View of Impact of global warming on potato late blight: risk, yield loss and control

Impact ^of global warming ^on potato late blight

risk, yield loss and control

Timo Kaukoranta

AgriculturalResearch Centreof^{Finland, Institute} of^PlantProtection,FIN-31600Jokioinen,Finland

The impactof climate warming ^on yield losses caused bypotato late blight and onthe need for disease control was studiedby constructingmodels fortiminglateblight epidemics anda^{model of} potatogrowthconstrainedbylateblight. Empirical modelspredictingthe date ofplanting andemer- gence of potato werebasedonthermal time,andamodel predicting the date of late blightoutbreak was based onthermal timeonrainy days. Experiments were ^conductedover ³years under ambient and elevated (+3°C) temperatures to obtain parameter values for the growthmodel. Potato emer- gence is predicted to occur^at 631 degree days accumulated above O°C after the 16-day running meantemperature inspringexceeds O°C.Ablightoutbreak ispredicted tooccurwhen the effective temperature sum accumulated above B°C after potato emergence, ondays with at least0.1 mm ^of precipitation, achieves 156 degree days. In theprediction of the outbreak the maximumdailyaccu- mulation of temperature is limited to 10 degree days. A preliminary sensitivity studycarried out at one^siteinsouthern Finland suggests that over^{arange ofI to}3°Cwarming, theperiod duringwhich lateblight needs to be controlled by fungicide applicationswould be 10-20 days longerper I°C^of warming. The increase in yield lossofunprotected crops would be of the same magnitude as ^the increaseⁱⁿyield potential,around2t/ha^ofdrymatter per I°Cofwarming.

Key words'.Phytophthora infestans,^climaticchange,riskevaluation, model

ntroduction

Predicted future climate warming and the rise in carbon dioxide concentrations (IPCC 1990) are generally believed ^toaffect the risks ofpest and disease damage to agricultural crops (e.g.

Coakley 1988,Harrington and Stork 1995). In northernlatitudes, the risks would be expected

to increase with warming, because lowtemper- aturesand the long winter currently reduce the survival,number of generations per year, repro- ductionrateand activity ofmostof thepestsand pathogens attacking crops during the growing season.

The economic importance of potato late blight (Phytophthora

infestans

^{(Mont.)de Bary)}

might change significantly, _as warming would

©Agriculturaland Food ScienceinFinland ManuscriptreceivedFebruary 1996

Vol. 5(1996):30-

probably affect both the yield potential ofpota- toand late blight epidemics. The yield potential is likely to increase with warming. Damage caused by the pathogen might also increase, if warming allowed epidemics to start at least as early _asthey do in thecurrentclimate.

In this study, robust models have been de- veloped to estimate the impact of global ^warm- ing on yield losses caused bypotato late blight and totime the planting and emergence ofpota- toand the outbreak of the epidemic. These mod- els_were coupledtoa simple crop growth model to estimate the reduction in green leafarea and tuber yield. ^Asensitivity test was conductedto study the effect oftemperature changes on the date of blight outbreak, the need for fungicide use, and yield losses.

Timing ^the epidemic

Definition of ^the problem and

selection of methods

Observations and models of the effect of weath- er on the reproduction of P.

infestans

^{and the}

progress of theepidemic abound (e.g. Crosier 1934, Beaumont 1947. Bourke 1955, Wallin 1962, Ullrich and Schrödter 1965, Smith and Seager 1974, Krause etal. 1975). Common to all the models considered successful in predict- ing the disease in a stable climate is _arequire- ment for dataon the duration of periods of high relative humidity (exceeding 75% or90%) and for temperatures during these periods. Howev- er, the duration of high relative humidity is not applicable in this study, because the models de- velopedweretobe applied nationally in Finland (cf. Carteretal. 1996),and daily dataonrela- tive humiditywere notavailableoverthe whole country. Instead, the approach employs simple, empirical models. It is accepted that the models do^notachieve the short-term accuracy of the best existing prediction models.

