synoptic climatology of the long-distance dispersal of white pine

ORIGINAL PAPER

Synoptic climatology of the long-distance dispersalof white pine blister rust II. Combination of surfaceand upper-level conditions

K. L. Frank & B. W. Geils & L. S. Kalkstein &

H. W. Thistle Jr.

Received: 31 August 2007 /Revised: 14 March 2008 /Accepted: 15 March 2008# ISB 2008

Abstract An invasive forest pathogen, Cronartium ribi-cola, white pine blister rust (WPBR), is believed to havearrived in the Sacramento Mountains of south-central NewMexico about 1970. Epidemiological and genetic evidencesupports the hypothesis that introduction was the result oflong-distance dispersal (LDD) by atmospheric transportfrom California. This study applies a method to identify theatmospheric conditions favorable for rust transport andinfection. An upper level synoptic classification (ULSC)identifies patterns of upper-level flow favorable for thetransport of rust spores from a source to a target. Transportdata are coupled with data for surface conditions favorablefor infection at a designated target. A resulting calendarlists likelihood classes for establishment by four-times-dailyobservations during a dispersal season from April throughJuly in the years 1965 to 1974. The single most-favorableperiod for transport and infection at the New Mexico sitewas identified as 1–15 June 1969. Five additional sites inthe western United States with susceptible white pine

populations and known infestation status were then evalu-ated to verify the model. Only the infested sites exhibit anestablishment likelihood of “high” or “very high.” Thissuggests that the methodology correctly identifies locationswith elevated establishment likelihood. Finally, likelihoodsat nine additional points in the southwestern United Statesare determined and used to map regional patterns oftransport, infection and establishment. The ULSC com-bined with appropriate surface meteorological data could beused to further investigate transport and infection, identifyother areas at risk, assess the potential for gene flow ofWPBR and evaluate long-distance dispersal of otherpathogens.

Keywords Cronartium . Invasive species .

Atmospheric transport . Synoptic climatology . Aerobiology

Introduction

Invasions of exotic species pose serious economic andecological threats to managed and natural ecosystemsworldwide (Mack et al. 2000). The 100-year, expandingepidemic of white pine blister rust (WPBR), caused byCronartium ribicola J.C. Fisch., in North America demon-strates how an important plant pathogen spreads to newenvironments (Geils 2001; Samman et al. 2003). WPBR isnative to Asia on white pines (Pinus, subgenus Strobus),ribes (currants and gooseberries, Ribes), and several otherplant genera. Hunt (2003) concludes the rust was probablymoved west by trade—first to Europe (circa 1850) and later(late 1800s through early 1900s) to numerous locations inNorth America. Although human activities mediated andthen mitigated the original North American introductions,the rust continues to spread (presumably) by atmospheric

Int J BiometeorolDOI 10.1007/s00484-008-0158-3

K. L. Frank (*)Center for Climatic Research, Department of Geography,University of Delaware,Newark, DE 19716, USAe-mail: [email protected]

B. W. GeilsRocky Mountain Research Station, USDA Forest Service,Flagstaff, AZ 86001, USA

L. S. KalksteinDepartment of Geography and Regional Studies,University of Miami,Coral Gables, FL 33124, USA

H. W. Thistle Jr.Forest Health Technology Enterprise Team, USDA Forest Service,Morgantown, WV 26505, USA

transport (Geils et al. 2003). The potential for epidemicexpansion to other vulnerable regions and for continuedgene flow remains an unresolved threat (Hamelin et al.2000).

Numerous attempts to quantify and model the expansionof invasive species are documented. Such expansion mayoccur as either the sum of many short steps (through localspread) or by a few great leaps [long-distance dispersal(LDD)]. The diffusion concept developed by Gregory in1945 (see Waggoner and Aylor 2000) remains at the core ofmodels proposed by Andow et al. (1990), Caswell et al.(2003) and others. This approach relies primarily upon thestudy of demography and dispersal gradients. Clark et al.(2003) and Muller-Landau et al. (2003) suggest thatalthough LDD may be infrequent and of variable length itmerits closer examination. For many pathogenic fungi,LDD contributes significantly to the establishment ofdisease outbreaks (Brown and Hovmoller 2002; Nagarajanand Singh 1990). The critical stages of transport andinfection are (1) production of spores at a source and theirescape from the canopy, (2) aerial transport and survival,and (3) deposition, germination, infection and reproductionon a suitable host (Aylor 1986; Aylor 2003; Isard et al.2005). Mims and Mims (2004) report several recentexamples of viable fungal spores transported long distancesin smoke. Other examples for LDD of plant pathogensinclude coffee leaf rust (Bowden et al. 1971), peanut rust(Van Arsdel 1973), sugarcane rust (Purdy et al. 1985), bluemold (Davis and Monahan 1991), soybean blister rust(Munoz et al. 2004) and soybean rust (Isard et al. 2005).These studies examine aerial dispersal by air-flow patternsacross continental or oceanic distances. Other approachesfor the study of LDD include climate-matching (e.g.,Yonow et al. 2004) and landscape genetics (e.g., Hamelinet al. 2000).

