Verticillium Wilt of Deciduous Fruit Trees		AG0162
Bill Washington, Knoxfield Updated: January 2006
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Verticillium wilt, also known as blackheart, is caused by the fungus Verticillium dahliae. The disease is found world-wide and affects many plants, including both woody and herbaceous species. Apricots are the most commonly affected fruit tree, but other stone fruits such as peach, nectarine and plum may also be infected. Nut trees including chestnut, almond, pistachio and walnut are also susceptible. Infections of apple and pear trees are rare.

Symptoms

The disease is usually observed in early summer as a progressive loss of leaves from infected limbs, starting at the base of each branch. Leaves may become yellow and dull in appearance before dropping prematurely. By late summer, only a tuft of leaves may remain at the tip of a severely affected branch (figure 1). Occasionally leaves may show a true wilt, and when the death of these leaves is very rapid they may remain attached to the plant for several weeks. An entire tree may show these symptoms, or infection may be confined to one side, or even one branch, of the tree.

The most important diagnostic symptom of this disease is found when the trunk, branches or twigs are cut open to reveal the internal wood. Diseased wood shows in the cross section as a series of light to dark brown, irregularly shaped spots that sometimes merge into a ring of stained tissue. In mild cases of infection, wood staining may occur in the absence of any leaf symptoms.

Economic importance

The disease is serious for commercial growers, for although affected trees seldom die quickly, they may remain stunted and unproductive for many years. There is also a strong probability that replacement trees will be attacked in due course. The fungus is widespread and particularly prevalent on land that has been repeatedly cropped with susceptible crops such as potatoes, tomatoes and strawberries, which are alternative hosts for the fungus.

Disease cycle

The fungus persists in the soil for many years in the form of resting-bodies (micro-sclerotia), and as a root-parasite of many crop plants and weeds. Mycelium enters a healthy plant by penetrating the root hairs, or through any wound. The fungus then grows into the water-conducting wood, in which it forms spores. These spores are carried upwards, with the flow of water, to the stem and leaves, giving rise to a continuous strand of infected wood from the invaded root to the branches and leaves vertically above it.

• New blocks of stone fruit should only be planted on land that has been under grass for several years. Avoid planting stone fruit into sites that have had a history of crops such as potatoes, tomatoes or strawberries. If fruit trees have to be planted in soil known to be infested with Verticillium, then plant apples or pears, as these species show marked resistance to the disease.
  
  Apricots grown on plum rootstocks also show some resistance to the disease, and may be planted where other options are not available.
  
  
• Avoid intercropping blocks of young stone fruit with tomatoes, potatoes, peppers, strawberries or melons, as these susceptible crops may increase the population of Verticillium in the soil.
  
  
• Suppress weeds after planting or sow the block down to grass and clover, to prevent a rapid increase of Verticillium in the soil. Important weed hosts of Verticillium belong to the families Chenopodiaceae (fat-hen), Solanaceae (nightshade), and Amaranthaceae (red-root amaranthus).
  
  
• Remove seriously affected, unproductive trees and as much of their roots as possible. The area should then be fumigated or otherwise treated to kill any remaining fungus before replanting.
  
  
• Avoid subjecting young plants to water stress, root damage and excessive quantities of fertiliser.