In this article we will discuss about various plant diseases and its control.
1. Diseases of Teak:
i. Bacterial Wilt:
Bacterial wilt in teak is caused by the pathogen Psedomonas solanacearum which attacks species belonging to as many as 17 families in the tropical and sub-tropical parts of the world. In the initial stages, small patches of brown tissue appear between the veins of the leaves.
They may form a network to give a scorched appearance to the leaf. Wilting of the leaves occurs and even young plants may wilt. As a result of this wilting, the plant up to pole stage becomes yellow. Discoloration occurs in vascular tissues of the roots. The main root and lower part of stem become soft, turn black and may decay in advanced stages.
Nurseries must have good drainage and they should not be located in waterlogged sites. Plants that have previously been affected by this wilt must be removed and burnt. Weeding must be done carefully to avoid root injuries which form the main avenue for infection. It is advisable to sterilize the soil of nurseries where this disease is prevalent. In extreme cases, the site may be abandoned and a new site selected.
ii. Leaf Rust:
Leaf rust in teak is caused by Olicea tectonae which occurs all over its natural range in India, particularly in teak nurseries. It attacks the leaves generally between September and February. Uredia develop profusely and rarely produce Telia. They cover the lower surface of the leaves with a rusty coat while the upper surface has a grayish appearance.
Survey conducted for rust disease in teak indicated that plantations under high humid conditions suffer more damage. Rust severity, rate of disease development and maximum defoliation is higher in 3-6 year old teak plants. In forests, opening of the canopy helps to prevent rust disease spread. Tridemorph @ 1ml/lit is effective and two sprays at an interval of 15 days may be followed.
iii. Powdery Mildew:
In India, powdery mildew in teak is caused primarily by Uncinula tectonae. Phyllactinia corylea and Phyllactinia guttata also cause damage. Pathogen attacks the upper surface of the leaf and adheres to it by serrated appressoria. Haustoria penetrates up to only the upper epidermal cells. Conidia are dispersed by wind, particularly on hot days and cool nights.
The infected leaves are coated with a dull white mycelium some time dotted with black fruiting bodies. Soon after attack, most leaves become infected and they may fall down. However, no serious damage is caused though the process of photosynthesis is retarded due to the coating on the leaves. The affected leaves are shed during the normal period of leaf fall.
iv. Phanerogamic Parasites:
In India, the mistletoe Loranthus longiflours, Dendrophthoe falcate are common on teak trees in different parts of the country. Initially, the infection causes mutilation or death of branches or tops of the trees. In case of heavy infection, the trees, particularly up to pole crop may even die or growth is severely retarded. Sanitary cutting of the affected branches below the point of infection is useful. Heavily infested trees may be felled. Spraying of chemical herbicides in early summer when teak is leafless is recommended.
2. Diseases of Sal:
i. Polyporus Root Rot:
In sal, polyporus root rot occurs almost all over the range of the host species. Though its occurrence is less common under relatively drier conditions and fairly common in localities with heavy rainfall, it has been recorded from all states bearing natural sal forests.
Polyporus shoreae has an immense capacity for infecting both healthy and injured or damaged roots. Due to the infection, the tree may die back; its top may become broken or it may even be uprooted in severe windstorms. In later stage, it also causes white pocket rot in the bark and sapwood though the heartwood largely remains unaffected. Large trees show symptoms of top dying as their remaining healthy roots are unable to meet the water requirement of the crown. This dying of branches gradually extends downwards leading the death.
Control burning reduces soil moisture and checks the growth of weeds which ultimately helps in lowering the incidence of this disease. All dead, dry, diseased and dying trees must be removed from sal areas where the incidence is high.
ii. Heart Rots:
In high sal forests, heart rots establish in trees from the trunk and as butt infections. In Coppice sal forests, infection usually establishes on the stool and progresses up the tree when the heart rot develops in coppice shoots. Heart rot in sal is caused by Fomes caryophylli, Fomes fastuosus, Hymenochaete rubiginosa, Trametes cubensis and Trametes incerta. Punk knots, swollen boles and presence of sporophores, wounds, branch stubs, dead branches etc. are the symptoms of heart rot disease.
Frost Injury to trees may be lessened by suitable adjustment of the canopy so that large gaps are not created. In uneven aged stands, suppression is prevented by canopy adjustment. Fire control measures help in preventing fire injury, thus lowering the incidence of heartwood rot. All trees injured by fire and frost and those with significant symptoms of decay must be removed in cultural operations. Trees to be felled must be lopped so that while falling their branches not damage the standing trees.
