Rubber Diseases

CROP PROTECTION


Abnormal leaf fall disease

Fig. 1 Affected pod

The infection is first noticed on green pods, remaining on the tree or fallen on ground. If you observe such infected pods you can see water soaked lesions of dull grey colour, rotting and oozing of latex and cheesy coating on the surface (Figure 1). The cheesy coating is due to numerous fungus sporangia produced on a fluffy fungal growth. These sporangia are blown over by the wind and rain splashes and reaches the leaves. On the leaves the infection is more common on the petioles. The infected region on the petiole shows water soaked lesions which turn to dark brown or black with a drop of latex coagulated in the centre. The affected leaves fall off even when they are green. Have you noticed green leaves fallen on the ground forming a green carpet during rainy season? If you observe such leaves you can see the symptoms on the petiole. Sometimes the mid rib and lamina are affected and show dull green to black colour. The fungus also infects the growing shoots and young twigs which lead to rotting and die back of shoots. In young plantations, shoot rot leads to suppression of growth and delay in maturity of the trees.

Powdery mildew disease

On mature rubber, the disease occur when the trees refoliate after annual wintering. The young brown as well as light green leaves when infected gets covered with white powdery masses. The infected leaves shrivel, curl and fall off leaving the petioles on the plant, giving a broomstick like appearance. The fallen leaves form a black carpet on the plantation floor. Sometimes petioles and young twigs are also infected. When semi-mature leaves are infected they remain on the trees with dark brown necrotic patches. The inflorescence and young fruits are also affected, thus reducing the fruit set and availability of seeds. Plants growing in shade are more prone to the disease. Severe infection leads to die back of twigs. Repeated defoliation and refoliation leads to depletion of nutrient reserves and such trees may dry off.

Corynespora leaf disease

On the leaves minute spots (pin head lesions) develop when the leaves are very young and enlarge with the growth of leaves. The spots are circular or irregular with a brown or papery centre surrounded by a dark brown ring and yellow halo.  The papery centre may fall off forming shot holes. When the disease is severe the leaf veins are affected and the toxin produced by the fungus spread along with veins giving dark brown colour. These appear as a fish borne or a railway track marked on a map. Severe infection on midrib causes leaf blight. The blighted leaves may remain on branches for some more time giving a burnt appearance. The branches which are exposed to sunlight are often more severely affected. Severe disease incidence leads to defoliation. Repeated defoliation and refoliation leads to die back of infected trees.

Colletotrichum leaf disease

Tender leaves during the first ten days of expansion are most susceptible to infection by C. acutatum. Infected leaves become distorted, turn black, shrivel and fall off leaving the petioles on the plant for a short period. Infection usually starts at the tip of the leaves and spreads towards leaf base. Often the leaf tip and margins becomes necrotic and the remaining portions of leaf become wavy. Spots on the infected leaves are small, circular and brown with a narrow brown margin surrounded by yellow halo. When the leaf becomes mature the spots became thick and raised forming conical projections. Sometimes the centre of the spots dry and fall off leaving a hole. Lesions are observed on the green portions of stem also causing shoot die back.

Bird’s eye spot

When infection is on tender brown leaves disease appear as dark brown water soaked spots. The leaves become black and get shriveled and distorted. Severely infected leaflets fall off leaving the petioles. When light green coloured leaves are infected minute yellow spots which later develop narrow reddish brown margin and a white papery centre are observed (Figure 5). This look similar to eyes of birds based on which the disease was named. Numerous spots are often observed on each leaflet. Mature leaves are not infected. Elongated brown stripe like lesions occur on infected mid ribs, petioles and green shoots. The underside of lesions on leaves develop a chocolate brown colour due to presence of numerous spores of the fungus. The spores are disseminated by wind, rain or irrigation splashes and movement of workers in nurseries. The defoliation due to the disease weakens nursery seedlings grown for budding.