Regional late blight epidemics follow a roughly exponential growthcurve. Initially the curverises very slowly after the emergence of thepotato,but afterasufficient accumulation of inoculum the curve turns sharply upwards for fields notprotected against late blight. Theturn is marked by the general appearance of the first symptoms,after which the haulm of susceptible varieties_canbe destroyed in 1 ^to3 weeks. Re- gional and annual variations in the timing of symptomappearancearemainly governed by the weather, which determines the time of planting, the length of the emergence period and the^rate of pathogen multiplication after potato emer- gence. Other factors affecting the variationare theamountof initial inoculum carriedoverfrom the previous season (Hirst and Stedman 1960, Croxall and Smith 1976) and the resistance of varieties.

For the purposes of this study the epidemic wasdivided into three phases: the period before

symptom appearance,the period of rapid destruc- tion of thehaulm,and the period when all of the haulm is dead. The objectivewas^todevelop sim- ple models for predicting the time of blightout- break, which is here definedasthe time of gen- eral appearance of first symptoms. As epidem- ics canonly^startafterpotatoemergence, mod- els for timingpotatoplanting and emergenceare also needed.

The date of planting and of emergence of potatois predicted by models based on thermal time,and the date ofblight outbreak byamodel based on thermal time and precipitation. Base temperaturesand the thresholds of the effective temperature sum were estimated by minimizing the sum of squared deviations of the observed dates from those predicted. A Pascal ^computer program was used _tosearch for_parameter val- ues by iterating through all possible values ^at stepsof I°C or 1 degree day. Daily weather data used for estimating the_parameters of the mod- elswereobtained from the Finnish Meteorolog- ical Institute(FMI) and extracted from the data- base managed by the Agricultural Research Cen- tre (Maatalouden tutkimuskeskus 1989).

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Kaukoranta, T:Impact

of

global warmingon^potato late blight: risk,yield loss and control

Models for predicting planting ^and emergence of potato

The time of planting for maincrop varietieswas assumedtodepend onthe effectivetemperature sumaccumulated in spring (from 1 March) after the 16-day running mean temperature exceeded a certain base temperature. Sixteen days was found to be the optimum period for smoothing the spring temperatures, toavoid the possibility of_{a brief,} early warm spell unrealistically ad- vancing the estimated planting date(T. Carter, pers.comm., 1995). Planting dates of 145 varie- ty trials with758 observations on emergence (several varieties)carried outby the Potato Re- search Institute, Lammi, ^Finland, during 1983-

1994 (Perunantutkimuslaitoksen koetuloksia 1983-1994) and in 1977, 1979 and 1980 by Hämeen Peruna Oy (Perunakoetuloksia 1977,

1979, 1980)wereusedtofind basetemperatures, between O°C and 5°C, and threshold tempera- ture sums. The experimental sitesareshown in Fig. 1.

With the minimization procedure itwasesti- mated that planting takes place after 337 degree days have accumulated above O°C (DDO). De- velopment from planting to emergence was found torequire 294 DDO. Both models explain 48% of the variation in data. As both models had the_samebasetemperature,theywerecombined

to giveamodel which predictspotato emergence 631 DDO after the 16 day mean temperature ex- ceeds O°C in the spring. Model predictions are compared with observations in Figs 2a and 2b.

A model for timing the outbreak of the epidemic

The date of outbreak of late blight (Day_sym⁾was predicted_on the basis oftemperatures accumu- latedover wet days after_potato haulm emer- gence. To reduce thepotential error caused by the high spatial variation in daily precipitation, the precipitation was transformedto a discrete scale with two possible values:^< 0.1 mm and

>0.1 mm. On days with^atleast0.1 mm of pre- cipitation (p.>0.1) therate of multiplication of the disease is governed by the daily averagetem- perature (T.). Its effecton the^rate is described by an effective temperature sum model with a base temperature T_band anupper limit fordaily Fig. 1.Locations of potatovarietytrialsin 1977-1994,au- tomatic weather stationsin 1993-1994and lateblightmon- itoringsitesin 1975-1994.

