Olive verticillium wilt: symptoms, causes and effective control methods

Olive verticillium wilt is a vascular disease caused by the fungus Verticillium dahliae. It is the most serious soil-borne fungal disease affecting olive trees. Klebahn first reported the fungus on diseased dahlia plants in Germany in 1913. Verticillium dahliae is one of the most significant vascular pathogens of higher plants, with a broad host range.

It affects forest trees, fruit trees, legumes, field crops, vegetables, ornamentals, and weeds. Zachos first identified and studied the disease in olive trees in Stylida, Fthiotida, Greece, in 1952. Since then, researchers have observed it throughout the country, particularly in areas with olive groves.

Table olive varieties, as well as young trees, are generally more sensitive than mature trees. Olive verticillium wilt causes significant damage to susceptible varieties such as Tsounati, Kalamon, and Amfissis. Even the practically resistant variety, Koroneiki, can be infected under favorable conditions for the pathogen.

What are the symptoms of olive verticillium wilt?

The symptoms of olive verticillium wilt can sometimes be confused with those of hadrobacteriosis and root system diseases, as they are similar in different hosts of the pathogen. The main symptoms include

• leaf wilting,
• necrotic yellow-brown leaf spots that lead to leaf fall,
• brown discoloration of the xylem vessels and
• stunted growth.

Olive verticillium wilt can affect trees of all ages, both in olive groves and nurseries. The disease usually appears sporadically, affecting individual trees, but rarely occurs in a generalized form, affecting entire groups or groves.

Olive verticillium wilt occurs in two forms

The first is the acute wilt, or apoplexy syndrome, and the other is the slow decline syndrome.

Acute wilt or apoplexy syndrome:

Apoplexy usually occurs in nurseries on young olive seedlings or in young field trees. The leaves of some branches curl or twist, then dry quickly without falling off, remaining attached for a long time.

Slow drying syndrome:

Slow decline develops gradually and often affects one side of the tree. The leaves of infected branches become chlorotic (yellow), eventually drying and falling, leading to defoliation and branch dieback. This form develops slowly from late spring to early summer and is characterized by leaf and inflorescence necrosis and gradual drying of branches.

How olive verticillium wilt spreads and what conditions favor it

The fungus survives in the soil and can infect olive trees year-round. Infection occurs through the root system and is more severe at higher inoculum levels. The inoculum persists and increases in the soil through the cultivation of susceptible crops (such as potatoes and tomatoes) or the presence of host weeds.

The fungus thrives at temperatures between 21°C and 27°C, while its growth stops above 30°C. For this reason, olive verticillium wilt is a serious pathogen in warmer regions such as southern Europe and the Mediterranean basin.

Methods of Control

The primary methods for controlling Verticillium wilt include the following:

1. Before establishing the olive grove, it is essential to confirm whether plants that host the pathogen have been grown in previous years. It is advisable to avoid planting in such fields.
2. The use of resistant varieties, such as the American Oblonga rootstock, provides excellent resistance to the pathogen, along with Koroneiki and Manzanilla.
3. Soil disinfection. You can disinfect the soil using either steam or the sun.
4. The ways to improve soil include a. balanced fertilization; b. liming acidic or neutral soils to create pH conditions unfavorable to the fungus; c. reducing irrigation, as excessive soil moisture increases inoculum levels; and d. avoiding intercropping with susceptible host plants.

Prevention and integrated management of olive verticillium wilt

Preventing olive verticillium wilt is the most critical step in managing the disease. The use of healthy planting material is crucial, as seedlings from infected nurseries can transfer the pathogen to new areas. The fungus that causes the disease grows best in wet soil, so limiting watering, especially in the spring, helps keep the disease from spreading.

Also, removing and destroying infected branches significantly slows the spread of inoculum. In cases of severe infection, replacing susceptible varieties with more resistant ones is recommended. The combined application of cultural, biological, and preventive measures creates unfavorable conditions for the pathogen, strengthening the long-term sustainability of olive groves and reducing the likelihood of reinfection by olive verticillium wilt.