What damage does tea blister blight cause to the growth of tea plants

Tea blister blight, also known as blister-like leaf blight and leaf gall disease, is an important disease of young buds and leaves. It is distributed in all tea-growing regions across the country and mainly damages tender leaves, tender stems, new shoots, flower buds, fruits and leaf stalks. When tender leaves are infected, small translucent spots appear at first, pale yellow to reddish brown, then expand into round spots with a diameter of 0.3-1.25 cm. The front of the lesion is sunken, pale yellowish brown to dark red, and the back is raised, forming steamed-bun-shaped blister spots, on which there is grayish white, pink or gray powdery material. Later the powder disappears, and the raised part shrinks to form brown dead spots, with a grayish white ring around the edge, resembling a cake, hence the name tea blister blight. In severe cases, there are several or dozens of obvious lesions on one leaf, which later dry up or form cankers. Diseased leaves are mostly twisted or deformed, and the tea leaves are distorted, folded in half or rolled up irregularly. When leaf stalks and tender stems are infected, they swell and twist, and in severe cases the new shoots above the diseased part die or break off. This disease has a great impact on tea quality.

The pathogen is Exobasidium vexans, a fungus of the subphylum Basidiomycotina. The white powdery substance on the lesions is the hymenium of this fungus, formed by many basidia gathered together. The basidia are cylindrical to club-shaped, narrower at the base, slightly rounded at the tip, single-celled, with 2-4 small sterigmata at the tip of each basidium, and one basidiospore borne on each sterigma. The basidiospores are oval and produce a septum before germination, becoming two-celled; upon germination, each cell produces one germ tube and invades the host.

The pathogen overwinters or oversummers as mycelium on living diseased leaves. In early May of the following spring or in autumn, when the average temperature is 15°C-20°C and the relative humidity is above 80%, basidiospores can form. The basidiospores are spread by wind to tender leaves or new shoots, begin to germinate when they encounter water droplets, and after invasion undergo an incubation period of 3-18 days, forming new lesions, on which the hymenium then grows. After the basidiospores mature, they disperse again and cause multiple secondary infections. The disease occurs at different times in tea regions across the country: in the East China and South China tea regions it occurs from March to May and from September to October, in the Southwest tea region from July to November, and in the Hainan tea region in February and from September to January of the following year.

In summer, the fungus oversummers on tea plants. Under suitable temperature and humidity conditions, basidiospores begin to germinate in 2 hours, and the germination rate reaches 70%-80% in 24 hours. The germinated basidiospores produce 2 germ tubes, which extend from both sides of the spore. The germ tubes invade directly through the epidermal cells of the host. At a temperature of 18°C-20°C and a relative humidity of 80%-95%, the incubation period is 10-13 days, and at 20°C-23°C it is 8-10 days. Spores begin to be produced 4-6 days after symptoms appear, and one infection cycle takes 12-19 days. In Sichuan, 7-8 secondary infections can occur each year, and in Guizhou 15 can occur. When the temperature is above 31°C and there is continuous light for 4 hours, the growth and development of basidiospores are inhibited, and the disease stalls. Tea gardens with little sunshine, long duration of dew, much fog and high humidity are prone to the disease; excessive or over-application of nitrogen fertilizer, excessive picking and pruning, extensive management and many weeds lead to severe disease.