Voi 5(1996):311-327.

Kaukoranta, T: Impactof^{globalwarmingonpotato} late blight: risk, yield loss and control

effectivetemperatureT_{m x}.The outbreak is pre- dicted^tooccur onthe day (Day_sym⁾when theac- cumulatedtemperature onrainy days (ETS) ex- ceeds athreshold requirement ^(ETSsym):

(1) ETS=SjMAX {O, (MIN{T,T_max) ^-T_b)).

i=l..Day_{J sym},p>o.l7 r i

The negative prediction model of Ullrich and Schrödter (1965), developed in Germany and recently validated in the Nordic countries

(Hansen and Holm 1991, Hansen etal. 1995), wasused for setting the initial values of the pa- rameters T._b’, T_max’, ETS_sym^. The program code of‘ ~

the modelwasprovided by E. Friis and J. Hansen of the Foulum ResearchCentre, Denmark. Con- trol _structuresof the programwere modified_to

suit the needs of this work_but, in essence, the negative prediction modelwaskept in its origi-

nal form.

The initial values of the basetemperature(T_b)

and maximum effectivetemperature (T_max⁾were estimated by running the negative prediction model with 3-hourly dataon temperature and relative humidity (RH) recorded by automatic weather stations in 1993-1994atsites shown in Fig. 1 (Ilmatieteen laitos 1995). The modelruns indicated that if the daily average temperature was below 7°C on a rainy day, the daily output value of the negative prediction modelwasgen- erally around zero, indicating that late blight does notmultiply on those days. As the temper- ature rosefrom 7°Cto 15-21°C,theoutputval- ueincreased.

Fig 2. Comparisonof thepredictedand observed daysof (a)planting,(b) emergence and (c) appearance of the first late blight _symptoms. 13points superimposed in 2aand 371 points in2b.

AGRICULTURAL AND FOOD SCIENCE IN FINLAND

The final values ofT.,_b’ T_maxand ETS_sym were determined using the minimization procedure with dataon the appearance of first late blight symptoms, daily mean temperature and daily precipitation. The functiontobe minimizedwas the sum of squared deviations of the observed dates ofsymptom appearance from the dates of outbreak predicted by the model.

The dataon symptom appearance consisted of 55 observations on unsprayed plots of cv.

Bintje. Twenty-six observations were derived from the results of mainly unpublished fieldex- periments conducted during 1974-1991 in plots measuring 2xlo m. Twenty-nine observations weremade during 1992-1994 in unsprayed plots measuring lOx 10m, where the emphasiswas on accurate observation of the appearance date of firstsymptoms.The field experiments and ob- servations _were conducted by the Institute of Plant Protection and research stations of the Agricultural Research Centre (Satakunta, SouthOstrobothnia), the Institute of Potato Re- search, Lammi, and Päijät-Häme Agricultural Institute, Asikkala. The observation sites are shown in Fig.

1.

As the observations weremade by several people and during 1974-1991,not specifically intended^torecordan accuratedate ofsymptom appearance, the symptoms were probablynot always reported immediately after they appeared.

The outbreak of late blightwas estimatedto require 156 degree days accumulated above B°C (T_b⁾ on days withatleast 0.1 mm of precipita- tion after potato emergence(DDB). The maxi- mumdaily effectivetemperature(T_mx⁾waslim- ited to 18°C.

The models predicting the emergence day and the outbreak _{were run} for 55 combinations of year and site using the daily average tempera- ture and precipitation measured^at these sites.

The predicted dates of the disease outbreak are compared with the observed dates in Fig. 2c. The combined models could discriminate between early and late outbreaks,but _alarge_part ofvar- iation in the date of disease outbreak remained unexplained, asthe coefficient of determination was 0.31.