Felling debris consisting of lops and tops must not be left in the forest. Severely affected stands may be worked on basis of the pathological rotation age. Pathological rotation is the age at which the progress of decay is equal to the rate of formation of new wood and this may differ from the rotation age.
Coppice forests of sal are severely affected by heartwood rot decay due to the vulnerable stools. Prevention and control of this disease in coppice stands include: lowering the rotation age; dressing the stool with chemical; and girdling old abandoned stumps so as to kill them.
3. Diseases of Shisham:
i. Wilt Disease:
This disease is caused by the soil inhabiting fungus Fusarium solani. The adverse effects are yellowing and death of leaves and subsequent defoliation. Young affected trees die off after an intense attack. Waterlogging favours the incidence of this disease. Silty and clayey soils favour growth and infection by this pathogen.
Selection of the proper site for raising shisham plantations is the most important preventive measure. The soil should be light textured, with adequate soil moisture and good drainage. Soil drenching with carbendazim, around the infected trees is recommended.
ii. Ganoderama Root Rot:
Disease is caused by Ganoderma lucidum both in natural forests and plantations. The pathogen spreads rapidly in the system of trees growing on light textured soils. The trees may die within a short span. On the other hand, the spread is relatively slow in heavy textured soils. It spreads through root contact and hence it damages pure plantations very quickly, resulting in the death of trees.
The diseased trees present a stag headed appearance and die after a few years. Selection of suitable site, removal of the affected individuals and sanitations are the recommended measures. This disease can be prevented by removing residual roots and stumps before raising a new crop. Isolation trenches also help to prevent the spread of this pathogen.
Ganodrma lucidum also causes root rot in Dalbergia latifolia natural forests as well as plantations. The symptoms and control measures are same as that of Dalbergia sissoo.
4. Diseases of Khair:
Ganoderma Root Rot:
Root rot caused by Ganoderma lucidum is serious disease in many khair plantations and coppices. This pathogen is normally endemic in natural forests though it does not cause any serious adverse effects there as a certain level of resistance is noticed. There is a poor development of lateral roots in the early stages.
The infected plants are killed in the first few years of a plantation and their small mass of roots disintegrates quickly with no effect in the further spread of the disease. With increasing age, the chances of lateral root contact become more. Infection is through root contact and injured surface in plantation.
Mechanized soil working and removal of residual roots and stumps are considered major prevention and control methods. Isolation trenches are dug for preventing root contact between the healthy and diseased individuals. These trenches are about 1 m deep, 1.5 m long (across the line) and up to 0.5 m wide.
5. Diseases of Neem:
Nursery Diseases:
i. Rhizoctonia Leaf Web Blight:
This disease affects the foliage of young plants. It is caused by Rhizoctonia solani. Development of greyish brown blotches that increase in size with the advancing fungal hyphae and then engulf the entire leaf blade. The fungal hyphae joins the infected leaves giving the appearance of spider’s web and hence the name of the disease. The leaflets or the entire pinnae become detached prematurely. However, the detached pinnae remain clinging to the stem for quite some time, being bound together with the fungal hyphae and presenting a web by appearance.
ii. Colletotrichum Leaf Spot and Blight:
This disease is caused by Colletotrichum gloeosporioides which initiates leaf spots that increase rapidly in size, covering large areas of the leaf surface. The affected leaves present a blighted appearance and there is premature defoliation. The growth is retarded due to the premature defoliation. Under nursery conditions treatment with Bavistin @ 1% is highly effective.
iii. Alternaria Leaf Spot and Blight:
This disease is caused by Alternaria alternate which affects the foliage of seedlings.
iv. Pseudocerocopora Leaf Spot:
This disease is caused by Pseudocerocopora spp. It produces brownish infection spots interspersed with white patches. The fungus sporulates on the upper leaf surface and produces conidia which appear grayish in mass. The heavily infected leaves turn pale and are shed prematurely.
v. Bacterial Leaf Spot:
Disease is caused by pseudomonas viticola.
vi. Other Diseases:
Leaf spot (collectotrichum capsic), blight and stem rot (Sclerotium rolfsii) and leaf wilt (Fusarium solani).
6. Disease of Sandalwood:
Spike Disease:
The spike disease occurs only in sandal trees growing in south India. It has not been reported from other regions or other species of the genus Santalum. The disease is also prevalent in Tamil Nadu. There is a drastic reduction in the size of the leaves, both in length and breadth. The internodes are shortened so that the leaves become crowded together. The leaves show a tendency to stand out stiffly from the branches.
This gives a rosette appearance to the affected branches. The leaves appear like spikes on the branches and hence the term spike disease. At times, the infected shoots tend to show continuous apical growth, becoming drooping and giving a pendulous appearance to the spike. The affected trees may perish after a period of 12 to 24 months.
