Introduction
Phytophthora alni is a host-specific, highly aggressive soil and waterborne pathogen, which causes root and collar rot of Alnus (alder) species. All European alder species, i.e., black alder (A. glutinosa), gray alder (A. incana), Italian alder (A. cordata), and green alder (A. viridis), and the North American red alder (Alnus rubra) are highly susceptible (Jung and Blaschke 2006, Gibbs and others 2003). The susceptibility of other North and South American and Asian alder species is currently unknown. P. alni was shown to be a recent inter-specific hybrid between P. cambivora and another species closely related to P. fragariae (Brasier and others 1995, 1999, 2004). There are 3 subspecies of P. alni, with markedly different aggressiveness to common alder (Brasier and others 2004; Brasier and Kirk 2001). The disease was first detected in 1993 in Southern Britain (Gibbs 1995), and has since been confirmed in 12 other European countries and across the UK (Figure 1) (Brasier and Jung 2003, 2006; Gibbs and others 1999, 2003; Jung and Blaschke 2004, 2006; Schumacher and others 2005; Streito and others 2002;) (Orlikowski, L. personal communication. July 2006, Pathologist, Research Institute of Pomology and Floriculture, Pomologiczna 18, 96-100 Skierniewice, Poland). Moreover, P. alni is likely present in Czech Republic, Spain, and Switzerland because typical symptoms and mortality of alders are reported from these countries. The disease occurs mainly along riverbanks, but also in orchard shelterbelts and forest plantations (Gibbs 1995; Gibbs and others 1999, 2003; Jung and Blaschke 2004; Streito and others 2002).
Disease symptoms include abnormally small, sparse, and often yellowish foliage and crown dieback (Figure 2). Other symptoms are early and often excessive fructification and tongue-shaped necroses of the inner bark and cambium. Necroses can extend up to 3 m from the stem base and are marked by tarry or rusty spots on the surface of the outer bark (Figure 3) (Gibbs and others 1999, 2003; Jung and Blaschke 2001, 2004) (www.baumkrankheiten.com/gallery/docs-en/alder_dieback/index.html). On riparian sites, P. alni has caused mortality as high as 70 percent in some locations. Disease incidences of 50 percent and high mortality rates were common (Gibbs and others 2003, Jung and Blaschke 2004, Streito and others 2002). The pathogen was shown to be widespread in alder nursery fields (Jung and Blaschke 2004, Schumacher and others 2005). Infected plants seldom showed symptoms, which made control efforts difficult.
A thorough investigation of disease pathways in Bavaria demonstrated that in most infested river systems, P. alni was introduced via infested young alder plantations established on the river banks or on forest sites that drain into the rivers (Jung and Blaschke 2004). Once introduced, P. alni spreads downhill with water run-off and downstream with streams and floods. Many infected alders were planted in afforestations of former agricultural land and also on wet sites in woodlands to stabilize steep slopes and banks of white water rivers (Jung and Blaschke 2004). These plantings increased the risk of infestation of riparian sites with the increased length of the river and upstream tributaries.
The rapid proliferation of P. alni throughout Europe probably resulted from the increased importance of alders in afforestation activities on wet sites and on former agricultural lands. The combined anthropogenic impacts of outplanting infected nursery stock and utilizing contaminated river water to irrigate nursery fields were contributing factors (Brasier and Jung 2003, Gibbs and others 2003, Jung and Blaschke 2004). P. alni may also be passively transported on bare-root nursery stock because it is able to adhere to and infect the fine roots of alders as well as adhere to other nonhost tree species exposed to the pathogen.
As short-time control measures, coppicing of infected alder trees and stools is recommended along water courses (Gibbs 2003), but not in infested forest plantations (Jung and Blaschke 2006). Some survivors in highly infested common alder stands were shown to be less susceptible to P. alni than declining trees, and in the long term, a resistance screening program may help to sustain alders as major components of riparian and swamp forests (Jung and Blaschke 2006).
The Exotic Forest Pest Website, which is sponsored by the North American Forest Commission lists P. alni as a high risk pest to North American forests for its potential to: (1) adversely affect the economic trade of alder trees and (2) affect the environment; specifically by changing forest composition, reducing wildlife food and habitat, increasing soil erosion, and changing soil composition due to alder’s nitrogen-fixing capabilities (Cree 1999).
An investigation of the conditions present at 434 sample locations in forested areas in Bavaria (Figure 4) was conducted using classification tree analyses and a tenfold cross validation to estimate the error. Classification and regression trees are a nonparametric iterative approach to compare all possible splits among the independent variables using a partitioning algorithm that maximizes the dissimilarities among groups. Classification trees are best used with binary data and regression trees with continuous data. Advantages of using decision trees such as classification and regression trees include the nonparametric nature of the model, ease of interpretation, and the robustness of the test (De’ath and Fabricius 2000). Decision trees have been successfully developed recently for: (1) modeling landscape dynamics of the spread of Phytophthora ramorum (Kelly and Meentemeyer 2002), (2) mapping hemlocks via tree-based classification of satellite imagery and environmental data (Koch and others 2005), (3) predicting the presence and absence of lichen and past fires in Jalisco, MX (Reich and others 2005), (4) developing a spatial model for estimating fuel loads in the Black Hills, SD. (Reich and others 2004), and 5) developing a methodology to predict oak wilt distribution in Minnesota and Texas (Downing and others 2007).
In this study, rules from the classification tree were used to create the potential distribution surface for Bavaria. Another potential distribution model, a Multi-criteria model (Eastman 2001, Eastman and others 1995), was created for the globe using both the rules from the Bavarian classification tree, and additional parameters established with expert knowledge. Global susceptibility surfaces, such as the P. alni surface, may be used to illustrate the need for a pathway approach to regulate nursery stock and for host species resistance testing. Multi-criteria models have been helpful to produce pest-risk maps for forested land in the United States (Krist and others 2006).
Encyclopedia ID: p3329