The spread of WPBR displays a range of spatial scalesindicative of different frequencies and processes. Althoughthe spores produced on ribes, which infect white pines, areeasily killed by light, heat and desiccation, they are stillcapable of spreading the rust as far as several kilometers(Van Arsdel 1967; Van Arsdel et al. 2006). The thick-walled and pigmented spores produced on white pines,which infect ribes, often disperse tens of kilometers andare even capable of surviving for days while travelinghundreds of kilometers (Mielke 1943). Recently discovereddisjunct outbreaks of WPBR suggest LDD to south-centralNew Mexico (Hawksworth 1990), western South Dakota(Lundquist et al. 1992), central North Dakota (Draper andWalla 1993), northeastern Nevada (Vogler and Charlet2004), southern Colorado (Blodgett and Sullivan 2004),central Utah (B.W. Geils and D.R. Vogler, manuscript inpreparation) and several additional locations in NewMexico (D. Conklin, personal communication). These

outbreaks are in relatively unpopulated regions and suggestintroduction by atmospheric rather than anthropogenictransport. The surveys of Smith and Hoffman (2000) andGeils et al. (2003) reveal that WPBR populations in interiorwestern North America are widely scattered, and that not allmountain ranges with host populations have detectablelevels of WPBR. The remote and incomplete distribution ofWPBR in this region implies that meteorological conditionsrelated to either transport or infection may limit LDD ofthe rust.

Various synoptic climatological approaches are used toexplain epidemic introductions. Many approaches focus oneither environmental suitability at the deposition site or onthe pathway of aerial transport. For example, Yonow et al.(2004) identify the potential world-wide range of a plantpathogen by matching critical climatic variables at siteswhere the pathogen occurs to others with climatic similar-ity. Fujioka’s (1996) failure to account for the distributionof WPBR in Oregon and California from climatic variables,however, indicates such an approach can be inhibited bylack of data at appropriate spatial and/or temporal scales foreither the climate or the pathogen. In contrast, Van Arsdelet al. (1961) is one of many examples where climate (andother factors) significantly explains how variation inenvironmental suitability determines rust distribution andseverity. Holly Kearns (2005) used observations of climateand WPBR distribution from Wyoming (where the diseasehas occurred for decades) to predict its potential distributionin Colorado (where it has recently invaded). Aylor (2003)incorporates transport and survival along with suitabilityinto dispersal models for several plant pathogens. Heconcludes that LDD may be more often limited byestablishment at the deposition site than by conditions oftransport.

Although the Sacramento Mountains of south-centralNew Mexico had long been recognized as highly vulner-able to WPBR (Bureau of Plant Industry 1921), its firstdiscovery was reported by Hawksworth (1990) severaldecades after the rust was known in the nearest populationsof susceptible pines in California, Idaho and Wyoming.Genetic analysis (Hamelin et al. 2000) concluded that theNew Mexico rust population had recently experienced ademographic bottleneck and was more similar to popula-tions in California than other potential sources. Van Arsdelet al. (1998) reported the oldest known infections on pine inthe Sacramento Mountains and dated introduction to circa1970. Even though the rust was widely distributed in theSacramento Mountains by 1985 (Geils 2000; Conklin2004), information on its source, dispersal pathway andestablishment would be useful for understanding thepotential for other, disjunct outbreaks (Geils et al. 2003)and gene flow (Hamelin et al. 2000), and the impacts ofclimate change on rust ecology and evolution.

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This study applied an upper level synoptic classification(ULSC) (Frank et al. 2008, this issue) to identify upper-level flow patterns likely to transport WPBR spores fromSierra and Cascade source regions to target regions in theGreat Basin, the southwestern United States and southernRocky Mountains and to determine when these patternsoccur. The resulting upper-level likelihood calendar wascoupled with surface data to determine when, and howoften, conditions for transport are followed by conditionsfavorable for infection, the combination of which isrequired for establishment. Initially, the methodology wasapplied and tested with a single target site in theSacramento Mountains, New Mexico. Subsequently, tovalidate the model, the methodology was applied to fiveadditional sites in the southwestern United States withknown WPBR infestation status. Finally, the susceptibilityof nine more points was explored to provide a morecomplete view of the potential vulnerability of white pinepopulations in the region.

Methods

Criteria derived from information on the distribution andepidemiology of WPBR were applied to meteorologicalobservations and used to group and rank periods forlikelihood of LDD. Information on WPBR described thepotential source and season of spore production, longevity,and temperature and humidity requirements for germinationand infection (Van Arsdel et al. 2006). Patterns character-izing upper-level air flow were generated by the ULSC(Frank et al. 2008, this issue). Corresponding surfaceconditions were extracted from the NCEP/NCAR Reanal-ysis Project (Kalnay et al. 1996) for the April to July seasonof spore release in the years 1965 to 1974. Results ofanalyses of these data for likelihood of upper-level transportand of surface conditions favorable for infection werecombined to produce a calendar of establishment likelihoodand summary tables.

Upper-level likelihood scenarios

The ULSC considers four meteorological variables: geo-potential height, specific humidity, and u- and v-windcomponents, at the 500 hPa level over North America fromthe NCEP/NCAR Reanalysis Project data set (Kalnay et al.1996; NOAA-CIRES 2002). This data set is generated fromthe application of mathematical data assimilation andforecasting models to historical weather data from numer-ous sources (UCAR 2003). The result is a global, gridded(2.5°), four-times-daily data set with a period of recordbeginning 1 January 1948. The 500 hPa level is selected forthis analysis because it is generally midway between the

levels of divergence and non-divergence; this level iscommonly thought of as representative of the steeringcirculation for surface systems. These data, for the period1965–1974, were subjected to principal component analysisto standardize and reduce the data set, and an averagelinkage clustering algorithm then identified groups ofobservations with similar flow patterns. The procedureyielded 16 clusters. The flow patterns identified by theULSC typify all patterns expected to be observed over thestudy area, and the resulting cluster calendar showed thatthe clusters are generally temporally continuous. Subse-quent classification of additional observations through a z-score method produced acceptable results, indicating thatadditional observations may easily be incorporated into theULSC calendar by this means. Data analysis that identifiedpatterns of similar upper-level conditions and how thepatterns were best fit to each observation are describedfurther by Frank et al. (2008, this issue).

