Primary Root Rots
The primary root rots are the most dangerous diseases of tea plants. The root rot diseases are caused by some soil borne fungi. The pathogens are mostly present in the vegetative materials in the soil and they attack the tea bushes when they come in contact with the roots of the tea bushes.
The characteristic feature of the primary root rots is that they do not show any diseased symptoms and suddenly the bushes die. There is no control measure to save the diseased bushes but to uproot them.
The pathogens are normally present in those areas where the planting is done without proper soil rehabilitation. It is, therefore, stressed that big trees or the shade trees should be killed by ‘Ring Barking’ before uprooting and replanting tea instead of felling the trees so that no fresh vegetative materials remain in the soil.
During the two years rehabilitation period the pathogens would die due to shortage of food. Grasses are used to rehabilitate the soil as the fibrous roots of the grasses cannot harbor any soil pathogen.
Brown Root Rot
Brown root rot is caused by the fungus, Fomes noxius Corner. This is the first root disease discovered in tea which occurs in India, Bangladesh, Sri Lanka, China and Indonesia.The disease is usually found in more than 3 years old plants, but younger plants may also be affetcted.Roots of affected bushes are encrusted with soil, sand and stone particles, firmly held by a brown mycelium. The mycelium also grows upto main stem along with thin white film. Irregular rings or reticulations are formed by hard brown lines in light yellow coloured lines. In the advanced stages of the disease, the brown ring decays forming a honey comb-like structure.
Charcoal Root Rot
The disease causing fungus, Ustilina zonata (Lev.) Sacc=U.deusta (Fr) Petrak. It is the most harmful root disease of tea in N.E. India and Bangladesh.The disease occurs also in South India, Sri Lanka and Indonesia and sporadically in Africa. It attacks all kinds of tea bushes usually above three years. It spreads through root contact with the diseased woody material present in the soil and through wind borne spores. There are no visible symptoms on the surface of the affected root but if the bark is removed characteristic delicate fan-shaped patches of dull white, silky mycelium may be seen spreading over the diseased wood.
Sometimes, it is seen that part of the mature bush affected by this disease dies while the other side remains apparently healthy and functional.
Red Root Rot
Red root rot is caused by the fungus, Poria hypolateritia (Berk) Cooke is disease of economic importance in South IndiaSri Lanka and Indonesia. Reddish rhizomorphs appear on the root surface and the roots present a reddish or reddish brown appearance. Underneath the rhizomorphs the root surface appears whitish. On washing with water, the rhizomorphs take brighter colour with the appearance of whitish patches of conidial conglomerations. When cut with a sharp knife, brownish dots appear on the wood. The cut surface of the old roots has water soaked appearance.
Black Root Rot
Black root rot is common in N.E.India particularly in the hills. It also occurs in Indonesia, Japan, South India and Central Africa.
The fungus (Rosellinia arcuata Petch) forms irregular black, cob-webby adherent cords of mycelium and small isolated black dots and dashes on root surface. Mycelium is woolly, purplish-grey to black in colour. Small white or black star-like markings with dashes and dots also develop on the wood. During fructification black spherical bodies like grains of shot grow side by side to form a crust. Spreading source is similar as in charcoal stump rot.
Tarry Root Rot
Tarry root rot is caused by the fungus Hypoxylon asarcodes (Theiss) Mill is not a common disease. The fungus does not produce any external symptom on the affected roots. A few centimeter of the stem from collar upwards shows a black smooth, hard, effused and adherent encrustation.