Diseases of Fruit Plantation Medicinal and Aromatic Crops

Blister blight

Causal organism: Exobasidium vexans Massee.

• This disease was first reported from Assam in 1868 which further spread to most other tea growing areas of North and South India.
• It also occurs in Burma, Indonesia, Japan, Malaya, Sri Lanka and Taiwan. Since 1946 it has become severe year after year in most of the tea gardens of South India causing heavy damage to the industry.
• The succulent growth of the plants developing after pruning is highly susceptible to infection. Temperature above 24°C is fatal to the pathogen and result in disappearance of this disease.
Symptoms

• The first symptom is the appearance of small, pale or pinkish spots on the leaves.
• These spots are round from the very beginning and enlarge upto about an inch in diameter.
• Young shoots are very much affected.
• On the upper surface of the leaf, spot becomes light green in colour and depressed into a shallow cavity while the under side bulges correspondingly forming a blister like swelling. The lower bulged surface is covered by white growth of the fungus.
• Later the blister turns dark brown and shrinks to flattened patch.
• Old leaves of four weeks and above are immune as only the young leaves are circular in shape but become elongated along the midrib.
• When many blisters appear near the margin or apex and coalesce, much distortion and curling of the leaf may be caused.
• The infection passes on to the petiole and young succulent stem which results in serious damage. On the stem, spots without blisters are formed.
• The fungus eventually penetrates and damages the stem.
• The leaves and buds above the point of attack wilt and wither.
• The leaf yield may be reduced and vitality of the tea bush is affected finally.
 
• Under severe attack, the bush may be permanently injured and growth impaired.
Etiology

• The mycelium is confined to the blistered areas on leaves.
• They are septate and collect in bundles below the lower epidermis.
• Later, by rupturing the epidermis a continuous layer of vertical hyphae are projected on the surface of spot.
• The fungus produces two types of spores viz., conidia and basidiospores.
• The conidia are most abundant, borne singly at the tips of long stalks.
• Conodia are hyaline, elliptical, straight or slightly curved and measure 12 - 21 x 4.5 to 6 µm. Basidia are formed on the surface in larger number but never form a continuous hytmeniurn. They are intermingled with 'conidial stalks and sterile hairs.
• Basidium is long, club shaped with usually two short sterigmata at the end, each bearing a basidiospore.
• The basidia are ovate to oblong, hyaline and 30 - 90 x 2.3 - 4.5Mm.
• Mode of spread and survival
• The fungus completes its life cycle within 11 to 28 days and several generations of spores are produced in a season.
• It produces conidia and basidiospores in the same blister.
• Spores are air-borne.
• The perpetuation of the fungus appears to be through the pre-existing infected bushes.
• No dormant or resting stage is known.
Epidemiology

• Relative humidity plays an important role in occurrence of epidemic of blister blight.
• When relative humidity remaining below 80 per cent for 5 days, the rate of infection decreases, but if it is above 83 per cent for 7 to10 days, the infection is moderate to serious.
• The disease is favoured by cool day and nights with wet or humid conditions.
• Severe outbreaks occur in under shades or adjacent to jungle or wind breaks or in damp, low lying areas where mist persists.
• The succulent growth of the plants, developing after pruning, is highly susceptible to infection. Temperature above 24 °C is fatal to the blister blight disease, a fact which accounts for the periodical appearance and disappearance of this disease.
• Cool, moist, relatively still air favours infection.
• Moist and shaded atmosphere are therefore conducive to the development of severe attacks.
Management

• Removal of infected leaves and shoots by pruning and their destruction have been recommended.
• Spraying bushes with Bordeaux mixture or copper oxychloride has been found to be effective. Copper fungicides have been widely used often in combination with nickel salts which are having a slight eradicant effect.
• A mixture of 210 g of copper oxychloride + 210 g of nickel chloride sprayed in one hectare at 5 days interval from June to September and 11 days intervals in Oct-Nov gives economic control.
• Spraying the bushes with a mixture of 420 g copper oxychloride and 27 g Agrimycin 100 in a hectare gives better control over the disease, compared to the treatment with copper oxychloride alone at the same dose.
• Among the organic fungicides, chlorothalonil gives protectant and therapeutic effect. Triademorph, Mancozeb, Baycor, Bayleton and Pyracarbolid also offer good disease control.
• Disease control achieved with triademorph @ 340 and 560 ml /ha is satisfactory under mild and moderate rainfall conditions.
• Spraying schedule can be regulated according to weather conditions.
• Dry leaves are less prone to infection than damp ones and exposure to the sun for only one hour at a temperature of 29.5 °C is lethal to the fungus.
• The fungicidal spray should be stopped until the average daily hour of sunshine for the previous five days has dropped below 3 3/4h.
• Spraying with the systemic fungicide, (Atemi 50 SL) @ 400 ml/ha or bitertanol 300 EC (Baycor 300 EC) @ 340 ml/ha at weekly interval controls the blister blight effectively.