Using the ULSC, four-times-daily observations of upper-level flow patterns for the April to July season in the years1965 to 1974 were ranked by their likelihood of air flowpassing directly from a source region in the Sierra-Cascaderanges to the target location in the Sacramento Mountains.Additional ULSC patterns for the April to July season inthe years 1975 to 1990 were considered for likelihood of airflow passing from source regions in the Sierra, Cascade andBitterroot Ranges to the Sacramento Mountains and 14additional targets in the area (Fig. 1). In this analysis,upper-level-flow patterns were classified and ranked on a1–4 likelihood scale for flow leading from source regions toa target site using knowledge-based visual interpretation.Each ULSC pattern was classified from low to very highfor likelihood of potential upper-level transport andassigned a corresponding ranking from 1 to 4. To representthe persistence of transport favorability, each observationwas then assigned the value of the average of the three four-times-daily observations centered on the observation. Sincethese sums were divided by 3, the average could artificiallyinflate the score of an observation with an unlikely upper-level pattern in transition between flows with higherlikelihoods from different sources. These transition obser-vations were identified and the artificially inflated scoresreduced.

Surface likelihood scenarios

From the NCEP/NCAR Reanalysis Project, four-times-daily observations of surface temperature and relativehumidity data were obtained for the spore season (April–July) in the years 1965 to 1974 at the grid point (32.5°N,105°W; Fig. 1, Sierra Blanca) representative of the initialinfection site identified in the Sacramento Mountains andwhich included the approximated time of initial infection

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(Van Arsdel et al. 1998). Correspondence between Reanal-ysis data at the surface (NOAA-CIRES 2002) and recordsfrom the Cloudcroft, New Mexico, COOP weather station(NESCIS 2003) confirmed that the Reanalysis data wererepresentative of the actual surface conditions at areaswhere rust had established and intensified. Due to theremoteness of the target locations, meteorological data aregenerally not available. Reanalysis data were selected foruse over other data sets because they consider historicalweather data from numerous sources and are available on afine temporal resolution. The data were obtained fordispersal seasons in the years 1975 to 1990 to expand theinvestigation to a longer period that might encompasspotential establishment at all 15 target sites identified inFig. 1.

Epidemiological criteria of time, temperature and hu-midity for germination and infection were applied to thesurface data. WPBR spores deposited on ribes leaves mightsurvive a maximum of 22 days, and spores might germinateand infect if maintained for 6–24 h in saturated air above13°C (Van Arsdel et al. 2006). Because leaf-wetnessinstruments have serious limitations and other coarse-resolution weather models have provided acceptable esti-mates of wetness information at the micro-scale (Seem etal. 2000), a hypothesis that synoptic data could be used toassess micro-scale surface conditions was provisionallyaccepted. A relative humidity of 90% at the observation

point was considered sufficient to indicate a saturatedcondition (necessary for germination) at the level of theribes leaf.

Periods that satisfied meteorological criteria for theSacramento site ranged in duration from a single observa-tion (there were many) to 66 h (only one period of thisduration was observed during the 1965–1974 study period).It was assumed that longer periods of favorable surfaceconditions are more conducive to spore germination/infection. Therefore, each observation falling within afavorable period of one observation or more was given avalue equal to the reciprocal of a value calculated by thethird-degree polynomial fit to the plot of the frequency ofextended periods of favorable surface conditions. That is,there were 64 single-observation periods but only nine18-h-long periods of favorable surface conditions duringthe study period, so the weight for a favorable period of18 h in duration was approximately 1/9 while the weight fora period of 6 h in duration was only about 1/64. A third-degree polynomial was chosen over a linear or second-degree polynomial because it would more closely fit thegenerally log-normal distribution of the frequency plot.

Observations occurring soon after deposition wereassumed to be more likely to result in infection than laterobservations, and no infection was considered possible22 days or more after deposition, so each observation wasassigned a value equal to the sum of the scores of the 21

Fig. 1 Fifteen points in thesouthwestern United States forwhich likelihood of white pineblister rust (WPBR) infestationwas calculated are shown (tri-angles) over a map indicatingWPBR infestation status. Datacompiled by B.W. Geils, 19June 2006

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subsequent days’ observations, weighting each observa-tion’s score with a linearly decreasing multiplier, rangingfrom 1 for the immediate subsequent observation to 0 forthe observation 22 days later. Finally, these weighted valueswere classified into four ranks similar to those of the upper-level values: (4) greater than 26.4 (four standard devia-tions): very high; (3) 26.4 to 13.2: high; (2) greater than 0but less than 13.2: moderate; (1) 0: low.

Coupling of upper-level and surface likelihood scenarios

Scores for upper-level transport and surface infection weresummed for each observation and classified to identify therelative likelihood of establishment for each observationduring the examined dispersal seasons. Threshold values of6, 5 and 4 were selected to place the summed transport andinfection scores into four nominal classes for likelihood ofestablishment (the combination of transport and infection).These classes were very high for a score >6, high for ascore 5.1–6, moderate for a score 4.1–5, and low for a score≤4. The stipulation that a score be greater than 6 rather thanequal to 6 was chosen to avoid classifying an observationas very high when either the upper level or surface termwas less than high.

A sensitivity test was undertaken to determine if thesethresholds were appropriate. Increasing the thresholds, from5.0, 4.0 and 3.0, respectively, did not affect the number ofobservations placed in the ‘high’ and ‘very high’ classesbut increasing the threshold by only 0.1 moved nearly 17%of the days from the ‘moderate’ to the ‘low’ class.Additional increase of the thresholds to 0.3 above thebaseline had no effect on the observations placed in any ofthe classes. The frequency distribution for the 0.1 elevatedthresholds was deemed to be the most robust and thesethresholds were used for the final classification.