A branch or tree that is fully affected by the spike disease does not bear flowers or fruits. Even if flowers appear before the branch is affected by this disease, they may develop into fruit but seeds may abort and not form fully. Flowers exhibit phyllody in infected branches. There is a significant accumulation of starch in the branches and leaves of diseased sandalwood trees.
Recent researches suggest that spike disease is caused by Phytoplasma like organism. This disease belong to the yellow group of plant diseases in which there is reduction in leaf size, development of witches broom and phyllody. Spread is also through weak-flying insect vectors that may be carried over long distances by strong winds. These insect vectors include leaf hoppers Jassus indicus and Moonia albimaculata.
The following measures should be adopted for prevention and control:
i. Sanitation:
The affected individuals or their parts may be removed and burnt.
ii. Host Selection:
A number of host species impart resistance to spike disease in sandalwood. It may be raised in association with such hosts (Redgram or Neem).
iii. Insecticides may be sprayed to control.
iv. Genetic Resistance:
Spike-resistant sandal may be evolved through selection of apparently resistant sandal trees in a heavily spiked area. There are two types of resistance to spike disease in sandal: Autogenic resistance which is largely independent of the nature of the associated host plant. Acquired resistance is built up by sandal if it parasites certain species of hosts and certain environmental conditions are provided.
7. Disease of Casuarina:
Stem Wilt:
Trichosporium vesiculosum causes stem wilt in casuarina. Numerous spores are produced by the pathogen which is dispersed by wind. Wounds caused to the stem or branches serve as channels for infection. The spores carried by wind attack the trees through these wounds or injuries.
The pathogen spreads to the shoot and root system of the tree after infection. After this, there is secondary spread of the pathogen in the plantation through root contact and root grafting. Casuarina trees have a well-developed network of roots and thus the disease spreads very rapidly in the plantation.
Symptoms of wilting like paling of needles which dry up completely after some time. Formation of spore mass beneath the bark results in the development of blisters on the bark. After some time, these blisters burst or rupture, exposing the black powdery mass of spores on the upper surface of the bark and sporulation also occurs on the roots.
Diseased trees may be found in groups as the secondary spread of this pathogen is by root contact. Prevention of primary infection by minimising all forms of injuries to the trees is important. Isolating the diseased trees by means of trenches and groups of affected trees may thus be isolated. All the infected trees must be removed as early as possible.
8. Diseases of Eucalyptus:
1. Pink Disease:
Eucalyptus pink disease is caused by the pathogen Corticium salmonicolor. Repeated die back in young plants occurs which lead to their death. Infection occurs, in trees which may have escaped being affected by this pathogen initially, at a later stage in the form of cankers.
There occurs girdling of branches and the main stem of young plants towards the later part of the rainy season. The parts lying above girdled portion may die, causing many epicormic branches to develop. One of these becomes the leading shoot which is again infected and killed in the next season. High summer rainfall (over 250 cm) and warm tropical conditions may cause epidemics.
The pathogen Corticium salmonicolor occurs in four growth forms on the host tree viz.:
i. Pustule:
These appear as pinkish cellular sterile bodies having a diameter of up to 0.1 cm on the branches and main stem soon after infection at the commencement of the monsoon season.
ii. Cobweb Mycelia:
These are thin white arachnoid mycelia produced on the bark and main stem by the infected parts. They originate either from the pustule or from the edges of the pink incrustation.
iii. Necator:
These are orange to reddish fruit bodies having a diameter of about 0.2 cms. They develop on the side of the attacked branch which is exposed to the sun.
iv. Pink Incrustation:
These are light pink, thin incrustations on the bark occurring as the perfect form of the pathogen. This develops in autumn on the shady portion of the stem and branches. Later on the incrustation spreads all around the stem.
Individuals showing a fair degree of resistance to this disease in an infected plantation could be identified and selected for breeding programme for introduction on a large scale. Eucalyptus torelliana is found resistant to this disease and may be used in crossing programme. Spraying of systemic fungicides like Trade morph @ 0.1% may be done on young plantations before and during the rainy season. Diseased individuals must be removed from the plantation as early as possible.
2. Seedling Diseases:
i. Seedling Blight:
Seedling blight is caused by Cylindrocladium clavatum, C. quinqueseptatum and C. scoparium. Damage to the leaves and in extreme cases there may be complete defoliation. Premature defoliation slows down the growth of the seedlings.
ii. Cercospora Eucalypti:
It causes damage to the leaves of Eucalyptus seedlings in the nursery.
iii. Puccinia Psidii:
It attacks the foliage and young twigs of seedlings up to 1 year in age. It may destroy the whole nursery crop.
iv. Botrytis Cinera:
It causes damping off and leaf spots in nursery.
v. Oidium Eucalypti:
Powdery mildews on Eucalyptus seedlings growing in the nursery are caused by this pathogen.
