Tree Damage Alert No 91
Water Relief!
In February the days lengthen noticeably; memories of the glorious autumn colours fade. Now reflect on the hot dry late summer of 2003 and its significance for trees in 2004.
The Young at Risk
Newly planted trees will have been very vulnerable to water stress. Attention to post planting care early in the growing season will have been crucial to the successful establishment of last year’s plantings. Thorough weed control should have assisted survival although adequate irrigation may have been essential on some sites. If roots failed to become well established into the backfill soil it will become apparent when, after an initial flush, the foliage wilts and trees may die – check last year’s shoot extension growth and the dormant buds. Costs of site clearance, where failures occur, and new trees and shrubs will be very high!
Above average rainfall in November and January has made good some of the soil moisture deficit. Significant snow fall may be needed to complete the task. But prolonged periods of lying snow result in unprotected trees, even middle aged trees, being at risk of severe rabbit damage.Pathogens. Pathogens.
Pathogens.
A soil moisture deficit may have resulted in established trees suffering stress and root death. Root pathogens (e.g. some Armillaria species (Honey fungus)) are able to invade roots and subsequently kill the weakened tree. Even species of Armillaria such as the normally weak pathogen A. gallica are able to kill stressed trees. Trees killed by drought or other causes may become colonised by Armillaria increasing the inoculum, and the possibility of further losses in the future.
Another disease to look out for next summer is Sooty bark disease (SBD) of Sycamore (Acer pseudoplatanus) caused by Cryptostroma corticale (see Tree Damage Alert 22). Historically, hot dry summers are followed by outbreaks of SBD. The first major outbreak in Britain was in London in 1948 after the hot summer of 1947. The last outbreak was 1996, again 1995 was an exceptionally warm, dry summer. The fungus is a ‘latent pathogen’ being present on dead wood and within living, healthy Sycamores. When trees are drought stressed the fungus can break out to invade large areas of bark and wood causing death of branches and trees.
Christmas trees also suffer from their share of diseases. Firs can be particularly susceptible to Phytophthoras. These spread from plant to plant through surface water and attack the roots. Look out for wilting, foliage discoloration, necrotic roots, and stem and/or root collar resinosis. To discourage Phytophthoras avoid farmyard manure and ensure planted areas are thoroughly drained. Pines can suffer from needle yellowing (Cyclaneusma minus), pine needle rust (Coleosporium spp.) and pine twisting rust (Melampsora pinitorqua). Silver fir (A. alba) has been affected by needle browning and needle cast but the cause is, as yet, unknown.
Insect Pests
The effects of a long, hot and dry summer on insects vary greatly from species to species. Certain insects can be quick to take advantage of trees suffering drought stress. Bark beetle species (e.g. Tomicus piniperda on pines, Ips cembrae on larch and Polygraphus poligraphus on spruce) can become very damaging, as can a number of wood-boring species (e.g. Tetropium gabrielli, the larch longhorn beetle and Sesia apiformis, the hornet clearwing moth on poplar). These species are normally considered secondary pests, healthy trees are not generally significantly damaged. When trees become stressed, however, these insects are capable of invading more effectively and may cause the death of trees that would otherwise recover.
The Horse chestnut leaf miner, Cameraria ohridella, which has spread rapidly across Europe in recent years, was first discovered in Wimbledon in 2002. Its population development and rate of dispersal increase in hot dry weather. The warm temperatures last summer will have aided the moth’s establishment in this country and certainly by September, 2003, many Horse chestnuts growing along the edge of Wimbledon Common had been extensively defoliated. Furthermore, by the end of the year, the moth had been reported from a number of other London Boroughs and at least two locations outside the capital!
During 2004 we will be watching developments. Please let us know your experiences!
Brain Greig, AAIS, Farnham, and Christine Tilbury, Forest Research, Farnham.
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This is one in an occasional series of Tree Damage Alerts produced for the benefit of the arboricultural profession and issued by the Arboricultural Advisory and Information Service.