Utgivare: SLU Konsulentavd./växtskydd (numera: SLU Info/Växter)
Redaktör: Djurle A.
Författare: Nevalainen S., Uotila A.
Adress: The Finnish Forest Research Institute, Box 68, SF-80101, Joensuu 10, Finland; The Finnish Forest Research Institute, Unioinkatu 40A, SF 00170 Helsinki 17, Finland
Titel: The susceptibility of Scots pine to Gremmeniella abietina
Nummer (ISBN, ISSN): ISSN 0042-2169
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NEVALAINEN, S. & UOTILA, A. 1984. The susceptibility of Scots pine to Gremmeniella abietina. Växtskyddsnotiser 48: 3-4, 76-80.
The most important environmental factors increasing host susceptibility are frost damage, shading, cool and rainy growing seasons, the harmful effects of topography and unsuitable edaphic conditions. According to an Finnish inventory, there were clear differences in disease susceptibility among Scots pine clones in a seed orchard and differences were also found among provenances in two plus three progeny trials. Transfer to north slightly increased mortality. So far environmental factors seem to be more important in affecting disease susceptibility. The degree of adaptation of trees to the prevailing environmental conditions is most important.
Gremmeniella abietina (Lagerb.) Morelet causes diseases to conifers through Europe, Northern America and Asia. Pinus species seem to be the most susceptible among conifers, although species of Picea, Abies, Larix and Pseudotsuga have also been seriously damaged (Donaubauer 1972, Roll-Hansen 1972). Examples of severe epidemics of Gremmeniella on Scots pine, Pinus sylvestris L., in Scandinavia have been reported e. g. by Kohn (1964) from Sweden between 1950-1960, by Roll-Hansen (1967) from Norway in the 1960's and by Kurkela (1967) and Norokorpi (1971) from Finland in the 1960's. In the past few years there has been a severe epidemic of Gremmeniella in the Scots pine stands of South Finland. At least twenty thousand hectares suffered from the disease.
Gremmeniella abietina can be described as a facultative parasite. Thus the infection requires that the trees are susceptible. Wind borne ascospores or splash-dispersed conidia infect the bud or shoot scales from May to September. The fungus penetrates the host during autumn and winter (Lang and Schutt 1974, Siepmann 1976). Infection is also possible through wounds in the stem and branches (Roll-Hansen 1964, Sletten 1917) and in small seedlings the infection may happen through needle epidermis (Skilling and Krogh 1970).
Gremmeniella abietina is suggested to have at least three continentally disjuncted physiological races (Dorworth and Krywienczyk 1975). Two different morphological and physiological races exist e. g. in Finland (Uotila 1983).
Environmental factors, such as weather conditions are important in predisposing conifers to infection by Gremmeniella. Both the amount of inoculum and the susceptibility of the host may increase. It is also useful to know the role of genetic variation in susceptibility, because this can give us a possibility to use resistant planting material in the future.
The effect of temperature and humidity is often connected with the winter hardiness and the lignification processes of pine tissues, which in turn affect the susceptibility to frost damage. Frost damage has been considered to be the primary cause of the differences in resistance to Gremmeniella (Dietrichson 1968), although after a severe frost damage no fungal infection is necessarily needed to kill the trees (Venn 1970, Pomerleau 1971). Frost hardiness seems to correlate well with the high autumn dry-matter content of the needles.
The damages to the nursery seedlings in Finland in the 1960's were perhaps more due to the cold weather conditions (or the nutrient contents in the soil) than the fungal invasion (Kurkela 1967).
Incomplete lignification can result from several combinations of weather factors. A warm summer followed by a rainy, cool growing period is perhaps the worst situation for pine: the height growth is good, but the winter hardiness is low, and at the same time the amount of inoculum may increase.
Edaphic factors often work together with climatic conditions. In several cases the disease incidence has been greater on fine-textured soils, which formerly may have been spruce stands. On such sites, the roots may suffer from anaerobic conditions. Disease is sometimes more severe in topographic depression. In such places, the trees are more often damaged by frosts. Moreover, the spores may recirculate, thus intensifying the amount of inoculum (Dorworth 1974). Sometimes the disease has been found to be more frequent on higher levels above the sea (Roll-Hansen 1972). This might be due to the extreme climatic conditions plus the harmful effects of the snow cover.
The effect of shading as a predisposing factor is due to the weakening of the trees in overly densed stands and the effect of greater and prolonged humidity on amounts and survival of spores (e. g. Gremmen 1968). Read (1966, 1967, 1968) found that the disease on Corsica pine in Great Britain was more severe on north facing slopes. In addition to the differences in humidity conditions in different aspects, he believed that the disease was due to the loss of soluble carbohydrates during the periods of low light intensity.
In general, the photoperiod and temperature are the most important factors determining the annual cycle of forest trees. Actively growing plants seem to be able to resist the penetration of the fungus into living tissues. At least in very small seedlings, the susceptibility is different in different phases of development (Hamnede and Dunberg 1983).
After the last epidemic in Finland, two plus three progeny trials and one seed orchard were inventoried in order to find out the differences in disease susceptibility between trees of different origin:
1. Loppi (670), 21 years old, 30 provenances from Southern and Middle Finland, 3 provenances from Siberia. Planned by the Finnish Foundation of Forest Breeding.
