Adelges abietis (eastern spruce gall adelgid),  A. cooleyi (Cooley spruce gall adelgid) and A. tsugae (hemlock woolly adelgid)
Nymphs are soft-bodied and wingless. Mature nymphs, winged. Wingless are visible only with hand-lens. For example hemlock woolly adelgid is often buried within a cottony mass of wax.

Adelgids are found on Norway spruce; occasionally Colorado blue, red, and white spruce, several species of spruce & hemlock, Colorado blue, Engelmann, oriental, Sitka, white spruce,Douglas fir and are widely distributed in the United States.

Gall adelgids are characteristic cone-like galls. Stunting of twigs and death of new growth. Heavy infestations give tree a ragged appearance. Eastern spruce galls are near bases of twigs. Woolly adelgids; On hemlock, base of needles look like snow.

They have Complex life cycles often involve alternate host species.

Two typical cycles are;

1. abietis: Overwinters as partially grown female (stem mother) which matures in early spring. Female lays eggs when new buds are ready to break. Eggs hatch within 10 days, and nymphs begin to feed on new needles, moving to bases of needles. As feeding continues, nymphs cause abnormal twig growth and gall development. Gall tissues grow around young insects, protecting them until galls crack open during mid-to-late summer. When galls open nearly mature nymphs crawl out onto a needle and transform to winged egg-layers. Eggs are deposited near tips of needles. Eggs hatch and partially grown nymphs attach themselves to a terminal twig, where they spend the winter.
2. cooleyi: Overwinters as mature female on spruce. Female matures in spring to become a stem mother; lays eggs on tips of laterals. After eggs hatch nymphs migrate to new spring growth where they feed at the bases of needles. By mid-summer galls open and the insects move to needle tips and transform into winged females. These fly to another spruce or to the alternate host, Douglas fir, where eggs are laid on needles. A generation of woolly adelgidis produced.

Physalospora obtusa (fungus)

Trees affected include Ailanthus, alder, American holly, apple, boxelder, black locust, chinaberry, crabapple, crape myrtle, dogwood, flowering quince, honey locust, maple, mountain ash, oak, pecan, Persian walnut, persimmon, sassafras, Atlantic white cedar, Austrian and eastern white pines and found mainly in the United States: Northeast, southeast, central California.

Warm, rainy weather in early spring at bud break. Inoculum source from dead and infected twigs and branches on tree all contribute .

This disease is noticed by wet, bleeding cankers on stems and branches of alders, with twig dieback. May cause fruit rot, cankers, cone and seed rot on conifers, and twig blights.

It overwinters as perithecia on infected and dead branches. Ascospores released in spring, during and following wet weather, when conditions are right. These spores invade bark wounds, germinate in the bark, and form pycnidia which provide inoculum throughout the summer, until cool fall weather favors perithecial development in bark and fruit.

Ambrosia beetles are beetles of the weevil subfamilies Scolytinae and Platypodinae (Coleoptera, Curculionidae), which live in nutritional symbiosis with ambrosia fungi and probably with bacteria.
The beetles excavate tunnels in dead or live trees in which they cultivate fungal gardens, their sole source of nutrition.

Ambrosia beetles are  sometimes referred to as Shot Hole Borers.

After landing on a suitable tree, an ambrosia beetle excavates a tunnel in which it releases spores of its fungal symbiont. The fungus penetrates the plant’s xylem tissue, digests it, and concentrates the nutrients on and near the surface of the beetle gallery.
The majority of ambrosia beetles colonize xylem (sapwood and/or heartwood) of dying or recently dead trees, but some attack live trees.

Species differ in their preference for different parts of trees, different stages of deterioration, and in the shape of their tunnels (“galleries”). However, the majority of ambrosia beetles are not specialized to any taxonomic group of hosts, unlike most phytophagous organisms including the closely related bark beetles. One species of ambrosia beetle, Austroplatypus incompertus exhibits eusociality, one of the few organisms outside of Hymenoptera to do so.

