Dutch elm disease (diagnostics)

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Sordariomycetes
Subclass	Sordariomycetidae
Order	Ophiostomatales
Family	Ophiostomataceae
Genus	Ophiostoma

Scientific Name

Ophiostoma ulmi

Scientific Name Synonyms

Ceratocystis ulmi

Ceratostomella ulmi

Pesotum ulmi

Common Name

Dutch elm disease

Author: Clarissa Balbalian, Mississippi State University

Reviewed by: William MacDonald, West Virginia University

Pathogen

Ophiostoma ulmi is the ascomycete fungus that caused the original Dutch elm disease pandemic in the 1920's - 1940's. However, a more aggressively pathogenic species Ophiostoma novo-ulmi Brasier, has displaced O. ulmi and is the cause of the current pandemic. Ophiostoma novo-ulmi is divided into a North American subspecies (O. novo-ulmi subsp. americana) and a European subspecies (O. novo-ulmi subsp. novo-ulmi).

Sexual Stage

Perithecia are black and flask-shaped. The setose bases of the perithecia range from 75-150 μm wide. Perithecial necks are long (160-640 μm). Asci are evanescent. Ascospores are hyaline, one-celled, and reniform in shape, about 4.5-6.0 μm x 1.5 μm. Conidia often exude in a sticky droplet perched atop the perithecial neck. The sexual stage is rarely found in nature.

Asexual Stages

Asexual spores are of three types.

1. Hyaline, single-celled, ovoid conidia that are 2.0-5.0 μm x 1.0-3.0 μm and borne in a white drop atop a black synnema, forming a coremium.

2. Conidia measuring 4.0-6.0 μm x 2.0-3.0 μm are borne in clusters on short denticles, directly on the vegetative mycelium. These conidia are hyaline, single-celled, elongated, pyriform and often have a convex curve on one side.

3. The fungus also produces yeastlike single-celled, hyaline spores of variable size that form by budding.

Symptoms and Signs

Ophiostoma ulmi causes symptoms typical of vascular wilt.

Foliar Symptoms

Wilting of green leaves progresses to yellowing and browning, and leaf abscission is common. Crown symptoms range from branch flagging to complete canopy involvement, depending on where the fungus invades the tree.

Vascular Symptoms

Brown discoloration is usually associated with the current year's growth ring (xylem) in branches and stems.

Insect Signs

Smaller European elm bark beetle (Scolytus multistriatus Marsham) preferentially feeds in the crotches of small twigs and can cause these twigs to fall. Brood galleries are formed under the bark, parallel to the grain of the wood. Larval feeding galleries are horizontal to the grain. The adult beetle is a shiny, dark reddish-brown color and about 1/8 inch long. The underside of the posterior portion of the beetle is concave and the beetle has an obvious abdominal spine-like projection that points toward the rear of the insect. Larvae are small, white, legless grubs that are about the same size as the adult beetle.

Native elm bark beetle (Hylurgopinus rufipes (Eichhoff)) differs from smaller European elm bark beetle in its feeding site preferences, breeding gallery orientation and morphology. Native elm bark beetle feeds on large branches and trunks. Brood galleries are horizontal to the grain of the wood and larval feeding galleries are parallel to the grain. Adults of the native elm bark beetle are dull, brownish-black and less than 1/8 inch long. The native elm bark beetle lacks the concave shape and abdominal projection of the smaller European elm bark beetle.

Ecology and Spread

Dutch Elm Disease Cycle

Ophiostoma ulmi is a vascular wilt pathogen of the Ulmaceae, primarily elm species. Spores of the fungus are vectored by bark beetles in the species Hylurgopinus and Scolytus and also spread from tree to tree via root grafts. The fungus overwinters in diseased and dead trees, which are favored breeding sites for the bark beetle vectors. The coremial stage of the fungus often colonizes the brood galleries of the beetles, and the perithecial stage may occasionally be found there as well. Sticky spore droplets adhere to the adult beetles as they emerge from the tree. The newly emerged beetle vector feeds on healthy trees, inoculating the xylem in the feeding wounds with spores of the fungus.

The most efficient North American vector is Scolytus multistriatus (the smaller European elm bark beetle), a non-native species that feeds in twig crotches, so most infections begin in twigs. In North America's northeastern region the native elm bark beetle Hylurgopinus rufipes is the more prevalent vector. However, because the native elm bark beetle preferentially feeds on large branches and smaller trunks, where bark is thicker, the likelihood of successful inoculation of the xylem is reduced. The feeding habit of native elm bark beetle makes it a less efficient vector and results in infections that begin in major branches.

The yeastlike stage of the fungus moves with the sap throughout the tree via the xylem. Hyphal growth also helps the fungus spread between vessels. The fungus usually spreads from the inoculation site to the roots during the first season of infection. The fungus proliferates in the roots and then is drawn up by the sap flow, leading to a systemic infection that kills the tree within a growing season. The fungus also can spread from tree-to-tree via root grafts.

