The important diseases of tea in North East India are:
|Primary Diseases||Secondary Diseases|
|i) Leaf diseases|
|Blister blight||Grey blight|
|Black rot||Brown blight|
|ii) Stem diseases|
|Dieback (Seed decay)||Poria branch canker|
|Thorney stem blight|
|iii) Root diseases|
|Charcoal stump rot||Violet root rot|
|Brown root rot||Diplodia disease|
|Red root rot|
|Black root rot|
Blister blight is a seasonal disease and can spread very rapidly by means of air borne spores. Mature tea in general and teas recovering from pruning in particular suffer from blister blight. The fungus attacks young succulent growth on all teas especially when the environment is foggy, moist and cool. A single spore can produce a blister when deposited on tender shoots in about 10-21 days from the time of infection.
|Tea growing areas||Periods of outbreak|
|Darjeeling||July to September|
|Assam||March to May and November to December|
|Dooars||September to November|
This disease can be controlled by adopting chemical as well as cultural control strategies simultaneously. Some of the predisposing factors are dull, humid, cloudy and cool weather prevailing for several days, heavy shade etc. In such conditions prevails, monitoring of blister prone sections should be thoroughly undertaken. The shades should be thinned.
Normally 4-6 rounds Copper are necessary in Darjeeling. In Assam and Dooars, the spraying can be discontinued once the damp weather condition is over and it becomes hot and sunny for a few consecutive days. The order of priority for chemical treatment is:
nurseries > pruned young tea > early medium pruned tea > light pruned tea > unprune tea.
The disease is caused by fungi Corticium theae and Corticium invisum. Both the fungi produce similar effect on the bush and some times occur together. They attack the maintenance leaves, causing gradual deterioration in the health of the bush and consequent loss of crop. The disease is first noticed in May/June/July although germination from the sclerotial stage starts in March/April. The fungus remains active right up to September and then over winters in the sclerotial form in the cracks and crevices of the stem. Transformation in to sclerotial stage is a means for survival during the unfavourable weather condition. When favourable condition prevails, they germinate and re-infect.
Cultural control :
- Pruning or skiffing of the severely affected sections.
- Improvement of aeration by lopping side branches and ‘matidals’.
- Thinning out of the dense shade.
- Improvement of drainage.
- Alkaline wash after pruning.
- Adoption of shorter pruning cycle in chronic and severely affected sections.
Chemical control :
Thorough drenching of all the stems and decomposing pruning litters below the bush with copper oxychloride (COC) one week after the first spell of rainfall in February/March. Spray two blanket rounds of COC or Copper hydroxide or Hexaconazole at 15 days interval during May-June. Spot spraying of these two fungicides should continue till September if the disease incidence is severe. Application of two monthly rounds of COC or Carboxin during winter will inhibit sclerotia formation.
Control by bioagents:
Two fortnightly rounds of 10% Bacillus subtilis broth can be applied in lieu of chemical fungicide during both active and dormant phase of the disease. Spraying should be directed towards small stems and under surfaces of the leaves, which bear fungal pathogens.
Branch canker and Thorny stem blight
Branch canker and Thorny stem blight are important stem diseases of tea prevailing in plains of Assam and Darjeeling hills, respectively. Branch canker is caused by Poria hypobrunnea and the causal organism of thorny stem blight is Tunstallia aculeata. These pathogens are typical wound parasites and enter the tea plants normally through heavy pruning cuts. Hence it is necessary to protect the fresh wounds with protective paints to stop the entry of these pathogens.
For an effective control of these diseases careful removal of all dead wood at each pruning and subsequent protection of the pruning cuts with protective paints/sprays is necessary. These protective paints may be bituminous paint, COC or Trichoderma bioagents. During recent times Trichoderma treatment has been found to be the best method of protecting the pruning cuts from the damage of both Poria and Tunstallia. For painting of large pruning cuts soon after rejuvenation and medium pruning 80 litres of 20% Trichoderma spore suspension using 16L or kg of the bioagent formulation/ha is required (considering 8000 plants/ha). For spraying of entire bush frame after light pruning/cut across pruning, 400 litres of 5% Trichoderma spore suspension using 20L or kg of the bioagent formulation/ha is required using a NMD 60/450 nozzle.
It is a disease of young stems mostly, the ultimate symptoms of which normally manifest on the leaves. The causal organism of this disease is Cephaleuros parasiticus, an alga, causes severe damage especially to the young teas. Tissues of the stem are killed in patches and cause dieback. The leaves of the infected branches variegate with yellow patches. The alga produces brick red or orange coloured fructifications in patches on the infected stems of the tea bush.
