One of the prime objectives of forestry is to grow forest crops free from diseases and to obtain a profitable harvest. In the event of an outbreak of a disease, control measures that have to be adopted should be economically justifiable and ecologically sustainable.
Direct Measures to Control Forest Disease:
i. Sanitation:
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Sanitation is the process that reduces or eliminates the initial inoculum from which the disease develops. Such inoculum may be present either on principal hosts or on collateral or alternate hosts. Corlicium salmonicolor, the cause of pink disease, occurs on rubber and many other hosts and attacks Eucalyptus planted in sites under suitable conditions of development of the disease. Control by sanitation requires an early detection of the disease in the forest and taking suitable steps to remove the disease inoculum before it can spread the disease.
ii. Eradication:
Eradication may be done mechanically or through use of weedicides, thereby keeping the other host plant of economic importance free from the disease. In natural forests, fungi normally remain endemic. Root and butt rot fungi have restricted growth on roots but quickly colonise residual roots and stumps after clear-felling.
The large inoculum potential thus builds up and becomes a potential threat to susceptible hosts later planted in such areas. Control of root rot may be obtained by removal of residual stumps and roots prior to planting either by raising mechanised plantations or through extraction by physical means.
iii. Isolation Trenches:
Once a root disease establishes in forest, it spreads centrifugally from the infection centre to adjacent trees. For pathogens which do not have a free spread through the soil, the disease is mainly communicated to adjacent healthy plants through root contact or root graft. In such cases, the spread of the disease can be checked by isolating diseased plants by trenches.
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iv. Chemical Control:
Chemical control of forest diseases is generally prohibitively expensive and, therefore, not practicable except probably in nurseries and plantations where the incidence of the disease and the damage are high. Fungicidal chemical may act as prophylactics in preventing infection. A systemic fungicide may be used and the chemical is distributed throughout the system of the host.
A fungicide will have its desirable effect if it attacks the pathogen at the weakest point in its life cycle and at the same time proves nontoxic to the host at the dosage used. The fungicide must adhere well to produce a lasting effect. For this, stickers as well as spreaders are added to the fungicide. To control a disease of aerial plant parts by a fungicide, the timing of application is important. Mancozeb, Copper oxychloride, Carbendazim, Tridemorph and Triazoles are commonly used.
Indirect Measures to Control Forest Disease:
i. Choice and Improvement of Site:
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Natural stands are adapted to the site they occupy. Such forests, therefore, generally grow healthy as long as the stands are properly managed. Suitable selection of site is important for plantations, particularly of exotics, which are far remote from their natural range of distribution. Good site is necessary for maintaining stand vigour to resist attack due to diseases.
An extreme case of species conversion may be seen in the high rainfall areas in Karnataka and Kerala States in South India where semi-evergreen and evergreen natural forests are clear-felled and planted with Eucalyptus. The situation becomes aggravated with large scale mortality in Eucalyptus due to pink disease resulting in large gaps in plantations.
ii. Choice of Species:
Natural forests have inherent slow growth and low productivity. Also, such forests are of mixed composition where only some species are economically valuable. Indigenous species are often not capable of making full economic use of the sites. Exotics should possess the ability to thrive in their new environment for which a great deal of care is needed in selection of suitable species. Plantations may be raised either pure or in mixer.
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A pure stand is, however, ideal for a pathogen to build up to epidemic proportions since infection is direct and rapid from tree to tree. Mixed stands may be raised in plantations where the species can be in intimate, strip, group or block mixers. In using clonal stocks in planting, it is advisable to plant a number of clones in mixture and limit the area occupied by each clone.
iii. Silviculture and Management Practices:
Natural forests which are worked on sound silvicultural practices and properly managed, do not normally suffer from any serious disease. Fire, which is a regular feature in forests, is the principal cause of injury to trees through which decay fungi establish in the tree. In an uneven-aged stand, suppression may occur to a certain degree to the understory crop under the shade of dominants. In forest raised for timber, a high incidence of decay in the heartwood causes serious loss in yield of timber.
