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Since the beginning of the twentieth century, oak decline has been a serious and frequently occurring disease of European oak forests (Delatour, 1983; Oleksyn & Przybyl, 1987; Hartmann et al., 1989). Being an episodic phenomenon of local or regional importance in the past, oak decline, in its current phase, has been going on since the beginning of the 1980s, and is occurring all over Europe (Delatour, 1983; Hartmann et al., 1989; Siwecki & Liese, 1991; Luisi et al., 1993). Above-ground symptoms include dieback of branches and parts of the crown, formation of epicormic shoots, high transparency of the crown, yellowing and wilting of leaves and tarry exudates from the bark (Siwecki & Liese, 1991; Luisi et al., 1993), all symptoms indicative of water stress and poor nutrition. Mortality rates may be up to five trees per hectare per year (Hartmann et al., 1989). In Bavaria, 28% of the oaks showed severe crown damage (crown transparency > 25%) in 1999 and severely damaged oak stands with mortality due to unknown causes were found throughout all growth regions (Anonymous, 1999).
Many causal agents such as frost, drought, air pollution, falling groundwater levels, silvicultural mismanagement, leaf defoliators, bark beetles, Ophiostoma and Ceratocystis species, bacteria, MLO and viruses have been discussed, but none of them account for more than local or regional problems (Delatour, 1983; Nienhaus, 1987; Oleksyn & Przybyl, 1987; Hartmann et al., 1989; Siwecki & Liese, 1991; Luisi et al., 1993; Ahrens & Seemüller, 1994; Schlag (1994, 1995; Ragazzi et al., 1995; Hartmann & Blank (1998).
In Spain and Portugal, a strong association was shown between the presence of Phytophthora cinnamomi, interacting with prolonged drought periods, and rapid mortality and decline of cork (Quercus suber) and holm oaks (Q. ilex), which display similar disease symptoms to declining pedunculate (Q. robur) and sessile oaks (Q. petraea) in Central and Western Europe (Brasier, 1993, 1996; Brasier et al., 1993a,b; Cobos et al., 1993; Gallego et al., 1999). In South-eastern France, P. cinnamomi was also isolated from cork and holm oaks. Its pathogenicity towards both oak species was proven and its possible involvement in the decline process proposed (Robin et al., 1998). The recovery of declining cork and holm oaks following trunk injections with potassium phosphonate further confirmed that Iberian oak decline is most probably a Phytophthora-related disease (Fernandez-Escobar et al., 1999).
In Central and Western Europe, deterioration of oak fine roots was observed in several studies (Näveke & Meyer, 1990; Vincent, 1991; Eichhorn, 1992; Blaschke, 1994; Jung, 1998; Thomas & Hartmann, 1998). Assessment of the root systems of declining and healthy oaks in 33 stands involving five European oak species all over Central Europe showed a progressive destruction of the fine root system, dieback of long roots and necrotic lesions on suberized and nonsuberized roots. These symptoms were observed in both healthy and declining oaks, but the damage was generally more severe in decline situations (Blaschke, 1994; Jung, 1998). Attempted isolations from fine roots and rhizosphere soil samples revealed the widespread occurrence of several Phytophthora species including P. cactorum, P. citricola, P. cambivora, P. gonapodyides, and the new species P. quercina in 29 out of the 33 stands (Jung, 1998; Jung et al., 1996). In Northern France, Galoux & Dutrecq (1990) isolated a Phytophthora species from fine roots of declining oaks, and Hansen & Delatour, 1999) demonstrated a diverse Phytophthora population including P. quercina in oak forest soils.
In several soil infestation tests, P. quercina and P. cambivora were the most aggressive species to root systems of young Q. robur plants (Jung, 1998; Jung et al., 1996, 1999a). In another test with various phytophthoras and a range of broadleaf tree species, P. quercina proved to be host-specific to the genus Quercus (Jung et al., 1999b).
In biotests, several of the isolated Phytophthora species released toxic substances into their culture medium that induced wilting and intercostal chlorosis and leaf necrosis in Q. robur cuttings (Jung, 1998): symptoms often observed in declining oaks in the field. Later, Heiser et al. (1999) showed for P. quercina, P. citricola and P. gonapodyides that these substances belong to the family of Phytophthora leaf necrotic proteins called elicitins.
