Beginner

Scientific Name: Leptosphaeria coniothyrium

Identification

• Fungus infects through wounds in primocanes.
• Infection is not easily visible on the surface of the cane; the epidermis must be scraped off to see the affected tissue.
• Infection grows beneath surface and dark brown cankers may appear around wounds.
• Diseased canes are often brittle, they may die during the winter or they may leaf out erratically in the spring.
• Side branches that grow from infected canes often die between flowering and fruiting because they lack structural support.

Often Confused With
Spur Blight
Raspberry Crown Borer

Period of Activity
Infection of primocanes is favoured by splashing rain when temperatures and precipitation increase in early spring. Symptoms are most evident the next spring.

Scouting Notes
Examine canes with visible wounds, mechanical damage or signs of die-back in spring and just before harvest. Use a sharp pocketknife to gently scrape away the bark of affected canes in the area where symptoms appear. Look for the zone where healthy tissue (whitish green) merges with the diseased tissue (brown). Check the crown and roots for borers, nematodes and diseases. Try to determine the cause of injury that led to the cane blight.

Thresholds
None established.  

Advanced

Scientific Name: Leptosphaeria coniothyrium

Identification
Cane blight is a serious raspberry cane disease in Ontario. Diseased canes may die during the winter or leaf out erratically in the spring and collapse at harvest.

The fungus, Leptosphaeria coniothyrium, infects through wounds in primocanes. It grows beneath the surface of the cane, eventually causing dark cankers around the wound. Although rarely evident from the surface, cane blight lesions are visible when the epidermis of an infected primocane is scraped off exposing the vascular tissues. Beneath the epidermis, a brown discolouration is visible spreading from the wound.

The fungus removes the cellulose of the vascular tissue. Canes become brittle and break easily the following spring. Fruiting laterals and side branches that grow from infected canes often die back just before harvest.

Often Confused With
Spur Blight
Spur blight often occurs simultaneously with cane blight, making it hard to distinguish the two. Spur blight produces distinct brown-purple, V-shaped lesions at the leaf spur, which turn silvery-grey during winter. Laterals growing from infected spurs may be weak or die.

Cane blight does not produce distinct lesions at leaf spurs. Brown spreading lesions caused by cane blight are visible under the surface of the cane by scraping away the epidermis. The entire cane dies back above the point of injury.

To accurately distinguish these two diseases, send samples of infected canes to a pest diagnostic clinic.

Raspberry Crown Borer
Both cane blight and crown borer cause canes to die back above the point of injury, often just before harvest.

Plants affected by crown borer are weak and easily snap off at the base. Close examination of the crown reveals frass, tunnelling and the presence of the small larvae. Fewer primocanes grow up from the crown where crown borers are active.

Canes affected by cane blight are healthy below the point of injury and weak or dying above. If the bark of an affected cane is scraped away, the underlying vascular tissue at the point of injury is brown instead of a healthy green. New primocanes are healthy and vigorous.

Biology
Cane blight is caused by the fungus Leptosphaeria coniothyrium. The fungus overwinters in  fruiting canes and old cane stubs infected the previous year. Small, pimple-like structures, called pycnidia, are found on the surface of these infected canes. When temperatures warm up and rain occurs in early spring, pycnidia produce and release conidia. Conidia are present until late fall and infect canes damaged during harvest or summer pruning. Pseudothecia are also embedded in the surface of infected canes and produce ascospores. Ascospores mature between April and May Both ascospores and conidia are dispersed by splashing rain and infect  primocanes through wounds.

Primocanes may be wounded by machinery, pruning, by scraping against trellis wires, or by wind.  Hoeing, herbicide damage, insect punctures, mechanical harvesters or abrasions against old cane stubs and spines can also damage primocanes and provide sites for infection. Wet weather facilitates the transport of fungal spores into the wound.

Initially, disease symptoms will not be visible on primocanes. By fall, brown, elongated lesions can be found underneath the bark of infected canes. These lesions are found in the vascular tissue of the plant and spread out from the site of the wound. The fungal pathogen removes cellulose from the vascular tissue around the wound which causes the cane to become brittle and weak.  If the lesions spread around the cane over the winter, they can completely kill the cane. 

Period of Activity
Primary ascospores are released in the spring and early summer, while secondary spores, or conidia, are released from spring until late fall.

The pathogen enters primocanes, or first-year growth, through wounds. It spreads up and down the vascular tissue, which causes the cane to become brittle and weak. Symptoms are most evident the next spring. Severely infected canes do not leaf out in the spring because they are brittle and snap or break easily. Infected canes may leaf out and then die back before harvest.

Scouting Notes
Watch for symptoms of die-back in fruiting canes in spring and just before harvest. Examine canes with symptoms for wounds or mechanical damage. Use a sharp pocketknife to gently scrape away the bark in the area where symptoms appear. The diseased vascular tissue beneath the bark is brownish in the affected area, compared to whitish green healthy tissue.  Look for the zone where healthy tissue merges with the diseased tissue. Check the crown and roots for borers, nematodes and diseases, which compound the problem. Try to determine the cause of injury that led to the cane blight.

Thresholds
None established.  

Management Notes

• Prevent injury to primocanes caused by rubbing against trellis wires, machine harvesters or summer pruning.
• Prune old canes close to the ground, because primocanes can be wounded when they rub against old cane stubs. 
• Where cane blight is a problem, avoid pruning out old fruiting canes immediately after harvest. While this practice can help reduce other cane diseases by reducing inoculom and improving air flow in the row, it can favour development of cane blight if primocanes are damaged. Damage to primocanes is less likely to occur if pruning is delayed until canes are dormant. 
• Prune out and destroy old fruiting canes close to ground level when canes are dormant.
• Avoid pruning or tipping when plants are wet or just before a rain.
• Avoid overhead irrigation during critical times for infection,  to reduce splash dispersal of conidia.