2. Kuorevesi (159/1), 23 years old, 36 provenances from Southern and Middle Finland. Planned by the Department of Forest Genetics, the Finnish Forest Research Institute.
3. Oitti seed orchard, 25-27 years old, 25 clones from Southern Finland. Owned by Keskusmetsälautakunta Tapio.
In progeny tests the trees were classified in three damage classes: healthy or slightly infected, diseased, dead or heavily infected. In the seed orchard the craftings were healthy or diseased. The results were tested using the twoway analysis of variance.
In the Loppi experiment the Finnish progenies were only slightly infected whereas the Siberian provenances were totally destroyed. It also seemed that the trees in Myrtillus site type were more diseased than in Vaccinium type.
At Kuorevesi 5-60% of the trees were dead or heavily attacked. The differences among progenies were statistically significant. The seed transfer did not affect very much to the mortality if the transfer was shorter than 200 km (fig. 1). However, transfer from south to north increased the mortality slightly.
Fig. 1. The effect of seed transfer to Gremmeniella mortality in 23 years old Scots pine progeny test at Kuorevesi.
In the Oitti seed orchard the differences between clones were clear (fig. 2), but only the lower branches were infected. Clones E 726 A and 710 D were most susceptible: they were infected to upper whorls than the others.
Fig. 2. The percent of slightly diseased crafts in clone in Oitti seed orchard.
The slight differences in resistance among the plus tree progenies, and clear differences among the seed orchard clones can be explained on the basis of the population genetics of Scots pine. The plus tree progenies are genetically not as homogenous as the clones in the seed orchard. The father tree is unknown in open-pollinated progenies. The pollen flies long distances, and only 0.8-3.0% comes from a single neighbouring tree even in a seed orchard (Rudin and Ekberg 1982). There is a clinal variation among the populations of Scots pine, and the variation within a population is wide, also when survival is concerned (Eriksson 1982, Eriksson et al. 1976).
The results of this inventory support the earlier findings of Dietrichson (1968) and Björkman (1972) that the southern provenances are more susceptible than the local, and that the local provenances are less resistant than the northern ones. Dietrichson also found a positive correlation between disease susceptibility and the earliness of growth cessation and the relative growth rate. Repo (1983) found clear differences in the electric impedance of needles of different Scots pine provenances. The northernmost provenance, which probably start their hibernation earlier and have better winter hardiness, had lower impedances.
There have been efforts to explain the differences among the provenances with biochemical factors. Studying P. nigra clones, Stephan and Scholz (1979) found a positive correlation between the contents of myrcene and limonene monoterpenes and disease resistance. Also buffering capacity of bark tissues was positively correlated with the resistance to Gremmeniella.
The broad host spectrum of Gremmeniella abietina, its diversity and the complex effect of environmental factors cause some difficulty in determining the real resistance mechanisms of Scots pine against the disease. Probably there are genetical differences in disease resistance even within the local pines. Many problems are still left, if we want to start a breeding program on the basis of these differences.
Gremmeniella resistance is likely horisontal resistance and when perennial plants are concerned the only acceptable type for resistance breeding would be horisontal (Simmonds 1983, Stelzer et al. 1983). As to field observations, provenance trials can give valuable information, but only if the natural inoculum has been even and sufficient, and thus artificial inoculation experiments are needed. We need screening methods, which can be succesfully repeated without the disturbing effects of the environment and the developmental phase of the host plant. The differences among the clones may be different in different environmental conditions and that is why the testing for resistance should always be performed in the climatic zone in which the plants will be used (Martinsson 1975).
So far it seems that the environmental factors are much more important in determining the susceptibility of Scots pine to the Gremmeniella infection. The genetic variations in disease susceptibility often reflect genetically controlled differences in the adaptation of the trees to their environment. The degree of adaptation is most important. We have enough examples of bad damages to trees introduced into unsuitable environmental conditions (eg. the Siberian or Polish provenances of Scots pine when planted in Finland). The outbreak of epidemics can be made less probable by many silvicultural practices: for example by selecting seed of right origin, by right-timed thinnings, by removing the diseased trees, by burning the diseased logging waste, by tilling the soil for better air conditions and by using species other than pine in places most liable to Gremmeniella infection.
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NEVALAINEN, S. & UOTILA, A. 1984. Tallens mottaglighet för Gremmeniella abietina. Växtskyddsnotiser 48: 3-4, 76-80.
De viktigaste miljöfaktorerna som ökar tallens mottaglighet för G. abietina är frost, beskuggning, kylig och fuktig vegetationsperiod, ogynnsam topografi och olämpliga edafiska förhållanden. Enligt en finsk inventering fanns tydliga skillnader i mottaglighet mellan olika kloner av tall (Pinus silvestris L.) i en fröodling. Det fanns även skillnader mellan provenienser i två avkommeförsök. Vid förflyttning norrut ökade plantornas dödlighet något. Miljöfaktorerna tycks dock vara av större betydelse för mottagligheten för sjukdomen ifråga. Den största betydelsen har trädens anpassningsförmåga till rådande förhållanden.