Guidelines for Verticillium Control

Verticillium wilt diseases of plants occur worldwide but are most important in temperate zones. More than 200 species of plants can be attacked by these diseases including tomato, eggplant, pepper, potato, chrysanthemum, asters, fruit trees, strawberries, raspberries, roses, alfalfa, maple and elm.

When a verticillium test is done on soil in the Pest Diagnostic Clinic, it is V. dahliae that is being counted. The dark microsclerotia (overwintering fruiting bodies) that the fungus produces are readily seen in culture plates. V. dahliae has a broad host range. It is assumed that one will see damage to most fruit and vegetable crops if the V. dahliae count is high.

There are in fact two species of Verticillium that may cause wilt diseases; Verticillium albo-atrum and V. dahliae. Both verticilliums naturally occur at low levels in soils and can attack susceptible crops when these are planted. V. albo-atrum grows best at 20 - 25 ^oC, while V. dahliae grows better at slightly higher temperatures at 25 - 28 ^oC. Both verticilliums will overwinter as mycelium in perennial hosts, plant debris and vegetative propagative parts. However, V. dahliae will persist longer in the soil due to the microsclerotia, while V. albo-atrum does not produce these structures. Inoculum levels build up when susceptible hosts are planted repeatedly on the same land. Both fungi can be spread in contaminated seed, propagative plant parts, by wind, water and soil. Both exist as various strains. The host range, virulence and other characteristics of these strains vary considerably. Some strains have a very broad host range while others are quite host specific and some of the host ranges overlap.

Verticillium counts are done primarily for soils in which solanaceous crops such as potatoes, tomatoes, eggplant or peppers are to be grown. Verticillium counts are used in conjunction with soil counts for plant pathogenic nematodes to determine whether or not soil fumigation is necessary. Verticillium counts will give only a rough estimate of the possible damage to the crops if they are not solanaceous.

The reported criteria are guidelines for solanaceous (potatoes, tomatoes, pepper, eggplant) only. They include counts for Verticillium dahliae only, and do not include Verticillium albo-atrum. For crops other than solanaceous ones, please consult your OMAF crop technology staff for further information on control measures.

• Low Risk 0 - 5 colonies*
• Moderate Risk 6 - 12 colonies
• High Risk > 12 colonies

* Based on Michigan State University criteria for control measures of Verticillium dahliae in potato.

The following guidelines are recommendations for solanaceous crops and should only be used as a GENERAL reference for other crops. The criteria are based on studies conducted by the Ridgetown College and Michigan State University. The numbers indicate levels at which crops will be damaged and control measures are warranted. Use control measures if you have:

• >2,000 root-lesion nematodes (Pratylenchus sp.)/kg of soil
• >1,000 root-knot nematodes (Meloidogyne sp.)/kg of soil
• >12 V. dahliae colonies/g of soil
  
• >200 root-lesion nematodes (Pratylenchus sp.)/kg of soil
  AND
  >6 V. dahliae colonies/g of soil
  
• >100 root-knot nematodes (Meloidogyne sp.)/kg of soil
  AND
  >6 V. dahliae colonies/g of soil
  
  

FUMIGANTS

Fumigation is expensive and the environmental conditions have to be optimal in order for the fumigant to be effective. Many but not all products have active ingredients that perform as nematicides, fungicides, insecticides and/or herbicides. Growers should consult OMAF guides, OMAF technology staff, chemical dealers and/or product labels for effective rates and precautions. For products registered for the control of nematodes and/or verticillium, contact the Pest Management Regulatory Agency, Health Canada.