Distribution and Severity of Alder Phytophthora in Alaska1
G.C. Adams,2 M. Catal,2 and L. Trummer3
In Alaska, an unprecedented dieback and mortality of Alnus incana ssp. tenuifolia has occurred which stimulated an effort to determine causal agents of the disease. In Europe, similar dieback and mortality of Alnus incana and Alnus glutinosa has been attributed to root rot by a spectrum of newly emergent strains in the hybrid species Phytophthora alni. The variable hybrids of P. alni were grouped into three subspecies: P. alni ssp. alni (PAA), P. alni ssp. multiformis (PAM), and P. alni ssp. uniformis (PAU). From 2007 to 2008, we conducted a survey of Phytophthora species at 30 locations with stream baiting as used in the 2007 national Phytophthora ramorum Early Detection Survey for Forests in the United States. Additionally, Phytophthora species from saturated rhizosphere soil beneath alder stands were baited in situ using rhododendron leaves. We discovered PAU in rhizosphere soils in 2007 at two sample locations in unmanaged stands hundreds of miles apart, on the Kenai Peninsula and near Denali National Park. PAA was reported to be the most aggressive and pathogenic to alders and PAM and PAU were significantly less aggressive than PAA, though still pathogenic. To ascertain whether PAU was of restricted distribution due to recent introduction, or widespread distribution, we extended the survey in 2008 to 81 locations. Intensive sampling was conducted at five alder stands exhibiting dieback and 10 alder genets per location were excavated to expose nearly the entire root system for evaluation of the severity of root rot, ELISA detection of Phytophthora in diseased roots, and isolation of Phytophthora species. At intensive sites, four bowls each containing 500 ml samples of saturated rhizosphere soil were baited by floating three detached leaves of Rhododendron spp. for a 2-week period. Leaves were rinsed and sealed in plastic bags and shipped to the laboratory where leaf tissues were placed in PARPH-V8 agar selective for Phytophthora spp. Phytophthora spp. were identified from DNA sequence of the ITS-rDNA region. The survey yielded some species newly reported for the U.S., including P. aff. gallica,and an undescribed species in Clade 8C closely related to P. ramorum and P. foliorum, and other undescribed species. The Clade 8C species was of restricted range in our isolations, and all 20 isolates were from one location. The species was of interest to researchers developing systems for detection of P. ramorum. Thirty-three isolates of PAU were identified out of approximately 600 isolated and sequenced Phytophthora spp. PAU was collected from 11 geographically distributed stands. Only one isolate was obtained from bait floating in a water course (the Tanana River) out of 81 watercourses sampled. Soil isolates were from four plots in southcentral Alaska along the Kenai and Russian Rivers, and seven plots in the interior, including a plot in Fairbanks, three plots between Delta Junction and Fairbanks along Highway 2, two between Slana and Tok along Highway 1, and one near Denali National Park on Highway 3. PAU was widely distributed and difficult to isolate. Severity of root rot was low, with less than one diseased root discovered per genet, on average. Root rot does not appear to be a significant contributor to the dieback and mortality of alder in Alaska.
Species of alder (Alnus) were exhibiting a dieback and mortality more severe and extensive than has been recorded in history in Europe and North America. Study of the mortality in Europe was well underway by 1993 (Gibbs and others 1999) and it was usually described as exhibiting long linear cankers on stems caused by Valsa oxystoma Rehm 1875, and root rot caused by Phytophthora species. Valsa oxystoma is prominent in the pathology literature in Europe, having a long association with, and periodic and extensive mortality of Alnus (Tabeuf 1895). The stem canker, though dramatic, has been in recent times discounted as being a primary pathogen in the disease etiology of alder mortality in Europe. No clear explanation for discounting the stem canker, however, has been given. Dieback was often recorded associated with drainages having ephemeral or variable water flow which suggested water stress was a predisposing environmental factor (Webber and others 2004). Various additional episodes of dieback and mortality of alders have been documented over the years, especially in Europe (Cech and Hendry 2003). These include damage due to hydrological extremes.
