Forestry & Forest Sciences · Forest Protection, Wildlife & Rangelands · also Forest Protection (diseases), Tree Diseases, Forest Pathology
Diseases of forest trees, nursery stock and wood: fungal, bacterial, viral and phytoplasma pathogens, root rots, wilts, rusts and decay, and their management.
In the exams: ICAR AIEEA-PG Group 09 (tree diseases, wood decay and discolouration, integrated disease management); IFoS Forestry Paper II Section B; Korea Forestry Engineer written subject Forest Protection
Forest pathology is the study of diseases of forest trees and of the decay and staining of wood. A disease is a sustained disturbance of a tree's normal functioning, caused by a pathogen (a fungus, oomycete, bacterium, virus, phytoplasma or nematode), by a parasitic plant, or by an abiotic factor such as frost, drought or pollution. Diseases kill seedlings in nurseries, deform and stunt young plantations, rot the roots and heartwood of mature trees, and in their most dramatic forms, when an introduced pathogen meets a host with no resistance, they can remove a tree species from whole landscapes, as chestnut blight did to the American chestnut and Dutch elm disease to the elms of Europe and North America.
This subject covers the concepts of disease and the disease triangle; nursery diseases such as damping-off, leaf spots and blights; root and butt rots; wilts and cankers; rusts and other foliar diseases; phytoplasma diseases, with sandal spike as the main South Asian example; mistletoes and other parasitic plants; wood decay by white, brown and soft rot fungi, and discolouration and sapstain; abiotic damage from frost, drought, pollution and wind; integrated disease management and forest health monitoring; and invasive forest pathogens and plant health regulation. Insect pests, many of which interact with diseases as vectors or predisposing agents, are covered in forest-entomology; wood preservation against decay in wood-seasoning-preservation; and fire in forest-fire-science.
Forest pathology is examined in the ICAR AIEEA-PG forestry group (tree diseases, wood decay and discolouration, integrated disease management), in Unit 8 of the ICAR AICE-JRF forestry syllabus, in Section B of Paper II of the Indian Forest Service forestry optional (forest protection), and in the Forest Protection subject of Korea's Forestry Engineer qualification. Questions ask for the causal organisms, hosts, symptoms and management of major diseases, the differences between types of decay, the biology of rusts and phytoplasmas, and the principles of disease management and quarantine.
Fungicide registrations differ by country and change often. This manual describes classes of control and principles, not products or doses; any chemical use must follow national registration, labels and regulations.
Plant pathologists distinguish disease from injury. Injury is damage caused by a brief event, such as a branch broken by wind or a cut made by an axe. Disease is a continuing process of abnormal function, caused by a persistent agent. Biotic (infectious) diseases are caused by living pathogens that can spread from plant to plant; abiotic (non-infectious) diseases are caused by environmental factors such as extreme temperatures, drought, waterlogging, nutrient deficiencies or toxic pollutants, and do not spread. Many tree problems involve both: stress from drought or pollution predisposes trees to attack by pathogens and insects, a pattern often called decline.
Fungi cause the majority of tree diseases. They grow as microscopic threads (hyphae) forming a mycelium, reproduce by spores and include the decay fungi, rusts, mildews, canker and leaf-spot fungi and many root pathogens. Oomycetes (water moulds), such as Pythium and Phytophthora, resemble fungi in growth form but belong to a different group of organisms; they produce swimming spores and thrive in wet soils, causing damping-off and root rots. Bacteria cause wetwood, some cankers and leaf spots. Viruses are less important in forest trees than in crops. Phytoplasmas are wall-less bacteria that live in the phloem and are spread by sap-sucking insects. Nematodes attack roots in nurseries, and the pine wood nematode kills pines. Parasitic flowering plants, such as mistletoes and dodder, are also treated as disease agents.