- The predisposing factors to be identified and corrected are i) poor drainage, ii) low soil fertility, particularly potash, iii) improper soil acidity, iv) inadequate shade and v) continuous use of green crops like Tephrosia candida, T. vogelli etc.
- Pruning of severely affected sections.
In susceptible areas young teas should be sprayed with four rounds of COC/Copper hydroxide during mid April to mid July. First two rounds at 15 days interval and the subsequent rounds at monthly interval. Young shade trees/ green crops in nurseries should be painted with COC to avoid fresh infection.
2% MOP and Urea can be sprayed as a rehabilitary measure in the severely affected sections. Spraying of fungicides should be directed towards the young stems and laterals bearing rusty fructifications.
Dieback of Primaries and seed decay
During the last couple of years, dieback of primaries in certain succulent TV clones and seed decay in some biclonal seed stocks have been causing serious concern. The causal organism of these maladies is identified as Fusarium solani, a parasitic fungus. Generally this pathogenic fungus is predominant in the humid temperate climate of North East India. The susceptible clones and seed stocks to Fusarium solani are recorded as TV 19, TV 23, TV25, TV 26, TV 29, S3A1 parent of seed stock 491 and both the parents of seed stock 520. Symptoms of these maladies are as follows :
On primaries : Blackening of the petioles of leaves, which gradually extends to the nodes and inter-nodes, followed by wilting of the primaries. White cottony fungal growth is seen on the dying tissues, which turns brown at maturity accompanying with development of small pink perithecia of Necteria
On seeds : Blackening of fruit carp followed by immature cracking and dropping of seeds. Seed becomes light pinkish with powdery coverage due to the fungus sporulation in the later stage of infestation.
Several chemicals have been evaluated for controlling Fusarium solani both in laboratory as well as in field. In these trials Carbendazim, Hexaconazole and Tridemorph and their combinations were found effective in controlling the pathogen.
Primary root diseases
The most common primary root diseases of tea plants in the plains of North East India are “Charcoal stump rot” and “Brown root rot”.”Black root rot” is occasionally found in Darjeeling gardens while Red root rot is of rare occurrence. The root diseases can spread by means of direct contact or from left over diseased woody material. However, the spread of Charcoal stump rot and Black root rots may take place by air borne spores as well. The diseased materials may also be carried by rain to healthy teas. Unfortunately, there is no known method of detecting root infection by examining the above ground portion of the plant. Depending on the age and size of the bush it may require six months to four years to kill a infected plant. The tea roots remain in close proximity and hence by the time a dead plant is noticed, it is possible that its immediate neighbours have also become infected. Adoption of proper cultural practices is a vital point for control of primary root diseases as no effective chemicals are available at the moment to control these pathogens.
- When the disease occurs on individual bush or in patch, the diseased plant together with a ring of apparently healthy bushes surrounding it should be uprooted removing all bits of roots.
- When a large area is infected the diseased patches should be isolated by 90 -100 cm deep and 30 cm wide trench surrounding the area until uprooting can be undertaken. This trench should be connected to the nearest drain.
- In case of old tea, due for uprooting, only the diseased plant is to be uprooted and the whole area should be kept isolated by an encircling trench as described above. The area should be uprooted two years before replanting.
- All vacant areas thus created should be kept under a green crop for two yearsbefore infilling or replanting.
Biological control :
Trichoderma bio agents have been found effective in controlling primary root diseases namely, Charcoal stump rot and Brown root rots. Planting pit treatment for infilling and replanting, 10% spore suspension should be used for treating the excavated soil of planting pits @ 100ml/pit. Alternatively, compost or well rotten cattle manure should be mixed thoroughly with the spore suspension and the treated manure should be mixing thoroughly with the excavated soil of the planting pit @ 200g/pit. The treatment will require 30L or kg of Trichoderma biocide/ha.
For top dressing around infills already planted in mature tea (1 year or below) should be sprayed around the collar region on the round @ 100 ml/plant using 5% spore suspension during Apr/May or Sep/ Oct before the soil gets dry. Forking of soil is a must before spraying.
Secondary root diseases:
Violet root rot and Diplodia
Violet root rot caused by Spherostilbe repens and attacks of Diplodia sp. on weak plants are the common secondary root diseases of tea. Violet root rot disease is very common on stiff clayey waterlogged soils. It can easily be controlled by improvement of drainage. Diplodia fungus can live on dead stem and root tissues, which are deficient in reserved starch material. Healthy plants are not affected by this fungal pathogen. No chemical treatment is necessary for controlling both these diseases.