In Europe, the common alder, A. glutinosa, and other alders, primarily A. incana ssp. incana, were exhibiting Phytophthora collar rot, root rot of fine roots, and tar spots on stems with dieback. Collar rot and tar spots were above ground symptoms associated commonly with Phytophthora diseases of trees. Tar spots were locations where a break in bark overlying infected cambium exudes a black liquid, like those observed in sudden oak death (SOD), caused by Phytophthora ramoum. These symptoms have been associated with the newly emergent species of Phytophthora, P. alni Brasier and S. A. Kirk 2004, which was the causal agent of the lethal root and collar disease of alder species in Europe. In Europe, alder Phytophthora was well documented as a lethal root and collar disease of alder in the United Kingdom (U.K.), France, Germany, Austria, Hungary, Italy, and the Netherlands. Considerable research has followed the discovery of the newly emergent alder Phytophthora because it was found to have arisen from hybridizations between other Phytophthora species (Brasier and Kirk 2001; Brasier and others 2004). P. alni has three variants which vary in their virulence and pathogenicity. P. alni ssp. alni (PAA) appears to be the most aggressive and pathogenic to Alnus species. The other two, P. alni ssp. uniformis (PAU) and P. alni ssp. multiformis (PAM), appear to be significantly less aggressive than PAA, though still considered pathogenic. The PAA variant was typically considered the primary agent killing alders in Europe. PAU and PAM variants were not well understood for their role in causing alder mortality. PAU has been detected across Europe and now was detected in Alaska. PAU was often found in soil, asymptomatic plants, and areas where PAA does not occur. Ioos and others (2006) suggested that PAU and PAM might have existed for a long time on or in the vicinity of alder trees before the recent emergence of large-scale death of alder in Europe. Thus the occurrence of PAU or PAM in the past might not have been noticed because of the lack of conspicuous symptoms or death of whole trees.
Of particular interest was that the parent species in the hybridizations were not pathogenic on alder. For hybridizations to occur in nature and result in a jump to a new host was of great interest to plant pathologists studying plant epidemics caused by newly emergent or recently introduced plant pathogens, such as P. ramorum.
Beginning in 2000 in North America, widespread and serious branch dieback and mortality of thinleaf alder, Alnus incana ssp. tenuifolia, was reported by land managers and others in the southern Rocky Mountains and, beginning 2003, in Alaska (Worrall 2009). Thinleaf alder ranges from the Arctic south to Arizona, and from the Pacific Coast east to central Alaska and the Rocky Mountains (Furlow 1979). In Alaska it may occur near sea level, while in the southern Rocky Mountains it was limited to higher elevations (approximately 3000 m) and riparian areas. Its primary value was in stabilizing soils and in shading and cooling streams, thereby improving fish habitat. It was a keystone nitrogen fixation species with the Frankia symbiont. In Alaska, less serious canker and dieback was observed in A. sinuata and A. crispa, species that more commonly occur in highlands and in the interior.
The widespread and serious branch dieback and mortality increasingly raised concerns about the future of the alder riparian ecosystem. We became involved as collaborators in studies to quantify the extent and severity of dieback and mortality in Alaska and the southern Rocky Mountains from southern Wyoming to northern New Mexico, to identify which pathogens might be potential causal agents of disease, while others assessed the remaining potential direct and indirect causal factors of the epidemic. A major impetuous throughout the study was the concern whether an introduced pathogen was spreading and becoming established in North America.
The broad range goals of our 3 years of research were to improve our understanding of the cause of alder mortality in Alaska by documenting symptoms, symptom severity, and signs of disease on alder suffering dieback and mortality; compare them to those in healthy stands; isolate and identify plant pathogens; and participate in completing Koch’s postulates with select pathogens. Throughout the widespread mortality, stems of the trees have exhibited narrow linear cankers, approximately 100 cm long by 2 to 7 cm wide (fig. 1). The cankers have been correlated with the dieback and mortality (Worrall 2009) and the canker surfaces usually have been entirely covered with densely aggregated ascostromata or, less often, conidiomata.
Figure 1—Typical canker on stem of A. incana spp.
tenuifolia, common in stands with dieback and
mortality (photo by J.J. Worrall).
