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Beginner

Scientific Names
Verticillium albo-atrum, V. dahliae

Identification

• Symptoms resemble drought stress.
  
• Infection and severe disease occurs mostly in first year of growth.
  
• Outer leaves develop brown necrotic areas between veins and on the edges during early spring.
  
• Interior leaves of diseased plants usually remain alive and blue-green in colour.
• Infected plants wilt rapidly under stress.
  
• Diseased plants may occur singly or in patches.

Often Confused With
Fertilizer Burn
White Grub Injury

Period of Activity
In new plants, symptoms appear when runners are beginning to form. In older plants, symptoms appear just before picking time.

Scouting Notes
Make a map showing areas of affected plants. Note any variety specific symptoms. Carefully dig up affected plants. Cut crowns lengthwise and look for discoloured vascular tissue. Confirm disease at a diagnostic laboratory to verify the presence of Verticillium. Above-ground symptoms are hard to distinguish from other root infecting fungi.

Thresholds
None established.

Advanced

Scientific Names
Verticillium albo-atrum, V. dahliae

Identification
The Verticillium fungus destroys water-conducting tissue, therefore above-ground symptoms will often resemble drought stress. Foliar symptoms often are characteristic. The outer leaves turn brown and wilt, and the inner leaves may wilt but remain a bluish-green. Brownish to bluish-black streaks or blotches may appear on the runners or petioles. New roots that grow from the infected crown are often dwarfed with blackened tips. Crowns when cut lengthwise, may show blackening or browning in the vascular tissue. Daughter plants attached to infected mother plants also show symptoms.

In serious outbreaks of the disease, large numbers of plants may wilt and die rapidly. In minor outbreaks, an occasional plant or several plants scattered over the entire planting may wilt and die.

Often Confused With
Fertilizer burn (Both can cause older leaves to turn dry around the edges. Verticillium is likely to occur in circular patches in the field. Pattern of injury from fertilizer burn is more general across the field.)
White grub injury (Plants injured by white grubs show severe root pruning and grubs are often present in adjacent soil.)

Biology
There are two species of Verticillium: V. dahliae and V. albo-atrum. Both attack a wide range of broadleaf plants including strawberry. The incidence of Verticillium wilt is highest in strawberry fields that were previously planted with potatoes, tomatoes, peppers and eggplants.

The disease is introduced into uninfected soil by seeds, tools and machinery, or infected transplants. Once established, Verticillium can survive in the soil without hosts for many years. The fungus overwinters in soil or plant debris as dormant mycelium or microsclerotia (black speck-sized bodies). Optimal conditions for development are wet soils, and cool, overcast weather interspersed with warm, bright days. Under these conditions microsclerotia germinate and produce hyphae (thread-like fungal structures), which penetrate the root. Infected plants wilt because the hyphae invade and destroy water-conducting tissue in the plant.

Period of Activity
Verticillium wilt is usually more of a problem in the planting year. In new plantings, the first symptoms appear about the time runners begin to form. If conditions are wet, the disease will be most severe in late summer or fall. In fruiting plantings, the symptoms usually appear just before picking time.

Scouting Notes
Make a map showing areas of affected plants. Note any variety specific symptoms. Carefully dig up affected plants. Cut crowns lengthwise and look for discoloured vascular tissue. Confirm the presence of Verticillium at a diagnostic laboratory. Above-ground symptoms are hard to distinguish from other root-infecting fungi.

Thresholds
None established.

Management Notes

• Verticillium also attacks potato, tomato, pepper, eggplant, raspberry, and peaches. Avoid planting strawberries following these crops, or where these crops or strawberries have been grown in the last 4- 5 years .
  
• Some varieties are more susceptible than others, but none are immune.
  
• Use tolerant or resistant varieties where the disease has been a problem.
  
• Pre-plant soil fumigation in the year before planting will control Verticillium wilt. Application rates of fumigant are higher for Verticillium than for nematodes.
  
• Plant disease-free stock from an accredited plant propagation program.