In stand with high incidence of decay, it is, therefore, advisable to lower the rotation age. Infestation by dwarf mistletoe is favoured by selective cutting while in cleared cutting, the infestation is less. A pure even-aged stand may suffer from injury due to frost and from excessive insolation resulting in sun scorch. An even-aged crop is also liable to wind damage which is accentuated by thinning and pruning.
iv. Cultural Practices:
Timely thinning with a view to provide spacing is therefore important as it increases plant vigour which in turn reduces damage from diseases. Diseased trees are likely to be randomly distributed for which reason thinning has to be irregular. On the contrary, stumps left after thinning in conifer stand may be colonised by Fomes annosus.
Such stumps serve as infection centres for spread of the disease in plantations. It is also done for trees in parks and avenues to remove dead and diseased limbs create openings for sun light or make clearance for overhead communication lines.
v. Biological Control:
Biological control involves the use of an innocuous organism to prevent attack by a pathogenic organism. Control of Armillaria mellea root rot in tea bushes is possible. Likewise, colonisation of fresh conifer stumps soon after thinning by Peniophora gigantea, an innocuous fungus, prevents stumps colonization by the pathogen Fomes annosus.
vi. Resistance Breeding:
Resistance is the ability of an organism to hinder the development of a disease. There may be active resistance where the host reacts to the attack by a pathogen or passive resistance due to qualities inherent in the host. A pathodeme is a host population in which all individuals have a given resistance in common, while a pathotype is a pathogen population in which all individuals have a given pathogenicity in common.
If in a series of inoculations, the amount of disease shows a differential interaction between pathodemes and pathotypes, the resistance and pathogenicity are called vertical, which involves mechanisms which are within pathogen’s capacity for change. In the absence of any such interaction between pathodemes and pathotypes, the resistance is termed horizontal, where the mechanisms involved are beyond pathogen’s capacity for change.
In vertical resistance, the inheritance is governed by single gene and, therefore, easy to manipulate in a breeding programme. Horizontal resistance is controlled by polygenes and, therefore, difficult to manipulate genetically. The effectiveness of horizontal resistance does not break down. Vertical resistance, however, is liable to breakdown.
The objective of disease resistance improvement is to select or breed trees to establish a host-parasite relationship to yield an economic harvest. The freedom from infection may either be due to escape or to inherent character in the presence of genes for resistance in the host or absence of gene for infectivity in the parasite.
The procedures for resistance breeding include selection of such disease-free individuals and testing their progenies by raising them in a heavily diseased area or through inoculations which will eliminate the escapers and ensure the heritability and selection of the resistance factor. Artificial inoculations, where massive spores or mycelium are used as inoculate; can be so severe that they assess only immunity and not relative resistance.
This should, therefore, not be relied upon fully. During selection, due consideration should be given to other desirable characters in the tree necessary for tree improvement. Parents and progenies should be tested against all known biotypes of the pathogen by exposing to natural infection and to inoculation. The improved stock may be out planted providing means of testing at the same time resistance under different environmental conditions favourable for disease development.
Individuals that have not developed disease resistance can be salvaged during improvement cuttings. Further propagation of these trees may be done by establishment of grafts in seed orchards. Intra- specific crosses between the selected individuals may increase the degree of resistance in the progenies. Seed orchards yielding such progenies will thus form valuable basis for further propagation of a resistant stock.
Though genetic resistance has been recognized in a large number of forest tree species, remarkably little is known about the nature of host resistance. Hairiness in leaves can provide complete protection against some virus vectors such as jassids and whiteflies. In Castanea, high concentration of pyrogallic tannins in the bark of genotypes resistance to Endothia parasitica may be responsible for retarding the growth of the fungus. Enzymatic activity is responsible for resistance of oaks against Microsphaera alphiloides.
A close correlation between bark moisture and disease development exists in canker diseases in trees, as in willow due to Cryptodiaporthe; in poplar due Fusarium and in western hemlock due to Cephalosporium. In absence of much data on the nature of host resistance, however, breeding forest trees for producing disease resistance clones is a potential way for disease control. In Populous, significant progress has been possible because of the variation in disease resistance between and within the natural species, wide range of genotypes and ease of vegetative propagation for purpose of clonal multiplication.