Indices for likelihood of conditions favorable fortransport, infection, and establishment were computed forall 15 sites from observation frequencies for upper-level,surface, and coupled conditions, respectively, and groupedby favorability classes, very high, high, moderate and low(data for six sites are presented in Tables 2 and 3). Thisresulted in 12 frequencies for each point. A weightingscheme and summation was employed to combine thefavorability class information and result in only threeindices at each site: one for upper-level transport, one forsurface conditions and one for likelihood of germination. Inthe weighting process, the frequency for a moderate classwas multiplied by 1/7, high by 2/7, and very high by 4/7(i.e., 1/7 + 2/7 + 4/7=7/7, preserving the relative frequencyof moderate + high + very high observations to lowobservations while factoring in differences among moder-ate, high, and very high classes). The weighted class-frequencies were then added together to obtain, for each

site, one index representing transport, another index forinfection, and third for establishment (Table 5). Finally, forgraphical displays that illustrate regional trends, indiceswere krieged and plotted as continuous gradient maps(Figs. 2, 3, 4).

Results

Assessing the model and likelihood of LDD

The synoptic model for likelihood of LDD of WPBR wasdeveloped and tested in three phases over several years. First,the procedure was implemented and evaluated for the onlyinfestation in the region known at the time—the SacramentoMountains. Model performance was judged by identificationand ranking of likely episodes for establishment compared tothe previously published age of oldest infections. Next, fiveadditional targets were added: two sites that were subse-quently discovered to be infested and three sites wheresearches as of 2007 have still failed to find establishedinfestations. Model performance was judged by its discrim-ination between sites differing in presence/absence of pineinfection. Finally, another nine sites were included tocomplete a regular grid across a region of interest. Veryrecent discoveries of young, spot infestations provided theopportunity to further assess model predictions against fieldobservations using a case-history approach.

Sacramento mountains 1965–1974

The ULSC used cluster analysis to identify 16 patterns ofsimilar upper level flow, named for dominant season andbarometric structure (Frank et al. 2008, this issue). Onlyfour of these clusters, however, are rated favorable for LDDof WPBR to the Sacramento Mountains (see Frank et al.2008, this issue, Figs. 2 and 4). A cluster typified as asummer-trough (ST) pattern provides conditions with avery high likelihood for transport. A trough in the westernUnited States generates a zone of higher wind speedspassing over the source region with a strong, direct flow tothe target. This pattern occurs on 14.8% of the observationsin the April–July spore season. Two other clusters, thewinter-trough-ridge (WTR) and the summer-trough-ridge(STR), are rated as high for transport likelihood. The WTRpattern is also characterized by a trough in the westernUnited States, but flow is weaker and traverses farther southin the transit from the source to the target. The WTR isobserved 2.2% of the time during the spore season.Although the STR flow over the southwestern UnitedStates is more zonal than for ST or WTR, and the area ofstronger winds is further north, transport likelihood is stillrated as high. This pattern occurs on 19.5% of the

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Fig. 3 Likelihood of surfaceconditions favorable for infec-tion by WPBR spores in thestudy area during the April–Julyspore season in the years 1975–1990

Fig. 2 Likelihood of upper-level transport of WPBR sporesto the study area during theApril–July spore season in theyears 1975–1990

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observations during the spore season. The observationswith a winter-zonal-warm (WZW) pattern are rated as onlymoderate for transport. This pattern occurs 7.9% of the timeduring the spore season and is distinguished by nearlydirect upper-level flow from the west to east, but displaysweaker upper-level winds that reduce the likelihood oftimely source-to-target transport. All other ULSC patternsare rated low for transport likelihood because of either veryweak flow or poor alignment.

Classification of four-times-daily meteorological obser-vations for WPBR transport and infection indicates thatunfavorable conditions occurred much more frequently thanfavorable conditions. For transport only, observations ratedas very high were relatively frequent, accounting for 13%of those in the 1965–1974 study period. Coupling withsurface infection requirements, however, greatly reducedthe frequency of favorable conditions for rust establishment(transport and infection): 80% low, 13% moderate, 6%high, and only 0.7% very high. The observations with veryhigh ratings occurred most frequently (>60%) in the laterhalf of the dispersal season (June and July). The 33observations rated as very high resulted from five differentcombinations of upper level and surface conditions, andoccurred as five episodes of variable length in only four ofthe years in the study period (Table 1). There were briefepisodes in June 1972 and July 1971, a longer episodebeginning the end of June 1968, and a 2-week period inJune 1969 when conditions remained very high for multipleperiods exceeding 24 h.

The synoptic pattern for June 1969 (Table 1) began witha trough-ridge flow and moderate-to-high likelihood fortransport; on 4 June the first of several transitions occurred.The following period of 5–7 June was dominated by an STpattern with very high likelihood for transport. After a fewdays (8–10 June) when a period of low likelihood fortransport prevailed, a short period (11 June) of STdeveloped with a high likelihood for transport. For thenext few days (to 15 June), an STR persisted andmaintained a high likelihood for transport. The patterns,duration, and sequences of synoptic conditions for June1969 had also occurred in nearly every year examined. Thedistinctive feature of June 1969 was the coincidence of anextended period favorable for transport with developmentof unusually persistent surface meteorological conditions(66 h beginning at 0600 hours on 14 June) that providednearly continuous favorable conditions for infection. Theweather in the Sacramento Mountains for the followingmonths of July, August and September was typical of theregion and adequately cool and wet for the rust to infectwhite pines and thereby establish a persistent infestation,with infection alternating between ribes and pines.

