''Candidatus'' Phytoplasma fraxini is a species of

phytoplasma Phytoplasmas are obligate intracellular parasites of plant phloem tissue and of the insect vectors that are involved in their plant-to-plant transmission. Phytoplasmas were discovered in 1967 by Japanese scientists who termed them mycoplasma-li ...

, a specialized group of bacteria which lack a cell wall and attack the

phloem Phloem (, ) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This transport process is ...

of plants. This phytoplasma causes the diseases ash yellows and lilac witches' broom.

Taxonomy

''Ca.'' Phytoplasma fraxini was first described in 1999 as the causal agent of ash yellows and lilac witches'-broom. It belongs to the 16 Sr group VII, subgroup A.

Distribution

The pathogen is found in Central and North Eastern USA and Central and Eastern Canada. It was reported from Chile for the first time in 2011. It was then reported from a range of trees in Colombia. In 2017 it was reported infecting

grapevine ''Vitis'' (grapevine) is a genus of 81 accepted species of vining plants in the flowering plant family Vitaceae. The genus consists of species predominantly from the Northern Hemisphere. It is economically important as the source of grapes, bot ...

in Iran.

Hosts

''Ca.'' Phytoplasma fraxini can infect a wide range of species of ''

Fraxinus ''Fraxinus'' (), commonly called ash, is a genus of plants in the olive and lilac family, Oleaceae, and comprises 45–65 species of usually medium-to-large trees, most of which are deciduous trees, although some Subtropics, subtropical specie ...

'' (ash) including: white ash, narrow leaved ash, bunge ash,

European ash ''Fraxinus excelsior'', known as the ash, or European ash or common ash to distinguish it from other types of Fraxinus, ash, is a flowering plant species in the olive family Oleaceae. It is native throughout mainland Europe east to the Caucasus ...

Oregon ash ''Fraxinus latifolia'', the Oregon ash, is a member of the ash genus ''Fraxinus'', native to western North America. Description ''Fraxinus latifolia'' is a medium-sized deciduous tree that can grow to heights of in height, with a trunk diamete ...

black ash Black ash is a common name for several plants and may refer to: * ''Acer negundo'', native to North America * ''Fraxinus nigra ''Fraxinus nigra'', or the black ash, is a species of ash native to much of eastern Canada and the northeastern United ...

, manna ash,

green ash Green is the color between cyan and yellow on the visible spectrum. It is evoked by light which has a dominant wavelength of roughly 495570 nm. In subtractive color systems, used in painting and color printing, it is created by a combi ...

, pumpkin ash,

blue ash ''Fraxinus quadrangulata'', the blue ash, is a species of ash native primarily to the Midwestern United States from Oklahoma to Michigan, as well as the Bluegrass region of Kentucky and the Nashville Basin region of Tennessee. Isolated populati ...

, Tianshan ash, Urapan and velvet ash. A number of ''

Syringa ''Syringa'' is a genus of 12 currently recognized species of flowering plant, flowering woody plants in the olive family or Oleaceae called lilacs. These lilacs are native to woodland and scrub from southeastern Europe to eastern Asia, and wid ...

'' (lilac) species and hybrids are recorded as hosts including: ''Syringa x diversifolia'', ''Syringa x henryi'', ''Syringa x josiflexa'', Hungarian lilac, nodding lilac, (cut-leaf lilac), Korean lilac, ''Syringa x nanceiana'', ''

Syringa oblata ''Syringa oblata''The Plant List http://www.theplantlist.org/tpl1.1/record/kew-356313Fiala, John L. "Lilacs: a gardener's encyclopedia", 2nd ed. copyright Timber Press 2008. rev. and updated by Freek Vrugtman. First ed. published 1988, copyright ...

'', ''

Persian lilac The name Persian lilac is commonly used for two different woody plants. * ''Syringa'' × ''persica'', a shrub in the olive family (Oleaceae) * ''Melia azedarach'', a tree in the mahogany family (Meliaceae Meliaceae, the mahogany family, is a fl ...

'', ''Syringa x prestoniae'', '' Syringa pubescens'', Chengtu lilac, '' Syringa tomentella'', late lilac,

common lilac ''Syringa vulgaris'', the lilac or common lilac, is a species of flowering plant in the olive family, Oleaceae. Native to the Balkan Peninsula, it is widely cultivated for its scented flowers in Europe (particularly the north and west) and Nort ...

and Yunnan lilac. In Canada, a few species of ''

Prunus ''Prunus'' is a genus of flowering plant, flowering trees and shrubs from the family (biology), family Rosaceae. The genus includes plums, cherries, peaches, nectarines, apricots and almonds (collectively Drupe, stonefruit). The genus has a cosm ...

