Spur blight

Symptoms typically appear on young canes in late spring as purple or reddish-brown lesions centered around the base of the leaf petioles.

As the disease progresses, these lesions expand and merge, eventually turning into a dark brown or charcoal-like color as the epidermis dies.

A characteristic sign of advanced infection is the longitudinal cracking of the cane bark, which makes the plant brittle and prone to breakage.

By late summer and autumn, numerous tiny black dots known as pycnidia develop on the infected surface, representing the fungus's fruiting bodies.

Severe infestations cause leaves to yellow and drop prematurely, while affected buds may fail to sprout in the following spring.

The causal agent of spur blight is the fungus Didymella applanata (anamorph: Phoma complanata), which primarily attacks the stems of raspberries and blackberries.

The fungus survives the winter as mycelium within infected canes and as pseudothecia, which release ascospores during favorable spring conditions.

Infection spreads throughout the growing season via pycnidiospores, which are easily disseminated by splashing rain and high-humidity air currents.

The pathogen is opportunistic, often invading plant tissue through natural openings such as leaf axils or wounds created by pests and physical handling.

Understanding the life cycle of this fungus is crucial for timing fungicide applications to interrupt the spread of the disease effectively.

Spur blight thrives in cool, wet environments, particularly during periods of frequent rainfall and high relative humidity.

Dense, overcrowded plant canopies restrict airflow, keeping stems wet for longer periods and creating an ideal microclimate for fungal infection.

Excessive nitrogen fertilization promotes rapid, succulent growth that is highly susceptible to tissue invasion by the fungal pathogen.

Insects like the raspberry cane midge contribute to the spread by causing physical damage that serves as an entry point for Didymella applanata spores.

Poor orchard hygiene, such as leaving infected plant debris on the ground, provides a constant source of inoculum for subsequent growing seasons.

This disease significantly impacts yield by weakening the canes, leading to winter injury and potential death of the fruiting wood.

Reduced vigor in the canes results in lower berry production, poorer fruit quality, and overall decline in the lifespan of the raspberry plantation.

By compromising the vascular system of the stem, the fungus interferes with nutrient translocation, further stunting the development of lateral shoots.

The economic impact of spur blight is cumulative, as persistent infections lead to a degraded plantation that requires expensive rejuvenation.

In cases of heavy infestation, the cumulative loss of fruit and plant health can render a commercial operation unprofitable.

Effective management begins with sanitation: promptly removing and burning old or infected canes immediately after harvest to reduce inoculum levels.

Improving air circulation through proper pruning, thinning, and weed management is essential for keeping the stems dry and preventing infection.

Chemical control involving copper-based or systemic fungicides should be applied strategically, particularly during the early growth stages.

Integrated pest management is vital to suppress insects that cause the mechanical damage necessary for the fungus to colonize the plant.

• Use disease-free planting stock.
• Space plants adequately for airflow.
• Apply balanced mineral fertilizers.
• Monitor for early infection signs.