In this study, the relationships between the root condition and crown status of healthy and declining oaks, the population of soilborne Phytophthora species and site factors, especially geological substrate, soil pH and soil texture, were investigated, with the aim of improving understanding of the role of Phytophthora pathogens in oak decline in Central Europe.
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- Materials and methods
It was shown that Phytophthora species are widespread on a range of different geological substrates with sandy-loamy, loamy, silty or clayey soils with a mean pH (CaCl2) between 3·5 and 6·6. P. quercina was by far the most frequently isolated species and had the highest plasticity concerning geological substrate and soil type, texture and pH. In addition, P. quercina was isolated more frequently from declining (63·1%) than from healthy oaks (23·7%) on these sites. This latter finding is in contrast to the results of Hansen & Delatour (1999) and Delatour et al. (2000) who found, in four stands in north-east France, no evident association between the presence of P. quercina and other Phytophthora species and the decline status of oaks. It seems most likely that differences in sampling methodology are responsible for this discrepancy between the Bavarian and the French results. In both French studies, the number of trees and stands examined, as well as the size of the plots, was relatively small. On small plots, healthy and declining trees probably share more or less the same rhizosphere soil. Therefore, a similar Phytophthora population, regardless of the crown status, can be expected. In the present study, comprising a larger data base with 217 oaks in 35 oak stands, such neighbourhood effects were avoided by sampling trees with a scattered distribution in the stand instead of sampling all trees on relatively small defined plots. Also Hansen & Delatour (1999) and Delatour et al. (2000) stated that their sampling, which was aimed at evaluating the population of soilborne Phytophthora species, was inadequate for supporting firm conclusions about correlations between crown status and the Phytophthora population in the soil.
In general, the epidemiology of Phytophthora-induced fine root diseases is considered to be multicyclic (Erwin & Ribeiro, 1996). Phytophthoras can increase and disseminate their inoculum from low, nearly undetectable levels during a relatively short time of favourable environmental conditions. On the other hand, it can take decades of inoculum build-up and progressive fine root destruction before a mature tree begins to show noticeable above-ground symptoms (Tsao, 1990). Newhook (1988) pointed out the importance of latent infections and excess soil moisture, and mentioned that ‘the margin between outward health and disease can depend on the state of balance between rootlet death and rootlet replacement’. According to these statements it can be concluded that the presence of P. quercina in the rhizosphere of healthy oaks does not argue against its possible involvement in oak decline nor does it necessarily mean that these trees will fall into decline. However, it could be shown that oaks with P. quercina or other Phytophthora spp. in their rhizosphere displayed on average 20% higher crown transparency and significantly higher levels of fine root damage than oaks without Phytophthora. The statistical analysis of the isolation results revealed that oaks with P. quercina or other Phytophthora spp. in their rhizosphere have a significantly higher probability of exhibiting severe above-ground disease symptoms than oaks without Phytophthora spp.
These findings indicate that the high in vitro aggressiveness of at least P. quercina and P. cambivora as shown by soil infestation tests (Jung et al., 1996, 1999a, 1999b; Jung, 1998) might also exist under favourable environmental conditions in the field. In addition, significantly higher levels of fine root damage in declining than in healthy oaks were found in the Phytophthora-infested stands, resulting in significant correlations between crown transparency and all root parameters assessed. All root parameters with the exception of root rot were negatively correlated with crown transparency, confirming that the fine root status is of great importance for the overall crown status in Phytophthora-infested stands.
No Phytophthora species were recovered from soil samples of stands on sandy to sandy-loamy sites with a mean soil-pH ≤ 3·9. On these sites the correlation coefficients between crown transparency and all root parameters, as well as the differences between declining and healthy oaks for all root parameters assessed, were markedly smaller and less significant than in the Phytophthora-infested sites. In some stands, the health of the fine root system was even better in declining than in healthy oaks (data not shown), indicating that crown damage may have occurred before root damage.
The present findings are in accordance with main results of the study of Thomas & Hartmann (1998) who, in northern Germany, found clear differences in fine root density and biomass between healthy and damaged oaks on a clayey and hydromorphic site, but not on a sandy site. However, on another clayey site damaged oaks were not inferior to the healthy ones with respect to root density and biomass. In a later investigation on the occurrence of soilborne Phytophthora species in northern German oak stands, a strong association of P. quercina with declining oaks growing in disease pockets was found in the first two stands (T Jung & G Hartmann, unpublished data).