South American Leaf Blight

The initial infection of the fungus is on 4 – 9 day old leaves on which conidia develop a greyish or olive green masses on the lower surface (Figure 6). The spores are disseminated by wind and rain splashes and cause rapid spread of the disease. The young infected leaves get distorted crinkled and shriveled and may often fall off. On the infected leaves that get mature, lesions containing pycnidia of the fungus are formed on the opposite side of the conidial lesions. On upper surface of fully mature leaves globose, carbonaceous and superficial stroma are formed often crowded or in rings around lesions. Ascocarps develop during wet weather and help in survival of the pathogens. Infection of flowers, fruits petioles and young shoots are also common. This may lead to die back of branches. Yield reduction upto 90 per cent has been observed due to South American Leaf Blight. Cultivated rubber clones are not resistant to the disease.



Stem diseases

Stem and the tapping panel are affected by many diseases. These may lead to tree loss and crop loss

Pink disease

Pink disease is the most serious stem disease of affecting mainly the young rubber trees. It is observed during the monsoon period. Wet weather and high relative humidity are favourable for disease development. The stem with brown bark is affected anywhere from the base to branches. In young plants the forking region is more vulnerable. The early symptom of infection is the superficial growth of mycelia which resemble cobweb and glistens in sunlight (Figure 7). The mycelium grows covering all around the stem, ramifies the bark tissue and damages it. When the pathogen damages the latex vessels, the latex exudes and the affected bark gets rotten and dries up. When the bark rots all around the stem translocation is affected. The leaves turn yellow and dry up. The dried leaves remain attached to the branch. New sprouts develop from below the affected region. When the infection progresses light pink coloured pustules appear in parallel lines all over the bark. Orange or red pustules develop on the portion exposed to sunlight. These contain the spores. In the advanced stage pink encrustations appear on the lower shaded regions of the affected branches. The disease gets its name from this stage. The pink encrustations contain the basidia of the fungus on which basidiospores are produced. During dry weather the mycelia remain dormant and became active when the favourable season returns.


Black stripe

The symptom appear first as sunken lesions which may rot later. Infected bark dries up and gets adpressed to the wood. On scraping the lesions vertical black lines extending into the renewed and untapped bark are observed (Figure 8). The disease got its name from this symptoms. The infected dried bark may fall off exposing the wood. This results in uneven bark regeneration and cause difficulty in tapping.


Patch canker

Patch canker disease is observed on any part of the stem on trees of all ages during the monsoon period. This is caused by Phytophthora and or Pythium sp. The symptom appear as exudation of latex or a purple liquid. The latex coagulates beneath the bark to form a pad at the loci of infection and consequently the bark bulges out (Figure 9). When the pad is removed a foul smell is emitted. The wood below the pad is often discoloured.


Dry rot

Dry rot, is caused by Ustulina deusta. This disease is observed near the collar region, on the main trunk of even on the branches during the rainy season. The symptom appears as exudation of latex. Later fruiting bodies of the fungus appear as soft circular to irregular, grayish white ear lobe like structures which later join together and became grey or black and brittle (Figure 10). The fungus penetrates the wood and spreads inside. As a result the wood becomes soft and the trees breakdown. Black double lines formed by the fungus are seen inside the wood. The fungus penetrate the bark through wounds and cracks and hence the disease incidence is more following heavy wind.



Root diseases

Let us now consider the diseases affecting roots of rubber trees. Root diseases occur only sporadically in India. Hence only a few trees in a plantations are affected. But if left neglected, they can cause considerable reduction in tree stand. Trees are affected irrespective of age. These diseases spread by root contact. The infected tree stumps serve as source of inoculum.



Brown root disease

It is the major root disease of rubber in India. It is caused by the fungus Phellinus noxius. It occurs in most of the rubber growing countries.

The symptoms appear as loss of shiny appearance of leaves drooping, yellowing and buckling (curving) of leaflets and retardation in growth of trees. The leaves later turn to reddish yellow and dry up. Occasionally flowering and fruiting is observed. The trees die bark. On the infected roots the surrounding soil and stones from a hard and thick coating (Figure 11). Yellowish brown patches of fungal rhizomorphs are visible on the surface of roots. Cortex appear as brown, mottled with white patches wood shows brownish discolourations and brown lines. On old infected stumps large, hard, brownish purple brackets of fungus with grey underside develop. The spores produced on these structures are blown over by wind which spreads the disease. The spread is more often through root contact with infected roots.

