Fruit tree black ball fungus
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Fruit tree black ball fungus
Fruit tree black ball fungus is a fungal disease of the oak family which is spread by planting of the affected plant with or without the use of susceptible grafted trees. The disease was first recognised in the United Kingdom in 1984. There is no cure and the only control measure is the removal of infected plant material from a site.
Distribution and host range
The disease can be introduced to areas where there is a high density of (closely related) conifers (conifers is any of several tree species having cone-bearing stems, usually having pointed leaves and used for producing timber. “Pin oak” or “American black oak” is the example often given in the British Isles) but sometimes a conifer will be brought from outside the area and become infected. The disease has also been recorded in young plantations and has been reported on eight species of grafted fruit trees.
In nurseries it is known that the disease can be transmitted to native British and European species, to those growing in the Mediterranean, and to species growing in eastern North America.
The fungus is known to infect the leaves of close relatives of the English oak such as white oak, sassafras, Eastern red oak and sassafras.
It has been found that cuttings of a European white oak that is grown in the Netherlands are more likely to show the symptoms than either planted cuttings or those taken from within the host country. It has been found that this is due to the host cuttings arriving with leaves that have already been infected.
The optimum temperature for the fungus to grow in the United Kingdom is 25 ,°C.
Identification of the organism
An experienced horticulturalist can identify this disease within a short time on cuttings from the trees affected by it.
The agent causing fruit tree black ball (Valsa sclerotior) can be identified in nursery cuttings and adults by observing the disease symptoms, the presence of a microscopic fungus in the cause, and its place of origin.
The symptoms include:
Cuttings taken from infected adults may show brown streaks or bands in the petiole and/or on the leaf surface.
Leaves may be covered with a coating of black or brown coloured material.
Stems may show red or purple discoloration and the presence of white patches.
Severely affected trees have wilting leaves and crowns that may be covered in mould and slimy material.
Identification of the agent is confirmed if there is a microscopic fungus present on the leaf surface and on the brown fungal material, which is described below.
Several microscopic fungi can cause similar symptoms, but none have so far been isolated from the infected trees.
Host range of the fungus
The fungal pathogen has been recorded to be present in the following countries and regions:
Isle of Wight
A survey of key areas within England was undertaken and showed that it was widespread across most of the southern half of England from Hampshire in the south to Norfolk and Suffolk in the east and London in the north.
More than 10 new plant introductions of unrelated species to regions in northern North America, southern Brazil, southeast Australia, southwest Africa and Southern France, have been recorded in recent years.
Types of conifer hosts
The symptoms of fruit tree black ball may vary according to the species of conifer used as a host tree. While the disease has been known to affect trees other than the English oak, no symptoms have been observed in any other commercially grown British and European conifer. It has been noted that high rainfall may bring out the disease.
Fruit tree black ball causes little damage to the host plant, but because of the high cost of eliminating affected trees, it is important to understand the disease.
The spores, which are believed to be produced in plant debris in late summer, can spread by:
Spores produced by infected tree fall to the ground. They can be spread by winds from trees and are spread by wind as the spores drift on fallen leaves. The spores can then be spread in midwinter by spring-thawing and during the growing season by rain, irrigation, machine harvesting and pruning operations.
Spores which may have survived in harvested plant material may be distributed in the produce and used to infect new areas. In 1989 an unrecorded number of apple trees, estimated at 10,000 ,pounds (4533 ,kg) were found in Canada.
Spread by insects, which has occurred in the Mediterranean where Valsa sclerotior has been recorded to be carried on gregarious bark beetles.
The spores of the fungus can survive for over a year on conifer residues, in the root zones and on soil and may be spread from one area to another by propagating plant material that may carry the disease.
Fruit tree black ball progresses through three different stages of development:
Persistent. In this stage the growth of the fungus is not yet serious enough to kill the tree or cause the tree to wilt.
Early infection. While most of the tree appears unaffected and healthy, the stem tissue is not as strong as the leaves. This stage occurs from early spring to late summer.
Severe infection. A serious attack occurs during late summer to early autumn, when the leaves become covered in a coating of fungal material and eventually die.
Management of the disease
Management of fruit tree black ball depends on the stage of the disease and how early it can be detected and stopped.
Early management can be done at an earlier stage than the standard management used by the trade, in order to avoid further spreading of the disease. To