Effects of Host Variability on the Spread of Invasive Forest Diseases

Abstract

Biological invasions, resulting from deliberate and unintentional species transfers of insects, fungal and oomycete organisms, are a major consequence of globalization and pose a significant threat to biodiversity. Limiting damage by non-indigenous forest pathogens requires an understanding of their current and potential distributions, factors affecting disease spread, and development of appropriate management measures. In this review, we synthesize innate characteristics of invading organisms (notably mating system, reproduction type, and dispersal mechanisms) and key factors of the host population (namely host diversity, host connectivity, and host susceptibility) that govern spread and impact of invasive forest pathogens at various scales post-introduction and establishment. We examine spread dynamics for well-known invasive forest pathogens, Hymenoscyphus fraxineus (T. Kowalski) Baral, Queloz, Hosoya, comb. nov., causing ash dieback in Europe, and Cryphonectria parasitica, (Murr.) Barr, causing chestnut blight in both North America and Europe, illustrating the importance of host variability (diversity, connectivity, susceptibility) in their invasion success. While alien pathogen entry has proven difficult to control, and new biological introductions are indeed inevitable, elucidating the key processes underlying host variability is crucial for scientists and managers aimed at developing effective strategies to prevent future movement of organisms and preserve intact ecosystems.

Spatio-temporal analysis at landscape scale of the Iberian oak decline epidemic caused by Phytophthora cinnamomi

“…An exception was tree canopy cover, which was found to be associated with an increased risk of shortrange infections. Host abundance has long been recognised as an important factor in the spread of forest diseases (Holdenrieder et al, 2004; Ostfeld and Keesing, 2012; Prospero and Cleary, 2017). In particular, it was also found to play an important role in the dispersion of other Phytophthora species in forest ecosystems.…”

“…At larger scales, one would also expect the onset of other dilution effects related to habitat fragmentation, notably at sites where isolated oak forest stands are surrounded by agricultural crops (Huang et al, 2015; Prospero and Cleary 2017). However, no evidence for differences in the respective rates for connected and unconnected stands were found here.…”

“…Both are key features in disease epidemiology, but they are poorly characterized for IOD and for other forest diseases. Although host diversity, host connectivity, and host susceptibility have been identified as main factors driving the spread of invasive forest pathogens (Prospero and Cleary, 2017), in communicable diseases a major role is also played by abundance and behaviour of the disease vectors (Fodor, 2011). In turn, these two features depend on stand and landscape characteristics.…”

“…It has been generally suggested that a high host diversity is more likely to decrease (rather than increase) the disease risk (Keesing et al, 2006). This reduction is attributed to a dilution effect (susceptible host density reduction) in those habitats where abundant and widespread competent hosts exist and where the pathogen transmission system is frequency dependent (Keesing et al, 2006; Haas et al, 2011; Prospero and Cleary, 2017). In contrast, an amplification effect (risk increase) may occur when the pathogen has a wide range of hosts and when the ecosystem at hand includes abundant reservoir hosts which protect and transmit the pathogen more efficiently than the primary or focal host (Keesing et al, 2006; Prospero and Cleary, 2017).…”

“…This reduction is attributed to a dilution effect (susceptible host density reduction) in those habitats where abundant and widespread competent hosts exist and where the pathogen transmission system is frequency dependent (Keesing et al, 2006; Haas et al, 2011; Prospero and Cleary, 2017). In contrast, an amplification effect (risk increase) may occur when the pathogen has a wide range of hosts and when the ecosystem at hand includes abundant reservoir hosts which protect and transmit the pathogen more efficiently than the primary or focal host (Keesing et al, 2006; Prospero and Cleary, 2017). For example, in California forests, Phytophthora ramorum causes decline symptoms and mortality in oak and tanoak trees, whereas California bay laurel trees ( Umbellularia californica ) are considered to be relatively tolerant hosts.…”