Adult walnut twig beetles carry spores of the "Geosmithia morbida" fungus, which grows profusely around the pupal chamber of the beetles. Following emergence from trees the beetles subsequently tunnel into branches and trunks of walnut for production of egg galleries or overwintering shelters. The fungus is introduced into the tree during this wounding where it subsequently germinates and grows.

The fungal mycelium initially colonize tissue immediately surrounding the beetle galleries. However, in less than a month black, oval-shaped, inky cankers extend considerably beyond the galleries and may reach more than 3 cm in length in susceptible hosts (e.g., black walnut). In the beginning these cankers develop in phloem and tissues formed by the cork cambium. The affected area is very shallow and never show the ‘open-faced’, perennial, target-shape typical of many canker diseases of trees (e.g., Nectria canker). Instead in TCD the bark remains firmly attached to the canker face making the necrotic areas very difficult to observe. Branch cankers usually are not visible until the outer bark is shaved to expose the beetle tunnels, although during late stages of the disease a dark amber stain may form on the bark surface in association with the cankers.

Each time a beetle tunnels into a tree a canker is initiated. Cankers also may continue to expand and penetrate into the cambium of the tree. Each such injury destroys the phloem and robs the tree of its ability to store and move nutrients. As TCD progresses cankers coalesce to further girdle branches greatly restricting nutrient movement. As the tree declines, more bark beetles are attracted and more cankers are formed.

Eventually the enormous number of beetle attacks and subsequent canker formation overwhelms and kills the tree. Thousand cankers is a progressive disease and its effects result from the culmination of a large number of relatively small cankers over a period of time. Just as a thousand cuts was once used as a form of human execution in Imperial China, black walnuts are subjected to death by thousands of branch and trunk cankers produced by infection from the "Geosmithia" fungus.

In end stages of the disease external symptoms become visible. Leaf yellowing on the exterior of the crown is often the first symptom and may originally be restricted to a single branch. However, as the cumulative effects of the girdling progress increasingly large areas of the tree are affected. Sudden leaf wilting, ultimately involving large limbs, characterizes end stage thousand cankers disease. In susceptible hosts, trees are almost always killed within 2–3 years after external symptoms of leaf yellowing are first observed.

The progress of thousand cankers will vary due to several factors, notably the susceptibility of the host. There appears to be a considerable range of TCD susceptibility among various "Juglans" species with "Juglans nigra" (black walnut) being particularly susceptible. Conversely, Arizona walnut ("Juglans major") appears to be quite resistant to the disease, with bark beetle attacks largely limited to small diameter branches, the fungus growing to a very limited extent, and effects of the disease rarely, if ever, progressing to involve large areas of the tree. Similarly southern California walnut ("Juglans californica") and little walnut ("Juglans microcarpa") may show fairly high resistance. Northern California walnut ("Juglans hindsii") and the commercial nut-producing Persian (English) walnut ("Juglans regia") apparently show various degrees of intermediate TCD susceptibility.

The foamy bark canker is a disease affecting oak trees in California caused by the fungus "Geosmithia pallida" and spread by the Western oak bark beetle ("Pseudopityopthorus pubipennis"). This disease is only seen through the symbiosis of the bark beetles and the fungal pathogen. The bark beetles target oak trees and bore holes through the peridermal tissues, making tunnels within the phloem. The fungal spores are brought into these tunnels by the beetles and begin to colonize the damaged cells inside the tunnels. Symptoms of the developing fungus include wet discoloration seeping from the beetle entry holes as the fungus begins to consume phloem and likely other tissues. If bark is removed, necrosis of the phloem can be observed surrounding the entry hole(s). As the disease progresses, a reddish sap and foamy liquid oozes from entry holes, thus giving the disease the name Foamy bark canker. Eventually after the disease has progressed, the tree dies. This disease is important because of its detrimental effects on oak trees and its ability to spread to several new Californian counties in just a couple years.

Leucostoma canker is a fungal disease that can kill stone fruit ("Prunus" spp.). The disease is caused by the plant pathogens "Leucostoma persoonii" and "Leucostoma cinctum" (teleomorph) and "Cytospora leucostoma" and "Cytospora cincta" (anamorphs). The disease can have a variety of signs and symptoms depending on the part of the tree infected. One of the most lethal symptoms of the disease are the Leucostoma cankers. The severity of the Leucostoma cankers is dependent on the part of the plant infected. The fungus infects through injured, dying or dead tissues of the trees. Disease management can consist of cultural management practices such as pruning, late season fertilizers or chemical management through measures such as insect control. Leucostoma canker of stone fruit can cause significant economic losses due to reduced fruit production or disease management practices. It is one of the most important diseases of stone fruit tree all over the world.