A symptom is a visible reaction of the host: wilting, yellowing (chlorosis), dead tissue (necrosis), leaf spots, blight, canker, dieback, galls, witches' brooms, stunting, resin or gum flow. A sign is the pathogen itself or its products: fungal fruiting bodies (conks, mushrooms, pustules, cups), mycelial fans, rhizomorphs, bacterial ooze. Diagnosis combines symptoms, signs, the pattern of damage in the stand, the site and weather history, and laboratory isolation or molecular identification. Koch's postulates set the classical test for proving that an organism causes a disease: the organism must be consistently associated with the disease, isolated in pure culture, cause the same disease when inoculated into a healthy host, and be re-isolated from the inoculated host.
Some of the most important forest health problems have no single cause. Decline diseases are gradual, progressive deterioration of trees over years, involving several factors acting in sequence. Manion's decline spiral describes three groups: predisposing factors, which are long-term stresses such as unsuitable site, old age, soil compaction or chronic pollution; inciting factors, which are short-term events such as drought, frost or defoliation; and contributing factors, which are secondary organisms such as bark beetles, borers, canker fungi and root rots that finally kill the weakened tree. Oak declines in Europe and North America, dieback of shisham in South Asia and some eucalypt diebacks have been interpreted in this way. Recognising a decline matters because treating only the last organism, for example spraying against beetles, will not solve a problem whose roots lie in site or climate.
Disease develops only when three elements coincide: a susceptible host, a virulent pathogen and a favourable environment. This is the disease triangle; adding time, and sometimes human activity, gives the disease pyramid or tetrahedron. Management works by changing any side of the triangle: planting resistant hosts, reducing the pathogen, or making conditions less favourable. The disease cycle describes the sequence of events in a pathogen's life: survival (in soil, debris, alternate hosts or infected trees), dispersal (by wind, water, insects, animals, soil or people), infection (through stomata, wounds, roots or directly through tissues) and colonisation, followed by reproduction. Monocyclic diseases complete one cycle per season, so they build up slowly; polycyclic diseases, such as many leaf spots and rusts, complete several cycles per season and can spread explosively.
Damping-off is the death of seeds and young seedlings caused by soil-borne pathogens, chiefly the oomycetes Pythium and Phytophthora and the fungi Rhizoctonia and Fusarium. Pre-emergence damping-off kills seeds or germinating seedlings before they appear above the soil, showing as gaps in the beds. Post-emergence damping-off attacks young seedlings at or near soil level: the stem becomes water-soaked and constricted, and the seedling topples and dies, often in expanding patches. Seedlings are susceptible until their stems become woody. Damping-off is favoured by wet, poorly drained, heavy or alkaline media, high seed density, excessive shade and watering, high nitrogen and warm humid weather.
Management combines cultural and, where justified, chemical measures. Nursery beds and containers should use well-drained, light growing media; media can be sterilised by solarisation (covering moist soil with clear plastic in hot sunny weather), steam or other approved methods; seed should be sown at the correct density and not too deeply; watering should be adequate but not excessive; shade should be reduced once seedlings emerge; and diseased seedlings and their surrounding soil should be removed promptly. Seed treatment with approved fungicides or biological agents such as Trichoderma, and drenches of registered fungicides on affected beds, are used where losses are serious, following national registrations.
Leaf spots and blights are common in nurseries because seedlings are crowded, watered often and kept humid. Leaf spots are localised lesions; blights are rapid, extensive killing of leaves and shoots. In tropical eucalyptus nurseries and clonal propagation units, leaf and shoot blights caused by Cylindrocladium (now often placed in Calonectria) can cause heavy losses, especially in mist chambers. Web blight caused by Rhizoctonia binds dead and living seedlings together with mycelial threads in humid conditions. Pestalotiopsis and Colletotrichum (anthracnose) cause leaf spots on many species. Powdery mildews coat leaves with white fungal growth, and rusts appear as orange or brown pustules. Management relies on spacing, ventilation, avoiding overhead watering late in the day, removing infected material, hygiene in propagation units and, where necessary, registered fungicides.
Root rots caused by Fusarium, Phytophthora and Macrophomina, and root-knot nematodes (Meloidogyne), which form galls on roots, stunt nursery stock and can carry disease into plantations. Using clean media, disinfested containers and water, and inspecting roots before dispatch are essential; diseased stock should never be planted out. Healthy, vigorous nursery stock is the foundation of disease management in plantations.
