The isolation of Phytophthora was successful in 13 of 63 samples (20.6%). The lack of success in obtaining isolates from all samples may be attributed to a number of factors.
First, sampled trees may not have been infected. Phytophthora species are not the only biotic organism causing stem lesions on trees. For example, damages caused by Neonectria spp. (the beech bark disease pathogen) closely resemble those of Phytophthora (Cale et al., 2015) (Fig. 13).
N. coccinea was isolated from several trees in this study and those trees may have been visually
mistaken for those caused by Phytophthora. However, Jung (2009) found that N. coccinea is an important secondary agent on trees already infected by Phytophthora species and argued that
Phytophthora is likely a primary agent in the whole beech bark disease complex.
Figure 13. Beech in Bokskogen, from which Neonectria coccinea was isolated
Another possible source of error is that Phytophthora could have been missed during the sampling of soil, since soil microbial communities often are heterogenic (Rodríguez Molina et al., 2000).
Since most Phytophthoras are hemibiotrophic, (i.e. organisms that live off of healthy, moribund or recently dead host tissue) (Oßwald et al., 2014), the age of the lesion seems to be a key factor for successful isolation and culturing of Phytophthora from infected tissue. Furthermore, the activity of Phytophthora is seasonal (Cooke et al., 2007), and detection of Phytophthora through baiting of soil following dry periods can be increasingly difficult (Anderson, 2006). Finally, the selective media contained several chemicals, including hymexazol, which is used to exclude e.g. Mortierella-‐‑, and Pythium species (Martin et al., 2012). Hymexazol has been reported to inhibit growth of some Phytophthora species, including P. cinnamomi, P. palmivora and P. citrophthora (Drenth & Sendall, 2001). Though none of these were species were found in this study, it cannot be excluded the possibility that certain chemicals in the selective media could affect the isolation success. Further testing in this regard is needed to optimize the selective media.
Potential impact of forest Phytophthoras in southern Sweden
As of today, eight forest Phytophthora species have been identified in Swedish forest settings. Since several of those are capable of infecting multiple tree species, including those native to Sweden (Table 3), the potential impact if becoming established in nature could probably be rather
extensive. Since many Phytophthora species are being spread from nurseries across Europe (Jung et al., 2015), it is likely that more Phytophthora species already have, or will be introduced to Sweden in the near future.
Table 3. Known pathogenicity of the forest Phytophthora species found in Sweden and the most common tree species in Götaland (% of total volume). S = The Phytophthora species can infect the host species and/or has been found in soil in close proximity to the host species. Sp = B. pendula, Sr = Q. robur, Sg = A. glutinosa.
G = No data was found regarding susceptibility on tree species level, but members of the host genera have proven to be susceptible to the Phytophthora species and/or the Phytophthora species have been found in soil in close proximity. ALN = P. alni; CAC = P. cactorum; CAM = P. cambivora; GON = P. gonapodyides; PLU = P.
plurivora; PSE = P. pseudosyringae; QUE = P. quercina; SYR = P. syringae. 1 (Skogsdata, 2015), 2 (Jung et al.,
2015; Keča et al., 2015; Redondo et al., 2015a; Orlikowski et al., 2011; Jung & Burgess, 2009; Jung et al., 2009; Lakatos & Szabó, 2009; Jönsson et al., 2003; Thomas et al., 2003; Brasier, 1999; Jung et al., 1996).
Tree species Volume 1 Phytophthora species 2
(mill. m3sk) (%) ALN CAC CAM GON PLU PSE QUE SYR
Picea abies 420 45.3 S S S
Pinus sylvestris 280 30.2 S S
Betula pendula, Betula pubescens 102 11 Sp G Sp
Quercus robur, Quercus petraea 36.1 3.9 S S S S Sr S Sr
Alnus incana, Alnus glutinosa 24.5 2.6 S Sg Sg Sg S Sg Sg
Fagus sylvatica 21.9 2.4 S S S S S S
Populus tremula 19.8 2.1 G G G
Sum 904.3 97.5
Impacts of Phytophthora to forest ecosystems are difficult to assess fully. Examples range from large-‐‑scale destruction of entire ecosystems by highly virulent and invasive P. cinnamomi in Australia (Weste, 2003), to more passive presence of forest Phytophthoras with little to no affect to tree health (Hansen & Delatour, 1999).
Phytophthora species are primary invaders and can infect healthy hosts (Oßwald et al., 2014; Sturrock et al., 2011). The expression of disease caused by Phytophthora species can be visualized through the plant pathological paradigm known as the disease triangle (Scholthof, 2007) which illustrates the interactions between the environment, susceptible host/genotype and the pathogen, to cause disease (Fig. 14).
Whether or not disease is expressed on the tree depends on the interaction between the particular Phytophthora-‐‑species, i.e. in terms of its virulence, the host tree species susceptibility, (Oßwald et al., 2014; Werres, 1995), and environmental conditions such as temperature, soil moisture and drought that may affect disease development (Redondo et al., 2015a; Oßwald et al., 2014; Martin et al., 2012; Sturrock et al., 2011; Jung, 2009). The cumulative damage to the tree also depends on which secondary agents colonize the trees (Jung, 2009). Assessment of potential impacts are further complicated by the fact that Phytophthora species distributions and/or virulence and their respective host interactions may be altered as a consequence of climate change.
Figure 14. The disease triangle, illustrating disease as an expression of interactions between the host, environment, and pathogen (source: Manion, 1981).
Predicted climate scenarios for Sweden (2071-‐‑2100) suggest an increase in mean annual
temperatures ranging from 3°C to 7°C (Kjellström et al., 2014). The predictions for precipitation are though more variable and uncertain, but generally precipitation is expected to increase, locally up to 40% (Kjellström et al., 2014). It is also likely that local flooding events and drought will increase because of extreme climate events. In Skåne, mean annual temperatures during the reference period (1961-‐‑1990) was -‐‑0.6°C in winter and 15.4°C in summer (Ohlsson et al., 2015). Climate scenarios predict increasing winter and summer temperatures of between 3°C to 5°C in Skåne (Ohlsson et al., 2015). The mean annual rainfall for the reference period was 748 mm, which is expected to increase with 15-‐‑25% (Ohlsson et al., 2015).
Most Phytophthora species can grow at temperatures between 1°C and 35°C (Erwin & Ribeiro, 1996), but their optimal growing temperature often ranges between 15°C and 30 °C. (Martin et al., 2012). Considering the predicted climate scenarios, conditions for Phytophthora species will likely improve in the future. A warmer climate will reduce the number of days with temperatures of inactive Phytophthora growth, and prolong the season of optimal growing temperatures. A wetter climate will also favor the activity of zoospores, which can move towards new hosts in wet soils only (Judelson & Blanco, 2005).