Currant anthracnose

Symptoms initially appear in early summer as minute, irregular, dark brown spots on the leaves. These spots gradually expand and may become angular, reflecting the venation of the leaf.

A distinctive feature is the development of small, shiny, slightly elevated pustules (acervuli) in the center of the necrotic spots. These pustules contain the conidia and are often visible to the naked eye.

Stems, petioles, and fruit stalks are also susceptible, showing small, elongated, dark, depressed lesions. These lesions can cause girdling, leading to the death of the distal parts of the affected tissue.

Defoliation is the most severe symptom. Heavily infected leaves turn yellow and drop prematurely, often leaving the currant bushes nearly bare by mid-summer, long before the natural leaf fall.

• Tiny, dark brown necrotic spots on leaves.
• Shiny, pimple-like fruiting bodies in the centers of spots.
• Premature yellowing and leaf shedding.
• Depressed lesions on stems and petioles.

The disease is caused by the fungus Drepanopeziza ribis, an obligate parasite that specifically targets members of the Grossulariaceae family, particularly black and red currants. It is a widespread pathogen in temperate climate zones.

The fungus survives the winter primarily in fallen leaves, where it develops its sexual stage (apothecia). During rainy spring weather, spores are ejected and carried by wind and splashing water to the young foliage of the host plants.

The asexual stage, consisting of conidia produced in acervuli, is responsible for the rapid spread of the disease throughout the growing season. Multiple infection cycles can occur within one season if environmental conditions are favorable.

The pathogen infects the host through stomata, leading to tissue necrosis. This disruption of the leaf structure directly impairs the plant's ability to carry out photosynthesis efficiently.

Laboratory diagnosis is performed by identifying the typical lenticular conidia under a microscope, which are found within the gelatinous masses on the surface of the leaf spots.

The development of anthracnose is highly dependent on humidity and rainfall. High atmospheric humidity and frequent rain events are critical for spore dissemination and successful infection of plant tissues.

Optimal temperatures for the fungus range between 15°C and 20°C. While development slows down during extremely hot and dry weather, the pathogen persists and can cause new infections once cooler, humid conditions return.

Poor air circulation within dense bushes creates a microclimate with prolonged leaf wetness, which is highly conducive to disease outbreaks. Proper plant spacing and pruning are essential to mitigate these risks.

The presence of overwintered leaf litter under the plants is the main factor for primary infection. If these leaves are not removed or buried, the fungal spores are perfectly positioned to attack new growth in the spring.

Nutritional imbalances, such as excessive nitrogen application, can produce succulent growth that is more susceptible to fungal penetration, highlighting the importance of balanced fertilization.

The primary damage is caused by severe premature defoliation. Without a functional leaf canopy, the plant cannot synthesize enough carbohydrates, which leads to weak growth and poor fruit filling.

Yield reduction is significant, as berries on infected bushes are often small, lack proper sugar content, and may drop before they reach full maturity due to the overall weakening of the plant.

Over several years, repeated infections cause progressive exhaustion of the currant bushes. The plant loses its vigor, produces fewer fruit buds, and becomes increasingly susceptible to winter injury and other environmental stressors.

The loss of foliage can trigger late-season regrowth, which is particularly dangerous because these immature tissues are highly vulnerable to frost, potentially leading to shoot dieback.

The economic impact involves both direct crop losses and the financial burden of implementing continuous fungicide control programs, alongside the potential need for premature replacement of infected plantations.

Sanitation is the most effective cultural control method. Collecting and destroying fallen leaves in the autumn removes the primary inoculum source, significantly reducing the pressure of the disease in the following spring.

Regular pruning is vital to keep the bush open, allowing for better air penetration and quicker drying of the foliage after rain. This practice disrupts the life cycle of the pathogen by removing its preferred environment.

Chemical control involves the application of appropriate fungicides during the growing season. Protective spraying is usually initiated before flowering or immediately after symptoms are spotted, following regional guidelines.

Integrated pest management (IPM) should be practiced by alternating products with different modes of action. This prevents the pathogen from developing resistance to specific active ingredients, ensuring long-term control effectiveness.

Planting resistant or tolerant cultivars is the most sustainable strategy for long-term anthracnose management. Breeding programs have successfully developed varieties that require minimal chemical intervention.