Another Species of Ambrosia beetle causing very serious concern and damage in California is the “Polyphagous shot hole borer (PSHB)”.

Symbiotic Relationship
Beetles and their larvae graze on mycelium exposed on the gallery walls and on bodies called sporodochia, clusters of the fungus’ spores. Most ambrosia beetle species don’t ingest the wood tissue; instead, the sawdust resulting from the excavation is pushed out of the gallery. Following the larval and pupal stage, adult ambrosia beetles collect masses of fungal spores into their mycangia and leave the gallery to find their own tree.
A few dozen species of ambrosia fungi have been described, currently in the polyphyletic genera Ambrosiella (mostly Microascales), Raffaelea, Ceratocystiopsis and Dryadomyces (from Ophiostomatales), Ambrosiozyma (yeasts), and Entomocorticium (Basidiomycota). Many more species remain to be discovered. Little is known about the bionomy or specificity of ambrosia fungi. Ambrosia fungi are thought to be dependent on transport and inoculation provided by their beetle symbionts, as they have not been found in any other habitat. All ambrosia fungi are probably asexual and clonal.[3] Some beetles are known to acquire (“steal”) fungal inoculum from fungal gardens of other ambrosia beetle species.

Euzophera semifuneralis.
The larvae of the American Plum Borer (APB) bore into the tree, leaving reddish orange frass and gum pockets called gummosis. The boring is most damaging to the scaffold crotches or graft unions of young trees. Vigorous trees will heal over, but with heavy, prolonged infestations, scaffolds may break with wind or a heavy crop.
The American Plum Borer (APB) is a cambium-feeding moth pest of fruit and ornamental trees.  It is the major borer pest of stone fruits and ornamental  trees causing up to a 33% decline in the life span of tart cherry trees there.
Significant numbers are also found in peach trees infected by canker diseases. Widely distributed throughout most of North America and parts of South America, it has an extensive range of hosts including at least 15 families of fruit, nut, ornamental, and forest trees, and a few woody annuals.b

Bark damaged by mechanical harvesters, which provides larvae an entryway to the cambium.
Male and female APB are identical. Wingspread ranges from 17-28 mm (2/3-1 inch. The forewing is narrow and somewhat triangular; the hind wing is broad and fringed on the trailing edge. The overall color of the moth is a light grayish brown.

Eggs are oval and covered with a network of triangular facets. They are dirty white when first laid but darken to pink, then deep red, as they mature. They are small (~/3 to ]/2 mm) and are laid singly or in small masses in or near the gummosis caused by bark wounds.

APB larvae range in color from grayish green to grayish purple; the head capsule, cervical shield, and anal plate are yellow to brown. The cervical shield has dark markings on either sid. In contrast, larvae of the peachtree borer and lesser peachtree borer, which may be found along with APB larvae, are creamy white with a yellowish-brown to dark brown head capsule. In addition, APB larvae have two rows of crochets (hook-like spines) at the tips of the abdominal prolegs, while sesiid larvae have only one row. Long primary setae are apparent on APB larvae, but not on the sesiids.
APB larvae feed on cambium. They can only enter the cambium layer through openings created by mechanical damage, diseases, sunscald, winter injury, etc. Entry is most commonly gained through splits in the bark of tart cherries caused by mechanical harvesters cankers in peaches caused by Cytospora spp. and other pathogens and black knot cankers in tart cherries and plums.

Because APB larvae feed horizontally, they may eventually girdle the trunk or scaffolds. However, damage may go unnoticed because the outer bark usually remains intact.
APB can also spread plant pathogens. Larvae may contribute to the enlargement of Cytospora cankers and ovipositing females can carry spores from one tree to another.

Apiognomonia veneta — (fungus), A. quercina, Gleosporium aridum (the imperfect stage)

Anthracnose has many host trees but is very common on Sycamore which is its primary host of A. veneta, while oak is attacked by A. quercina. Ash hosts Gleosporium aridum.
It is found throughout the Continental United States and southern Canada.

