Verticillium dahliae
By Luis Gómez-Alpízar
A class project for PP-728
Introduction
 
Verticillium dahliae , a soil borne pathogen, belongs to the fungal class Deuteromycetes (Fungi Imperfecti), a group of fungi, which do not have a known sexual stage.   V. dahliae has a wide host range. Over 300 woody and herbaceous plantspecies are known to be susceptible to this fungal pathogen. The disease, Verticillium wilt, is problematic in temperate areas of the world,especially in irrigated regions. There are no curative measures once a plant is infected.
 
Host range and distribution
 
Over 300 woody and herbaceous plant species are known to be susceptible to V.dahliae including tomato, eggplant, pepper, potato, peppermint, chrysanthemum, cotton, asters, fruit trees, strawberries, raspberries, roses, alfalfa, maple, and elm.Resistant plants include all monocots, all gymnosperms, apple, crabapple, mountain ash, beech, birches, dogwood, hackberry, hawthorn, linden, honeylocust, oaks, sycamore, poplar, walnut, and willow. V. dahliae occurs worldwide but is more important in temperate zones.

Isolation

Since fungal structures are not visible on diseased specimens, a laboratory culture should be done to confirm the diagnosis of  V. dahliae. Small, thin pieces of infected vascular tissue can be placed onto a culture medium such a streptomycin water agarVascular discoloration and resulting colonies from infected tissue   or Sorensen's NP-10 medium and incubated for 4 days. When the fungus grows out of the vascular tissue it can be examined microscopically. A satisfactory sample for analysis should include several stem or branch segments, with attached leaves, showing a range of symptom severity if possible. Dead or dried out branches usually cannot be diagnosed reliably.

 
Verticillium assay is also done on soil, primarily for soils in which solanaceous crops such as potatoes, tomatoes, eggplant or peppers are to be grown. The dark microsclerotia that the fungus produces are readily seen in culture plates. It is assumed that the risk of damage for the crop is higher if the V. dahliae count is high. In potato and the other solanaceous crops the following criteria have been established in Michigan State University per gram of soil:
 

Low risk                0-5   colonies

Moderate risk       6-12  colonies

High risk               >12   colonies

Identification

 
Verticillium dahliae  belongs to the fungal class DeuteromycetesLine drawing of hyphae, condiophores and conidia of Verticillium   (Fungi Imperfecti), a group of fungi, which do not have a known sexual stage.The vegetative mycelium is hyaline, septate, and multinucleate.The nuclei are haploid in culture. Conidia are ovoid or ellipsoid and usually single-celled. They are borne on phialides, which are specialized hyphae produced in a whorl around each conidiophore. Photomicrograph of verticillate conidiophores and conidiaEach phialide carries a mass of conidia. Verticillium is named for this verticillate (=whorled) arrangement of the phialides on the conidiophore.

The fungus forms microsclerotia in dying tissue, which are masses of melanized hyphae.Photomicrograph of a microsclerotium


 
 
 
 
 
 
 

Symptoms and signs

 
Symptoms vary among hosts, and none is absolutely diagnostic. Premature foliar chlorosisChlorosis on cotton due to V. dahliae and necrosis and a tan to brown colored discoloration or streaking of the vascular system, however, are characteristic of all hosts. Symptoms of wilting are most evident on warm, sunny days. Microsclerotia formed in the dying tissue are frequently visible with a hand lens.

 

vascular discoloration on cottonmicrosclerotia in infected cotton stem

Ecology and Life Cycle
 

V. dahliae naturally occurs at low levels in soils and grows better at slightly higher temperatures 25 -28 oC. The fungus can overwinter as mycelium in perennial hosts, plant debris, and vegetative propagative parts.The fungus can survive for many years (10 years or more) in soil in the form of tiny, black, seed-like structures called microsclerotia. Microsclerotia can even form on and in the fine roots of many species of resistant plants without causing symptoms. Microsclerotia are stimulated to germinate by root exudates of both host and non-host plants. The fungus penetrates a root of a susceptible plant in the region of elongation and the cortex is colonized. From the cortex, the hyphae penetrate the endodermis and invade the xylem vessels where conidia are formed. Vascular colonization occurs as conidia are drawn up into the plant along with water. As the diseased plant senesces, the fungus ramifies throughout cortical tissue then produces microsclerotia, which are released into the soil with the decomposition of plant material.
 
Long distance dissemination of the pathogen occurs via infected seed tubers, and planting stock. In bare root or vegetatively propagated plants such ornamentals, a nursery may spread the fungus by selling non-symptomatic, but infected, planting stock. Once established in a field or landscape, spread of the pathogen occurs primarily by soil cultivation and movement of soil by wind or water. Inoculum densities and disease severity tend to increase from year to year when susceptible crops are planted.
 
Damage caused by V. dahliae to the plants is often more severe in fields infested with the root-lesion nematode, Pratylenchus penetrans . This nematode may increase the severity of the disease by altering the host physiology, thus making the plant more susceptible to damage. Symptoms may develop even when population densities of Verticillium and P. penetrans individually are too low to cause significant disease.
 
Control measures
 
Resistant or partially resistant cultivars of some susceptible plant species are available. Use resistant cultivars and pathogen-free plants whenever possible. Avoid fields previously used for susceptible crops (eg. tomato, cotton, potatoes, and strawberries). Remove and destroy any plants that exhibit symptoms of Verticillium wilt. Soil fumigation with high concentrations of metham-sodium or methyl-bromide is an effective (eradicates the fungus), but fumigation is expensive and hazardous control tactic. Soil solarization in sunny climates can be useful. Fungicides are generally not economical for control of Verticillium wilt.

Links to other sites:
APSnet.org  Lessons in Plant Pathology: Verticillium
University of Guelph  Control Guidelines: Verticillium

 
References:

1. Agrios, G.N.1990. Plant Pathology.  3th ed. Academic Press,San Diego, CA. 803p
2. Bruehl, G. W. 1987. Soilborne Plant Pathogens. Macmillan Publishing Company, London. 368p
3. C.M.I Descriptions of Pathogenic Fungi and Bacteria No 256
4. Dhingra, O.D. and Sinclair, J.B. 1985. Basic Plant Pathology Methods.CRCPress Inc., Boca Raton, Florida. 355p.
5. Tjamos, E.C., Rowe, R.C., Heale, J.B. and Fravel, D.R.2000. Advances in Verticillium Research and Disease Management. APS Press. St. Paul,MN.376p.

Acknowledgments
 
I would like to thank Dr. D. Shew for supplying slides for this web page. Some pictures were also taken from apsnet.org.

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Posted Spring 2001