Symptoms: It has been two to seven years after you planted your new cherry orchard. In June or July, your once healthy, vigorous young cherry trees suddenly develop yellow leaves and withered shoot tips on parts of the tree, while the rest of the tree continues to be green and vigorous. The affected scaffolds shed some bright yellow leaves, then as the season continues, other limbs on the tree develop the same symptoms. Other trees scattered around the orchard start losing scaffolds as the weather gets warmer.
It is highly likely that your trees have Verticillium Wilt.
Verticillium (vert – i – sill’ – ee – um, or “Vert” ) is a fungus that was introduced to central Washington and built to high levels in the soil during the production of potatoes or mint. Few fields in the Columbia Basin have not produced three or more crops of potatoes during the past 40 years, and most have grown this important crop several times. The key soil-borne disease of potato is Verticillium Wilt (early dying), which greatly reduces yields and quality if not carefully controlled, usually by careful soil fumigation after long rotations out of susceptible crops.
Unfortunately, this is a very persistant fungus once it has built to high levels in the soil. Use of the most effective fumigants, biological control efforts and long rotations will slow the attack by this pathogen for a growing season, but not much longer. This is sufficient to protect susceptible annual crops, but not susceptible perennial crops, such as fruit trees in the stone fruit group.
Many acres of cherries have now been planted on sites that once produced potatoes, and the incidence of Verticillium Wilt of cherries is increasing. Other “stone” fruits such as peaches, nectarines, apricots and plums are also damaged or killed by this pathogen. Apples and pears are not affected.
How the fungus and host interact:
The Verticillium fungus attacks the tree through the very young portions of its roots, penetrating and infesting the xylem (the water and mineral nutrient transport system.) Eventually the fungus plugs an ever-increasing percentage of the xylem tubes, which reduces the movement of water from the soil up through the tree. Every season, the young tree forms another ring of new xylem, hopefully keeping ahead of the fungus attack.
Depending on the number of Verticillium resting structures in the soil, the tree may be quickly overwhelmed in it’s first few years, and will start collapsing one or two scafold limbs at a time. At times, the tree is attacked while very young, but stays ahead of the damage until it sets the first heavy crop. The slowed growth that year may bring on symptoms as the fungus gets ahead of the tree.
What to do if this disease is affecting your oprchard:
The degree of damage to the orchard is quite variable. Most orchards continue to produce good or excellent crops, however, yields will be less than the potential due to a loss of bearing surface. On the other extreme, some acreage has been removed due to a high incidence of damage to very young trees.
The best management of this disease includes keeping the affected orchard growing well. Good fertility, weed control and irrigation may help the trees stay ahead of the fungus attack on the xylem tubes. New wood production is critical. The fungus builds up on various common weeds, but not grass, so weed control helps keep the level of Verticillium fungus at a lower level.
Parts of the tree that are recently dead or in the process of dying should be removed promptly to prevent the build-up of shot-hole bark beetles, which have a great affinity for tree parts that are in stress. These bark beetles can build up on the scaffolds afffected by the vert, then move into heathy wood, causing serious damage.