Raspberry spur blight

The fungus survives the winter primarily in the infected lesions of canes and on fallen leaves. During early spring, the mature fruiting bodies (perithecia) release ascospores into the air, which land on new plant growth.

The infection cycle continues as the fungus develops pycnidia, which produce conidia throughout the summer. These spores are spread by water splashes from rain or irrigation, facilitating the rapid colonization of new shoots.

Entry into the plant usually occurs through natural openings like stomata or through cracks in the epidermis. Wounds caused by insect pests or improper pruning act as major entry points for the fungal mycelium.

Understanding this life cycle is crucial for effective control. Fungicide applications are most effective when timed to coincide with the primary spore release and the rapid growth stages of the raspberry canes.

The first symptoms typically appear in early summer on the lower portions of the new primocanes. You will notice faint, light-brown or purplish lesions, often located around the nodes where leaf petioles attach.

As the disease progresses, these lesions enlarge and may coalesce, covering a significant portion of the stem. The bark in the affected areas often turns brown and may exhibit cracking as the fungus colonizes deeper tissues.

By late summer and into autumn, the characteristic diagnostic sign appears: tiny, black, pimple-like fruiting bodies known as pycnidia scattered across the discolored lesions on the bark.

Leaf symptoms are also present, manifesting as dark, V-shaped brown necrotic spots with yellow halos. In severe cases, these leaves will yellow and drop prematurely, further weakening the overall vitality of the cane.

In the following spring, infected canes often show a greyish, silvery discoloration. Buds in the infected areas frequently fail to sprout, or the resulting lateral shoots are stunted and weak, ultimately producing few or no berries.

The development of spur blight is highly dependent on environmental moisture levels. Persistent wet weather, heavy fog, and dew provide the necessary conditions for spore germination and active fungal growth.

Dense, overcrowded plantings inhibit airflow, creating a humid microclimate that allows the disease to flourish. Lack of proper thinning and canopy management is a primary driver of the infection's intensity.

Warm temperatures paired with excessive overhead irrigation can exacerbate the situation. When high humidity persists, the fungus can complete its reproductive cycle more quickly, leading to rapid spread within the plot.

Excessive use of nitrogen fertilizers promotes lush, succulent growth that is more susceptible to fungal entry. These soft canes are prone to cracking, which makes them easier targets for the pathogen.

Poor horticultural practices, such as failing to tie canes to a trellis or allowing excessive basal growth, prevent the canes from drying out, which significantly increases the risk of spur blight development.

The most severe impact of spur blight is the reduction of winter hardiness. Infected canes are significantly more likely to suffer from cold damage during winter, which can lead to widespread bud death or cane mortality.

Fruit yield is drastically reduced due to the destruction of productive fruiting laterals. In heavily infested fields, yield loss can exceed 50%, making the crop economically unviable for commercial growers.

The disease builds up an inoculum reservoir in the soil and on plant debris over time. This makes it increasingly difficult to manage the disease in subsequent years without drastic sanitary measures or replanting.

The fruit quality also suffers, as the plant struggles to distribute nutrients through the damaged vascular tissues of the infected canes. The berries may end up smaller, less flavorful, and prone to rot.

Spur blight also complicates the production of nursery stock, as plants sourced from infected blocks carry the pathogen, spreading the disease to new planting sites and causing long-term management challenges.

The foundation of effective management is starting with certified, disease-free nursery stock. Selecting resistant or tolerant cultivars can drastically reduce the dependency on chemical interventions throughout the season.

Good site management is essential. Proper pruning to thin out the canes, removing excessive basal shoots, and ensuring adequate row spacing significantly improves air circulation, which keeps the canes dry.

Sanitation is a non-negotiable step. Immediately after harvest, all old, infected, and weak canes should be pruned out and burned or removed from the site to destroy the fungus's overwintering sites.

Fungicide programs should target the early stages of growth. Applying appropriate protective treatments during the pre-bloom period helps prevent the primary infection and limits the secondary spread of the fungus.

• Always prune out old canes immediately after harvest.
• Maintain balanced fertilization to avoid overly succulent growth.
• Ensure proper trellis systems are used for cane support.
• Sanitize all pruning tools regularly to prevent spread.
• Keep the orchard floor clear of weeds to improve drying conditions.