Root and butt rots are caused by fungi that decay roots and the lower stem. They kill trees, cause windthrow by weakening anchorage, and reduce timber value by decaying the most valuable butt log. They usually spread underground from infected roots or stumps to healthy roots through root contacts and grafts, producing expanding patches (disease centres) of dead and dying trees. Some also spread by airborne spores that colonise freshly cut stumps. Because the pathogens can survive for decades in old roots and stumps, root diseases are long-lasting problems on a site.
Heterobasidion annosum in the broad sense (annosus root and butt rot) is the most damaging disease of conifers in the northern temperate zone, especially in thinned plantations. Its airborne spores land on fresh stump surfaces created by thinning or felling, colonise the stump and spread through root contacts to neighbouring trees. Treating fresh stump surfaces immediately after felling, with urea or borate solutions or with spores of the competing fungus Phlebiopsis gigantea (a biological control), prevents infection and is standard practice in many countries.
Armillaria species (honey fungus) attack a wide range of trees and shrubs worldwide. They spread by rhizomorphs, black bootlace-like strands that grow through soil from infected wood, and form white mycelial fans under the bark of infected roots and stems, with honey-coloured mushrooms in clusters at the base. Armillaria is often aggressive on stressed trees and on sites recently cleared of infected natural forest, where old stumps act as inoculum for planted trees.
In the tropics, root rots caused by Ganoderma (red and other root rots), Phellinus noxius (brown root rot) and Rigidoporus (white root rot) cause losses in plantations of acacias, eucalypts, rubber, teak and other species, and in amenity trees. Infection often comes from stumps and roots of the previous crop or natural forest. Ganoderma produces conspicuous bracket fruiting bodies at the stem base. Root rots have been reported among the factors in mortality of shisham (Dalbergia sissoo) in South Asia, together with wilt pathogens and site stresses.
Phytophthora cinnamomi, an oomycete, causes root rot and dieback in hundreds of plant species and has devastated jarrah forests and heathlands in south-western Australia, where it spreads in soil and water and on vehicles, machinery and footwear; hygiene and access controls are used to contain it. Management of root rots in general includes removing infected stumps and roots before replanting (stumping), planting less susceptible species on infested sites, treating stumps after thinning, avoiding root damage and soil compaction, maintaining good drainage, and in high-value situations using trenches to break root contacts. Mixtures of species reduce the spread of host-specific root pathogens.
Vascular wilts are diseases in which the pathogen invades the xylem, blocking water transport and causing wilting, yellowing, dieback and death. The host's own responses, such as tyloses and gums that plug vessels, contribute to the blockage. Dutch elm disease, caused by Ophiostoma ulmi and the more aggressive Ophiostoma novo-ulmi, is spread by elm bark beetles (Scolytus species) that breed in diseased elms and carry spores to healthy trees when they feed, and through root grafts; two pandemics in the twentieth century killed most mature elms in Europe and North America. Oak wilt in North America is caused by Bretziella fagacearum. Ceratocystis species cause wilts and cankers of many tropical trees, including Ceratocystis wilt of eucalyptus, acacia and mango, often entering through wounds. Fusarium wilts affect some tree species, and wilt has been reported among causes of shisham mortality in South Asia.
Pine wilt disease is caused by the pine wood nematode, Bursaphelenchus xylophilus, a microscopic worm native to North America, carried from tree to tree by longhorn beetles of the genus Monochamus (pine sawyers). Introduced into Japan in the early twentieth century, it has killed large numbers of native pines there and has spread to China, Korea and Portugal. Affected trees wilt and die within weeks to months of infection in warm summers. It is a major quarantine concern, and international wood-packaging rules (see forest-entomology) are partly designed to prevent its spread.