Therefore, one objective was to isolate the pathogenic fungi growing at the advancing margins of cankers on A. incana, isolate from stromata formed on the cankered tissues, and identify the isolated ascomycetes using morphology and molecular sequence homology. This objective necessitated a re-evaluation of the Valsa spp. (anamorphs Cytospora spp.) pathogenic on Alnus spp. using phylogenetic analyses and virulence among the characterized species compared on the host.
Initially, the project was funded by local U.S. Department of Agriculture, Forest Service (USDA FS), Forest Health Protection (FHP) contracts, but as the possibility increased that an exotic pathogen of concern may have been introduced, funding came from the USDA FS Forest Health Technology Enterprise Team (FHTET). The FHTET had just completed the modeling of risk factors for introduction, spread, and establishment of P. alni (lead by Marla Downing and posted on the Internet at http://www.fs.fed.us/foresthealth/technology/invasives_phytophthoraalni_riskmaps.shtml). Similarly, the Europeans had established a website on the potential risk of spread of P. alni into non-infested regions of Europe. The better studied epidemic in Europe was used as reference in modeling the risk to North American forests, and experts consulted included T. Jung (Germany). In Europe, Alnus species were utilized extensively in habitat restoration where nursery stock was routinely out-planted along river banks. Therefore, the introduction and spread of P. alni in North America was proposed as likely originating in nurseries and spreading by interstate trade in nursery stock with establishment of the pathogen occurring following out-planting, or resulting from propagules carried by watercourses from epicenters such as production nurseries. At the time we initiated our studies, P. alni had not been previously found in North America, although rumors had been heard of the occurrence of an isolate tentatively referred to as P. alni that had been found in a survey of nurseries in Minnesota. This incident was later reported by Schwingle and others (2007), but the species could not be confirmed as P. alni.
During these studies we collected data on edaphic factors, stem and genet size and density, and detailed landscape features. However, these latter measurements were for use in modeling and were not reported herein. The publication of Worrall (2009) thoroughly covers investigations of many ecological factors that have or have not been correlated with the epidemic disease in the southern Rocky Mountains, and the publication by the team of Ruess and others (2009) covers some ecological features of the Alaskan epidemic.
A second objective was to isolate Phytophthora spp. from roots, rhizosphere soils, crowns, and tar spots of alder as well as from adjacent watercourses and wetlands. Due to growing concern among pathologists and state and federal regulatory officials that cryptic invasion by the Phytophthora that devastates alder in Europe may be damaging Alaskan riparian forests, a more extensive survey for alder Phytophthora was conducted in 2007 to 2008. Phytophthora species were baited and trapped from a total of 81 sites across south central and interior Alaska (fig. 2). The objectives necessitated the identification and virulence testing of Phytophthora spp. reported as pathogenic on alder.
Particular interest in a need to know whether particular rivers or streams might be carrying the propagules of PAU and spreading the pathogen downstream was expressed by government agents. Therefore, once PAU had been discovered, it became essential to determine its location, distribution, and whether it caused serious damage to alder. The data generated from the study should address some questions concerning the biology and ecology of the newly discovered PAU. Providing this needed information to USDA Animal and Plant Health Inspection Service (APHIS) and Alaska Division of Agriculture personnel should aid them in determining whether they should pursue specific actions in regard to the finding of PAU in Alaska. The project was designed to provide some answers to the Figure 2—Map showing the distribution of extensive plots in alder stands and baited watercourses adjacent to alder stands in Alaska.
major concerns listed below:
What pathogen(s) are causing the stem cankers?
Are the stem canker pathogens native or introduced?
Can Koch’s postulates be reproduced with select canker pathogens on thinleaf alder?
What was the incidence and severity of Phytophthora crown and root rot in alders in Alaska?
Is PAU causing disease on alder in riparian ecosystems in Alaska?
What was the distribution of PAU among alders in Alaska?
What streams and rivers yield PAU inoculum and may be transporting the pathogen?
Is PAU a recently introduced pathogen or a long established member of the Alaskan riparian ecosystem?
Can root rot be reproduced with PAU, a new species related to P. ramorum, P. aff. gallica, and P. pseudosyringae on thinleaf alder, and Koch’s postulates completed?
Answering these questions concerning the biology of pathogens present in alder stands with dieback and mortality will improve decision making in the management of alder in Alaska.