Warm, rainy or foggy late winter and early spring weather, overhead irrigation; dead branches and twigs in crown; masses of infected leaves overwintering on ground all contribute to the disease.

Anthracnose is often considered to be a transition between leaf and stem disease. Spring buds do not mature because the tissue has been killed. During shoot elongation, infection causes wilting and death of growing shoots and immature leaves. The pathogen then moves into vein tissues of mature leaves, resulting in progressive necrosis along the mid-rib and veins; finally, it moves down the petiole into twig tissues where cankers develop on twigs or branches. The cankers are small sunken dead areas which usually occur at leaf scars and occasionally girdle the branch. Heavy infections cause defoliation by late spring or early summer. Repeated annual infections may be fatal to the tree, especially on ash.

The fungus overwinters in fruiting bodies in dead leaves, twigs and canker margins. It becomes active when the weather first turns warm in the early spring. During warmer, wet weather spores are released from the fruiting bodies and are blown or splashed onto new, re-emerging shoots and breaking leaf buds. Infected shoots and leaves are killed, and the fungus grows into the attacked twig, causing a canker. Spores produced in these shoots can infect nearby leaves, causing defoliation. Fruiting bodies are also produced on dead leaf tissue. Disease development is minimal during drier summer months. New twig infection may occur in the fall when infected leaves and some dead shoots are dropped

Some 29 genera of which Aphis, Microsiphum, and Myzus are among the most common.

Adults are about 1/8-1/4 in. long, rounded; two elongated projections arising from the abdomen.
Aphids are sometimes coated with white, cottony wax.

Aphids have many host plants and trees and are spread worldwide. Some are specific to one area.

Symptoms include heavy infestations with bud blight, curling and drooping of leaves, chlorosis or light yellow “pin prick” spots on foliage from feeding. Aphids usually cluster to feed on undersides of leaves, often more numerous on inside shaded portions of crown. Dried white skins cast off during molt. A clear, shiny liquid (honeydew) is often seen on the leaf surface of infested plants. Honeydew may drip and is attractive to ants. Black sooty mold forms on leaf surfaces growing on the honeydew.
Cool temperatures, vigorous foliage are favored.

Most will overwinter in eggs attached to bark crevices or protected plant parts. Nymphs hatch in spring to become wingless adults (stem mothers), which then produce live young (nymphs) in about a week. New generations are created weekly, depending on climate.

Three to five (3-5) weeks of conducive conditions can cause severe plant injury. In fall, males and females are produced. Females deposit fertilized eggs on protected plant parts to overwinter. Many are vectors for viruses or mycoplasma.

Venturia inequalis, V. asperata (fungus)
Plants affected are Apple, Crabapple, Pear, Firethorn and many other species of Malus throughout North America and Europe.

Cool, wet weather favors heavy leaf infection.

Symptoms first appear in the spring as spots (lesions) on the lower leaf surface, which is the side first exposed to fungal spores as buds open. At first, the lesions are usually small, velvety, olive green in color, and have unclear margins. On some crabapples, infections may be reddish in color. As they age, the infections become darker and more distinct in outline. Lesions may appear more numerous closer to the mid-vein of the leaf. If heavily infected, the leaf becomes distorted and drops early in the summer. Trees of highly susceptible varieties may be severely defoliated by mid-to-late summer.

The fungus, V. inequalis, survives the winter in the previous year’s fallen diseased leaves. In the spring, the fungus in the previous year’s leaves produces millions of airborne spores during rain periods in April, May and June. These are carried by the wind to young leaves, flower parts and fruits. Spores germinate in a film of water, the fungus penetrates into the plant and depending upon weather conditions, symptoms (lesions) will show up in 9 to 17 days. The fungus also produces a different kind of spore in these newly developed lesions. These spores are carried and spread by splashing rain to other leaves and fruits, where new infections occur. The disease may continue to develop and spread throughout the summer.

Seiridium unicorne
Arborvitae Branch Canker (Sieridium) is a serious disease, occurring on Oriental arborvitae

Canker diseases result in the formation of distinct, sunken lesions on the bark of woody plants and occurs on Oriental Arborvitae, Bald-cypress, Arizona, Italian and Leyland cypresses, and occasionally Junipers..