A canker is a localised dead area of bark and cambium on a stem or branch, often sunken, cracked or with callus at its edges. Annual cankers develop quickly and are then contained by the tree; perennial cankers expand year after year as the fungus and the callus alternate, forming target-like rings. Cankers that girdle the stem kill everything above them. Chestnut blight, caused by Cryphonectria parasitica, introduced from Asia, killed almost all mature American chestnuts in eastern North America in the early twentieth century; in Europe its impact has been reduced by hypovirulence, a virus that infects the fungus and reduces its aggressiveness. In tropical eucalyptus plantations, Chrysoporthe cubensis canker damages susceptible species and clones, and pink disease, caused by Erythricium (Corticium) salmonicolor, produces pink fungal crusts on stems and branches of eucalypts and other trees in humid areas, including Kerala; it is managed through resistant species and clones and removal of affected stems.
Management depends on the pathogen and its spread. Measures include planting resistant species and clones (the main approach for eucalyptus cankers); sanitation felling and prompt removal of infected trees and of beetle breeding material for vector-borne wilts; breaking root grafts; avoiding wounds during pruning and harvest and timing these operations to avoid infection periods; quarantine to prevent introductions; and, for valuable individual trees, injections or other treatments. Breeding for resistance has produced disease-tolerant elms and chestnuts for restoration.
Rust fungi (order Pucciniales) are obligate parasites: they can grow only in living host tissue. Many have complex life cycles with up to five spore stages: spermatia (pycniospores), aeciospores, urediniospores, teliospores and basidiospores. Heteroecious rusts need two unrelated hosts to complete their life cycle; autoecious rusts complete it on one host. Urediniospores are produced repeatedly during a season and spread the disease rapidly (the repeating stage). Rusts produce orange, yellow or brown pustules on leaves, needles, stems or cones, and some form galls and cankers.
White pine blister rust, caused by Cronartium ribicola, is a heteroecious rust native to Asia that alternates between five-needle pines and Ribes (currants and gooseberries); introduced to North America and Europe, it killed large numbers of white pines, forming stem cankers that girdle and kill trees. Control strategies have included removal of Ribes near pines and planting resistant pines. Poplar leaf rusts (Melampsora species) defoliate poplars prematurely, reducing growth in plantations; resistant clones are the main control, but the rust can overcome resistance, so clone diversity is important. Teak leaf rust, caused by Olivea tectonae, produces orange-yellow pustules on the undersides of teak leaves, especially in nurseries and young plantations, and causes premature defoliation. Myrtle rust, caused by Austropuccinia psidii and native to South and Central America, has spread to Australia and other regions, attacking many species of the myrtle family, including eucalypts.
Many other fungi attack leaves and needles. Powdery mildews, such as oak powdery mildew, form white superficial growth and can stunt seedlings. Needle casts and needle blights of conifers, such as Dothistroma needle blight, which has damaged Pinus radiata plantations in the southern hemisphere and pines in Europe, cause premature needle loss; Lophodermium causes needle cast in nurseries. Anthracnose diseases (Colletotrichum and related genera) cause leaf spots, shoot blight and dieback on many broadleaves. Leaf spots and blights caused by Cylindrocladium (Calonectria), Cercospora and other fungi affect eucalyptus, teak and other plantation species. Most foliar diseases are polycyclic and favoured by humid weather and dense, poorly ventilated stands.
Management of foliar diseases in plantations relies mainly on species and clone choice, matching species to climate (avoiding planting susceptible species in very humid zones), stand spacing and thinning that improve air movement, and in nurseries, hygiene and registered fungicides when needed. For heteroecious rusts, removing the alternate host near valuable stands can break the life cycle. Because many foliar pathogens evolve rapidly, deploying a diversity of resistant clones rather than one is safer.
Phytoplasmas are bacteria without cell walls, belonging to the class Mollicutes, that live in the phloem of plants and in the bodies of the insects that transmit them. They cannot yet be grown routinely in culture, so they are detected by electron microscopy or, now, by molecular methods based on DNA. They are transmitted by phloem-feeding insects, mainly leafhoppers, planthoppers and psyllids, and by grafting and vegetative propagation; dodder can also transmit them experimentally. Because they lack a cell wall, they are insensitive to penicillin but sensitive to tetracycline antibiotics, which can temporarily suppress symptoms.
