The fungus (Exobasidium vexans Massee) causing blister blight is an obligate parasite with no known alternate or collateral host. It attacks the young succulent leaves but not the mature leaves and stems of tea under cool, misty weather with little or no sunshine. It is presently the most injurious leaf disease in all tea growing countries in Asia. Another fungus, the net blister blight, Exobasidium reticulatum common in tea plantations of mountainous regions of Japan and Taiwan, is equally harmful.
Average weekly sunshine hours ranging from 2-10 hours, temperature from 100-230C, mean daily rainfall from 20-100mm and relative humidity from 60 100% favour the disease development.
A leaf wetness of 11 hours is optimum and 13 hours maximum for infection. The fungus reproduces through basidiospores, which are disseminated by wind. The wind borne spores on lodging on the surface of a susceptible host tissue germinate under favourable conditions and infect the host.
The pathogen requires only a short period to complete its life cycle. Depending on the ambient weather condition it may vary from 11 to 28 days. Hence, many generations of the fungus can be completed during the disease season. Sporulation may start 10-12 days after infection and spore discharge continues upto 8 days.
The symptoms first appear on the young expanding leaves as pale yellow or pinkish translucent spots. The spots gradually enlarge to a circular lesion of 3.0 – 12.5 mm or more in diameter. As the lesion develops further, the upper side is depressed into a shallow cavity and correspondingly the lower side becomes convex forming the typical blisters from which the blight receives its name. The upper concave surface of the lesion is smooth and shiny, while the lower convex surface is dull at first and powdery as it sporulates.
The affected leaves often become folded or irregularly rolled up owing to the development of lesions. A mature blister lesion can produce upto two million spores in 24 hours. Under severe infection, the affected leaves curl up and become distorted.
The fungus also infects young growing stems by producing slightly swollen velvety patches on the surface but do not form blisters. When the tender stem is affected, the entire shoot withers and results in dieback.
Black rot is caused by two species of the genus Corticium namely Corticium theae Bernard and Corticium invisum Petch which occur at the same time. Heat combined with humidity is the favourable condition for the fungal development. It is a common disease in the plains of India, Indonesia and southern part of China.
Mature leaves are the spots of infection but the fungus is also seen on stems. The fungal mycelium enters leaf through undersurface of petiole and spread. While Corticium theae shows a cord of mycelium, white in colour, on the stem which changes to pink and finally to dark brown with age but no such symptom is exhibited by Corticium invisum.
Corticium invisum produces irregular patches with slightly raised wavy margin on the leaves, which are accompanied by many small greyish white circular spot. Larger patches exhibit a mixture of brown yellowish to chocolate brown and grey upper surface with evenly brown and grey under surface.
Corticium theae on the contrary produce large patches of reddish brown colour on upper surface covering about half or more of leaf lamina while the under surface is light brown or greyish white, usually covered with a network of cream to brown mycelium.
Diseased patches on leaves look black and slimy when wet. The disease gradually fills the leaves resulting in deterioration of health. Dead leaves often remain attached to other leaves or to the stem by small cushions of mycelium. Fructifications appear as powdery white patches on the undersurface of apparently healthy green leaves.
Brown blight caused by the fungus, Glomerella cingulat or Colletotrichum camelliae Massee is a common foliar disease in India, China, Sri Lanka, Africa and other tea growing regions. Infestation starts at margin and spreads inwards, with the formation of patches, edges of the patches are sharply defined with a delicate concentric zone. Upper surface is yellowish to chocolate brown initially changing to grey from centre to outwards. Minute, black, scattered, dot-like fructification of the fungus appears a both surface of leaves.
The casual fungus of this disease is Pestalozzia theae. Diseased patch may occur anywhere in the leaf lamina. Patches on the upper surface are brown coloured with a greyish centre, roughly circular to oval in shape. On young leaves the patches are usually dark brown to almost black.
It is the only major algal disease of tea caused by the an intercellular alga, Cephaleuros parasiticus Karst.It causes severe damage to both old and young tea and is prevalent all over the tea areas. The alga causes branch die-back and hinders frame formation in affected young tea plants. The diseased leaves turn yellow, variegated and usually develops circular zones with purple margin. Leaves drop and the stem die back giving the bush a thin and weakly appearance.
Anthracnose caused by Colletrotrichum theae-sinensis (Miyake) Yamomoto is an important leaf disease of tea in north-east Asia such as Japan, China and Taiwan. This disease is prevalent in hilly areas and occurs in rainy season. The germ tube of conidia penetrates on the leaf and invades the leaf tissues. It produces large reddish-brown lesion on leaves and most of the affected leaves fall off.
Tea White Scab
It is another leaf disease caused by Elsinoe leucospila Bitan & Jenk in Japan and Phyllostica theaefolia Hara in China. Low temperature and high humidity are favourable for development of this disease. The disease produces minute circular spots on shoots which leads to defoliation.