Purple root disease

Purple root disease is observed more in North East India though mild scale occurrence of the disease has also been reported from South India. The disease is caused by Helicobasidium compactum. The infection is observed both in nurseries an in the main field. Infected plants are weak with sparce crowns. The tap root will be badly damaged and its bark often sloughs off exposing wood. Several adventitious roots arise with which the plant survives. Formation of a brown spongy fruiting body of the fungus around the collar of the infected plants is a distinct symptom (Figure 12).



Insect pests

Several insect pests attack rubber plant. Some wood borers attack rubber wood.

White grubs are the most serious pest in the nurseries. They are larvae of cockchafer beetles. These are fleshy grubs with C shaped body (Figure 13). They feed on roots of nursery plants. The most common species is Holotrichia serrata. The damage is more in nurseries near virgin forests. The damage is severe when the grubs in the third instar are abundant. This instar lasts for about 100 days. The grubs feed mainly on the taproot. The leaves of infested seedlings remain yellow and are sheds off. The whole plant dries off, later.

Bark feeding caterpillar

It is the most important pest of mature rubber tree in India. The common bark feeding caterpillar is Aetherastis circulata. The caterpillar of A. circulata build galleries on the trunk using chewn bark, faces and silk and live inside the galleries. Whenever they feed deeper, latex vessels in the bark break and latex oozes out. Later, pathogenic fungi like Phytophthora or Pythium enter through the wounds and cause rotting of tissues which may even lead to breakage of branches.

The caterpillars are brick red with broad head and thorax and a tapering body and 1.5 to 2cm long (Figure 13). The adults are white moths with dark dots on the forewing. The caterpillar stage lasts for about a month. This is a dry season pest. It disappears with the onset of rains. To control severe infestation, Fenvalarate dust (Fenval 4D) should be applied at the rate of 7 kg/ha using a power duster. Spraying fenvalerate 0.02% (Tatafen/Arfen 20 EC, 1ml/L) on to the trunk is also effective.

Termites

Termites live inside the soil. They sometimes attack the outer bark of rubber trees. The common species is Odontotermes obesus. They may also cause damage to dry mulch, shade baskets and dry or partially dried trees. Termites can be controlled by drenching the plant base and spraying on the mulch with chlorpyriphos 0.1% solution (Dursban or Tataban or Classic 20 EC, 5ml/L)

Slugs and snails

The slugs Mariaella dussumieri and snails Crypotazona bistrialis damage young rubber. They climb on the stems and feed on the terminal and auxiliary buds arresting growth of young plants. Shoots become compact duster with club shaped appearance (Figure 14). Occasionally they climb on mature trees and drink latex from the tapping cut and collection cups. They usually feed during night and during the day time remain concealed under multch or other materials lying on the ground. Their presence can be detected by the spring like excreta and glistening lines of dried slimy secretion.


Non parasitic maladies

The damage to rubber trees caused by sun light, cold, wind, fire, hail storm, water logging and deformities which are not caused by parasites are induced in this section.

Sun scorch

Damage due to sun scorch are more when the rubber plants are young. When the canopy closes, the plants are protected from sun scorch. Very small nursery plants get sun scorch at the collar region resulting in girdling. Such seedlings dry up.

In young plants the bark near the collar region facing south or south west gets damaged by sun scorch. The bark in that region dries up in budded plants the snag dries up leaving a depression above the bud union. When this faces the south or south west sun scorch becomes prominent. The bark dries up in the shape of a spearhead and hence the damage is also called spear head damage (Figure 15). Saprophytic fungi colonise the dried portion. If left untreated the tree may break at that region. Application of black panel protectant and wound dressing compounds on bark also leads to sun scorch if exposed to direct sunlight.

In the nursery, mulching prevents damage to the collar region. Providing shade basket or plaited coconut leaf provide protection in the first year of planting covering the stem with thatch grass or hey also is effective. From the second year whitewashing of the brown portion of the stem should be done during November/December to protect the plants from sun scorch in the summer. Lime is effective for white washing although china clay also can be used. The sun scorch affected portion of the stem should be scraped and washed with a solution of mancozeb 0.75% (Dithane/Indofil M 45 10g/L) and the wound should be covered with a wound dressing compound


Sprayers
The lever operated knap sac sprayer has a metallic or moulded plastic tank of 10 to 15 lire capacity (Figure 16). The pump assembly is fixed inside or outside the tank. While spraying it is pressurized by operating a lever. The spray lance is connected to a cut off valve to which the delivery hose is connected. The lance may have single or multiple nozzle. Knap sac sprayers used for spraying weedicides have a pressure regulator attached to the lance and are fitted with flood jet nozzle.