Phytoplasma diseases cause characteristic symptoms resulting from disturbed hormone balance and phloem function: yellowing; small, narrow leaves; shortened internodes; proliferation of shoots forming witches' brooms; transformation of flowers into leafy structures (phyllody); stunting; and decline and death. Examples in trees include Paulownia witches' broom in East Asia, ash yellows and elm yellows in North America, and spike disease of sandalwood in South India.
Spike disease of sandalwood (Santalum album) is the most important disease of sandal in India and has been known in Karnataka and Tamil Nadu since the late nineteenth century. Its name describes the symptoms: leaves become progressively smaller, narrower and stiffer, internodes shorten, and the leaves become crowded on the shoots so that branches look like spikes; the tree stops flowering normally and dies within a few years. The disease is caused by a phytoplasma and is spread by sap-sucking insect vectors and through the sandal tree's root connections (haustoria) with other plants. Because there is no practical cure, management relies on early detection and removal of affected trees to reduce spread, raising healthy planting stock from seed of healthy trees, and research into resistance. Antibiotic injections can produce temporary remission but are not a practical control.
General measures against phytoplasma diseases include using healthy, tested planting material; removing infected plants; managing insect vectors where practicable; controlling weeds and alternative hosts that harbour the pathogen and vectors; and breeding resistant material. Quarantine prevents spread in plant material between regions.
Several groups of flowering plants are parasites of trees. Hemiparasites are green and photosynthesise but obtain water and minerals (and some carbon) from the host through specialised connections called haustoria. Holoparasites lack chlorophyll and depend entirely on the host. Stem parasites attach to branches and stems; root parasites attach to roots. Sandalwood itself is a root hemiparasite, forming haustoria on the roots of many other plants, which is why it needs host plants in nurseries and plantations.
Leafy mistletoes of the family Loranthaceae are stem hemiparasites common on trees in tropical and subtropical forests and on roadside and orchard trees. In India, Dendrophthoe falcata is the most widespread species, attacking teak, mango, neem and many other trees. Birds eat the sticky fruits and deposit seeds on branches, where they germinate and penetrate the bark. Heavy infestation reduces vigour, causes branch dieback and deformity, and can kill trees. Control is by cutting off infected branches well below the point of attachment before the mistletoe fruits, and removing heavily infested trees in plantations; mistletoes are also important food sources for birds, so complete removal in natural forests is not always desirable.
Dwarf mistletoes (Arceuthobium, family Viscaceae, formerly Loranthaceae in the broad sense) are small, mostly leafless parasites of conifers in the northern hemisphere. They are among the most damaging forest pathogens in western North America, causing witches' brooms, reduced growth, poor wood quality and mortality. Their seeds are shot explosively from the fruit for several metres, spreading the parasite within stands. In the Himalaya, Arceuthobium minutissimum on blue pine (Pinus wallichiana) is a notable example. Control is by removing infected trees and creating stand conditions, such as harvesting infected overstorey trees before regenerating the stand, that prevent infection of the next generation.
Dodder (Cuscuta) is a leafless, twining stem holoparasite that forms dense yellow or orange tangles over hosts, including nursery seedlings and hedges. It can also transmit phytoplasmas and viruses. Control is by removing infested plants before dodder sets seed and keeping nurseries free of it. Root parasites such as broomrapes and Striga are mainly agricultural problems but illustrate the range of parasitic plants.
Wood decay is the breakdown of wood by fungi that use cellulose, hemicellulose and lignin as food. Decay fungi need food (the wood), moisture (wood moisture content above roughly the fibre saturation point, usually above about 20% for decay to proceed), oxygen and suitable temperatures. Removing any one prevents decay: dry wood (below about 20% moisture), wood saturated with water (too little oxygen), and wood treated with preservatives resist decay. Heartwood of naturally durable species contains extractives that inhibit fungi. In living trees, heart rot fungi enter through wounds, broken branches and dead roots and decay the heartwood, which is non-living, while the sapwood keeps the tree alive; hollow trees are often old trees with long-established heart rot.