'' have been recorded as hosts:

peach The peach (''Prunus persica'') is a deciduous tree first domesticated and Agriculture, cultivated in China. It bears edible juicy fruits with various characteristics, most called peaches and the glossy-skinned, non-fuzzy varieties called necta ...

and Pembina plum.

Diseases

Ash Yellows

This disease is relatively new so the disease cycle is mostly unknown, although it is thought that insect vectors are used as the mode of transmission. The disease is difficult to diagnose because some symptoms match those of stressed environmental conditions such as drought, flooding, or shallow soil. When trees become infected they can survive for many years or immediately die depending on the environmental conditions and health of the tree. The major symptom of this disease is

witches broom Witch's broom or witches' broom is a deformity in a woody plant, typically a tree, where the natural structure of the plant is changed. A dense mass of shoots grows from a single point, with the resulting structure resembling a broom or a bird ...

which causes branches in tufts. Unfortunately, there are not many strategies for controlling the disease besides avoiding areas where it is more prevalent.

Host and symptoms

Ash yellows refers to the disease that occurs on

ash Ash is the solid remnants of fires. Specifically, ''ash'' refers to all non-aqueous, non- gaseous residues that remain after something burns. In analytical chemistry, to analyse the mineral and metal content of chemical samples, ash is the ...

trees, usually white ash (''

Fraxinus americana ''Fraxinus americana'', the white ash or American ash, is a fast-growing species of ash tree native to eastern and central North America. White ash trees are threatened by the invasive emerald ash borer. The tree is highly valued as lumber. ...

'') and green ash (''

Fraxinus pennsylvanica ''Fraxinus'' (), commonly called ash, is a genus of plants in the olive and lilac family, Oleaceae, and comprises 45–65 species of usually medium-to-large trees, most of which are deciduous trees, although some subtropical species are ever ...

''). All ages and sizes of ashes are vulnerable to infection and symptoms will occur within three years of infection. Some of these symptoms include progressive loss of vitality, subnormal growth, and leaves that fail to reach normal size and are often light green to

chlorotic In botany, chlorosis is a condition in which leaves produce insufficient chlorophyll. As chlorophyll is responsible for the green color of leaves, chlorotic leaves are pale, yellow, or yellow-white. The affected plant has little or no ability to ...

. Additional symptoms include branch dieback, cracks in the bark, early color change in the fall, and premature death of trees. This disease is often diagnosed by the presence of a distinct symptom known as

. Witches broom occurs when there is an overgrowth of the branches of the host, resulting in a clumps of branches that resemble the head of a

broom A broom (also known as a broomstick) is a cleaning tool, consisting of usually stiff fibers (often made of materials such as plastic, hair, or corn husks) attached to, and roughly parallel to, a cylindrical handle, the broomstick. It is thus a ...

. This usually occurs near the soil line but can occur higher up in the tree as well.

Disease cycle

Not much is known about the disease cycle of ash yellows. It is unknown how exactly the ''Phytoplasma'' enters the tree and how it spreads but insect vectors are suspected. This is due to the fact that ''Phytoplasma''s are often spread in the

saliva Saliva (commonly referred as spit or drool) is an extracellular fluid produced and secreted by salivary glands in the mouth. In humans, saliva is around 99% water, plus electrolytes, mucus, white blood cells, epithelial cells (from which ...

ry secretions of insects during feeding. One likely candidate for spreading these ''Phytoplasma''s, also known as ''

Mycoplasma ''Mycoplasma'' is a genus of bacteria that, like the other members of the class ''Mollicutes'', lack a cell wall, and its peptidoglycan, around their cell membrane. The absence of peptidoglycan makes them naturally resistant to antibiotics ...

''-like organisms (MLOs), is

leafhopper Leafhopper is the common name for any species from the family (biology), family Cicadellidae: based on the type genus ''Cicadella''. These minute insects, colloquially known as hoppers, are plant feeders that suck plant sap from grass, shrubs, or ...

s because data shows they are the most common insect vector for MLOs. Once inside the host, the ''Phytoplasma'' attacks the tree's vascular system, specifically targeting the

sieve tubes. Additionally, the severity of the disease symptoms caused by the ''Phytoplasma'' varies. In one study that was conducted, 12 different strains of ''Phytoplasma''s belonging to the ash yellows group were tested and it was found that these strains varied significantly in aggressiveness and the impact they had on growth of the host.