Geographic Distribution

Ophiostoma ulmi was found throughout the range of elm in North America and also was present throughout Europe and Asia as well, but is reportedly rarely found now as it has been displaced by the more aggressive subspecies of Ophiostoma novo-ulmi. Ophiostoma novo-ulmi subsp. americana has been documented in Canada, and in the United States in the mid-Atlantic and northeastern states, the Great Lakes states, Washington and California. Subspecies americana also occurs in Europe (Germany, Ireland, Italy, Netherlands and Scandanavia).

Ophiostoma novo-ulmi subsp. novo ulmi occurs throughout Europe, as well as southwest and central Asia.

Management

Management of any pathogen is often dependent upon both cultural and chemical options. Consult your local extension specialist or agent for recommendations relevant to your particular host and state.

Sanitation

Sanitation is the most important part of Dutch elm disease management. Any cut elm wood, whether from healthy, dying or dead elms should be debarked, buried or burned to discourage colonization by bark beetles and prevent beetle emergence from infested wood.

Breaking Root Grafts

Mature elms that are within 25-50 ft. (9-15 m) of one another likely share grafted roots. Root grafts should be broken if Dutch elm disease is detected in one of the trees. Disrupting the graft before removing the infected tree is extremely important because once the trunk of the infected tree is cut, then the infested sap in the roots will be drawn into the adjacent healthy tree via intact grafts. Grafts can be disrupted mechanically with a 3-5 ft. blade (5 feet is best) or chemically by a professional pesticide applicator.

Pruning

New branch infections can be eradicated by pruning out the infected branch. If fungicides are being used in conjunction with pruning, then the fungicide should be injected before the branch is pruned. Because the infection may be more advanced than the foliar symptoms indicate, care should be taken to peel the bark from infected branches and examine the vascular tissue for staining. Infected branches should be removed at a branch fork at least 5 ft. (10 ft. is better) below the last visible sign of vascular streaking. If the branch is pruned during the growing season, then an application of tree paint to the cut surface is recommended to help prevent beetles from being attracted to the wounded tree. For pruning to be the most successful, constant monitoring of the canopy for new symptoms is necessary, followed by immediate excision of the affected branches.

Fungicides

Valuable amenity trees can be preventatively or therapeutically treated with fungicide injections. Therapeutic treatment is usually not effective if more than 10% of the canopy is visibly symptomatic. Fungicides are not curative and must be reapplied at label intervals for the life of the tree.

Fungicide Table derived from Haugen and Stennes, 1999.

Active Ingredient	Injection Style	Pros	Cons
Thiabendazole hypophosphite (Arbotect 20S)	Macroinjection into root flares	2-3 years of protection. Has shown long term protection when applied every 2-3 years at the highest label dosage.	Effective period of the chemical may be closer to 2 years in warm, southern climates with longer growing seasons. There are reports of foliar phytotoxicity and damage to the cambium and parenchyma cells surrounding the injection site.
Propiconazole (Alamo)	Macroinjection into root flares	Effective for preventative and therapeutic treatment. Preventative treatment may be effective for 2 years at low label rate and up to 3 years at the high rate. Requires dilution in less water than thiabendazole hypophosphite, making treatment faster.	Shorter residual activity than thiabendazole hypophosphite. Severe phytotoxicity at high dosages on small diameter trees treated early in the growing season
Debacarb plus carbendazim (Fungisol)	Microinjection canisters	Low phytotoxicity	Efficacy for preventative and therapeutic treatment is questionable due to low dosage rate of active ingredient.
Tebuconazole (Tebuject 16)	Microinjection capsules		Efficacy and phytotoxicity data not available
Copper sulphate pentahydrate (Phyton 27)	Macroinjection into trunk no more than 6 inches above the soil line.		Marginal efficacy as a preventative and therapeutic treatment. Defoliation is common but is usually followed by refoliation. Severe vascular tissue damage and discoloration at the injection site.

Remember: the label is the law.

Insecticides

Insecticides are a rarely utilized component of Dutch elm disease management. Beetle emergence can occur from April through October, making timing of insecticide application very important. Insecticides should be applied to the bark during the dormant season, before adult beetles emerge in the spring. Complete coverage of the bark is critical. Active ingredients labeled for use on elm and targeting native elm bark beetle and smaller European elm bark beetle include bifenthrin, carbaryl, permethrin and lambdacyhalothrin.

Resistant or Tolerant Trees

Planting resistant or tolerant elms is a viable way of keeping an elm component in landscapes. When selecting elms, consideration also should be given to elm yellows susceptibility. Elm yellows is an insect vectored phytoplasma for which there is no cure or treatment. The table below provides information on the susceptibility of select elm species, cultivars and hybrids to both Dutch elm disease and elm yellows. This list is not a comprehensive list of all commercially available elm cultivars and hybrids.