Effect of late blight ^on potato growth

Selection of methods

Rotemetal. (1983), Haverkort and Bicamumpa- ka (1986) and Oijen (1991) have shown that the effect of late blight on potato growth can be largely explained by the reduction in green leaf area over a growing season. The radiation use efficiency (RUE) of green foliage was found ^to be insensitive to the disease. A simple model, such _as those published by MacKerron and Waister (1985) and Jefferies and Heilbronn (1991), which describe leaf area expansion by thermaltime, calculate radiation interceptionas afunction of green leafareaand transform the intercepted radiation to dry matter by RUE, should thus be sufficient ^to estimate tuber dry matteraccumulation in both healthy and infest- ed crops.

Model structure

In the growth model adopted for this study, the interception of photosynthetically active radia- tion(PAR)by the plant canopy after haulmemer- gence iscomputed from the leafareaindex(LAI) and daily total radiation using an exponential light extinctioncurve. The form of thecurve is determined by anextinction coefficient(k).The intercepted PAR is converted into daily produc- tion of drymatterbyaconstantRUE. Part of the drymatterproduction is allocatedtotubers(DM) according to an allocationparameter (A). LAI and A are given _{as a} function of accumulated temperature above 5°C after emergence. LAI is assumed_toremain _constantuntilharvest, since under the conditions prevailing during a short growing period, long days and ample nitrogen fertilization, LAI does ^not generally decrease very much before the haulm is removed priorto harvesting. The daily growth of tubers is given by equation 2

(2) DM=(1 ^-exp(-k*LAI))^*PAR^*RUE ^*A Late blight affects growth by reducing LAI Vol. 5(1996):311-327.

linearly from its normal valuetozeroin 14 days after the predicted outbreak of epidemic.

Experiments and parameter estimation

The values ofLAI, RUE and A were estimated from field experiments conductedatambient and elevated temperatures at Jokioinen in 1993- 1995. Ambienttemperature was measuredat a height of2 m and elevatedtemperature condi- tions_werecreated by regulating thetemperature at2 m height inalarge greenhousetofollow the ambient temperature at a level 3°C above it. Temperatures under the ambient and elevated temperatureconditionswerenot essentially dif- ferent in the early stages of crop development, as the _{potato was} planted 3 _to 4 weeks earlier under elevated than under ambienttemperature conditions. The experimental^site,equipment and weather conditionsarereported in Hakala etal.

(1996).

Two varietieswereused ^inall experiments:

cv.Bintje (Netherlands) and cv. Pito (Finland).

The varieties differ in earliness undercurrentcli- matic conditions, cv. Bintje being arelatively early and cv. Pito a late type, with respective differences in the courseof LAI and tuber fill- ingrate overthe growingseason.Both varieties are fairly susceptible ^to late blight, though cv.

Bintje is clearly_more susceptible than _cv.Pito.

High grade seed for the experiments, aver- aging 30 mm in diameter, was produced by the Seed Potato Centre^atTyrnävä. Before planting the tubers were pre-sprouted for2 weeks^at5- 10°C. Five rows of bothcultivars,each8 m long and spaced 0.7 mapart, wereplanted in ambient and elevated temperature conditions. The dis-

tancebetween plantswas0.33 m. The cropswere fertilized with NPK fertilizer atarate of80 kg/

ha nitrogen and irrigated by drip irrigation lines along the_topofridges and by misting from above the crop. The mistwasproduced by passing_wa- ter through nozzlesatapproximately 2 1/minat an applied pressure of 500 kPa. The nozzles were assembled 1.5 m ^apart in parallel pipelinesat a height of 1.5 m.

Half of therow length was inoculated with P.

infeslans

^on dates considered representative of thestartof late blightatambient and elevated

Table 1. Dates ofplanting,inoculation andharvesting^of theexperiments inambient and elevated (+3°C) tempera- tures at Jokioinenin 1993-1995.