Environment

The disease typically occurs in woodlots and forests, home landscapes, and urban settings. The geographic origin of ash yellows is unknown but currently the disease is only reported to be in

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

. Some environmental factors that could contribute to the growth of the disease include mechanical damage, insect infestations, and

fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

. One major environmental condition that has been shown to amplify the growth of the disease includes drought.

Management

There is no known way to cure ash yellows, so it is important to avoid growing ashes where ash yellows is prevalent. Some management strategies include promoting species diversity to reduce plant stress and limit competition among the ash trees. In addition, it can be effective to remove infected trees with dieback and place

irrigation Irrigation (also referred to as watering of plants) is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has bee ...

systems so that during dry periods the ashes are not as susceptible to ash yellows. One final control method is to use

insecticide Insecticides are pesticides used to kill insects. They include ovicides and larvicides used against insect eggs and larvae, respectively. The major use of insecticides is in agriculture, but they are also used in home and garden settings, i ...

, however it is unclear if the use of insecticide to control leafhoppers is an effective way to prevent the spread of the ''Phytoplasma'' that causes ash yellows.

Importance

Ash yellows has been found to be present in the

U.S. The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 states and a federal capital district, Washington, D.C. The 48 contiguous ...

as well as some parts of

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

. In one study, the ash yellows ''Phytoplasma'' was discovered in 102 out of 106 ash populations that were sampled from six US states and three Canadian provinces. Within the ash populations that were sampled, 50% of the trees had crown die-back of 10% or more caused by ash yellows. In a different study that looked at the annual increase of ash yellows in six populations of white ash in

New York New York most commonly refers to: * New York (state), a state in the northeastern United States * New York City, the most populous city in the United States, located in the state of New York New York may also refer to: Places United Kingdom * ...

, the average annual increase in disease incidence was found to be 4.5%. This study also found that disease incidence was lowest in the two populations where other plant species were present. The reason that all of this is of importance is that wood from ash trees can be used to make a number of products including

baseball bat A baseball bat is a smooth wooden or metal Club (weapon), club used in the sport of baseball to hit the Baseball (ball), ball after it is thrown by the pitcher. By regulation it may be no more than in diameter at the thickest part and no more t ...

s and

furniture Furniture refers to objects intended to support various human activities such as seating (e.g., Stool (seat), stools, chairs, and sofas), eating (table (furniture), tables), storing items, working, and sleeping (e.g., beds and hammocks). Furnitur ...

. In addition to this ash trees also provide food and habitat to a number of animals including

cardinals Cardinal or The Cardinal most commonly refers to * Cardinalidae, a family of North and South American birds **''Cardinalis'', genus of three species in the family Cardinalidae ***Northern cardinal, ''Cardinalis cardinalis'', the common cardinal of ...

and

wood duck The wood duck or Carolina duck (''Aix sponsa'') is a partially migratory species of perching duck found in North America. The male is one of the most colorful North American waterfowls. Taxonomy The wood duck was Species description, formal ...

Lilac Witches'-broom

Lilac witches’-broom (LWB) is a disease of

lilac ''Syringa'' is a genus of 12 currently recognized species of flowering woody plants in the olive family or Oleaceae called lilacs. These lilacs are native to woodland and scrub from southeastern Europe to eastern Asia, and widely and commonly ...

s caused by ''Candidatus Phytoplasma fraxini''. This ''Phytoplasma'' was first identified as the causal agent of

Ash yellows ''Candidatus'' Phytoplasma fraxini is a species of phytoplasma, a specialized group of bacteria which lack a cell wall and attack the phloem of plants. This phytoplasma causes the diseases ash yellows and lilac witches' broom. Taxonomy ''Ca.'' ...

and has since been attributed to both diseases. Symptoms of the LWB ''Phytoplasma'' include

witch's broom Witch's broom or witches' broom is a deformity in a woody plant, typically a tree, where the natural structure of the plant is changed. A dense mass of shoots grows from a single point, with the resulting structure resembling a broom or a bi ...

s, shortened

internodes A stem is one of two main structural axes of a vascular plant, the other being the root. It supports leaves, flowers and fruits, transports water and dissolved substances between the roots and the shoots in the xylem and phloem, engages in pho ...

on new growth, twig dieback, overall loss of vitality and premature death.

References

{{Taxonbar, from=Q3901764 Bacterial tree pathogens and diseases Mollicutes Bacterial plant pathogens and diseases Ornamental plant pathogens and diseases Candidatus taxa