Ash dieback
The new disease on ash, assumed to be caused by a fungus, is widespread in Denmark. We have observed symptoms since 2003, but the first couple of years we thought they were caused by weather conditions. Based on the similarities between the disease in Denmark and Sweden, we have concluded that the cause is the same. Chalara fraxinea has been found on diseased ash trees in Sweden, and there are indications that this fungus may be the main cause of ash dieback. The decline of ash related to the symptoms described below is an important issue in the Nordic-Baltic Forest Pathology network PATHCAR.

Ash dieback 2008
As in 2007 bud burst of most tree species came early in Denmark due to a mild winter and warm spring, this year including ash. Dieback was still widespread, both in forests and in the landscape. Warm dry weather in April, May and June probably contributed to rapid dying of infected shoots. There are indications from clonal seed orchards that genetic variation in susceptibility to ash decline can be found. Research into these aspects have been initiated by Professor Erik Dahl Kjær and colleagues.

Ash dieback 2007
Many tree species had early bud burst in Denmark due to a mild winter and warm spring. However, ash flushed at the normal time, which in Denmark is late May and early June. Dieback was widespread as expected, both in forests, in the landscape and in some nurseries. Many trees had dead shoots in the upper crown or partial dieback of the crown. Other trees looked transparent or "patchy" because the leaves were not evenly distributed but sat in "lumps" at the end of branches. Fortunately, there were also healthy looking trees with few symptoms. Efforts were made to evaluate the frequency of the problem, especially in young stands. Impact of the disease is high in ash stands of 6-10 meters, because the dieback of the main stem will result in future timber having short length and lower quality.

Symptoms of ash dieback
The most visible symptom of the disease is top dying. The one and two year old shoots die either before flushing or during dry periods in summer. The problem is especially visible in ash with a height of 4-10 meters. I.e. in cultures, young forest stands (afforestation) and on road trees. But even on older trees symptoms are clearly visible in the crown. The dead shoots are caused by desiccation beacuse death of bark lower down prevents water transport, or are killed directly by the fungus.

Similar damages have been recorded in Sweden, Poland, the Baltic countries, Austria, Czech Republik, Slovenia and Germany. Recently, the first finds were made in Norway and mainland Finland. The influence of frost or drought as secondary factors is debated.

Wilting sypmtoms on  ash in Denmark	Wilting sypmtoms on   ash in Sweden		Bark necrose around shoot on young ash     in Denmark                in Sweden

Possible pathogen: Chalara fraxinea
This fungus was first described in 2006 based on ash dieback in Poland (Kowalski 2006). In Sweden the researchers at the Swedish Agricultural University have isolated many fungi from diseased ash trees. An unidentified fungus, which was among those fungi most commonly found in dead bark and other symptomatic tissue, also seemed particularly able to produce the same symptoms in tests of pathogenecity. By comparing their isolates of the unknown fungus with the Polish Chalara fraxinea isolates described by T. Kowalski, the Swedish researchers have concluded that it is the same fungus. Subsequently the fungus have been found in many countries. However, knowledge on infection biology is still lacking. In Denmark Chalara fraxineawas identified in 2007, when the fungus was isolated from samples taken from diseased trees in experimental stands in East Jutland and sent to Dr. Rimvis Vasaitis (Department of Forest Mycology and Pathology, SLU).

Ash dieback symptoms 2003-2006  The first enquiry on problems in ash came in 2003 and was followed in 2004 by several reports on wilting of current year shoots on young trees and thin crowns on older trees. In most cases the dieback was caused by necrotic bark lower down on the stem. In trees with symptoms in 2004 the bark had been killed between the 2003 and 2004 growth season, and in some cases after the cambium became active in 2004. This could be seen by the presence of a row of spring vessels just below the dead bark.

However, the real extent of the problem became visible in the spring of 2005, where many young stands had widespread top dying. Ash dieback was now reported from all over Denmark and from forests as well as road trees and in the landscape. In all cases where damaged trees were inspected, the bark had died before the growth season of 2004. No cases were found, where only shoots from 2004 had died, so there seemed to be no expansion of the disease. But Chalara fraxinea may have spread during autumn 2004, either in the bark of already infected trees or via spores, although we have no certain knowledge of the infection biology of the fungus.

In the spring of 2006 ash dieback occured again in Danish forests, in urban and rural areas and was also recorded in nurseries which spezialise in 3-4 meter tall ash for landscape purposes. This time the cambium had been killed in the autumn of 2005, and the necrotic part was usually at the base of the 2005 shoot or somewhere on the older part of the stem. On many of the damaged trees long necroses were centered around a young shoot.

There could now be no doubt that the cause of the disease in Denmark was not climatic factors, but had to be something else. In Sweden, researchers had already reached the conclusion that a fungus was involved. But they still lacked a name and knowledge about infection biology.

In the autumn of 2006 older stands shed their leaves early and there were brown areas on the leaves and the stalk indicating a possible entry point for the fungus. By early winter it was possible to see necroses and dead shoots on young ash trees. During the winter and spring 2006-07 we have received many reports on dieback in ash. We expect widespread damages to become visible after the ash trees have flushed.