Phytophthora lateralis (port-orford-cedar root disease).

Abstract

Abstract Genetics P. lateralis is in phylogenetic clade 8, defined by ITS DNA sequence (Cooke et al., 2000). The most closely related species is P. ramorum, the sudden oak death pathogen (Ivors et al., 2004). Other related species include P. hibernalis and P. foliorum; these species sometimes cross-react in molecular diagnostic tests designed for P. lateralis (Winton and Hansen, 2001). In much of Europe and in western North America, P. lateralis populations are very uniform in appearance and genetics, as expected of a clonally spread pathogen that is inbreeding or functionally asexual (McWilliams, 1999; 2000). In Taiwan and one small area of Britain, however, morphologically and genetically distinctive populations are established (Brasier et al., 2012). Isolates from Taiwan are especially variable, as might be expected of an ancestral, endemic population. In total, four distinct lineages have been described, including two from Taiwan, the invasive population found in North America and Europe, and the small UK population. Reproductive Biology P. lateralis reproduces primarily through asexual zoospores and chlamydospores (Trione, 1974; Englander and Roth, 1980). Zoospores are released from sporangia, sack-like hyphal appendages that hold about 20 zoospores each. Sporangia form in water or in saturated soil, on hyphae that grow from colonies in tree roots (or in agar). Zoospores are encased only in a membrane and are vulnerable to drying. In water they are motile with flagellae. They are chemotactic, following chemical gradients or root exudates. They remain active for about 24 hours. Zoospores encyst on root surfaces or after agitation, such as in a fast moving stream, and germinate with hyphae that may penetrate a host root and establish a new infection (Oh and Hansen, 2007). Chlamydospores are thick-walled asexual resting spores that form on hyphae in roots, foliage or in culture. They allow the pathogen to survive drying conditions, germinating with hyphae and sporangia when cool moist conditions resume (Trione, 1959). Oospores are sexual spores formed from the fusion of antheridia and oogonia. They are thick-walled and presumably function as resting spores in roots and foliage, as well as providing the population with genetic variation. They are very seldom seen in culture, however. Longevity P. lateralis survives for up to 10 years in infected root fragments in the soil (Hansen and Hamm, 1996). Population Size and Structure Populations of propagules free in the soil are small (Tsao et al., 1995). Environmental Requirements P. lateralis dies quickly from surface horizons exposed to sun as it cannot survive warm dry conditions (Hansen and Hamm, 1996). P. lateralis is active during cool, wet times of the year (Trione, 1959).