The disease can infect trees as young as 6 years-old, and infects trees throughout their lifespan. Diagnostic symptoms include crown yellowing and thinning, a distress crop of cones, red brown stained outer heartwood, and laminate decay (decay that separates along annual rings). The disease tends to occur in patches due to a primarily short range spread mechansism. Infected or decayed roots break close to the root collar forming “root balls.” Laminated root rot is frequently detected during ground survey when canopy openings and standing dead and fallen trees are observed. Signs of laminated root rot include the setal hyphae (tiny hairlike hyphae) between sheets of decomposing wood and also buff-colored mycelium on the outside of the roots.

Leaf curl is a plant disease characterized by curling of leaves, and caused by a fungus, genus "Taphrina", or virus, especially genus "Begomovirus" of the family "Geminiviridae". One of the most notable types is peach leaf curl, caused by the fungus "Taphrina deformans", which infects peach, nectarine, and almond trees. "T. deformans" is found in the United States, Europe, Asia, Africa, Australia, and New Zealand. It was first introduced in America in 1852 and has now spread all over the country.

Thousand cankers disease is a recently recognized disease of certain walnuts ("Juglans" spp.). The disease results from the combined activity of the walnut twig beetle ("Pityophthorus juglandis") and a canker producing fungus, "Geosmithia morbida". Until July 2010 the disease was only known to the western United States where over the past decade it has been involved in several large scale die-offs of walnut, particularly black walnut, "Juglans nigra". However, in late July 2010 a well-established outbreak of the disease was found in the Knoxville, Tennessee area. This new finding is the first locating it within the native range of its susceptible host, black walnut.

Black pod disease is caused by many different "Phytophthora spp." pathogens all expressing the same symptoms in cocoa trees ("Theobroma cacao"). This pathogen if left untreated can destroy all yields; annually the pathogen can cause a yield loss of up to 1/3 and up to 10% of total trees can be lost completely. With the value of the cocoa industry throughout the world being so large there are much research and control efforts that go into these "Phytophthora spp." pathogens.

This pathogen can be located anywhere on the cocoa trees but is most noted for the black mummified look it will give to the fruit of the cocoa tree. Staying ahead of the pathogen is the best means of control, the pathogen can be greatly reduced if leaf litter is not allowed to stay on the ground and if the pathogen gets out of hand chemical control can be used. This pathogen is mostly found in tropical areas where the cocoa trees are located and need rainfall in order to spread its spores.

Armillaria root rot is a fungal root rot caused by several different members of the genus "Armillaria". The symptoms are variable depending on the host infected, ranging from stunted leaves to chlorotic needles and dieback of twigs and branches. However, all infected hosts display symptoms characteristic of being infected by a white rotting fungus. The most effective ways of management focus on limiting the spread of the fungus, planting resistant species, and removing infected material. This disease poses a threat to the lumber industry as well as affecting recreational areas.

The first visible sign of a beech scale insect infestation is a woolly, white, waxy covering that the insect secretes. This sign can be observed covering small areas or most of the tree. The amount of waxy material observed depends on the population of the beech scale insect on that tree. The "Neonectria" fungi also show signs of its presence. An early sign is what looks like a bleeding spot on the tree. A reddish-brown fluid will ooze from the wound site, giving it this appearance. Later, perithecia will form around the dead spot, which is another sign of the disease.

Symptoms of beech bark disease can be observed in the foliage and on the bole of the tree. Foliage may become small, sparse and yellowed. Trees that display a thin, weak crown may persist for several years but may also die without displaying any symptoms. Noticeable symptoms on the bole are the cracking of the bark, the formation of cankers, and beech snap. Beech snap is a result of the fungi and insect weakening the wood, which makes it susceptible to being blown over by wind.

The symptoms of Cherry X disease vary greatly depending on the host. On cherry hosts symptoms can usually first be seen on the fruits, causing them to be smaller in size with a leathery skin. Pale fruit is common at harvest time. It is common for symptoms to first be seen in a single branch. The branch may lose its older leaves, and the leaves tend to be smaller with a bronzed complexion.

