in cutting nurseries, and both main and lateral roots can be affected. The pathogen invades from the cut end of cuttings or root wounds. Initially, light brown spherical protrusions appear, which gradually enlarge into tumor-like growths, small ones like millet grains and large ones like peas; multiple tumors often heal together into irregular large tumors. The tumorous growths are brown, lignified and hard, with a rough surface. After being affected, tea seedlings have fewer fibrous roots, and the aboveground part grows poorly or dies.
(2) Pathogen: The tea root canker pathogen is Agrobacterium, a bacterium. The bacterial cells are short rod-shaped, with 1 to 3 polar flagella. On ordinary culture medium, it forms gray-white round colonies. The optimum temperature for development is 25 to 29°C, and the lethal temperature is 51°C (10 minutes). It develops best at pH 7.3.
(3) Occurrence pattern: The root canker pathogen overwinters in soil or diseased tissue. When conditions are suitable the following year, it spreads and causes damage via water flow, soil insects, and farm tools. The pathogen invades from seedling wounds or cuts, grows and develops within the tissue, stimulates cells to divide rapidly, and produces cankers.
(4) Control methods: Strictly inspect seedlings, control underground pests, and reduce root wounds. When necessary, seedlings can be soaked in 20% lime water for 10 minutes for disinfection before transplanting.
3. Tea seedling root-knot nematode disease
(1) Distribution and symptoms: It is mainly distributed in warm tea-producing areas. This disease is caused by a very small nematode. When the nematode invades tea roots, it causes tumors to form on the roots, resembling soybeans, rapeseeds, etc., and varying in size. Affected root systems have no fibrous roots and are deformed; sometimes the root tip is thicker than the front part. The aboveground part of the diseased plant grows poorly, the plant is short, growth is weak, and leaves turn yellow. In dry seasons, leaves easily drop in large numbers and the plant dies.
(2) Occurrence pattern: Root-knot nematodes are aerobic organisms that prefer high temperature and dryness. They have multiple generations per year. When the soil temperature reaches 25 to 30°C and soil moisture is about 50%, in sandy soil on high terrain with loose soil structure, one generation can be completed in 20 to 30 days. Nematodes occur more in the surface soil than in the lower soil, and stop activity below 10°C.
(3) Control methods:
① Choose virgin land as the nursery. Land where crops such as peanut, cucurbits, beans, and sweet potato have been grown is not suitable for use as a nursery. In midsummer, the soil should be deeply plowed, exposed to the sun for 10 days, and then plowed again, which can kill nematodes in the soil.
② For seedlings already affected, increase the application of phosphate fertilizer and potassium fertilizer to improve their disease resistance. At the same time, disinfection with lime or applying 1:20 tea seed cake water into the soil also has a good control effect.
4. Tea purple root rot
(1) Distribution and symptoms: It is distributed in various tea-producing areas, but is more common in North China and East China. This disease mainly occurs at the seedling stage and adult plant stage, harming the roots or root collar. First, the fibrous roots rot and become brown or dark brown, then it spreads to the lateral roots, which after rotting become purple-brown. The surface of the diseased spots is covered with purple-brown thread-like material. On the surface of diseased roots there are hemispherical granular sclerotia; the base of the small stem plus centimeters is often surrounded by purplish-red mycelium, soft in texture and easy to peel off, and the root cortex is also easy to peel off. In severe cases, the aboveground part wilts, new shoot sprouting decreases, and the diseased plant dies.
(2) Occurrence pattern: Generally, the disease is more severe at the turn of spring and summer or the turn of summer and autumn when it is hot and rainy. Tea gardens with a high groundwater table, poor drainage, or excessively dry soil are prone to the disease. This pathogen can survive in the soil for many years and spreads and expands with farming operations, rainwater, underground pests, and root contact; long-distance spread can also occur when transporting infected seedlings and soil. Continuous cropping and a previous susceptible crop also make the disease more likely.
(3) Control methods:
Select sterile healthy seedlings. If diseased seedlings are found, dig them out and burn them promptly. After use, wash tools, then soak the remaining healthy seedlings in 20% lime emulsion for 1 hour, or soak the roots in 50% carbendazim 1000-fold dilution for half an hour before planting. For tea gardens with poor drainage and a high water table, pay attention to drainage and dig drainage ditches well before flooding. For land where sweet potato, potato, and fruit trees were previously grown and where this disease has occurred, disinfect with 70% quintozene, using 2.5 kg per mu mixed with fine soil for soil disinfection, or irrigate the soil with 30-fold formalin dilution. In the early stage of disease, irrigate the root collar with potassium permanganate or carbendazim 1000-fold dilution.