Rocker sprayer (Figure 17) is used in rubber plantations mainly to spray Bordeaux mixture. The sprayer has a pump assembly, operating lever a pressure chamber, suction and delivery hoses all fitted on a frame mounted on a wooden platform. When the lever is rocked, pressure (up to 36 kg/cm2) builds up in the pressure chamber. The suction hose is used to such the spray fluid from a vessel. The delivery hose is long (usually more than 30 m) and is connected by a trigger valve to a spray lance. Long bamboo lances of 2m length are used in rubber plantations. One or more nozzles may be set in different direction to ensure wide coverage.

Power operated sprayers

Use of power operated sprayers is more economic in rubber plantations as the spraying is rapid and needs less labour. The spray fluid is pressurized by using hydraulic, gaseous or centrifugal energy in such sprayers. Portable high and low volume power sprayers are used in rubber plantation.

The portable high volume sprayer (Figure 18) consists of a pump, air chamber, pressure regulator pressure gauge, strainer, filters, control value distribution system and power source which usually is a four stroke 3.5 to 4.5 HP petrol/kerosene engine. The pump of sprayers used on rubber plantation is usually double piston one with a discharge of 24 litres per minute. The delivery line may be single or double with heavy duty hose with a spray gun attached to it. The spray fluid can be discharged upto a height of 5 meters. So, young trees can be sprayed from the ground. On mature trees, the entire canopy can be covered from the first branching region.

Portable low volume power sprayer consists of a power source blower, pesticide tank and nozzle. The spray liquid is atomized by delivering it into air of high velocity. Such sprayers are called mist blowers or air blast sprayers.

The knap sack mist blowers are usually fitted with two stroke engines of 35 or 70 cc cylinder capacity. The sprayer is mounted on a vertical frame and is provided with belts and cushions to fit on to the back of an operator (Figure 19). The machine drives a fan which delivers 2.7 to 9.1 m3 of air per minute at a velocity of 175 to 320 km/h. The spray liquid is delivered to the air stream through a nozzle. The spray tank has 7 to 12 litre capacity. A part of the air from the blower is diverted to the spray tank to form an air cushion to push the spray fluid, through the delivery hose to the nozzle by gravity. A rotary pump and tree lance are required for vertical spraying on trees.

The micron sprayer (Figure 20) is the most widely used spryer on rubber plantations. It has a four stroke petrol or kerosene engine which develops 4.5 hp at 3000 rpm. The sprayer has a spray tank pump, delivery hose, nozzle, atomizer and a blower. The micron sprayer can deliver the fungicide mixed in oil up to a height of about 20 meters. The fungicide mixture is delivered near the atomizer by pumping it from the spray tank. The atomizer which rotates at a high speed due to the high velocity air from the blower splits the spray fluid into fine droplets and are carried up by the air blast from the blower. The micron sprayer is carried by four workers in between each planting rows at a speed of 2 to 3 km per hour. Spraying is effective during the morning when the air is still. By noon air movements occur and the spray particles get drifted. Hence desired height of spray cannot be achieved. One micron sprayer can cover about 5 hectares of mature plantation in a day using six labourers. The sprayer should be cleaned every day after use by spraying spray oil alone, keeping it in an open space. Care should be taken to maintain the engine oil at the specified level. Tractor mounted micron sprayers have larger blower. The power is provided by the pressure take off of the tractor. On steep terrains the mobility of tractor is difficult. Hence its use is limited to plain areas and gentle slopes.

Power operated dusters are commonly used in rubber plantations. The micron sprayer can be converted into a micron duster by altering the duster unit (Figure 21) which consists of the hopper, a reduction gear to drive the agitator inside the hopper, a metering devise to adjust the discharge of dust. The dust is fed from the hopper directly into the air current from the blower which is delivered through a vertical tube. The dust is blown up to a height of 25 meters. A micron duster can cover 10 to 12 hectares in a day.