Anthracnose and Bacteriosis of walnuts and their prevention

Walnuts (Juglans spp.) are valuable trees but can be affected by various diseases, among which anthracnose and bacterial diseases are some of the most widespread and serious. Identifying diseases, recognizing symptoms, and taking timely action are crucial for effective management, maintaining tree health, and ensuring quality harvests.

Anthracnose is a fungal disease (Marssonina juglandis) that affects all species of walnuts, manifesting on both the fruits and leaves. Initial symptoms include irregularly shaped, varying-sized yellow-red spots with a red-brown halo on the leaves. Severely affected leaves turn yellow, scorch, dry out, and fall off. Small black-brown spots appear on the fruits, which may lead to premature fruit drop. The disease develops rapidly in wet weather and temperatures between 10°C and 32°C. It overwinters in infected shoots and fallen leaves, spreading in the spring through wind and rain.

Bacterial disease is a bacterial illness affecting all green parts of the walnut tree, presenting as localized necrosis. Severely affected leaves become deformed and die. Fatty, darkening, and expanding brown spots appear on the shoots. These may penetrate deeply, reaching the wood and core or be confined only to the bark in later infections. Infected young fruits and catkins blacken and die. The bacterium overwinters in infected shoots, buds, and catkins. The disease is favored by cool and rainy weather during and after flowering.

Types of bacteriosis are bacterial blight, botrytis canker and the accumulation of dead branches and shoots.

• Symptoms of bacterial blight on walnuts: Black lesions on the fruits and sometimes the blackening of young, tender shoots.
• Botrytis canker and blight: Appear in late August and September with brownish nuts and partially killed branches. Occasionally, affected leaves are seen, but never necrotic lesions on the leaf blades.
• Another common symptom is the accumulation of dead branches and shoots (old wood). Many of these dead tissues are filled with pycnidia of Botryosphaeria and/or Phomopsis.

To effectively manage anthracnose and bacterial diseases, the following measures are recommended:

• Choosing Resistant Varieties: Planting varieties resistant to these diseases can significantly reduce the risk of infection. Examples of such varieties include Fernor, Lara, Ferne, Franquette, Izvor, and Chandler.
• Sanitary Pruning: Regular removal of infected shoots and branches reduces the sources of infection. Infected parts should be cut 15 cm below the affected area and burned.
• Managing Fallen Leaves: Collecting and burning fallen leaves prevents overwintering of pathogens. Alternatively, they can be buried in the soil at a depth of 10–15 cm.
• Copper-containing Fungicides: Applying copper-based products like Bordeaux mixture (2%) before and after flowering, as well as other copper fungicides, can provide protection against these diseases. Recommended products include Champion, Cuprocin, Funguran, etc.
• Improving Growth Conditions: Ensuring optimal growing conditions for walnuts, including proper irrigation and fertilization, can strengthen the tree's immune system and make them more resistant to diseases. 

In Bulgaria, control of bacterial disease on walnuts is carried out through agronomic practices such as pruning and destroying infected shoots, as well as winter treatments with copper-containing products. During the growing season, copper fungicides (Champion, Cuprocin, Funguran, etc.) are recommended.
To control Botrytis canker and blight, fungicides like Coprantol Duo (copper hydroxide + copper oxychloride) are used. The application rate is 350 g/ha, with 4 applications recommended at 7-day intervals.

Anthracnose (Marssonina juglandis):

Copper-based fungicides:

• Bordeaux mixture (2%) – before and after flowering
• Champion (copper hydroxide) – 300-350 g/ha
• Funguran (copper hydroxide) – 300-350 g/ha
• Cuprocin (copper oxychloride) – 300-400 g/ha

Fungicides with combined composition:

• Coprantol Duo (copper hydroxide + copper oxychloride) – 350 g/ha, 4 applications at 7-day intervals

Bacterial blight (Xanthomonas arboricola pv. Juglandis):

Copper-based products:

• Champion, Cuprocin, Funguran – applied before and after flowering

Biological products:

• Serenade AS (Bacillus subtilis) – for bacterial infection control

Anthracnose and Bacterial Blight:

Copper-based biological products (approved for organic farming):

• Bordeaux mixture (1-2%)
• Cuprocin 35 WG (copper oxychloride)
• Funguran OH 50 WP (copper hydroxide)

Biofungicides and biobactericides:

• Serenade AS (Bacillus subtilis)
• Bioformula based on Trichoderma spp.
• Bioprotection with propolis extract (natural fungicidal effect)

Application Recommendations:

• Spraying begins early in spring, before leaf emergence.
• Repeated after flowering and during the growing season in humid conditions.
• Interval between applications: 7-14 days, depending on weather conditions.
• In rainy weather, reapplication is recommended within 24-48 hours.

These practices help ensure effective disease control and prevent crop losses.

Bacterial blight, Botrytis canker, and anthracnose represent unique challenges, but careful monitoring, timely intervention, and proper orchard sanitation can help reduce their impact. Regular monitoring of walnut plantations and timely preventive measures are key to maintaining tree health and ensuring a high-quality harves.