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Depending on the type of spores involved, infection (spore germination and establishment on the shoot) occurs at different times of the year. If conidia are involved, infection usually occurs in early summer, whereas ascospores infect later during the growing period and late in autumn (Skilling, 1972; Gibbs, 1984; Laflamme and Archambault, 1990). The killing of vascular tissue inside the shoots occurs during winter and when the temperature is between +5 and -6°C (Marosy et al., 1989).
An infection of G. abietina might be detected in several ways. By early spring, before the start of bud flushing, the needles on infected shoots are easy to detach due to their dead vascular tissue. The shoot blight symptom occurs after the time of normal shoot elongation; typical reddish-brown needle bases (more brown on Pinus sylvestris and more red on P. contorta), gradually extending to the tip, followed by needle-cast as the result of dieback of these shoots. The shoot blight symptom is usually concentrated in the lower parts of the crown, but in Sweden the disease appears in the top of the tree. A characteristic yellow coloration of the xylem tissues can be seen (Read, 1967). In early summer, new stem cankers might occur; typically recognized as oval 'thumb marks' on the young pine bark (Witzell, 2001). Stem cankers might occur despite no other symptoms being present (Kaitera and Jalkanen, 1994). Old cankers grow quite fast, especially vertically, and some might be >20 cm long. At least on Pinus contorta, they are often covered with apothecia (Witzell, 2001). In early spring, cryptopycnidia (which are smaller than conventional pycnidia) can sometimes be found under the thin bark (Cauchon and Lachance, 1980). During spring or early summer the vegetative fruiting bodies (pycnidia), which are about 1 mm in diameter and shiny black or dark-brown, can be seen on the recently killed shoots. Two years after infection the sexual fruiting bodies, apothecia, are formed. These are sometimes seen on the same twigs where pycnidia occur, as the pycnidia are produced both one and two years after infection. The size of apothecia is about the same as for pycnidia, but the colour is more brownish and the structure somewhat hairy (at least not shiny). When air humidity is high (during or after rain) the apothecia open up and the light grey hymenium is exposed.
On Picea abies, pycnidia only develop on needle cushions. No apothecia have been found on this host. In northern Sweden, extreme abundance of apothecia on the introduced Pinus contorta is reported (Karlman et al., 1994; Hansson et al., 1996).
Adventitious buds sometimes occur at the base of dead shoots. If the attack is not too heavy, this might mean that the trees survive and adventitious buds develop below the point of dieback to provide new growth (Gremmen, 1972). However, in the heavy epidemics of 2001 in Sweden, entire Scots pine stands were rapidly killed without having any chance to recruit adventitious shoots.
Pine seedlings in the nursery should be inspected for orange to brown discoloration at the base of needles in early May. By July, needles and branch tips become brown. Needles fall from branch tips when the slightest pressure is applied. In young pine trees, green discoloration appears beneath the bark of dead branches. Stem cankers are rare but small branch cankers are commonly found. Throughout the year, but mostly in spring and early autumn, black pycnidia or light-brown apothecia should be visible at the base of dead needles or on dead branch tips.
The disease may be controlled in the nursery using the fungicide chlorothalonil applied about seven times from May to mid-August (Skilling and Waddell, 1970, 1974). In Swedish nurseries the fungicides propiconazole and azoxystrobin are used. On the forest scale, however, once G. abietina is established in a plantation it is almost impossible to control. The use of chemicals is not practical in plantation crops where careful selection of disease-free planting material, as well as selection of planting sites at some distance from infected plantations, are important considerations.
Systematic pruning of the lower four whorls in diseased red pine (Pinus resinosa) plantations in Quebec, Canada, reduced the incidence rate of the disease from 67% to 22% (Laflamme, 1999). To avoid many successive interventions, pruning the lower half of the crown whorls and even two-thirds, if necessary, is recommended in infected plantations less than 20 years old.
Brunchorstia dieback, caused by G. abietina, was reported to have devastated Pinus nigra var. maritima [P. nigra subsp. laricio] in Scandinavia in about 1880. In the UK, it occurs mainly on P. nigra var. maritima and only occasionally on P. sylvestris. It has also caused loss to Picea abies in continental Europe over the last century. G. abietina was first identified in North America in 1962 and since then has been an increasing threat to Picea, Pinus resinosa and P. sylvestris forests.
In Sweden the most severe epidemic ever started in 2001 and has seriously affected the forest industry regionally. Approximately 300,000 ha of managed productive Scots pine stands were severely affected. An economic evaluation of the early harvest of these only half-grown productive forests indicates that the net losses were over 100 million euro (M Persson and P Hansson, Swedish University of Agricultural Sciences, Umea, Sweden, personal communication, 2003). This figure does not include the losses due to the expected long-term depression in yield on moderately affected stands that were only thinned.
In a Finnish study, volume growth in slightly damaged Pinus sylvestris plots decreased by 22-42%, depending on disease severity (Riihinen and Uotila, 1992).
P. contorta logs with occluded cankers caused by the pathogen G. abietina gave kraft pulp with poor paper properties: it required more beating energy and resulted in paper with a low tear strength, air permeability, tensile stiffness, burst strength, and poor light-scattering properties (Ahlqvist et al., 1996). Thus, wood damaged by G. abietina should be separated and classed as low-grade raw material.
The disease is typified by death of the growing point and the apical needles of the lower branches of pine and spruce (although the disease started in the top of the crowns in Sweden in 2001). Under severe conditions all the foliage of the host may be affected and die. Thousands of hectares of 30- to 50-year-old, and some 70- to 80-year-old, Pinus sylvestris trees were killed during the 2001 epidemic in Sweden. It is most damaging to species that are grown towards the limit of their range and attacks are favoured by shaded conditions, by dense, badly aerated plantations in which humidity is high, and by weather damage, such as temperature oscillations during shoot elongation. The disease may kill young trees as well as reducing growth and causing distortion of older trees. It can also cause serious nursery loss.