The rootstock that the cherry is grafted onto can play a significant role in the disease symptoms seen. Rootstocks of Mahaleb cherry exhibit different symptoms from stocks of Colt, Mazzard, or Stockton Morello. When the scion is grafted onto Mahaleb, symptoms consistent with Phytophthora root rot can be seen. To distinguish between root rot and x-disease the wood under the bark at the graft union should be examined. If it is x-disease the wood at the union will have grooves and pits this causes a browning of the phloem and shows the cells in decline. This rapid decline is caused by the rootstock cells near the graft union dying in large quantities. Foliage begins to turn yellow and the curl upward and inward toward the leaf midrib. Trees infected with Mahaleb rootstock die by late summer or early the following year.

When Cherries are grafted onto Colt, Mazzard, or Stockton Morello rootstocks, there is a different range of symptoms. Affected leaves are smaller than normal and the foliage may be sparse. Dieback of shoot tips is common as the disease progresses. Fruit on branches are smaller, lighter, pointed, low sugar content, poor flavor, and a bitter taste.

Peaches are the next most common economic fruit host of the X-disease. Symptoms can be seen after about two months single branches will begin to show symptoms of their individual leaves. These leaves curl up and inward with irregular yellow to reddish-purple spots. These spots can drop out leaving “shotholes”. Leaves that are affected by the disease will fall prematurely. After 2–3 years the entire tree will show symptoms.

Laminated root rot also known as yellow ring rot is caused by the fungal pathogen "Phellinus weirii". Laminated root rot is one of the most damaging root disease amongst conifers in northwestern America and true firs, Douglas-fir, Mountain hemlock, and Western hemlock are highly susceptible to infection with "P. weirii". A few species of plants such as Western white pine and Lodgepole pine are tolerant to the pathogen while Ponderosa pine is resistant to it. Only hardwoods are known to be immune to the pathogen.

Cherry X disease also known as Cherry Buckskin disease is caused by a plant pathogenic phytoplasma. Phytoplasma's are obligate parasites of plants and insects. They are specialized bacteria, characterized by their lack of a cell wall, often transmitted through insects, and are responsible for large losses in crops, fruit trees, and ornamentals. The phytoplasma causing Cherry X disease has a fairly limited host range mostly of stone fruit trees. Hosts of the pathogen include sweet/sour cherries, choke cherry, peaches, nectarines, almonds, clover, and dandelion. Most commonly the pathogen is introduced into economical fruit orchards from wild choke cherry and herbaceous weed hosts. The pathogen is vectored by mountain and cherry leafhoppers. The mountain leafhopper vectors the pathogen from wild hosts to cherry orchards but does not feed on the other hosts. The cherry leafhopper which feeds on the infected cherry trees then becomes the next vector that transmits from cherry orchards to peach, nectarine, and other economic crops. Control of Cherry X disease is limited to controlling the spread, vectors, and weed hosts of the pathogen. Once the pathogen has infected a tree it is fatal and removal is necessary to stop it from becoming a reservoir for vectors.

Beech bark disease is a disease that causes mortality and defects in beech trees in the eastern United States and Europe. In North America, the disease occurs after extensive bark invasion by the beech scale insect, "Cryptococcus fagisuga". Through a presently unknown mechanism, excessive feeding by this insect causes two different fungi ("Neonectria faginata" (previously "Nectria coccinea var. faginata") and "Neonectria ditissima" (previously "Nectria galligena")) to produce annual cankers on the bark of the tree. The continuous formation of lesions around the tree eventually girdles it, resulting in canopy death. In Europe, "N. coccinea" is the primary fungus causing the infection. Infection in European trees occurs in the same manner as it does in North American trees. Though the disease still appears in Europe, it is less serious today than it once was.

Dead arm, sometimes grape canker, is a disease of grapes caused by a deep-seated wood rot of the arms or trunk of the grapevine. As the disease progresses over several years, one or more arms may die, hence the name "dead arm". Eventually the whole vine will die. In the 1970s, dead-arm was identified as really being two diseases, caused by two different fungi, "Eutypa lata" and "Phomopsis viticola" (syn. "Cryptosporella viticola").

Hosts associated with "Geosmithia pallida" include a number of tree species, including oak and other hardwoods, pine and spruce trees, depending on the beetle vector. In this case, the western oak bark beetles target live oak trees of western United States. Beetles tend to attack stressed trees that are already weakened from drought or injury. Symptoms causing branch dieback and tree death also include a cinnamon-colored gum seeping from multiple beetle entry holes on the bole, followed by a prolific, cream-colored foamy liquid. These symptoms, as well as signs (entry holes, larvae, beetles) of bark beetles, are key factors in diagnosis. Necrosis of xylem and phloem tissues underneath bark can be observed.