Cankers restrict water and nutrient movement and may ultimately lead to branch dieback and tree death.

The symptoms for this disease are frequently confused with winter-related injuries.

Symptoms of the disease are similar to Botryosphaeria canker. Elongated, flattened cankers form on small branches and main stems. Bleeding or resin formation in cankers is common. Multiple coalescing branch and stem cankers may cause branch dieback, or in some cases, tree death.

Armillaria mellea
AKA,  Oak Foot Fungus, Oak Root Rot
This fungus attacks most trees and is found world wide.
Trees infected by Armillaria sp. usually exhibit thinning crowns, declining growth increments, foliage chlorosis, distress cone crops, and heavy basal resin flow. Hardwoods may exhibit symptoms of crown decline, may appear to die suddenly, or may be wind thrown. Occurrence of white mycelia fans under the bark of roots and lower stems is diagnostic. Advanced decay is yellow, stringy, and malodorous. Rhizomorphs (black or brown shoe string-like structures) may be found under bark, and golden-colored mushrooms of the pathogen often appear around the bases of infected trees in fall. Infected trees commonly occur in distinct disease centers but also may have scattered distributions.

Armillaria root disease is often associated with trees under stress. It is most commonly found on sites with compacted soils where trees have been poorly planted, where many trees have been wounded, or where there has been a poor match of stock to site. When A. mellea is encountered on conifers, it is usually in an area where they are growing close to hardwoods.

Armillaria is often associated with forest and landscape trees. Symptoms of the disease are not clear-cut and may be similar to those caused by other diseases or environmental conditions. Commonly, an infected tree will show a gradual decline, evidenced by small, yellowish leaves, reduced growth, and dieback of branches. Trees which are chronically infected may wilt suddenly during periods of stress, such as drought. Infected trees can be wind thrown but most die standing; many are predisposed to bark beetle infestation.
It spreads between trees occurs most commonly via fungal growth across root contacts and to a much lesser extent by rhizomorphs. Rhizomorphs can grow short distances (up to a few feet) through soil to cause infection on nearby susceptible roots. Some trees can effectively resist the fungus at the time of initial infection by walling it off. However, if the tree subsequently becomes stressed or is cut, the fungus may break out of the callus tissues formed around infected areas on roots and spread rapidly in the wood. Once in a root, the fungus spreads proximally and distally within it. Fungal mycelia can survive for at least 35 years in old-growth stumps and roots before being replaced by other organisms. Survival of the fungus is influenced by stump size, tree species, and habitat type. Larger infected stumps provide a more substantial food base and longer survival. The fungus survives longer in stumps of resinous hosts than in those of nonresinous hosts. Spore spread can occur but is uncommon

These are Phytoplasmas, formerly called mycoplasma.

 Common hosts include white ash (Fraxinus americana) and greenash (F. pennsylvanica), blue ash (F. quadrangulata), black ash (F. nigra), and velvet ash (F. velutina). All Fraxinus spp. can be considered susceptible.

Ash yellows has been reported only in North America. The main range of the disease includes parts of 16 northeastern and midwestern states and the southernmost portions of the Canadian provinces of Ontario and Quebec. Ash yellows has also been found in two southwestern locations.

Environmental factors such as drought, excessive salt, or root damage have long been known to contribute to ash yellows.

Field diagnosis of ash yellows is sometimes difficult, however laboratory tests for ash phytoplasma are now available. Reduced growth, deliquescent branching, “witches brooms” and progressive decline are typical symptoms of ash yellows, but may also result from other factors.

The Phytoplasma moves thru sieve cells in the phloem, causing death of the inner bark. Progressive root loss results in chlorosis, stunting, decline and death. Leafhoppers carrying phytoplasma, transmit the pathogen while feeding on host trees. Symptoms may develop up to 3 years after phytoplasma is detected in ash phloem. Tolerance to ash phytoplasma is common.