Disease Profile

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What is Verticillium Wilt?

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Verticillium wilt is a vascular wilt disease caused by a fungus. The fungus may be one of five known species in the genus Verticillium [1]. Verticillium is found naturally in the soil and is common in Nevada. The fungus enters a tree through its root system and spreads throughout the outer layer of xylem, or sapwood of the tree [2][3]. Sapwood is the part of the tree’s vascular system responsible for transporting water [3]. As the fungus spreads systemically through a tree’s water vessels, it plugs them, and a tree’s crown begins to die back from a lack of available water.

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What types of trees are susceptible?

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We see it most commonly in Norway maples, but other susceptible trees include boxelder, tree-of-heaven, tuliptree, catalpa, elm, ash, black locust, lilac, russian-olive, horsechestnut, prunus species, goldenraintree, redbud, yellowwood, and sometimes linden [2][4].

 

Resistant Trees include apple, beech, birch, crabapple, fir, hawthorne, honey locust, linden, mountain-ash, mulberry, oak, pear, pine, poplar, spruce, sycamore, walnut, willow, zelkova, and some others [2].

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Consequences and Severity

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The disease may be fatal to some trees within one season. Others may take many years to succumb, or they may recover on their own [2]. However, there may be stunted growth, and branch dieback may result in canopy holes, misshapen crowns, and permanent structural damage [2].

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Diagnostic Clues

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Verticillium can only be confirmed by lab testing, but there are symptoms we look for to make a probable diagnosis [2]. During hot, dry periods, leaves will suddenly wilt and dry up [2]. Leaf scorch, or the browning of leaf margins, is present. If trees experience uniform symptoms of wilting or leaf scorch, it may only be drought stress or another abiotic disorder. If the wilting is nonuniform–symptoms on one side of a tree only or on select branches only–it’s more likely to be Verticillium.

 

Verticillium will cause vascular discoloration, which is darkening of the xylem [3]. The presence of discoloration is a strong indicator, but it could be the symptom of another disorder [3]. When cutting affected branches, look for a ring of discolored wood. Peel bark to see if there are longitudinal streaks on the sapwood, which would be gray, brown, or greenish [2]. 

 

Management

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Verticillium wilt can only be prevented by planting disease-resistant trees. It can be eradicated from the soil before planting, but it is very difficult. It cannot be eradicated once it is within a tree, but suppressing it may be possible with management. In most cases, it should be managed with cultural and mechanical controls. In high value trees, a systemic fungicide may provide additional help with suppression.

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Cultural and Mechanical Control

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Avoid planting susceptible species of trees in old vegetable gardens where the Verticillium fungus is most present [2]. Maintain healthy trees through mulching, properly fertilizing, and deep watering [2]. Prune out affected branches one foot below any discoloration [2]. Remove trees that are too far gone; they will not recover [2]. Prune out dead wood where targets may be impacted by branch failure.

 

Chemical Control

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Systemic fungicides

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Systemic treatment of PHOSPHO-jet, developed by Arborjet, can be applied by trunk/basal bark spray and foliar spray. It is absorbed and translocated throughout the entire tree by its vascular system, and remains effective for 6-12 months [5].

 

PHOSPHO-jet (containing the active ingredient Phosphorous Acid) inhibits fungal cells and increases a tree’s natural defense system which includes stronger plant cells, root development, and quicker recovery time [6][7].

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Biological Control and Biorational fungicides

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There are no biorational fungicides available that are effective against Verticillium wilt, but new research suggests that we may see biological control products in the future. A study in 2020 showed effective suppression of Verticillium wilt in olive trees using an antifungal bacterium called Bacillus velezensis XT1 [8]. Hopefully in the future we will be able to treat Verticillium within the soil, and within the vascular system of trees, with a biosafe product.

 

References

 

1. Plant pathogenic Verticillium species: how many of them are there? Dez J. Barbara, Emily Clewes. https://bsppjournals.onlinelibrary.wiley.com/doi/abs/10.1046/j.1364-3703.2003.00172.x

 

2. Verticillium Wilt of Shade Trees. https://extension.usu.edu/pests/ipm/notes_orn/list-treeshrubs/verticillium-wilt

 

3. Arborists’ Certification Study Guide. International Society of Arboriculture, 2022.

 

4. Plants Resistant or Susceptible to Verticillium Wilt. UC Davis. http://depts.washington.edu/hortlib/resources/ucdavis_verticillium.pdf

 

5. PHOSPHO-jet Label. Arborjet. https://arborjet.com/product/phospho-jet/

 

6. Anthracnose Treatments | Leaf Blight Solutions | Arborjet. https://arborjet.com/problems_solutions/anthracnose/

 

7. PHOSPHOROUS ACID FUNGICIDES EXPLAINED. John Hartman and Kenny Seebold. http://www.uky.edu/Ag/kpn/kpn_05/pn050613.htm

 

8. Castro D, Torres M, Sampedro I, Martínez-Checa F, Torres B, Béjar V. Biological Control of Verticillium Wilt on Olive Trees by the Salt-Tolerant Strain Bacillus velezensis XT1. Microorganisms. 2020; 8(7):1080. https://doi.org/10.3390/microorganisms8071080

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