White rot fungi, mostly basidiomycetes, degrade lignin as well as cellulose and hemicellulose. Decayed wood becomes bleached, lighter in colour and often stringy, fibrous or spongy, sometimes with dark zone lines. White rot is common in hardwoods. Some white rot fungi remove lignin preferentially (selective delignification). Many common bracket fungi on standing trees and logs cause white rot, for example species of Trametes and Phellinus. White rot fungi have industrial interest for biopulping and the breakdown of pollutants.
Brown rot fungi, also basidiomycetes, degrade cellulose and hemicellulose but leave lignin largely in place, modified. Decayed wood turns brown, shrinks and cracks into roughly cubical pieces (cubical checking) and crumbles to powder when dry. Strength is lost very early in brown rot, often before decay is visible. Brown rot is particularly common in softwoods and in wood in service. The dry rot fungus Serpula lacrymans, which can transport water through strands and attack relatively dry timber in buildings, is a brown rot fungus, as are Gloeophyllum and many others.
Soft rot is caused by ascomycete and related fungi in very wet wood or wood in contact with soil, and in conditions unfavourable to basidiomycetes, such as wood with high preservative loadings or in cooling towers. The fungi form cavities within the cell walls (in the S2 layer), and the surface of the wood becomes soft and darkens, while deeper wood may remain sound. Sapstain (blue stain) fungi, such as Ophiostoma, Ceratocystis and Lasiodiplodia, grow in the sapwood of freshly felled logs and green lumber, feeding on cell contents rather than cell walls and producing blue-black or grey discolouration. They cause little strength loss but reduce the value of timber, and they can be carried by bark beetles. Rubberwood and many light-coloured tropical hardwoods are highly susceptible and must be treated or dried quickly after sawing. Moulds produce surface discolouration that can usually be planed off. Chemical stains, such as iron stains in tannin-rich woods, are non-fungal.
Heart rot in living trees reduces the merchantable volume and strength of the most valuable part of the stem and increases the risk that trees break in wind, which matters both for timber and for safety in parks and along roads. Its presence is indicated by fruiting bodies (conks) on the stem, swollen knots, old wounds, fire scars, cavities and hollow sounds when the stem is struck. Increment borers, resistance drills and sonic tomography are used to assess the extent of decay inside a stem without felling it. Because heart rot generally increases with age and with the number of wounds, the proportion of defective trees rises in old stands, and in some forests the rotation is set partly to limit decay losses. In natural forests, hollow trees and decaying wood are also valuable habitat for birds, bats and invertebrates, so decay is not only a loss.
Decay and stain are prevented by rapid processing of logs, keeping logs wet (water storage or sprinkling) or drying sawn timber quickly, prophylactic treatment of green timber against sapstain, kiln drying, using durable heartwood or preservative-treated timber where decay hazard is high, and design that keeps timber dry in service. Details of treatment are in wood-seasoning-preservation.
Frost damages trees in several ways. Late spring frosts kill newly flushed shoots and flowers; early autumn frosts injure shoots that have not hardened; winter cold can kill tissues of species planted outside their hardiness range. Frost cracks are longitudinal splits in stems caused by rapid contraction of the outer wood in severe cold. Frost heaving lifts small seedlings out of the soil when wet soil freezes and thaws repeatedly. Frost pockets in hollows where cold air collects concentrate damage (see silviculture-principles). Heat and sunscald also injure trees: thin-barked stems suddenly exposed to sun by felling or pruning can suffer bark death on the sun-facing side, and very hot soil surfaces can girdle young seedlings at ground level.
Drought causes wilting, leaf scorch, premature leaf fall, dieback and death, and weakens trees so that bark beetles, borers and canker and root-rot fungi can attack them. Repeated or prolonged droughts, which are increasing in many regions with climate change, have caused widespread forest dieback. Waterlogging deprives roots of oxygen and causes root death, yellowing and dieback, and favours Phytophthora root rots. Salt damage occurs near roads treated with de-icing salt, in coastal areas and on saline soils.
