Timber producing forests grow mainly in areas of ample precipitation and the major species of such trees in Tamilnadu are Teak, Sandal, Rose wood etc that play important role in lives of people. The non-timber forest products mainly medicinal plants are also equally given importance, as they are the native sources of the forests. The biotic and abiotic interference has sometimes led to the diseases of the trees and growth retardation. Mostly there is direct attack by the insects, fungi etc but sometime disease has been made to explore symptoms of various diseases of important tree species analyse and suggest measures to prevent and control the diseases in order to save the forest wealth for the man kind.
To analyze and control the diseases and insect pests it is necessary to study their symptoms and causative agents. Based on the symptoms and causative agents the identification of diseases will be very helpful to control the insect or disease problems to maintain the health of forest from seedling to tree stage. In this study the problems were divided in to three parts viz.,
Hence, this study will be useful as a field guide to identify and control the insect and disease problems.
This part is emphasized the types of disease symptom, causes and identification. It also includes the role of biotic and abiotic factors that caused the diseases in forest trees. This guideline will be very useful to identify the diseases through symptoms for timely management of diseases.
The word pathology derives from the Greek word “pathos” meaning a suffering. Thus pathology defined as science of disease. In forestry this sciences used in understanding the nature of diseases in plants. Pathology also helps exploring the prevention and control measures of tree studies. The objective of this brochure is to explore of forest diseases, causes and identification with special reference to the important trees of Tamilnadu. The suggestive measures or treatments for controlling the diseases would be very useful to farmers, foresters and researches to maintain the health of forest from seedling to tree stage.
In general there are two types of diseases, i.e., (i) infectious and (ii) non – infectious.
Infectious diseases:
These diseases are the introduced ones, which are caused by infectious agents like fungi and bacteria.
These diseases are characterized by the absence of any infection agent and are generally caused by the external factors like edaphic factors and climatic factors like soil pH, soil minerals, forest, lightening and sun scorch etc.
Symptoms are the ultimate indications of abnormality of forest trees or plants. Consequently a pathologist relies upon a basic knowledge of normal tree anatomy and physiology in order to interpret pathological alternation of tissue and function. Because the complex of cells which compose the roots, stem and crown of living tree can be altered by infective agent as it accesses nutrition from the plant. Once the infective agent enters in to the tree it began to derive the substance and alter the host physiology in various ways. At least five vital process can be affected viz.,
Before diagnosing a tree for disease identification it is important to observe the tree thoroughly. First the tree must be observed morphologically in the terms of colour and size of the leaves, branches, bark, butt and roots. If necessary the bark may be peel off to observe the sapwood and heartwood. If the moisture content of the sapwood is less, or leaves or the needles fall on shaking are the reasons of drought or the tree under the problem of water translocation. These characters should be thoroughly noted otherwise there might be chance for misidentification. The fungal fruiting bodies if seen on the stem or root may be observed, collected and identified. The fungal fruiting bodies having the specific feature of pores beneath the surface it must be treated as a pathogen otherwise it could be beneficial ecto – mycorrhizal fungi. The feeder roots of the tree is the best observation for root rot, insect attack etc. If the trees feeder are dichotomous, thick and coloured are the identification of healthy roots and if are thin, loose and dirty coloured it is a diseased roots and must be treated for the problem.
Identification or diagnosis of disease on the visible alternation in plant morphology. The major types of plant characters are tissue alternations namely necrosis (death), hepertrophy (overdevelopment) and atrophy (under development). Symptoms are also indicators of infective agents that involve in some localized areas while others are systemic and are expressed by the entire tree. For. e.g. Stem cankers and leaf spots are localized necroses of respective tree parts while systemic wilts are symptomatic of a malfunction of the water conducting system of a tree. But multiple symptoms can sometimes overlap to give the illusion of systemic action by the pathogen. For e.g. a canker may completely girdle a stem causing death of stem, branches and foliage beyond that point. In this and other like instances, the canker is a primary symptom associated with the direct influence of the pathogen while the distal necroses that follow are secondary symptoms, those arising indirectly and apart from the locus of the infective agent.
