Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Sordariomycetes
Subclass	Hypocreomycetidae
Order	Glomerellales
Family	Plectosphaerellaceae

Scientific Name

Verticillium

Common Name

Verticillium wilts

Author: Alicyn Ryan, University of Florida

Reviewed by: Jeffrey Rollins, University of Florida

Pathogen

Verticillium dahliae is commonly found in temperate areas and infects a wide range of hosts that includes annuals, perennials, and woody plants. Over 200 dicotyledonous species have been identified as susceptible hosts of this pathogen. Agricultural hosts include spinach, strawberries, lettuce, and artichokes, among others (1). This a very difficult pathogen to control because it can persist in the soil for years without a susceptible host (2, 3).

Symptoms and Signs

Symptoms are not commonly used to identify this pathogen because of their non-specific nature. The symptoms also vary depending on the specific host that the pathgen is infecting. During an early infection on potato plants, chlorosis on the newer leaves develops and can become necrotic on one side of the leaf. This is due to the colonization of the pathogen forming conidia in the xylem vessel as water is being drawn up. Thus, the vascular tissue progresses to a tan color and wilting is observed on warmer days. The reduction of sugar movement within the plant causes a stunting that’s noticeable within the field. In some cases the tubers of the potatoes may become necrotic on the vascular ring. This disease can cause significant loses in yield due to reduced photosynthesis, if the plant survives. Black, thick resting bodies (i.e., microsclerotia) form on the necrotic tissue and over winter in the soil until germination is triggered by root exudates. The germinated microslerotia then penetrate the root tip and colonize the roots by forming hyaline mycelium that is septated and multinucleate. Microsclerotia can be seen with a hand lenses and can aid in the identification of the pathogen (2, 3).

Hardwood trees that are infected with V. dahliae exhibit chlorotic leaves that lead to necrosis, just as in potatoes. These leaves are also typically stunted due to the blockage of nutrients. Buds may not open in the spring, while fall coloring of the leaves may occur sooner than the natural time of coloring. The sapwood becomes discolored and necrotic, which can lead to complete branch death (2).

Ecology and Spread

The most common introduction of this pathogen in the field is by infected stock being planted. Potatoes, peppermint, and woody plants can easily spread V. dahliae without being symptomatic. Once the pathogen is in the field the proagules can be moved by wind, water, and soil. Tilling an infected field can lead to a broader spread of the pathogen. Optimal growth occurs in moist soils with a temperature range or 21-27° C. When Verticillium propagules are spread by wind, it has been demonstrated that the highest concentration is in the top 30 cm (12 inches) of the soil profile. However, propagules have been recovered from a depths as low as 41 cm (16 inches) (2, 3).

Geographic Distribution

Verticillium dahliae has a cosmopolitan spread that can infect many plant genera (4).

Management

Avoid use of infected soils that may have inoculum still present.
Control Pratylenchus penetrans activity within the field.
Avoid reservoir crop rotations like wheat, barley, and legumes.
Fumigate by shank injectors, irrigation, or with plastic mulch.
Prune any symptomatic tissue off with clean pruners.
Do not plant susceptible plants were V. dahliae has been an issue in the past.
Start transplants with disease free seedlings and inspect transplants.
Rogue any plants that appear to have wilting or one sided chlorosis.
Select resistant cultivars whenever possible like ‘Russet’ potatoes, ‘Murray Mitcham’ peppermint ,and ‘Jade Glen’ norway maples.
Consult your local extension specialist for legal and efficacious fungicide products available in your state. Remember, the label is the law and the product applicator is responsible for reading and following all chemical labeling (2, 3).

Diagnostic Procedures

Incubate fresh symptomatic material to allow production of spores within leaf, petiole, and fruit lesions. Diagnostic features include:

streptomycin water agar or Sorensen's NP-10 medium can be used and incubated for four days;
conidia are ovoid or ellipsoid and typically single-celled; and
conidia are born on phialides, which are produced in a whorl around each conidiophore with a mass of conidia (2).

Resources and References

1. Atallah, Z. K, Maruthachalam, and K. Subbarao. 2012. Sources of Verticillium dahliae Affecting Lettuce. Ecology and Epidemiology. Vol. 102, No. 11, 1071-1078.

2. Berlanger, I. and M.L. Powelson. 2000. Verticillium wilt. The Plant Health Instructor. DOI: 10.1094/PHI-I-PHI-I-2000-0801-01 Updated 2005. http://www.apsnet.org/edcenter/intropp/lessons/fungi/ascomycetes/Pages/VerticilliumWilt.aspx

3. Gómez-Alpízar, L. Verticillium dahliae. http://www.cals.ncsu.edu/course/pp728/Verticillium/Vertifin.htm

4. http://nt.ars-grin.gov/fungaldatabases/new_allView.cfm?whichone=FungusHost&thisName=Verticilliumdahliae&organismtype=Fungus&fromAllCount=yes

Acknowledgments

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)