Growing Knowledge

An aversion to verticillium With proper attention, verticillium wilt disease can be managed JERRY WEILAND

becomes contaminated (by soil or infest- ed hardwood mulch) or if roots grow from the container into infested soil. The pathogen is very common in agricultural fields and has an extremely wide host range. Verticillium dahliae infects hundreds of economically impor- tant plant species, including ornamental flowers (coneflower, dahlia), vegetables (potato, lettuce), fiber crops (cotton), crops for essential oils and flavorings (mint, bay), fruit crops (raspberry, strawberry), nut trees (pistachio), and ornamental trees and shrubs (maple, smokebush), among many others. Most knowledge about V. dahliae comes from the agricultural food and fiber crop industries for plant hosts such Verticillium dahliae is a fungus affecting certain susceptible plants, mostly when they are field grown. It starts in the roots and can work its way up through the vascular system, as potato, mint, strawberry, and cot- where infested plants will show discoloration. ton. Very little is known about how the pathogen affects woody plant species By Jerry Weiland and, as a consequence, disease manage- Verticillium wilt is a one of the most ment options for nurseries are limited. serious diseases affecting field produc- Nevertheless, we do know some tion of ornamental trees and shrubs. basic aspects of the pathogen’s biology The disease is caused by the soil- that dictate which disease control mea- borne fungus, Verticillium dahliae. (Note: sures will be effective. V. dahliae was first isolated from dahlias, The next few sections discuss which gave rise to part of the scientific important points about the V. dahliae name for this pathogen.) life cycle, its host range, and include a

It is rarely a problem in container- ▲ ized plants unless the potting media 34

An ongoing series provided by Oregon State University in collaboration with the United States Department of 50 ▲ Agriculture and in partnership with OAN FEBRUARY 2013 ▲ DIGGER 33 ▲ aversion to verticillium

description of common verticillium wilt It survives as tiny fungal structures and then larger roots to die. symptoms. The final section addresses called microsclerotia. These are dark, Once inside, the fungus may also disease management strategies that can microscopic, thick-walled cell masses colonize the plant’s water-conducting help minimize the damage caused by that are resistant to drying, flooding, tissues and produce tiny spores that this destructive pathogen. and temperature extremes. can quickly move up through the plant Microsclerotia germinate and in the sap stream. These spores lodge Life cycle produce hyphae (filamentous fungal in other parts of the tree or shrub. Verticillium dahliae is extremely strands) when roots of a host plant There, they plug the water-conducting well-built for survival. Once it is in the grow nearby. The hyphae then pen- vessels, thereby initializing the wilt soil, the fungus can endure for years in etrate into the root system and begin symptoms that give rise to the name of the absence of a suitable host. to spread slowly, causing the fine roots the disease. As the plants begin to die, micro- sclerotia are produced in the leaves,

J erry W eiland stems, seeds, and roots. These infected parts are then important sources of inoc- ulum for new infections. Infected leaves or seeds may blow into new areas, thus spreading the pathogen into previously clean field soils. Infected wood chips can also transmit the fungus.

A tale of two verticilliums Historically, V. dahliae has often been confused with a different species of Verticillium called V. albo-atrum. As a result, many older articles incorrectly listed V. albo-atrum as the causal agent of verticillium wilt in many woody plant species. However, the two pathogens are now officially recognized as distinct species with different host ranges. Verticillium dahliae is the most common species isolated from orna- mental and shade trees, and has a huge host range. Some of the most susceptible woody plant genera grown in the region include ash (Fraxinus), goldenrain tree (Koelreuteria), maple (Acer), redbud (Cercis), and smoke- bush (Cotinus). All conifers (pine, spruce, yews, firs, and others) and grasses are con- sidered resistant or immune, as well as birch (Betula), boxwood (Buxus), hackberry (Celtis), katsura tree (Cercidiphyllum), dogwood (Cornus), beech (Fagus), honeylocust (Gleditsia), sweetgum (Liquidambar), and syca- more (Platanus). A more complete list- ing of resistant and susceptible species should be consulted if you suspect V. Acute and chronic symptoms of verticillium wilt in sugar maple: leaf scorch and stunting (left), premature fall coloration and stunting (middle), healthy plant (right). dahliae is a problem in your fields. Diseases of Trees and Shrubs by Sinclair

34 FEBRUARY 2013 ▲ DIGGER J erry W eiland

This magnified image shows a single microsclerotum Premier Supplier of Nursery Products produced by the soilborne fungus Verticillium dahliae. The organism is 20 micrometers wide, which • Shipping Materials - Stickers, shelves and equals 2 hudredths of a millimeter. pallets made to your specs, pallet repair boards, shipping gates, tilt sticks, used pallets and Lyon offers one such guide. • Packing Supplies In contrast to V. dahliae, V. albo- - Shrink wrap, banding • Planting Stakes atrum has a much more limited host - Multiple sizes available range and has been mainly isolated • Treated Lumber from hops (Humulus lupulus), tree-of- - Multiple sizes available heaven (Ailanthus altissima), potato, and alfalfa. Therefore, if you have any soil analyses conducted for the pres- 503-434-5525 1726 SW Highway 18, McMinnville, OR 97218 • www.dstakemill.com 6152 ence of Verticillium in tree or shrub production fields, make sure that the results are based on V. dahliae and not V. albo-atrum.