In a pH (CaCl2) range from 3·5 to 3·9, Phytophthora spp. were isolated from heavy but not from sandy soils. A possible explanation could be that, in sandy soils, even after heavy rain, the period with free water is too short to allow sufficient production of sporangia and release of viable zoospores at these suboptimal pH values. The isolation of P. quercina from sandy to silty-sandy soils in northern Germany with mean pH values > 4·3 (T Jung, unpublished data) indicates that the importance of soil texture for the occurrence of Phytophthora spp. decreases with increasing soil pH.
The transformation of the absolute values of the root parameters into season- and weather-corrected relative values resulted in markedly higher correlation coefficients between crown transparency and almost all fine root parameters, thus proving it to be a suitable method for comparing the fine root status of oaks from different stands. For the correlation analysis of crown transparency and fine root condition, and for the calculation of differences in fine root condition between healthy and declining oaks as well as between Phytophthora-infested and Phytophthora-free stands, the parameters root damage estimated followed by fine root length/mother root length, fine root length/dry weight mother roots, and fine root tips/mother root length were the most suitable.
Isolation of P. quercina from various nonhydromorphic sites and from sites with periodically fluctuating watertables shows that this pathogen is well adapted to temporary dry conditions, possibly due to its particularly thick oospore walls (Jung et al., 1999a). The observed level of fine root damage indicates that in temperate climates, even on nonhydromorphic sites, the period of water saturation in the soil after heavy rain or during the cool season is long enough to allow production of sporangia, discharge and dissemination of zoospores and subsequent infection of fine roots.
In general, phytophthora diseases are more severe at high soil pH values (Schmitthenner & Canaday, 1983). In the present study, soil pH was a limiting factor for the occurrence of Phytophthora spp.. No Phytophthora species could be isolated from soil samples with a pH (CaCl2) < 3·4 (< 4·2 measured in H2O). This finding is in accordance with the results of an in vitro test on sporangial formation in nonsterile soil filtrate with pH (H2O) values ranging from 3·5 to 6·0. Most Phytophthora species tested, including P. quercina, P. cambivora and P. citricola, failed to produce sporangia at pH values < 4·0, and sporangial formation increased rapidly with increasing pH. Exceptions were P. cinnamomi, which produced sporangia even at pH 3·5, and P. syringae and P. gonapodyides, which had optimum sporangial production at pH 5·0 (T. Jung, unpublished data). These results confirm the observations of Ribeiro (1983) who noted that sporangia may not form at pH values < 4·0. According to Ribeiro, this is probably due to the inability of Phytophthora oospores to germinate at low pH. In addition, T Jung, (unpublished data) observed an increasing proportion of hyphal lysis and sporangial abortion with decreasing pH. It can be derived from the liming experiments of Muchovej et al. (1980), working on pepper blight caused by P. capsici, that an increase in soluble aluminium with decreasing soil pH and calcium concentrations can also contribute to an inhibition of sporangial formation of Phytophthora, thus decreasing disease severity.
The concentration of exchangeable calcium in the soil was also an important factor for the occurrence of Phytophthora. Soils of Phytophthora-infested oak stands had significantly higher calcium concentrations than soils of Phytophthora-free stands (28·4 ± 37·4 vs. 5·5 ± 4·6 p.p.m.). This might be due to both a linkage of the calcium content with other soil properties such as pH and clay content, and the essential role of calcium for the asexual reproduction and infection process of Phytophthora. External calcium has a stimulatory effect on sporangial production (Ribeiro, 1983), and minimum levels are required for zoospore taxis (Carlile, 1983), adhesion on solid surfaces (Donaldson & Deacon, 1992) and cyst germination (Xu & Morris, 1998). Furthermore, it is known that cations such as calcium enhance oospore germination (Ribeiro, 1983). Although there are examples of high calcium suppressing root rot caused by Phytophthora spp., Phytophthora diseases are generally considered as being more severe at higher calcium concentrations (Schmitthenner & Canaday, 1983).