Elms resistant or tolerant to Dutch elm disease and information on elm yellows susceptibility

Species	Cultivar	Resistant	Tolerant	USDA Hardiness Zone	Elm Yellows
Ulmus americana (American or white elm)	'Jefferson'		X	5-7	Susceptible
	'Lewis and Clark'		X	2-3	No Information
	'New Harmony'		X	5-10	Susceptible
	'Princeton'		X	4-7	Susceptible
	'Valley Forge'		X	4-9	Susceptible
Ulmus davidiana (David or Japanese elm)		X		5-9	Resistant
Ulmus parvifolia (Chinese or lace-bark elm)	'Allee'	X		5-8	Susceptible
	'Athena'	X		5-9	Resistant
	'Dynasty'	X		5-10	Resistant
	'Everclear'	X		4-9	No information
Ulmus pumila (Siberian elm)		X		4-9	Resistant
Ulmus pumila x Ulmus japonica	'Cathedral'		X	4-7	No information
Ulmus carpinifolia x Ulmus parvifolia	'Frontier'	X		5-7	Tolerant
Ulmus japonica x Ulmus pumila	'New Horizon'	X		4-7	No information
Ulmus 'Urban' x Ulmus wilsoniana 'Prospector'	'Patriot'	X		4-7	Tolerant
Ulmus glabra x Ulmus carpinifolia	'Pioneer'	X		5-8	Tolerant
Ulmus wilsoniana	'Prospector'	X		4-7	Tolerant
Ulmus japonica x Ulmus wilsoniana	'Accolade'	X		4-9	Resistant
Ulmus glabra x U. carpinifolia x U. pumila	'Homestead'	X		5-8	Resistant

Diagnostic procedures

Wilting leaves and discolored (brown) xylem are the characteristics used for field diagnosis of the disease. The perithecial and coremial stages of the fungus may be observed fruiting in beetle galleries of infected trees. Coremia production can be induced in the laboratory by incubating discolored vascular tissue in a moist chamber at room temperature. The perithecial stage is rarely observed.

The fungus can be isolated from twigs with discolored xylem by surface sterilizing the twig, removing the bark, and placing excised pieces of discolored xylem onto solid culture medium. O. ulmi grows on a wide variety of synthetic agars and agars containing plant extracts. Amending the culture medium with cycloheximide and antibiotics reduces bacterial competition.

Conidia will form on most fungal culture media at room temperature.

Coremia will form readily on sterilized elm wood that has been inoculated with fungal culture.

Perithecia also can be obtained on sterile elm wood, but both mating types are required for perithecial formation.

All structures also form on elm sapwood agar (ESA): 50 g. of elm sapwood (obtained by peeling and milling elm twigs up to 0.5 cm diameter), 15 g. of agar, 500 ml. water autoclaved for 15 minutes (Brasier, 1981). Shake during pouring to maintain dispersal of sapwood.

A selective medium for isolation and storage: Brasier recommends dissolving 0.2 g. of cylcoheximide in 100 ml of distilled water, then adding 50 ml of the cycloheximide solution to 1 L of malt agar and autoclaving 15 minutes. Cool the medium to about 45°C, add streptomycin alone or in combination with penicillin to make the final antibiotic concentration 0.1% 0f the liquid medium.

Resources and References

• Biggerstaff, C., Iles, J.K., and Gleason, M.L. 1999. Sustainable urban landscapes: Dutch elm disease and disease-resistant elms. Iowa State University Extension.
• Brasier, C.M. 1981. Laboratory investigations of Ceratocystis ulmi. In: Stipes, R.J. and Campana, R.J. (Eds.), Compendium of Elm Diseases (pp. 76-79). St. Paul, Minnesota: APS Press 76-79.
• Brasier, C.M. and Kirk, S.A. 2001. Designation of EAN and NAN races of Ophiostoma novo-ulmi as subspecies. Mycol. Res. 105 (5):547-554.
• Haugen, L. 1999. How to identify and manage Dutch elm disease. In Ash, C. (Ed.), Shade tree wilt diseases (pp. 37-51). St. Paul, Minnesota: APS Press.
• Haugen, L. and Stennes, M. 1999. Fungicide injection to control Dutch elm disease: Understanding the options. Plant Diag. Quart. 20 (2): 29-38.
• Sinclair, W.A., Townsend, A.M., Griffiths, H.M., and Whitlow, T.H. 2000. Responses of six Eurasian Ulmus cultivars to North American elm yellows phytoplasma. Plant Dis. 84:1266-1270
• Stipes, R.J. and Campana, R.J. 1981. Compendium of Elm Diseases. APS Press. St. Paul, Minnesota.

Acknowledgment

• National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30708 as part of "Diagnostic Image Series Development for Supporting IPM in the Southern Region" (USDA-NIFA-RIPM-003351)