Verticillium albo-atrum

By: Travis DeSavigny
PP 728 Soilborne Plant Pathogens
North Carolina State University

 

Introduction

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  

Polymerase 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).   

 

 

Verticilliate whorled arrangement of the phialides on the conidiophores of Verticillium species. Each phialide contains a mass of conidia.

Symptoms

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. 

Ecology and Life Cycle 

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.

Control

Sanitation practices 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 cultivars. 

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.

References

  1. Agrios, G.N., 1990. Plant Pathology. 3rd edition Academic Press, San Diego, California.
  2. 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.
  3. Jimenez-Diaz, R.M. 1986.  Lack of systemic colonization of alfalfa plants after inoculation of uninjured leaves with conidia of Verticillium albo-atrum Plant Disease70: 509-515.  
  4. 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. 
  5. Guidelines for Verticillium Control.  Pest Diagnostic Clinic: University of Guelph, Ontario.  http://www.uoguelph.ca/pdc/Control_Guidelines/Vertillium_Control_Guidelines.htm. 
  6. Kreitlow, 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, 1-15.
  7. 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.
  8. 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.