Although canker aging can only approximate time sinceinfection, it does provide an independent observation ofinfestation history (Kearns 2005). Considering the uncer-tainty of finding and correctly dating the oldest canker, themodal estimate of Van Arsdel et al. (1998) for theinfestation origin of 1970 implies “sometime from 1965to 1974.” If the dispersal model fits epidemiology and

Fig. 4 Likelihood of establish-ment of WPBR, the combinationof spore transport at upper levelsof the atmosphere and coinci-dent surface conditions favor-able for infection, in the studyarea during the April–July sporeseason in the years 1975–1990

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meteorology well, any of the observations rated very highfor establishment could be the actual origin. Considering,however, the continuity of the June 1969 observations, thisis most likely the origin of the Sacramento infestation. Thisdemonstrates the utility of the modeling approach foridentifying potential dates for origins of infestations.

Six target sites 1975–1990

Five more sites in the southwestern United States wereidentified for further evaluation of the model (Table 2). Thisprovided three targets with pine populations known to beinfested: the original target in the Sacramento Mountains(Sierra Blanca), a population in the Sangre de Cristo andWet Mountains in southern Colorado (La Veta), and a

population in the Jarbidge Mountains, Nevada (Elko), andthree targets where intensive searches have failed todiscover WPBR as of 2007: the plateaus of southwesternUtah (Bryce Canyon), the San Francisco Peaks, Arizona(Flagstaff), and the Spring and Sheep Creek Mountains,Nevada (Needles). Upper-level patterns were examined forlikelihood of transport to the new points, and surface datawere acquired for the period 1975–1990, inclusive of thetime when the rust is believed to have first reached the LaVeta and Elko target points.

The three infested sites had high or very high modeledlikelihood of establishment for the period considered,whereas sites not known to be infested were rated to haveonly low or moderate likelihood (Table 2). A detailedexamination of the calendar for the Elko site reveals nine

Table 1 Thirty three observations in the May–July 1965–1974 period were identified with ‘very high’ likelihood for white pine blister rust(WPBR) establishment in the Sacramento Mountains

Observation USLC patterna Upper level likelihood Surface likelihood Coupled likelihood

30 June 1968 12Z 2.2 3.33 3.00 6.331 July 1968 00Z 2.1 3.33 3.00 6.331 July 1968 06Z 2.2 3.67 3.00 6.671 July 1968 12Z 2.2 4.00 3.00 7.001 July 1968 18Z 2.2 4.00 3.00 7.002 July 1968 18Z 2.2 4.00 3.00 7.006 July 1968 06Z 2.1 3.33 3.00 6.332 June 1969 00Z 1 2.67 4.00 6.672 June 1969 06Z 1 3.00 4.00 7.002 June 1969 12Z 1 3.00 4.00 7.002 June 1969 18Z 1 2.67 4.00 6.673 June 1969 00Z 4 2.67 4.00 6.673 June 1969 06Z 1 2.67 4.00 6.673 June 1969 12Z 1 3.00 4.00 7.005 June 1969 00Z 2.2 2.33 4.00 6.335 June 1969 18Z 2.2 3.67 4.00 7.676 June 1969 00Z 2.2 4.00 4.00 8.006 June 1969 06Z 2.2 4.00 4.00 8.006 June 1969 12Z 2.2 4.00 4.00 8.006 June 1969 18Z 2.2 3.67 4.00 7.677 June 1969 00Z 2.1 3.67 3.00 6.677 June 1969 06Z 2.2 2.67 4.00 6.6711 June 1969 06Z 2.2 3.00 4.00 7.0011 June 1969 12Z 2.2 4.00 4.00 8.0011 June 1969 18Z 2.2 3.00 4.00 7.0012 June 1969 12Z 2.1 3.00 4.00 7.0012 June 1969 18Z 2.1 3.00 4.00 7.0013 June 1969 00Z 2.1 3.00 4.00 7.0013 June 1969 06Z 2.1 3.00 4.00 7.0015 June 1969 06Z 3.1b 3.00 4.00 7.0029 July 1971 12Z 2.2 4.00 3.00 7.0030 July 1971 18Z 2.2 3.67 3.00 6.674 June 1972 12Z 2.1 3.33 3.00 6.33

a 1 Winter-trough-ridge (WTR), 2.1 summer-trough-ridge (STR), 2.2 summer-trough (ST), 4 winter-zonal-warm (WZW), 3.1 transition-trough-ridge (TTR) (see Frank et al., this issue)b Not identified as likely for upper-level transport

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consecutive observations beginning 2 July 1979 00Z withhighly favorable establishment likelihood. The correspon-dence of this date with an independent estimate of theinfestation origin by canker age, 1975–1984 (Vogler andCharlet 2004), suggests that coupled conditions rated ashigh (not just very high ratings) may be sufficient forsuccessful rust dispersal. Although the possibility ofestablishment resulting from episodes rated moderatecannot be excluded, the infrequency of observations ofeven moderate rating at putatively non-infested sitesdemonstrates that the model distinguishes among sitesdifferentiated by infestation status.

Examination of the surface and upper-level rankingsbefore the coupling of the two scores shows that, separately,neither surface nor upper-level flow conditions are limitingfactors for rust establishment (Table 3). Infested and un-infested points show similar frequencies of occurrence ofsurface and upper-level conditions. For example, the un-infested Bryce Canyon site has higher frequencies of veryhigh likelihoods for both surface and upper-level flow thandoes the infested La Veta site. However, La Veta experi-enced 41 observations of high establishment likelihoodduring the study period while Bryce Canyon had none(Table 2). This suggests that the coincidence of favorableupper-level flow patterns with surface conditions conducivefor germination is the unique condition that permitsestablishment of a population by LDD.