Common Symptoms:

- Wet discoloration on bark

- Phloem necrosis

- Beetle entry holes

- Reddish sap oozing from entry holes

- Foamy liquid from entry holes

Sudden Death Syndrome (SDS) in Soybean plants quickly spread across the southern United States in the 1970s, eventually reaching most agricultural areas of the US. SDS is caused by a Fusarium fungi, more specifically the soil borne root pathogen "Fusarium virguliforme," formerly known as "Fusarium solani" f. sp. "glycines"."." Losses could exceed hundreds of millions of dollars in US soybean markets alone making it one of the most important diseases found in Soybeans across the US

Dead arm is a disease that causes symptoms in the common grapevine species, "vitis vinifera", in many regions of the world. This disease is mainly caused by the fungal pathogen, "Phomopsis viticola", and is known to affect many cultivars of table grapes, such as Thompson Seedless, Red Globe, and Flame Seedless. Early in the growing season, the disease can delay the growth of the plant and cause leaves to turn yellow and curl. Small, brown spots on the shoots and leaf veins are very common first symptoms of this disease. Soil moisture and temperature can impact the severity of symptoms, leading to a systemic infection in warm, wet conditions. As the name of this disease suggests, it also causes one or more arms of the grapevine to die, often leading to death of the entire vine.

Bleeding canker of horse chestnut is a common canker of horse chestnut trees ("Aesculus hippocastanum", also known as conker trees) that is known to be caused by infection with several different pathogens.

Infections by the gram-negative fluorescent bacterium "Pseudomonas syringae" pathovar "aesculi" are a new phenomenon, and have caused most of the bleeding cankers on horse chestnut that are now frequently seen in Britain.

Most of the SDS symptoms can be confused with other factors like nutrient deficiencies and some other diseases like brown stem rot and stem canker. Usually the first symptom seen is interveinal chlorosis, which is the yellowing of the plant material between the leaf veins. When leaves begin to die, puckering and mottling can also be observed along with the chlorosis. As severity increases, necrosis (death of cells) occurs and eventually these leaves will fall off, leaving only petioles left on the stem. If the conditions are right (cool and wet), these symptoms can appear suddenly, causing large yield reductions. Normally, this is seen in mid or late July around the time of flowering and pod production.

In addition to foliar symptoms, the stem of the soybean plant can show symptoms as well. If a soybean stem with SDS is split, the pith will be visibly white while the cortical tissue around the pith will be tan to light brown in color. If the pith is brown in color (or if the whole stem looks brown on the inside), it is likely that the plant has brown stem rot, rather than SDS

Along with the above ground foliar and stem symptoms, the roots usually show some kind of rotting and decrease in vigor compared to other healthy soybean roots. If soil conditions are moist, roots are also likely to show blue masses of spores (macroconidia) around the taproot just below the soil surface. Blue fungal masses, found along with the foliar and stem symptoms, are strong diagnostic indicators for SDS

Acute oak decline is a disease that infects oak trees in the UK. It mainly affects mature oak trees of over 50 years old of both Britain's native oak species: the pedunculate oak ("Quercus robur") and the sessile oak ("Quercus petraea"). The disease is characterised by the trees bleeding or oozing a dark fluid from small lesions or splits in their bark. Unlike chronic oak decline, acute oak decline can lead to the death of trees within 4 to 5 years of symptoms appearing. The number of trees affected is thought to number in the low thousands, with a higher number of infected trees being found in the Midlands. It is thought to be caused by a bacterium; it is currently not known which species is involved, but scientists are actively trying to discover what is responsible. At least three genera of bacteria are possibly responsible.

In some instances, the disease is accompanied by insects attacking the trees, too, particularly the oak splendour beetle, ("Agrilus biguttatus"). These are not thought to be the cause of the disease, but rather they are opportunistically taking advantage of already weakened trees; such infestations further weaken and can hasten the death of trees.

Velvet Blight is a disease that affects the stems, branches, leaves, fruits or trunks of plants and trees. This disease is primarily caused by three fungal species from the genus "Septobasidium": "S. bogoriense", "S. pilosum" and "S. theae".

It is known to affect mainly tea plants ("Thea" genus).

The most studied of these species is "S. bogoriense", most notably due to the work of Ernst Albert Gäumann. "S. bogoriense" is named after the Herbarium Bogoriense (Bogor, West Java, Indonesia) which is the place where it was first identified on the bark of an unspecified tree and named by E. Nyman on June 3, 1898. This species was also listed in Otto Warburg's Monsunia in 1900.