In Bavaria, as well as in other European countries, crown damage and mortality of oaks is markedly higher in older than in younger stands (Anonymous, 1999; Hansen & Delatour, 1999). This is possibly related to the observed decrease in plasticity and regeneration capacity of oak root systems with increasing age (Thomas & Hartmann, 1998; Becker & Lévy, 1982). It is proposed that, as a consequence, the balance is changing gradually from one of relative host tolerance to one of susceptibility to Phytophthora. Thus, under favourable environmental conditions, the infection rate of new fine roots by Phytophthora zoospores in mature oaks could exceed the rate of production of such roots, leading to a progressive destruction of the whole fine root system from year to year, and predisposing the weakened trees to secondary pathogens or droughts. The hypothesis that the susceptibility of the fine root system of oaks to Phytophthora is increasing with the age of the trees, resulting in a gradual inoculum build-up of Phytophthora, is supported by the isolation results in stand 29. In this stand, decline of oaks was restricted to mature trees, all of which were affected by P. quercina. In contrast, 25-year-old oaks, growing under the same site conditions in a part of the stand where the mature oaks had been cut 20 years previously, were healthy, and no Phytophthora was recovered (data not shown).
In some recent publications (Hartmann & Blank, 1998; Lobinger, 1999; Delb & Block, 1999) leaf defoliators have been considered as causal agents of rapid mortality in German oak stands. In the present study, the relationship between defoliation and presence or absence of Phytophthora spp. in the soil was investigated in stand 14 near Würzburg. In 1994 and 1995, the 45-to 50-year-old stand was completely defoliated by caterpillars of Tortrix viridana, Lymantria dispar and Thaumatopoea processionea. Subsequently, the oaks growing on the upper slope on a moderately fresh orthic luvisol recovered totally, whereas in July 1996 a dramatic mortality of about 75% of the oaks started at the bottom of the slope and in the depression on a stagno-gleyic luvisol with periodically fluctuating watertables. The absence of any Phytophthora species in the rhizosphere of oaks on the upper slope and a Phytophthora infestation at the bottom of the slope and in the depression indicate a synergistic detrimental effect of site conditions, fine root destruction by Phytophthora spp. and defoliation events. However, additional investigations are needed before general conclusions can be drawn.
Schütt (1993) and Schlag (1994) mentioned that oak decline is a complex of different diseases. This hypothesis was confirmed by the results of the present study. From the data it can be concluded that there are at least two different complex diseases being referred to under the name ‘oak decline’ in Central Europe: (i) Phytophthora-mediated oak decline and (ii) decline mediated by droughts, defoliations or miscellaneous causes.
Phytophthora-mediated oak decline occurs on sites with mean soil pH (CaCl2) values ≥ 3·5 and sandy-loamy to loamy, silty or clayey soil texture where soilborne Phytophthora species are strongly involved in the aetiology of oak decline by causing a fine root disease with highly significant correlations between crown transparency and most fine root parameters. Among all Phytophthora species isolated, P. quercina plays the major role because of its widespread occurrence, its high plasticity concerning site conditions and soil pH, and its host specificity and high aggressiveness to oak species (Jung et al., 1996, 1999a, 1999b; Jung, 1998). Because of the multicyclic nature of Phytophthora fine root diseases (Erwin & Ribeiro, 1996) and the strong dependence of the infection process on the presence of free water in the soil, it may be some decades from the point where rootlet death begins to exceed rootlet replacement to the appearance of first visible crown symptoms in mature oaks. From this point onwards the progressive destruction of the fine root system can cause a chronic dieback of the crown, weakening the oaks and predisposing them to a series of abiotic and biotic stress factors.
Therefore, in combination with high or periodically fluctuating water tables (Oosterbaan & Nabuurs, 1991; Ackermann & Hartmann 1992; Malaisse et al., 1993; Hartmann & Blank, 1998; Thomas & Hartmann, 1998), exceptional periods of heavy rain (Prpic & Raus, 1987; Varga, 1987; Block et al., 1995; Hartmann & Blank, 1998; Jung, 1998), summer droughts (Brasier, 1993; Delatour, 1983; Hartmann & Blank, 1998), heavy defoliations, often in combination with destruction of the second flush by Microsphaera alphitoides (Block et al., 1995; Lobinger, 1999; Hartmann & Blank, 1998; Jung, 1998), or favourable conditions for secondary pathogens (e.g. Armillaria melleasenso lato) or parasites (e.g. Agrilus biguttatus) (Hartmann & Blank, 1992; Lobinger, 1999), the disease incidence is markedly higher, and dramatic and rapid mortality, often of groups of trees, can occur.