Nine additional sites 1975–1990

To further examine the susceptibility of white pinepopulations in the southwestern United States, nine addi-tional target points were selected from the Reanalysis dataset, bringing the total number of target sites to 15. Thesepoints were: northwestern Utah (Dugway), east-centralNevada (Caliente), southeastern Utah (Blanding), south-western Colorado (Durango), northeastern Arizona (Win-slow), northwestern New Mexico (Grants), northeasternNew Mexico (Cline’s Corners), southeastern Arizona(Stafford) and southwestern New Mexico (Deming). Fourof these sites were eventually recognized as infested(Table 4). Infected ribes were found in 2005 on the West

Table 2 Likelihood of WPBR establishment for the April–July sporeseason in the years 1975–1990. Of these six sites in the southwesternUnited States, Sierra Blanca (NM), La Veta (CO) and Elko (NV) areknown to be infested and Bryce Canyon (UT), Flagstaff (AZ) andNeedles (CA) are known to be un-infested

class occurrences percent class occurrences percen tLow 6579 84.26% Low 6243 79.96%Moderate 919 11.77% Moderate 1565 20.04%High 185 2.37% High 0 0.00%Very High 125 1.60% Very H igh 0 0.00%

class occurrences percent class occurrences percen tLow 6544 83.81% Low 7782 99.67%Moderate 1223 15.66% Moderate 26 0.33%High 41 0.53% High 0 0.00%Very High 0 0.00% Very H igh 0 0.00%

class occurrences percent class occurrences percen tLow 4188 53.64% Low 5837 74.76%Moderate 3611 46.25% Moderate 1971 25.24%High 9 0.12% High 0 0.00%Very High 0 0.00 Very H igh 0 0.00%

Elko, NV

Bryce Canyon, UT

Flagstaff, AZ

Needles, CA

Sierra Blanca, NM

La Veta, CO37.5°°N, 105°W 35°N , 112.5°W

40°N, 115°W 35°N, 115°W

32.5°N, 105°W 37.5°N, 112.5°W

Table 3 Upper-level flow and surface meteorological conditionsfavorable for transport and germination, respectively, of WPBR for theApril–July spore season in the years 1975–1990. Of the six sites in thesouthwestern United States, Sierra Blanca (NM), La Veta (CO) andElko (NV) are known to be infested and Bryce Canyon (UT), Flagstaff(AZ) and Needles (CA) are known to be un-infested

class occurrences percent class occurrences percentLow 5451 69.81% Low 5110 65.45%Moderate 772 9.89% Moderate 2058 26.36%High 1137 14.56% High 217 2.78%Very High 448 5.74% Very High 422 5.40%






Elko, NV

Sierra Blanca, NM

Surface

Upper Level Surface

Upper Level Surface

La Veta, COUpper Level Surface

Upper Level

Upper Level SurfaceBryce Canyon, UT

Flagstaff, AZ

Needles, CA

Upper Level Surface

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Tavaputes Plateau in southern Duchesne County, Utah,(east of Dugway) but infected white pines have not beendiscovered (B.W. Geils, unpublished data). A smallpopulation of infected white pine was found in the JemezMountains of Sandoval County, New Mexico, (Grants) anddated to 1999 (D. Conklin, personal communication). In1999, an infestation was discovered by Dave Conklin onGallinas Peak, Lincoln County, New Mexico, (Cline’sCorners); the infestation was then several years old.Another small, localized but well-established populationof infected white pines was discovered (also by DaveConklin) in the New Mexico San Francisco Mountains ofwestern Catron County (nominally Safford). The infestation

was dated by canker age observations in 2007 to circa1995.

The four infestations at the added sites providedadditional data for testing minimum coupled conditionssufficient for establishment (Table 5). Although infre-quent (0.001), very highly favorable observations hadoccurred at Cline’s Corners of the Gallinas Peak infesta-tion. Highly favorable conditions were observed at Grants(0.007) and Safford (0.009) where the Jemez Mountainsand San Francisco Mountains infestations were discov-ered. Within the period of study, 1975–1990, no veryhighly or highly favorable observation were recorded forDugway where rust was found on ribes in 2005 (Tavaputes

Table 4 Seven WPBR infestations in the eastern Great Basin, southwestern United States and southern Rocky Mountains

Nearest sitea County, State Mountain range Originb Discoveryc

Elko Elko, Nevada Jarbidge Mountains 1975 2002Dugway Duchesne, Utah West Tavaputes Plateau 2005 2005La Veta Huerfano, Colorado Sangre de Cristo Mountains and Wet Mountains 1987 2003Grants Sandoval, New Mexico Jemez Mountains 1999 2006Cline’s Corners Lincoln, New Mexico Gallinas Peak 1997 1999Stafford Catron, New Mexico San Francisco Mountains 1995 2005Sierra Blanca Otero, New Mexico Sacramento Mountains and Sierra Blanca and Capitan Mountains 1969 1990

a Site corresponds to the point on the 2.5° latitude by 2.5° longitude grid shown in Fig. 1b Year of infestation origin, determined from age of oldest cankers and/or meteorological correspondence (West Tavaputes Plateau, collection ofinfected Ribes)c Year of discovery of infestation authenticated by collection and/or verification

Table 5 Indices for likelihood of long distance dispersal of WPBR as weighted frequency for the occurrence of favorable, synoptic conditions fortransport only (upper level), for infection only (surface condition), and coupled for establishment

Sitea Weighted frequency of occurrence b

Upper level Surface condition Coupled

Elkoc 0.380 0.002 0.066Dugwayc 0.303 0.003 0.042Caliente 0.264 0.002 0.038Bryce Canyon 0.155 0.008 0.029Blanding 0.144 0.004 0.026Durango 0.122 <0.001 0.021La Vetac 0.122 0.025 0.024Needles 0.181 0.001 0.036Flagstaff 0.076 0.005 <0.001Winslow 0.138 0.010 0.004Grantsc 0.118 0.017 0.011Cline’s Cornersc 0.077 0.049 0.010Safford c 0.101 0.013 0.003Deming 0.035 0.028 0.003Sierra Blancac 0.089 0.076 0.033

a Site corresponds to the point on the 2.5° latitude by 2.5° longitude grid shown in Fig. 1.b Frequency is proportion of 7,808, four-times-daily observations, April–July, 1975–1990 when conditions met requirements consideredminimally favorable for dispersal (better than low); frequencies for moderate, high and very high are summed with relative weights; see text fordescription of conditions and criteriac Associated with known rust infestations

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Plateau infestation). Moderately favorable observationshad been recorded there, however, with a frequency of0.292.