Leaf curl is distinctive and easily noticeable, and the severity of the signs depends on how early infection has occurred. Diseased leaves can usually be identified soon after they emerge from the bud, due to their red color and twisted shape. As the leaves develop, they become increasingly distorted, and ultimately thick and rubbery compared to normal leaves. The colors of the leaves change from the normal green to red and purple, until finally a whitish bloom covers each leaf. Changes in the bark are less noticeable, if at all. Fruit may fail to develop from diseased blossoms, or may be affected, showing a reddish color. Infected leaves fall early. The tree usually produces a second flush of leaves that is rarely diseased, except in an unseasonably cool and wet spring, because the fungus is not infectious at the normally higher temperatures in late spring and early summer.

Dutch elm disease (DED) is caused by a member of the sac fungi (Ascomycota) affecting elm trees, and is spread by elm bark beetles. Although believed to be originally native to Asia, the disease was accidentally introduced into America and Europe, where it has devastated native populations of elms that did not have resistance to the disease. It has also reached New Zealand. The name "Dutch elm disease" refers to its identification in 1921 and later in the Netherlands by Dutch phytopathologists Bea Schwarz and Christine Buisman who both worked with Professor Johanna Westerdijk. The disease affects species in the genera "Ulmus" and "Zelkova", therefore it is not specific to the Dutch elm hybrid.

The causative agents of DED are ascomycete microfungi. Three species are now recognized:

- "Ophiostoma ulmi", which afflicted Europe from 1910, reaching North America on imported timber in 1928.

- "Ophiostoma himal-ulmi", a species endemic to the western Himalaya.

- "Ophiostoma novo-ulmi", an extremely virulent species from Japan which was first described in Europe and North America in the 1940s and has devastated elms in both continents since the late 1960s.

DED is spread in North America by three species of bark beetles (Family: Curculionidae, Subfamily: Scolytinae):

- The native elm bark beetle, "Hylurgopinus rufipes".

- The European elm bark beetle, "Scolytus multistriatus".

- The banded elm bark beetle, "Scolytus schevyrewi".

In Europe, while "S. multistriatus" still acts as a vector for infection, it is much less effective than the large elm bark beetle, "S. scolytus". "H. rufipes" can be a vector for the disease, but is inefficient compared to the other vectors. "S. schevyrewi" was found in 2003 in Colorado and Utah.

Other reported DED vectors include "Scolytus sulcifrons", "S. pygmaeus", "S. laevis", "Pteleobius vittatus" and "Р. kraatzi". Other elm bark beetle species are also likely vectors.

The hosts for Leucostoma canker include stone fruits such as cultivated peach, plum, prune, cherry ("Prunus spp".), or other wild "Prunus" spp. It can also be found on apple ("Malus domestica"). Stone fruits are referred to as drupe, which are fruits containing a seed encased by a hard endocarp, surrounded by a fleshy outer portion.

Leucostoma canker symptoms differ depending on where on the tree infection takes place. Discoloration occurs in sunken patches on infected twigs. Light and dark concentric circles of narcotic tissue characterize this symptom, occurring near buds killed by cold or on leaf scars. Infections on the nodes are seen 2–4 weeks after bud break. As time passes, darkening occurs within diseased tissues, and eventually, amber gum ooze may seep from infected tissue. Nodal infections are particularly vulnerable in one-year-old shoots that develop within the center of the tree. If fungal growth persists without treatment, scaffold limbs and large branches will likely become invaded within a short time frame. Cankers occurring on branches that are the product of such infections will contain dead twigs or twig stubs at the canker’s center.

The most striking symptom of infection includes cankers located on the main trunk, branch crotches, scaffold limbs, and older branches. A symptom called “flagging” can be found on necrotic scaffold limbs. The cankers are parallel to the long axis of the stem and take on an oval shape. Normally, large-scale production of amber colored gum marks the first external symptom of such cankers. While gum production is the typical plant response to irritation, the gum secretion of Leucostoma occurs in bulk amounts. This gum darkens as time passes, gradually leading to the drying and cracking of bark; thus exposing the blackened tissue below.

As the tree continues to mature in the early growing season, the tree resists additional fungal penetration through the formation of callus rings surrounding the canker. However, the Leucostoma generally reinvades the tissue late in the growing season while the tree switches into dormancy. Due to the alteration of callus production and canker formation, cankers with circular callus rings are usually observed.

Foliar symptoms might develop from branch or twig infections. Symptoms include chlorosis, wilting, and necrosis. Signs include small black structures on dead bark which contain pycnidia.