By Luis Gómez-Alpízar
A class project for PP-728
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.
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 agar
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.
count is high. In potato and
the other solanaceous crops the following criteria have been established
in Michigan State University per gram of soil:
belongs to the fungal class Deuteromycetes
(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.
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
Symptoms and signs
Symptoms vary among hosts, and none is absolutely
diagnostic. Premature foliar chlorosis
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.
Ecology and Life Cycle
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
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
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.
to other sites:
Lessons in Plant Pathology: Verticillium
of Guelph Control Guidelines: Verticillium
Agrios, G.N.1990. Plant Pathology. 3th ed. Academic Press,San Diego,
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.
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