Susceptibility to Phytophthora ramorum and Inoculum Production Potential of Some Common Eastern Forest Understory Plant Species

Phytophthora ramorum is a destructive pathogen of oak and tanoak in California and Oregon (10,31,33) and also causes disease symptoms on a large number of other plant species (7,15). Quarantines have been in place since 2001 to prevent the spread of P. ramorum from infested areas (1,28). A federal order went into effect on 10 January 2005 in response to detections of P. ramorum in California, Oregon, and Washington commercial nurseries and was replaced by an enhanced interim rule on 26 February 2006 (42). On 27 February 2007, restrictions were placed on the interstate movement of nursery stock in nonquarantined counties in California, Oregon, and Washington; interstate movement of all other nursery stock from quarantined areas was restricted; and the lists of regulated plants and quarantined areas were amended (29). The current list of P. ramorum hosts (43) includes 45 proven and 72 associated plant hosts, all of which are now subject to regulation. The economic losses to growers associated with P. ramorum quarantine efforts have been high (4,10). The likelihood that P. ramorum will one day become established in the Eastern United States keeps growing, due to the steady occurrence of positive P. ramorum finds in Eastern states that have suitable hosts and climate for disease development, and which receive plants from California, Oregon, and Washington (2,10,38,46). Modeling has been performed to assess which areas of the Eastern United States are at highest risk of potential establishment of P. ramorum (20,37,44). Understory hosts susceptible to P. ramorum exist in California (11,30) as well as Oregon forests (12,14). However, there exist notable differences between California and Oregon in the epidemiological role played by California bay laurel, also known as Myrtlewood (Umbellularia californica) (13). California bay laurel is known to play a major role in driving epidemics in California forest ecosystems (5), whereas in Oregon, Myrtlewood is seldom infected (13). In both regions, foliar infection of various understory hosts is considered to serve as an important source of inoculum that may then be spread to oaks and tanoaks by various means such as rainsplash (6–8). Oaks originally were considered to represent a “dead end” epidemiologically because no sporulation had been observed to occur on them (30,33). However, Vettraino et al. (45) showed that P. ramorum was able to sporulate on coast live oak leaves, even though the role this phenomenon may play in the epidemiology of the disease may be far less that that played by the role of California bay laurel (8). Several species native to other parts of the world have been screened for susceptibility to P. ramorum. An important Australian native species, Pittosporum undulatum, is susceptible to P. ramorum (16). Also, an assortment of Mediterranean shrubs were susceptible to P. ramorum (26). Eastern U.S. oak forest ecosystems are comprised of many understory species, in addition to mature oak species, that could potentially become infected by P. ramorum should it be introduced in an area where climate was suitable for infection and disease development. Several studies have been performed that have evaluated some Eastern U.S. native species for susceptibility to P. ramorum. Tooley et al. (40) screened a number of ericaceous host species that included the following Eastern U.S. native species or cultivars thereof: Arctostaphylos uva-ursi, Gaultheria procumbens, Gaylussacia baccata, G. frondosa, Kalmia angustifolia, K. latifolia, Leucothoe axillaris ‘Greensprite’, L. fontanesiana, Pieris floribunda, Rhododendron arborescens, R. calendulaceum, R. carolinianum, R. catawbiense, R. maximum, R. minus, R. vaseyi, R. viscosum, Vaccinium angustifolium, V. corymbosum, and Zenobia pulverulenta. A large variation in susceptibility of these native species was observed, measured both by lesion areas produced by P. ramorum and in terms of chlamydospore numbers produced in host tissue. The susceptibility of seedlings of several oaks from the red and white oak groups including two Eastern U.S. native species, Quercus rubra (northern red oak) and Q. palustris (pin oak), was evaluated using stem inoculation with agar plugs of P. ramorum (32). Lesions in the bark of both species were longer than those developed in coast live oak (Q. agrifolia) and California black oak (Q. kellogii). Tooley and Kyde (39) also used stem inoculation to test seedlings of eight Eastern native oak species as well as sugar maple (Acer saccharum) and black walnut (Juglans nigra) ABSTRACT Tooley, P. W., and Browning, M. 2009. Susceptibility to Phytophthora ramorum and inoculum production potential of some common Eastern forest understory plant species. Plant Dis. 93:249256.