Synoptic climatology for LDD of blister rust in the greatersouthwest

In the greater Southwest, upper-level conditions favorablefor rust transport occur more frequently than surfaceconditions favorable for establishment (Table 5). The leastfavorable site for transport is Deming and the mostfavorable is Elko (index values of 0.035 and 0.380,respectively). Infestations occur near both of these sites,suggesting that upper-level transport is not a limitingfactor. The regional pattern of upper-level transportlikelihood is shown in Fig. 2. Upper-level transport like-lihood displays a peak in the northwest corner, high valuesin the extreme north and west, and lowest values at Demingand Flagstaff.

The least favorable site for infection is Durango, and themost favorable is Sierra Blanca (index values <0.001 and0.076, respectively). The infestation known to be nearest toDurango is about 2.5° east (in the Sangre de CristoMountains, Table 4); the oldest and most extensiveinfestation of the region is in the Sacramento Mountainsand adjacent ranges of the Sierra Blanca site. Figure 3shows the regional pattern of favorability for surfaceconditions. This pattern displays a peak in the southeastcorner, high values extending across the central NewMexico ranges into the ranges of south-central Colorado,and lowest values in the northwest corner.

Establishment of the rust requires the timely sequence oftransport and infection modeled as a coupled index (Table 5,Fig. 4). The least favorable site is Flagstaff, and mostfavorable site is Elko (index values <0.001 and 0.066,respectively). An infestation is well established in theJarbidge Mountains north of Elko (Table 4); no infesta-tion has yet been found in the forests around Flagstaff. Thecoupled likelihood pattern has a peak in the northwestcorner with high values extending across the western andnorthern portions (reflecting the influence of the upperlevel factor) and a second, lower peak in the southeasterncorner (reflecting influence of the surface factor). Lessfavorable areas are central and southeastern Arizona(reflecting low coupling where the two factors areindividually moderate). Overall, the map of the coupled,synoptic index accounts well for known infestations; thepoorest fits occur where infestations appear restricted tolocal micro-sites (e.g., the New Mexico San FranciscoMountains). The model identifies vulnerable areas ineastern and southern Nevada and across Utah asfavorable for spring transport and ribes infection byWPBR.

Discussion

Dispersal to the Sacramento mountains

From observations of heavy infection by a native rust onribes and knowledge of the climate at Cloudcroft, NewMexico, an early report from the Bureau of Plant Industry(1921) predicted that “if blister rust ever reached this region[Sacramento Mountains], it would flourish...[there].” Therust did reach the Sacramento Mountains and it nowseriously infests the adjacent White Mountains (SierraBlanca) and nearby Capitan Mountains (Geils 2000).

Several lines of evidence support the hypothesis that theinitial Sacramento infestation resulted from a singleintroduction in 1969 by aerial transport from California.The advanced age and remote location of cankers suggestsimportation and planting of infested pines or ribes isunlikely (known transplanting of seedlings from Idahooccurred after 1970). The genetic profile of the rust(Hamelin et al. 2000) is explained by a single introductionof a rust from California. By 1969, WPBR had been inCalifornia for several decades and had spread well southalong the Sierra Nevada (Kliejunas 1985; Kliejunas 2002).Although a flight from California is more than 1,200 km,the rust also appears to have completed other, somewhatshorter dispersals (Geils et al. 2003), and longer distancesfor other rusts are reported (Nagarajan and Singh 1990).Source regions such as Oregon, Idaho and Wyoming cannotbe ruled out; few, infrequent synoptic patterns (Frank et al.2008, this issue) would transport rusts from these locationsto south-central New Mexico.

In addition to transport of viable rust spores, depositionmust occur on a susceptible telial host, followed byinfection, development over the season by several otherspore stages, infection of white pine and development ofperennial cankers. The Sacramento Mountains are note-worthy in the Southwest United States for a large andextensive population of Ribes pinetorum, a susceptible ribescapable of supporting a large inoculum potential (VanArsdel and Geils 2004). These mountains also have, inclose proximity to the ribes, many very susceptiblesouthwestern white pine (Pinus flexilis var. reflexa). Othermountainous areas of the region also have some ribespopulations, white pine populations, exposure to westernand northern rust sources and spring-summer moisture; theintensities of these factors vary geographically (e.g., fewribes in some ranges well populated with white pines).

Upper-level flow and spore transport

The patterns of upper level flow identified by the ULSC(Frank et al. 2008, this issue) describe the potentialconnections between source and target locations. Although

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host and rust populations have patchy distributions at variouslandscape scales, they can be mapped at a county level usingdata from published literature, herbarium records, and fieldreports (Fig. 1, review completed by B.W. Geils, June 2006).County distributions are sufficient to identify source andtarget areas for a synoptic analysis with a spatial resolutionof a 2.5° grid. Given their proximity to western andnorthwestern sources, there is little surprise that sites inNevada and Utah are frequently exposed to favorableconditions for spore transport (Fig. 2). The relativelyfrequent occurrence of flow across the entire region suggeststransport alone may not be limiting, at least as far away fromthe source populations as central Colorado and eastern NewMexico. This agrees with the finding of Aylor (2003) thatLDD may be more often limited by establishment at thedeposition site than by conditions of transport.