Symptoms Symptoms of verticillium wilt usual- ly begin to appear on trees and shrubs in mid-to-late summer, and may occur more quickly on plants experiencing water or transplant stress. Therefore, ROOTING COMPOUND the best time to scout for the disease Soluble Concentrate is from summer to early autumn when symptoms of wilting and dieback are Continued excellence from our family owned company for over 30 years most apparent. There are two phases of symptoms: acute and chronic. Acute symptoms WOOD’S ROOTING COMPOUND 3 SIZES! occur in a relatively short time span Our unique formula uses only the highest quality (days or a few weeks) and include ingredients which results in instant absorption of wilting, leaf curling, premature fall both IBA & NAA to your cuttings. coloration, leaf loss, branch dieback, and death. Chronic symptoms occur 3 Convenient Sizes: over a longer period of time (over a 4 oz. / Pint / Gallon single growing season or across mul- tiple years) and are typified by slow- Call Today For A Distributor Near You growing, stunted plants with sparse or undersized leaves and twigs. 503-678-1216 In some cases, leaf scorch (brown- ing of leaf margins) or abundant seed P.O. Box 327 Wilsonville, OR 97070 production is observed. Both acute www.earthscienceproducts.com

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FEBRUARY 2013 ▲ DIGGER 35 ▲ aversion to verticillium

and chronic symptoms may occur on any detection of the pathogen in field a susceptible host. the same plant and the symptoms may soils should be considered a risk. Finally, soil solarization has shown either affect the entire plant or occur on It may also pay to know the prior some success in western Oregon only a few scattered branches. cropping history of the field. Fields with at reducing V. dahliae populations. If you cut into infected branches or a history of mint, raspberry, strawberry, However, given that solarization is most stems, the sapwood may show a dark potato or other vegetable (squash, toma- effective in the upper soil profile and greenish-black or reddish-brown streak- to, cabbage, etc.) should be used with that it does not completely eradicate the ing along the direction of the grain. caution as these crops are also hosts for pathogen, solarization may not be an This will appear as a circular or semicir- V. dahliae and can leave behind substan- effective option for the production of cular pattern in cross section. tial amounts of the fungus. susceptible tree and shrub species. In order to minimize the acciden- Management tal introduction of the pathogen, never Summary Avoidance is the best management move infested stock into fields where Verticillium dahliae is a challeng- strategy. Before planting a new field the pathogen is absent and inspect all ing pathogen for many ornamental with susceptible plant species, be sure incoming stock for symptoms of the dis- nurseries to deal with because it is to send soil samples to a private or ease. If possible, all equipment moved widespread, has a large host range, and public laboratory to test for the pres- from infested fields to clean fields survives for years in the soil. In fields ence of V. dahliae. should be cleaned and/or disinfested. where the pathogen is absent, avoid- As little as 1 microsclerotia per Once V. dahliae is present in ance is key and all attempts should be gram of soil (1 CFU/g or 1 ppg) has a particular field, the most practi- made to prevent the pathogen from been associated with disease. Therefore, cal option is to plant resistant tree being introduced. In locations where and shrub species. Susceptible plants the pathogen is present, resistant plant should never be planted in soils where species should be used. the pathogen is found. In some cases, resistant cultivars of normally suscep- References tible tree species are available, and Pinkerton, J. N., Ivors, K. L., Miller, these may also be a good option. M. L., and Moore, L. W. 2000. Effect Infected nursery stock (includ- of soil solarization and cover crops on ing the root system) should be culled populations of selected soilborne patho- and burned to destroy any potential gens in western Oregon. Plant Disease inoculum. Do not chip infected trees for 84: 952-960. mulch, as infested mulch can spread the Sinclair, W. A. and Lyon, H. H. pathogen to other locations. 2005. Diseases of Trees and Shrubs. Fumigation is effective at signifi- Second Edition. Cornell University cantly knocking back soil populations Press. Ithaca, NY. and thereby reducing disease incidence. Stoven, H., Owen, J., and However, V. dahliae is unlikely to be Santamaria, L. 2012. A safe sub- completely eradicated. As the soil popu- strate? Investigating the virulence of lations build back up over time, the Verticillium in hammer-milled shade Ben Piper 503-710-1778 infested fields will need to be periodi- trees when used as an alternative sub- cally refumigated. strate. Digger. August 2012: 121-127. Ken Doubrava 503-866-6116 Fungicides are largely ineffective, and do not cure infected plants. Crop Dr. Jerry Weiland is a research plant Jim McKay 503-710-1777 rotation does not work because of the pathologist with the Horticultural Crops pathogen’s ability to survive in the soil Research Laboratory, USDA Agricultural for long periods of time as microscle- Research Service in Corvallis, OR. rotia. The fungus can afford to wait for Jerry is also an Assistant Professor in the next suitable host to be planted. In the Department of Botany and Plant addition, V. dahliae is sometimes able Pathology at Oregon State University. His to persist on the roots of some weed group studies the biology, epidemiology, species and resistant hosts without caus- and management of soilborne pathogens

WILBUR-ELLIS Logo and Ideas to Grow With are registered trademarks of Wilbur-Ellis Company. K-1012-790 ing symptoms, thereby sustaining its on nursery crops. He can be reached at population in the soil in the absence of [email protected].

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