As said earlier, the major alternations of plant morphology are the basal identification for plant diseases. The necrosis, hypertrophy and atrophy are the major alternations of the diseased forest tree and express the symptoms according to the vital changes.
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Symptoms |
Causes |
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Canker |
Death of cambium, ooze out of resins or gum to heat or mechanical injury |
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Leaf spots |
Death of a few or many leaf cells angular or circular spots on the leaves due to fungal pathogen, air pollution, or toxic chemicals |
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Decay, Wood rot, Root rot |
Death of parenchyma cells in wood of roots and stems of living trees in white or brown colour due to fungal pathogens |
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Vascular wilt |
Death of parenchyma cells in wood of roots and stems of living trees by fungal pathogens |
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Blight |
Sudden dying of all part of the tree and blight in the leaves due to fungal pathogen |
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Die back |
Sudden dying of tree from top to bottom due to fungal pathogen, sudden temperature change, air pollution or drought. |
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Symptoms |
Causes |
|
Witches brooms |
Proliferation of adventitious buds due to dwarf mistletoes |
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Rusts |
Abnormality of tissues due to fungal pathogens |
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Galls |
Over growth of leaves, stems or roots due to insects, nematodes or fungi |
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Leaf blister |
Localized enlargements due to abnormality of cells |
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Symptoms |
Causes |
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Chlorosis |
Yellowing of needles or leaves due to absence of chlorophyll caused by pathogens, toxins, mineral deficiency, air pollution, drought, excess of water or chemical burns. |
|
Dwarfing |
Due to root disease, mineral deficiencies or myco plasmas. |
Yellows or Chlorosis is a very common symptom of plant disease used to describe any physiological disturbance or disease that results in the destruction or reduced synthesis or chlorophyll. Factors that can cause chlorosis include saline or alkaline soils, deficiencies or excess of essential mineral elements, toxic concentrations of pesticides, insufficient soil water, high or low soil and air temperatures, feeding roots by mites or insects and parasitic fungi.
The factors causing diseases are mainly due to pathogens, edaphic and climatic factors. These factors are played a very important role that causing the diseases.
Pathogen is a living agent, which cause the disease in living trees. There are two types of pathogen (i) biotic and (ii) abiotic pathogen.
The living agents of disease like fungi, bacteria, mycoplasma, viruses, parasitic flowering plants, nematodes and some insects. They are all capable of infection, meaning that they can become established as parasites within a living tree. The infectious natures of these pathogens are especially critical because they can spread from diseased to healthy trees by means of dispersal in growth or reproduction. However, non- infectious diseases, which are caused by abiotic factors, differ notably in lacking signs and for that reason are sometimes more difficult to diagnose. Chemicals in disease causation may derive from either biotic or abiotic origin. Bio – chemicals such as toxins enzymes and growth regulators are responsible for the mechanism by which pathogen, especially fungi to kills of cells for obtaining nourishment and modify tissues of the tree.
Non- infectious disease agents or abiotic pathogens are physical and chemical factors of the environment that are unfavourable for normal growth and development of a tree. In addition trees in a stressed or weakened condition from abitoic causes are often more susceptible to biotic pathogens. Therefore abiotic factors are concurrently as agents of pre disposition
Pathogens often cause nutrient deficiency like symptoms when they injure roots. Roots are the plants absorbing structures, which absorb water and mineral elements from the soil. Pathogenic injury to the root system may cause foliar symptoms, which may be very difficult to diagnose as the cause.
The nutritional condition of tree can dramatically affect its response to pathogen attack. Excess of certain nutrients can affect the physiological condition of a plant, which in turn alters response of the plant to disease. For. e.g. seedlings grown with excess of nitrogen are succulent and slow to harden off.
Soil pH has an important influence on availability of some mineral elements. Iron, Zinc, Manganese and Copper are less soluble in alkaline than in acidic soils a hydroxides. Phosphate is readily absorbed at pH of greater than 6.5. The variation in the pH levels in the soils results to plant disease as they favour to the pathogen or weaken the tree. It may also cause the vascular diseases and produces the symptom of Chlorosis.