Air pollutants injure foliage and weaken trees. Sulphur dioxide causes interveinal chlorosis and necrosis of broadleaves and needle-tip necrosis of conifers, and was responsible for severe forest damage near smelters and power stations. Ozone, a secondary pollutant formed from nitrogen oxides and hydrocarbons in sunlight, causes stippling and flecking on the upper leaf surface and reduces growth, and is now a widespread regional pollutant. Fluorides cause tip and margin necrosis. Acid deposition (from sulphur and nitrogen oxides) acidifies soils, leaching calcium and magnesium and mobilising aluminium; widespread forest decline in central Europe in the 1980s, known in Germany as Waldsterben, prompted major research and pollution controls. Nitrogen deposition can fertilise forests at first but cause nutrient imbalances and soil acidification over time.
Wind causes windthrow (uprooting) and stem breakage, especially in shallow-rooted trees, on wet or shallow soils, in recently thinned stands and along newly exposed edges; it also predisposes stands to bark beetles. Lightning splits stems and kills trees, sometimes in groups. Nutrient deficiencies cause characteristic symptoms such as chlorosis (for example iron or nitrogen deficiency) and dieback (boron deficiency in some eucalypt and pine plantations). Herbicide drift and mechanical damage from machinery are further abiotic causes. Diagnosis of abiotic damage relies on the pattern of symptoms (often uniform across species or related to a site feature or an event) and the history of weather and management.
Integrated disease management applies the same logic as integrated pest management (see forest-entomology): prevention first, monitoring, decisions based on risk and thresholds, and a combination of methods with least harm. Classical principles of plant disease control are exclusion (keeping the pathogen out, through quarantine and clean planting stock), eradication (eliminating the pathogen from an area, by removing infected plants, alternate hosts or inoculum), protection (shielding the host, for example by stump treatment or fungicides), resistance (using resistant or tolerant hosts) and therapy (curing infected plants, rarely practical in forests). Silviculture and resistance carry most of the weight in forest disease management.
Matching species and provenances to site and climate is the most important preventive measure: trees off-site are stressed and susceptible. Mixtures of species slow the spread of host-specific pathogens. Spacing and thinning improve air movement and vigour, reducing foliar diseases. Sanitation removes infected trees and debris that serve as inoculum. Stump treatment prevents infection by root rot fungi after thinning. Careful harvesting and pruning avoid wounds through which canker and decay fungi enter, and are timed to avoid periods of high spore release. Rotations can be shortened where heart rot accumulates with age. Nursery hygiene, healthy planting stock and drainage prevent many problems.
Breeding and selecting resistant trees has been highly successful in some cases: clones of eucalyptus resistant to canker and rust, poplar clones resistant to leaf rust, and white pines resistant to blister rust. Because pathogens evolve, deploying several resistant clones rather than one, and continuing to screen material, is safer than relying on a single resistant genotype. Biological control of tree diseases includes stump treatment with Phlebiopsis gigantea against Heterobasidion, Trichoderma in nurseries, and hypovirulence against chestnut blight.
Forest health monitoring tracks the condition of forests over time to detect problems early and assess trends. Methods include ground surveys of permanent plots, recording crown condition (defoliation or crown transparency, discolouration), mortality, damage agents and growth; aerial surveys and satellite remote sensing to detect dieback, defoliation and mortality over large areas; and targeted surveys for specific pathogens. The pan-European ICP Forests programme, established under the UN Convention on Long-range Transboundary Air Pollution in the 1980s, assesses crown condition and environmental factors on a network of plots across Europe; the United States Forest Service runs a national Forest Health Monitoring programme with aerial detection surveys. Monitoring data guide management decisions and policy, and provide evidence of the effects of pollution, climate change and invasive organisms.
