Spur blight

The first symptoms typically appear on young primocanes during late spring or early summer as small, dark brown or purple spots, usually concentrated around the leaf attachment points (nodes).

As the infection progresses, these spots enlarge, merge, and spread along the cane, often resulting in large, irregular necrotic lesions with a characteristic purple-brown color.

In late summer, tiny black specks called pycnidia become visible within the lesion area. These are the fruiting structures of the fungus and indicate active spore production.

The infected cortex tissue often cracks and peels away, leaving the cane weakened and prone to drying out. In severe cases, the entire node area may become completely girdled.

Leaves on infected canes may develop chlorotic or necrotic spots, followed by premature yellowing and dropping, which impairs the plant's overall energy production for the next season.

Spur blight is caused by the fungus Remotididymella destructiva (formerly known as Didymella applanata). It is a significant fungal pathogen that targets the stems, buds, and petioles of raspberry plants.

The fungus overwinters as mycelium and fruiting bodies within the tissues of infected canes and debris left on the ground, serving as a primary inoculum source for the following season.

During spring and early summer, moisture from rain or irrigation triggers the release of spores. These spores are disseminated by splashing water, wind, and potentially insects to nearby healthy canes.

The pathogen primarily infects through the nodes where leaves attach to the stem. Once it penetrates the epidermis, the fungus colonizes the cortex tissue, disrupting the plant's vascular integrity.

Throughout the growing season, the fungus continues to produce spores (conidia), allowing for multiple cycles of infection and rapid spread across the planting site during humid conditions.

Spur blight development is highly dependent on humidity. Extended periods of rainfall or high relative humidity (above 80%) are crucial for the germination and spread of spores.

Dense, overcrowded planting schemes restrict air circulation, creating a trapped, moist microclimate at the base of the canes that is perfect for fungal proliferation.

Temperatures between +18°C and +25°C provide the most favorable environment for the growth of the fungus and its rapid colonization of cane tissues.

Excessive nitrogen fertilization promotes overly lush, succulent vegetative growth, which is significantly more susceptible to infection compared to stronger, more lignified canes.

Failure to prune old or damaged canes provides a continuous reservoir of inoculum, making it difficult to prevent the disease from spreading year after year.

Spur blight causes direct losses by weakening canes, which often leads to the death of lateral shoots that are supposed to bear fruit in the following season.

Infected canes exhibit poor overwintering capacity and are highly susceptible to cold injury and desiccation during winter, frequently resulting in significant stand loss.

Fruit quality is also severely impacted; berries on diseased plants are often smaller, show poor color development, and have reduced shelf life, making them less marketable.

If left unmanaged, the pathogen can cause a rapid decline in the vitality of a raspberry plantation, shortening the productive lifespan of the site by several years.

The combination of cane damage and physiological stress makes the plants more prone to secondary infections and infestations by other pests, creating a cumulative negative effect.

The primary control strategy involves good canopy management, including proper spacing and thinning of canes to ensure adequate sunlight penetration and air movement.

Sanitation is critical; all infected or dead canes should be pruned out immediately after harvest and removed from the field to reduce the pathogen population.

Fungicide applications are most effective when timed correctly, particularly during the early growth phase and after the harvesting period, to protect new primocanes.

Selecting resistant or less susceptible cultivars is a highly recommended long-term management strategy to minimize the reliance on chemical control methods.

• Implement a precise irrigation system to minimize water contact with canes.
• Maintain balanced nutrient levels, focusing on potassium for cane hardening.
• Keep the orchard area free of weeds that could block airflow around the base.