Oak decline mediated by droughts, defoliations or miscellaneous causes occurs on sites with a mean soil pH (CaCl2) ≤ 3·9 and sandy to sandy-loamy soil texture and is generally a chronic disease without consistent correlations between crown and root status. On these sites, Phytophthora species have not been isolated and therefore cannot be considered as being involved in the decline process. The hypothesis is put forward that, on these sites, defoliations (defoliation-mediated oak decline), droughts (drought-mediated oak decline) or miscellaneous agents such as Collybia fusipes (Marçais & Delatour, 1996; Marçais et al., 1998) or extreme winter frost and late winter frost (Hartmann et al., 1989) are weakening the trees and predisposing them to attacks by secondary pathogens (e.g. Armillaria mellea) or parasites (e.g. Agrilus biguttatus). On these sites, as on Phytophthora-infested sites, repeated defoliations or prolonged droughts can lead to rapid mortality, mainly because of the low water retention capacity of the sandy soils. In these cases mortality often starts on, or is even restricted to, the shallowest and sandy parts of the stand or the upper slope.
All these biotic and abiotic stress factors often act synergistically, and can also accelerate Phytophthora-mediated decline of oaks, as described above.
The distribution of the disease in the field is often a good indication of the type of aetiology, but can also be misleading. For instance, decline and mortality in groups of trees can be indicative of both a Phytophthora-related disease beginning with inoculum build-up around the most susceptible trees, resulting in a heavier attack on adjacent roots of neighbouring trees and build-up of disease pockets, or of drought stress on shallow and sandy microsites. Another example is the slow scattered decline of oaks that is characteristic of damage by Collybia fusipes (B Marçais, personal communication), but also of Phytophthora-mediated decline in stands with small-scale changes between sandy and clayey soils (e.g. stands 20 and 22 in this study).
According to Manion (1981) and Manion & Lachance, 1992) a decline is a gradual, general loss of vigour caused by the interaction of a number of interchangeable, specifically ordered biotic and abiotic factors, which often ends in the death of trees. Following this definition, Central European oak decline is an excellent example of a decline-type disease of varied aetiology
It should be emphasized that there may certainly be stands or even sites where dieback and mortality of oaks is not complex, but can be explained simply by the action of only one biotic (defoliation, Phytophthora on permanently or periodically wet, moderately acid to neutral soils, Collybia on sandy acid soils) or abiotic factor (drought on sandy shallow soils, extreme winter frost, late winter frost). By definition these single-cause diseases should not be called oak decline. On the other hand, even in these situations, transitions to more complex interactions of different factors exist, which are most often due to changing microsite conditions within single stands.
The isolation of Phytophthora species from rhizosphere soil samples of 88 out of 129 oak stands (68·2%) in 11 European countries (Germany, France, England, Scotland, Sweden, Luxembourg, Poland, Switzerland, Italy, Slovenia and Hungary), and similar relationships between the occurrence of Phytophthora spp. and soil pH and texture (T. Jung, unpublished data), indicate that the conclusions from the present study may have general validity for Central and Western Europe as well as for parts of Southern and Northern Europe. Furthermore, Iberian oak decline is also considered to be caused by an interaction of fine root damage by Phytophthora (here P. cinnamomi), climatic perturbations such as prolonged droughts and episodes of unseasonal heavy rain and flooding, site factors, and secondary pathogens or parasites (Brasier, 1993, 1996).
Considering the epidemic extent and the long duration of oak decline in Central and Western Europe in its current phase, the question arises of whether any environmental changes have occurred during recent decades that have unbalanced the host–parasite relationship between the oaks and the naturally occurring soilborne Phytophthora species. Excess anthropogenic nitrogen input into forest soils (Nihlgard, 1985; Kreutzer, 1991; Mohr, 1994; Thomas & Kiehne, 1995), the rise in mean winter temperature of 0·03°C per year, and a seasonal shift of precipitation from summer to winter in Central Europe and parts of Western and Northern Europe (Rapp & Schönwiese, 1995; Schönwiese et al., 1994) have already been discussed as potential triggering factors by Jung et al. (1996, 2000) and Jung (1998). Results of two recent studies further support these hypotheses: sporangial production is enhanced with increasing concentrations of nitrate in soil filtrates for P. quercina and five other Phytophthora species (T. Jung, unpublished data), and in Bavaria after analysing all forest monitoring data between 1984 and 1997, Mayer (1999) found that the probability of oaks showing severe crown symptoms (crown transparency > 25%) is 1·6 times higher following wet winters with average temperatures more than 2°C above the long-term average.