Nature + Green  

 

City Trees

Diseases and Pests


In general, only limited space is available to trees in inner-city areas, especially street trees. A condensed, sealed off root area is particularly detrimental to the trees’ vitality; mechanical injuries permitting the entry of wood-destroying fungi are a frequent additional problem. Road salt, natural gas leaks and dog urine also damage the trees. Weakened and damaged trees are especially susceptible to disease and pests.

The street-tree status report (in German) shows that in 2005 about one-third of the inner-city Berlin street trees more than 15 years old had damaged crowns.

Examples of the many pests and diseases affecting Berlin’s trees:
  • Spider mites attack especially large-leaf lindens almost every year, with hot, dry years favoring their increase. Infested trees will display yellowed leaves in the lower-crown area as early as June. In cases of heavy infestation, the yellowing can continue up into the crown. This causes progressive browning and drying up of the leaves, leading to early foliage loss.
     
  • Aphids can be found particularly on young leaves and shoots. Lindens come under stronger attack by aphids in hot, dry years. Sooty molds feed on their sticky excretions, known as “honeydew,” leaving blackened leaves. Sticky and soiled surfaces under linden trees – often on cars parked below them – are another unpopular consequence.
     
  • Sycamore anthracnose in plane trees is caused by a fungus (Apiognomonia veneta); symptoms include brown spots along leaf veins. If trees are affected, young shoots will wither and dry up in the spring; scaffold branches are not attacked.
     
  • The horse-chestnut leaf miner (Cameraria ohridella), an insect first discovered in Macedonia in 1985, was not found in Berlin until 1998. Horse chestnut trees are their special target. The feeding activity of the leaf miner larvae destroys leaves from the inside, indicated by light-brown spotting. Heavy infestation causes premature foliage loss, and years of attacks will weaken the tree.
     
  • The horse-chestnut scale (Pulvinaria regalis) was first identified in Berlin in 2000. The larvae of this insect take root on leaves and branches, where they suck out the sap. The horse chestnut scale prefers horse chestnuts and lindens, and can be recognized by a white, cotton-like substance on the trunk and, in cases of heavy infestation, on branches.

  • The ascomycete fungus Splanchnonema platani is a weak parasite of plane trees and causes rapid branch dieback. This disease, known as Massaria, was identified in Germany for the first time in 2003 after a hot, extremely dry summer. Even larger, less vital branches can be affected, with all or part of them dying quickly. Broad, discolored (light violet to light red) areas of bark on the upper surface of branches are one sign of infestation. Dark fungus spores later turn these areas a patchy black. Rapid wood-rot on the top side of the branch follows, but since the lower side is not yet affected, the branch stays leafy. Stronger, previously unremarkable branches with full foliage can then die off within just a few weeks, creating a hazard.

  • White mistletoe (Viscum album) is a semi-parasitic perennial that grows on the branches of certain host trees and, over time, can produce bushes of up to one meter in diameter. The plant is an important winter food source for birds, which spread its seeds. Berlin’s Plant Protection Office has recorded an increasing occurrence of mistletoe on deciduous trees in the Steglitz-Zehlendorf borough since 1987. Because urban street trees are weakened by exposure to various types of damage, they are susceptible to mistletoe colonization. However, although mistletoe has gained a recent reputation as being harmful to its host, there is still no proof that it actually kills trees. Since mistletoe has been at home in Berlin and Brandenburg for a long time now, its current expansion signals a natural change. Changes of this kind are not unusual in urban environments, but are in fact typical of nature in cities.

    An information sheet (in German) published by the Senate Department for Urban Development, the Berlin State Commissioner for Nature and Landscape Conservation, and Berlin’s Plant Protection Office can tell you more.