3. Ganoderma Root Rot:
It is caused by Ganoderma lucidum in different species of Eucalyptus. It is prevalent in areas where the previous crop is affected by Ganoderma. The residual stumps and roots serve as infection centers. It then spreads by root contact. Removal of residual stumps and roots of the previous crop before planting prevents disease.
However this is possible only in the case of mechanized plantations. Eucalyptus may be raised in mixture with resistant species so as to prevent the spread of the disease through root contact. Affected trees may be isolated by digging isolation trenches around them.
4. Phytophthora Root Rot:
Root rot in many tree species including Eucalyptus is caused by Phytophthora cinnamomi. It spreads through roots or even soil particles and results in decay of the root system.
5. Canker:
Diaporthe cubensis causes canker in various species of Eucalyptus, though some species are resistant to this pathogen. Young plants are attacked at the root collar causing discolouration, girdling and even death. There is development of attacks around the base of the dead branch stubs.
Initially there develops longitudinal cracks in the bark, while open cankers form later on having a length of more than 1 m. There is exudation of gum from these cankers. In tracts prone to this pathogen, only resistant species should be planted. The diseased individuals must be identified, isolated and removed from the plantation as early as possible.
6. Gummosis:
Gummosis or the exudation of gum kino from Eucalyptus trees may be due to severe frost which causes injury to the cambium, resulting in gummosis. Fires too have been attributed to injure the cambium, thereby causing the exudation of gum kino. Due to this disease, there occurs swelling and splitting of the bark at various parts.
This is accompanied by the exudation of gum kino which is a shining, golden brown coloured viscous to semi- viscous liquid. It hardens as brownish or red lumps soon after exposure to air. There is lateral displacement of the medullary rays and the ray cells are broadened at the region of the injury. The right species of Eucalyptus must be planted on a particular site. Control burning helps to minimize injury to trees by fire.
9. Diseases of Bamboos:
1. Leaf Diseases:
i. Leaf Blight:
It is caused by Rhizoctonia solani and the infected leaves have profuse mycelial growth of the fungus. It causes premature defoliation.
ii. Leaf Rust:
It is caused by a number of pathogens such as Dasturella divina, Dasturella bambusina, Puccinia xanthosperma and Tunicospora bagchii.
iii. Leaf Spots:
Leaf spots are caused by a number of pathogens such as Ascochyta bambusina, Balladyna butleri, Corticium koleroga, Drechlsera rostrata, Phyllachora bambusae, Phyllachora graminis, Phyllachora malabarensis and Phyllachora shiraina.
2. Bamboo Blight:
This is one of the most destructive bamboo diseases in India caused by the pathogen Sarocladium oryzae. The pathogen also causes sheath blight in rice and bamboo acts as collateral host. Burning of debris in clumps in April and May before the onset of the rains, application of certain chemicals as soil drench and removal of blighted culms are recommended.
3. Rhizome and Culm Rot:
i. Rhizome Bud Rot:
This disease is caused by Fusarium and Pythium spp.
ii. Basal Culm Rot:
This disease is caused by the pathogen Fusarium moniliforme and occurs in culms that have just emerged from the soil.
iii. Culm Rot:
This disease is caused by two species of Fusarium and develops on wounds of culms caused by the sap sucking insects.
4. Rhizome and Root Rots:
i. Ganoderma lucidum:
This is a serious root rot pathogen affecting bamboos and other trees. It is wide spread in Bambusa bambos in which there occurs lateral spread of the disease through root contact.
ii. Merulius Eurocephalus:
This pathogen causes severe losses in various species of Bambusa and Thyrsostachys. Sporocarps develop at the base of the culm and also on humus around the infected clumps.
iii. Poria Rhizomorpha:
It occurs as a saprophyte in the soil, forming abundant rhizomorphic stands on decaying roots and other vegetable matter. It becomes a parasite on poorly drained soils, spreading through rhizomorphs.
5. Decay in Bamboo:
A number of pathogens cause decay in bamboos both in standing culms and also in bamboos in storage and use. They also affect the rhizomes. Important decay causing pathogens in bamboos are Amauroderma rugous, Daedala Flavida, Fomes spp., Ganoderma lucidum, Guepinia ramose, Polystictus spp., Sphaerostilbe spp., Trametes corrugata and Tremellodon gelatinosum.
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