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Symptoms
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Dieback of green alder (Alnus viridis) - twigs, branches and stems, mostly beginning after budburst (wilt); characteristic are long, stripe-shaped bark necroses, mostly located on only one side of the tree and commonly with numerous pustules of microfungi (Valsa, Ophiovalsa); these fruiting bodies rarely produce spores. Crown dieback extending downwards on the stem.
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Impact
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This rather local phenomenon can be observed after winters with low amounts of snow and could develop into a widespread problem as a consequence of climatic changes. Especially, if the period of snow coverage is significantly shortened in spring, the alders, which require high amounts of water, suffer from drought stress (a 6 months duration of snow cover is optimal for the alders). The drought stress causes the bark necroses, where the microfungi develop. According to experiments by pathologists in Trentino the microfungi are probably not pathogenic (infection tests were negative) – indicating that they are at most weak parasites.
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Control
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Careful diagnosis in the field, to identify the reason for the damage: watch for the downwards extension of the necrosis, check the base of the stem, which often remains healthy to produce coppices in the next year. The phenomenon can be mistaken for Phytophthora-disease of alders: watch for „tarry spots“ (=dried exudates)! Phytophthora-stem necroses always grow upwards on the stem, mostly from the roots or the stem base.
Hygienic measures are usually not necessary: if possible, alders showing dieback should be coppiced to avoid extension of rot fungi into the roots.
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| Hosts |
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Alder;
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| Affected plant parts
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Leaf;
Shoot/Twig/Branch;
Stem;
Stem;
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Characteristic narrow striped bark necrosis with numerous fruiting bodies (Valsaceae)
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Declining stand of green alder (foto LFD Tyrole)
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Declining stand of green alder (foto LFD Tyrole)
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Declining stand of green alder in Italy (Trentino)
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Characteristic narrow striped bark necrosis with numerous fruiting bodies (Valsaceae), Italy, Trentino
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Sectoral rot of the wood below the necrosis
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