Year

1993 1994 1995

Ambient temperature

Planting 1Jun 23May 29May

Inoculation 29Jul 2 Aug

Harvest 1 29Jul 2 Aug 24Jun

Harvest2 17Aug 23 Aug 10Aug

Harvest3 8 Sep 7 Sep 20 Sep

Elevated temperature

Planting 10 May 25 Apr 2 May

Inoculation 8Jul 22Jun 30Jun

HarvestI 8Jul 22Jun 30Jun

Harvest2 4 Aug 18Jul 20Jul

Harvest3 8 Sep 25Aug,7Sep 4 Sep Fig.3.Set up of plots foronecultivar andonetemperature condition. Plants shown with circles. Harvest times indi- catedingrey tone. Two adjacent plants in a row with the sametone form aharvesting plot. Background shading shows thehealthy and inoculated^areas.

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global warmingon^potato late blight: risk,yield loss and control

temperatures (Fig. 3). The other half waskept free of late blight by applying Ridomil MZ(met- alaxyl⁺mancozeb)orTattoo (propamocarb HCL

+mancozeb) atintervals of 1to 2 weeks. Crops under ambienttemperatures were ^not inoculat- ed in 1995. The canopies werethoroughlywet- ted by misting in the evening, thena homoge- nized mixture of mycelium, spores andwaterwas sprayed ^onto the leaves with a bottle-sprayer.

From 10_to 20 leaves were kept covered with

plastic bags for thenext 12hours to ensure ^that atleast some leaves would be infected. For the next2 days the cropswere misted between0900 and2000 hours every 15 minutes for 10 seconds atatime. The mistingwas continued duringmost nights from 2000 ^to2400 hours until the death of the inoculated foliage. The dates of planting and inoculation_aregiven in Table

I.

Inoculation of leaves with P.

infestans

^suc-

ceeded well. Symptoms appeared 3 to 4 days Fig. 4. LAI of healthy crops as a function of degree days after emergence, base temperature 5°C, at ambient and elevated (+3°C) temperatures in 1993-1995. Bars indicate standard ^errorsof the mean with95% probability.

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^{globalwarming onpotato} late blight:risk,yield loss and control

Fig. 5.Tuber dry matter accumulation inhealthy and diseased crops following emergence at ambient and elevated (+3°C) temperaturesin 1993-1995.

after theinoculation, bothon the leaves kept in plastic bags and on those kept ^wet by misting.

The misting provided conditions under which disease lesions grew fast and blight spread toall plants ^not protected by fungicide.

Todetermine the dates whennewtubers ^start- ed growing, stolon ends were occasionally ex-

posed at two tofive places for both cultivars and

temperatures. The day on which the tips of^sto- lons were clearly expandedwas marked as the day of tuber initiation. Dry^matteraccumulation in healthy and diseased crops was measured by harvesting four plots three timesa seasonin both healthy and inoculatedareas. The layout of the

AGRICULTURAL AND POOD SCIENCE IN FINLAND

harvesting plots is given in Fig. 3. Each plot consisted of^twoplants in a row, separated by a guard from surrounding plots in the same row which were harvested atdifferent times. The plants were separated into tubers, roots, ^stems and leaves, cut into small pieces, driedat60°C for24-48 hours in paper bags and weighed. The dates of harvesting aregiven in Table

1.

LAI in the healthyarea wasmeasured with a portable plant canopy analyzer(typeLAI-2000, LI-COR Inc.,USA).Good estimates of LAIwere not obtained after the second harvest,whichwas indicated by the high variability in the below-

canopy readings of the plant canopy analyzer.

The variabilitywascaused by holes in thecano- py created by harvesting and by lodging in the plots ofcv. Bintje under elevated temperatures in 1994. Four_toeight _separateestimates ofLAI, each of themmeansof eight below-canopy meas-

urements,weretaken for each cultivar and ^tem- perature level. The estimates taken_at one time were averagedto get a single value of LAI for each cultivar and temperature level.

Thecourse of LAI as afunction ofaccumu- latedtemperatureabove 5°C(DDS) afteremer- gence during the three seasons 1993-1995 is shown in Fig. 4 for cvs. Bintje and Pito. The highest values ofLAI, attained between400 and 500 DDS, were approximately 2.5 to 4 for cv.