Introduced pathogens have caused some of the most severe forest losses on record because their new hosts have not co-evolved with them and lack resistance. Chestnut blight eliminated the American chestnut as a canopy tree; Dutch elm disease killed most mature elms in Europe and North America; white pine blister rust devastated five-needle pines; Phytophthora cinnamomi transformed ecosystems in south-western Australia; ash dieback, caused by Hymenoscyphus fraxineus from Asia, has killed large numbers of European ash since the 1990s; Phytophthora ramorum causes sudden oak death in California and Oregon and has killed larch plantations in Britain; myrtle rust has spread through Australia and elsewhere; and the pine wood nematode has killed pines in East Asia and Portugal. The rate of new introductions has increased with global trade.
The most important pathway for invasive pathogens is the trade in live plants for planting, including nursery stock and ornamental plants, which can carry pathogens in roots, soil and foliage without obvious symptoms. Other pathways include logs and sawn timber with bark, wood packaging material (especially for vector-borne pathogens such as pine wilt), soil on vehicles and equipment, and seeds. Once established in nurseries, pathogens can be spread rapidly through the plant trade within a country.
Plant health regulation operates through the International Plant Protection Convention (IPPC) and its International Standards for Phytosanitary Measures, implemented by national plant protection organisations (see forest-entomology for ISPM 15 and India's plant quarantine law). Measures include pest risk analysis, import requirements and prohibitions for high-risk plants and wood, phytosanitary certification, inspection and testing at borders, pest-free areas and places of production, post-entry quarantine, and emergency measures after detection. Many scientists argue that inspection of plants for planting cannot detect symptomless infections reliably and that stricter controls on this pathway are needed.
When a new pathogen is detected, responses include surveys to define its extent, eradication by destroying infected and nearby hosts if the outbreak is small, containment through movement controls, research into resistance and management, and, if the pathogen becomes established, long-term programmes such as breeding resistant trees (as for elms, chestnuts and ash) and changing species choice. Foresters, nursery managers and the public all have roles in early detection; awareness of unusual symptoms and prompt reporting are essential.
Learn the disease triangle and Koch's postulates, then the major diseases by group with causal organism, host, symptoms and management: damping-off (Pythium, Phytophthora, Rhizoctonia, Fusarium), Heterobasidion and Armillaria root rots, Dutch elm disease, pine wilt, chestnut blight, white pine blister rust, teak rust, sandal spike, mistletoes, and the three types of decay. Practise distinguishing white, brown and soft rot and sapstain by appearance and cause, and abiotic from biotic damage by pattern. In descriptive answers, structure each disease as cause, symptoms, cycle and management.
A forester five years in diagnoses disease problems in nurseries and plantations, organises hygiene in nurseries and propagation units, treats stumps after thinning where root rot is a risk, removes sources of infection, chooses resistant species and clones, protects felled timber from stain and decay, monitors forest health on plots and reports unusual symptoms that may indicate invasive pathogens. Practical skills include recognising symptoms and signs, sampling for laboratory diagnosis and keeping records.
A senior forester sets disease management policy: species and clone deployment for resistance and diversity, nursery standards and certification of planting stock, responses to outbreaks and invasive pathogens with plant health authorities, and the integration of forest health monitoring into planning. The senior forester weighs the cost of prevention against the risk of large losses and coordinates with research institutions.
A senior researcher studies pathogen biology, epidemiology, host resistance and its genetics, molecular diagnostics, biological control, climate effects on disease and invasion pathways; develops monitoring tools and resistant material; and advises on plant health regulation and emergency responses.
No fungicide products, doses or schedules are given, because registrations differ by country and change. Scientific names follow current usage where known, but fungal taxonomy changes frequently (for example Cylindrocladium and Calonectria, Corticium and Erythricium), so older literature may use different names. The causes of shisham mortality and the vectors of sandal spike are described only in general terms because published accounts differ. Detailed identification of decay fungi, laboratory methods and disease threshold values are not included. Insects are in forest-entomology and wood preservation in wood-seasoning-preservation.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
These notes are free to read here, with no account.
What the Laisha app adds:
Free for ten days, then a subscription. We would rather write that here than have you find it later.