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A. errabunda sensu stricto (anamorph Discula umbrinella) causes leaf spot and shoot dieback on Fagus sylvatica. On the upper leaf surface, the leaf spots are brown and roundish or irregular in outline, whereas on the lower leaf they are olive-green. In summer and autumn, many roundish pustules (the acervula of the anamorph, D. umbrinella) form on the spots. The perithecia of the teleomorph A. errabunda form on senescent leaves or leaves that have fallen to the ground.
A. quercina (anamorph Discula quercina) essentially causes leaf spot and shoot dieback although symptoms vary depending on the oak species infected, the vegetative stage of the tree at the time of infection, the tree organ affected, and the climate.
In Mediterranean countries the first symptom that appears on the leaves shortly after the annual resumption of growth is small, brown spots. These spots increase in size and become reddish-brown, irregular in outline, and bounded by the leaf veins (angular spots); when more nearly round these spots measure 12-22 mm in diameter. The spots may become confluent and cover the entire leaf surface between the veins. However, a part of the leaf blade remains green. When the spots cover 70-80% of the leaf blade, the leaf withers, becomes papery and hazel-brown, and the leaf blade becomes twisted. The entire expanding foliage in the crown is often destroyed.
Older leaves are also affected, but symptoms remain limited to brown spots, which spread but do not occupy the entire leaf area.
The infection often passes through the leaf stalk to the shoot, although the shoot can also be infected directly. If the infection completely surrounds the shoot, it dies. The infection also proceeds to the twigs, on which a canker forms, 8-10 cm long and slightly sunk into the wood. Twigs so infected will die (shoot dieback). The cones are also often infected, becoming covered with small, black specks.
Repeated infections over the years, always entailing the death of leaves, shoots and twigs, gradually cause very extensive crown thinning and ultimately the death of the tree. The perithecia of the teleomorph A. quercina form on the leaves, whereas the acervula of the anamorph D. quercina differentiate on the cankers.
The symptoms as they appear in European countries are described, albeit in a general way, by Grove (1937), who states that in more than usually rainy years, especially in the spring and summer, the incidence of the disease increases as it did in England, UK, in the summer of 1980.
In North America, symptoms vary on the many susceptible oak species. On Quercus alba, which is probably the most sensitive to this infection, symptoms manifest themselves in a number of ways: rapidly spreading blight of leaves and shoots, with browning of young leaves during the growth period; a large and irregularly shaped portion of the leaf blade dies off, and the leaf blade becomes twisted, but part of the leaf area remains green; and small necrotic spots form on the adult leaves.
On Quercus phellos and Q. laurifolia the spots are often very numerous, small, brown or black, and surrounded by a halo of green tissue. On Q. nigra and Q. falcata the spots are larger and often surrounded by a yellow ring. When infection is severe the apical shoot dies and folds in upon itself. On Q. palustris and Q. shumardi the spots on the upper leaf blade are round, reddish-brown, varying in size and surrounded by a pale-green halo, whereas on the lower leaf blade they are often concave. On Q. virginiana, which is not a very sensitive species, leaf spots also form, but at a very low density, 6-10 per square inch compared with 100-150 spots per square inch on more sensitive species. On Q. durandii the density of spots is even lower than that on Q. virginiana.
Further details can be found in Grove (1937), Parris and Byrd (1962), Morelet (1989) and Ragazzi et al. (1999c).
Any methods of control must be applied to trees in the nursery or to young trees in the field. On adult trees, the general decline caused by A. errabunda on beech and A. quercina on oak, involves other organisms and is therefore difficult to control.
In the nursery and arboretum it is good practice to gather up the leaves, twigs and branches that fall to the ground, to keep the trees in good condition by proper watering and fertilizing, and to ensure good ventilation.
Although it remains doubtful whether A. quercina is transmitted by seed, it is nevertheless strongly advisable to use healthy seed (checked to be free of the mycete) or seed from healthy trees. Any beech or oak trees that become infected in adult stands must be eliminated.
Chemical treatments are rare. Only in cases where anthracnose has defoliated oak trees for 3-5 years in succession is thiophanate-methyl applied to start vegetative growth, with repeated treatment after 7-10 days. Additional spraying may be carried out when the growing season is cool and humid (see http://www.entomology-umn.edu/cues/dx/CB/oanthab.htm, the site of the Center for Urban Ecology and Sustainability, The University of Minnesota, USA).
One promising means of control is to promote fungi that are biological antagonists against A. quercina and occur on the same organs on the oak tree that yield the disease agent.
The economic impact of A. errabunda and A. quercina is hard to assess. Many oak species that become infected have no specific uses and there is no market for them. Other species are used for timber but precise data on losses to these trees caused by infections from pathogenic agents are not accessible, if only because the infections they cause do not attack the trunk and therefore seem economically insignificant. It should nevertheless be remembered that repeated infections, with their defoliations in consecutive years, lead to a lower growth rate and, hence, a lower annual increment. For the owner of a stand so affected this certainly means an economic loss, which is aggravated by the cost of removing the dead wood.
In urban and periurban parks the removal of leaves fallen in consequence of infection by species of Apiognomonia is without doubt a burden on the city administration.