By: Travis DeSavigny
Soilborne Plant Pathogens
North Carolina State University
Verticillium albo-atrum is a soil-borne pathogen belonging
to the class Deuteromycota (Fungi Imperfecti; no known sexual stage).
V. albo-atrum has a limited host range. The most important hosts
of this pathogen include hops, alfalfa and cotton (2,3,8). The
pathogen infects the host and causes yellowing and wilting adversely
affecting the host plants fitness and yield and ultimately reducing the economic
value of the crop (1).
Host Range and Distribution
Verticillium wilt caused by the
pathogen Verticillium albo-atrum has been a major disease of alfalfa
in Europe for many years (6).
The disease was not recognized in the United States until 1976 on alfalfa (4).
The host range of V. albo-atrum
is limited. Some of the most susceptible
plant species include alfalfa and hops. V. albo-atrum
occurs worldwide but is most severe in the temperate regions preferring
temperatures ranging from 20-25ºC. The pathogen
exists in various strains with considerable variation of virulence and
host range (5).
Identification and Isolation
The pathogen is
not readily visible on the diseased tissue and must be cultured to be
accurately identified. A common procedure for identification
includes cutting 2-4 cm sections of diseased stem tissue, placing them
on a water agar or ethanol-stretomycin agar (ESA) for 3-5 days at 20-21ºC.
The fungus will grow out of the vascular tissue onto the medium
and can be examined microscopically. Under close examination
one can see the verticilliate whorled structures on the mycelium
(Figure 1). Unlike some another Verticillium
strains, V. albo-atrum does not form microsclerotia.
|Maple tree infected by Verticillium
albo-atrum. photo: Steve Enebak
chain reaction (PCR) primers have also been designed that are able to accurately
differentiate between the different Verticillium species. This
enables rapid, quantitative analysis of the pathogen (7).
arrangement of the phialides on the conidiophores of
Verticillium species. Each phialide contains a mass of conidia.
Symptoms of Verticillium
wilt include yellowing and defoliation on a few branches to massive wilting of
the entire plant. From there the edges of the leaves tend to
roll inward and the foliar wilting ensues. If the plant is
severely infected the foliage will turn brown. Discoloration
of the vascular tissue can be seen when an infected plant is cut longitudinally.
Severe infestation will V. albo-atrum can lead to serious injury
or death of the host.
The roots show no
external discoloration or decay distinguishing it from other diseases
showing similar foliar symptoms such as Phythophthora root rot.
Conidia of V.
albo-atrum form abundantly on senescent or necrotic plant parts and
are able to travel large distances through the air (4). The fungus
penetrates the tissue of a susceptible host and in some cases the
infection becomes systemic whereas others appear to be localized
infections. The conidia that do become systemic
penetrate the plant tissue and occupy the xylem vessels where conidia
are produced. Vascular colonization then occurs as the conidia move up
the plant xylem vessels along with water.
Once colonized the
pathogen travels up the vasculature of the plant plugging the xylem
with mycelium and hyphae growth reducing the amount of water that
reaches the upper regions of the plant. This lack
of water results in the wilting symptom associated with the disease.
The infection sites soon produce more conidia increasing the
inoculum density in the area.
and resistant cultivars are successful ways of controlling disease
incidence due to Verticillium albo-atrum. This is done by
removing infected crop debris from fields and planting resistant
Crop rotation, with
non-hosts is another method used as a successful control method since V.
albo-atrum is unable to survive in the soil for long periods of
time. Two-three year
rotations with cereals are a common practice in alfalfa growing regions.
V. albo-atrum can also be controlled
by chemical applications, such as methyl bromide, 1,3-dichloropropene,
and related C3 hydrocarbons + chloropicrin (Telone + Picfume).
Recently, some of the chemicals have been banned (methyl
bromide) and some other are being restricted and/or applications very
costly to the growers. Furthermore, the chemical
fumigation methods are very weather and temperature sensitive.
This makes fumigation difficult, offering only small windows of
opportunity in some regions for effective application.
- Agrios, G.N., 1990.
Plant Pathology. 3rd edition Academic Press, San Diego, California.
- Clarkson, J.M. and J.B.
Heale 1985. Pathogenicity and colonization studies on wild-type and
auxotrophic isolates for Verticillium albo-atrum from hop.
Plant Pathology 34 (1): 119-128.
R.M. 1986. Lack of systemic
colonization of alfalfa plants after inoculation of uninjured leaves
with conidia of Verticillium albo-atrum Plant Disease70:
- Graham, J.M., R.N.
Peaden, and D.W. Evans, 1977. Verticillium wilt of
alfalfa found in the United States. Plant
Disease Reporter, 61: 337-340.
- Guidelines for
Verticillium Control. Pest Diagnostic Clinic: University of Guelph, Ontario. http://www.uoguelph.ca/pdc/Control_Guidelines/Vertillium_Control_Guidelines.htm.
K.W. 1962. Verticillium wilt of
alfalfa. A destructive disease in Britain and Europe not yet observed in
the United States. Crop Res. ARS, 34-40,
- Nazar, R.N., X. Hu, J.
Schmidt, D. Culham, and J. Robb, 1991. Potential use of PCR-amplified
ribosomal intergenic sequences in the detection and differentiation of
Verticillium wilt pathogens. Physiological and
Molecular Plant Pathology. 39, 1-11.
- Schnathorst, W.C. and D.E. Mathre,
1966. Host range and differentiation of a severe form of Verticillium albo-atrum
in cotton. Phytopathology, 56: 1155-1161.
- I would like to thanks
Dr. David Shew (North Carolina State University) and Scott Enebak
(University of Auburn) for use of the images.