Surface conditions and infection of ribes

In general, deposition of rust spores and the followingstages in the rust life cycle are processes determined by amicroenvironment of temperature and moisture that usuallyvaries over very short distances (leaf boundary layer or insun vs in shade). However, in this case we are interested inrelatively long periods of very high humidity in a regionallydry climate zone. When synoptic data suggest conditionsare favorable for infection, the conditions ought to occurwidely across the area. Additional episodes favorable forinfection are likely only in special habitats such as narrow,wet canyons. Although there is a low probability that a rustspore released at 1,000 km distance will arrive at a site,probability of infection is even lower if there are fewerribes leaves. The synoptic model does not explicitly includeassessment of the abundance of ribes, which varies greatlyfrom location to location within a synoptic site (see Kearns2005). Conditions favorable for infection are generallyuncommon in the region, but there is significant geographicvariation and micro-site exceptions exist.

Coupled conditions and initial establishment of blister rust

The time limitations on viability of rust spores require that,for a spore to establish an initial infestation on ribes,conditions suitable for germination and infection mustfollow soon after deposition. Coupling upper-level andsurface conditions into a single metric identifies and ranksthe favorability of this sequence of events. Comparing therelative frequency of these events across a region thenprovides a means of assessing the risk of LLD at a synopticscale. There remains, however, uncertainty over theminimum conditions necessary for establishment. Tworesults support an assertion that a sequence of highly orvery-highly rated observations satisfy these minimum

conditions. First, in a comparison of infested and putativelynon-infested sites, ratings of high or better occurred only atthe infested sites (Table 2). Second, for the Sacramento andJarbidge Mountains infestations, the coupled calendars’high and very high observations agree with the estimates oforigin year by canker age. Very highly and highly favorableobservations are consistently recorded for the infested sitesElko, La Veta, Grants, Cline’s Corners, Safford, and SierraBlanca, but these events are rare (<0.02). On the WestTavaputes Plateau in 2005, B.W. Geils found infected ribesin a wet cove of the Dugway site. This site had notpreviously (1975–1990) experienced periods identified ashighly or very highly favorable for establishment, butmoderate observations had been frequent (0.292). Althoughindividual moderately favorable conditions may not iden-tify specific times when establishment could occur, manymoderately favorable periods could be an indication thatsuitable conditions are sometimes met in the best localhabitats.

A coupled index based on the weighted frequencies oflinked upper level and surface conditions provides areference for assessing the LDD risk of blister rust. Thecoupled index and its component indices help to understandthe transport and infection processes that result in either theintroduction of the rust to a new area or the long distancemovement of genes from a donor to a recipient population.A better understanding of the production of spores and theirescape from the canopy would help to refine the model.Other factors, such as the abundance and proximity ofsusceptible hosts and the frequency of favorable meteoro-logical conditions for continuing reproduction of the rust,are also important for determining the spread and intensi-fication of an infestation at the stand and landscape levels.

Applications

The synoptic climatology model for LDD simplifies manybiological requirements for successful infestation by WPBRbut still provides a useful display of vulnerability at aregional scale. Because the Arizona San Francisco Peakshave susceptible populations of white pine and ribes inclose proximity to each other (and to motivated forestpathologists), this area will receive continued monitoringfor early detection. The synoptic model, however, suggeststhat a lack of rust in the Peaks is not sufficient reason toforgo continued reconnaissance in the “drier” Great Basinand “more distant” Southern Rockies, where sites are more,not less, vulnerable to infestation. Susceptible populationsof white pines and ribes also occur in Mexico, but theirvulnerability to WPBR is unknown. The synoptic modelprovides a tool for assessing the potential of LDDintroducing blister rust in to Mexico. Over time, globalclimate changes and, consequently, the genetic connections

Int J Biometeorol

between metapopulations are altered. The synoptic modelprovides a framework at a sufficiently large and long scaleto develop a means to assess an important component offorest health in response to climate change.

Afterword

Since this paper was submitted, scouting for additionalinfestations in the southwestern United States has contin-ued. By the end of 2007, several additional, very small,infested areas had been discovered, but some areas, such asFlagstaff, appear to remain uninfested. D. Conklin and B.W. Geils (unpublished data) located more trees, infected fora long period in the San Francisco Mountain infestation aswell as other, small and recent infestations about 40 km and100 km to the southeast. D. Conklin has also locatedseveral additional, infested locations near to the originalJemez Mountain discovery. These may indicate thatsecondary spread has already occurred. R.S. Danchockphotographed a single canker in the Zuni Mountains about30 km southwest of Grants, New Mexico; we have not yetdetermined if this single canker is part of an enduringinfestation. The apparent lack of perennial cankers at WestTavaputes Plateau, Utah, suggests that this is another casewhereby infection of ribes was not followed by infection ofpine. None of these observations, however, refute thegeneral conclusions of the synoptic model—that withinthis Southwestern, synoptic region, transport of rust sporesis nowhere limiting but, where conditions for infection aresuitable, varies from widespread in the southeast to easternportions to only localized elsewhere.

Acknowledgments The authors thank Frank Hawksworth forfinding the rust; Gene Van Arsdel for insisting it came fromCalifornia; Dave Conklin and John Popp for showing us where itwas; Mary Lou Fairweather for challenging us to explain why not theSan Francisco Peaks; Detlev Vogler for encouraging us to see it as adynamic, interacting, and evolving member of a complex pathos-ystem; Paul Zambino for comments on the manuscript; and RichardSniezko for sharing Danchock’s find. This work was funded by theUnited States Department of Agriculture, Forest Service, ForestHealth Protection, Forest Health Technology Enterprise Team Project#TD.01.M01 and Rocky Mountain Research Station (03-JV-11221605-297), and University of Delaware, Department of Geography, Centerfor Climatic Research. Development of the ULSC and its applicationto the Sacramento outbreak was performed by Katrina Frank in partialfulfillment of a Ph.D. in Climatology.

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