Bintje and 3.5to4 forcv. Pito,respectively. Once the maximum was reached, LAI remained rela- tivelyconstantuntilatleast 800^to900 DDS. In

1994, the LAI of both cultivars remained clear- ly loweratambient than^atelevatedtemperatures

throughout theseason.

Tuber dry matter accumulation is given in Fig. 5 as afunction of days after emergence. In Fig. 6, the fraction of total plant dry^matterstored in tubers is given as a function of accumulated temperature after emergence(DDS). Values are based _on the harvest measurements on the healthy crops pooled over 1993-1995 ^at both temperature levels. Each value plotted in the fig- ure is a mean of four harvesting plots. The zero values of the fraction show when the first stolon ends were found expanded. The tubers of both varietiesare seen to startgrowing _atpractically the same time,^on average after 160 DDS.

The harvesting intervals were too long and harvests were sometimes too late to allow the accurate determination of the date after which all newdrymatter went totubers.^Fromthe cal- culated ratios of the rise in tuber dry weight to

the rise in total dry weightover aharvesting in- terval and the LAI observations,it wasdeduced that all assimilates wereallocated^totubers ap- proximately after 450 DDS incv. Bintje and af-

ter550 DDS in cv.Pito.

Fig. 6.Fraction of total dry matterfound intubers as afunction ofdegree-daysafter emergence, base temperature +5°C. Pooled data^over 1993-1995and at both temperature levels.

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RUE of healthy crops was estimated by di- viding the dry matter accumulatedover a har- vesting interval by the intercepted PAR. The in- tercepted PARwascalculated from the observed LAI andanextinction coefficient (k) of 0.55,as reported by Firman and Allen(1989). A single value of k was used, as the growth model em- ployed is not sensitive to small changes in the extinction coefficient. PARwasderived from the total daily radiation(EMI, Jokioinen)byassum- ing that the proportion of PAR in the total radia-

tion is 0.5 (Monteithand Unsworth 1990).The reduction in PAR due tothe plastic film above the experiments wastaken intoaccount by mul- tiplying PAR by 0.6 (Hakala ^etal. 1996).

The total drymatteraccumulatedover a har- vesting interval is plotted against the calculated

intercepted radiation in Fig. 7.^A very clear lin- earrelationship is observed between the inter- cepted PAR and the drymatteraccumulation. For each mega joule of intercepted PAR the crops pro- duced between 3.41 and 3.67 grams ofdry ^matter.

Fig. 7.Relation of the observed totaldrymatteraccumulation and estimatedintercepted PARat ambient and elevated temperaturesin 1993-1995.Radiation ^use efficiency(RUE) is theslopeof the least squares regression line (also shown).

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Verification of the loss prediction ^method

Totest whethera model witha singleset of pa- rameters wasrobust enough ^togive yield losses reasonably similar^tothoseobserved,the growth modelwas run using the conditions measured in the experimentsatambient and elevatedtemper- atures in 1993-1995. Because crops were not inoculated_atambienttemperatures in 1995 and lodging probably reduced the growth of cv.

Bintje during the last harvesting interval in 1994, these observationswere^notincluded incomput-

ing growthreduction.

The value of the extinction coefficient (k) was set^to0.55 and RUEto3.54. The proportion of drymatter allocated_to tubers (A) and LAI were given asfunctions of daily mean tempera- tureaccumulated above 5°C(DDS) after_emer- gence. The tuber allocationwas0overthe inter- val 0-160 DDS and increased from 0to 1over the interval 160-450 DDS (cv. Bintje), or 160- 550 DDS (cv.^Pito). LAI increased linearly from 0 to3 overthe interval 0-450 DDS ^(cv. Bintje), or from 0 to3.5 over the interval 0-550 DDS (cv. Pito).The computationswere startedatthe emergence dates observed in the experiments.

Late blight was set to start5 days after inocula- tion and^toreduceLAI linearly^tozeroin 14 days after the ^start.

Comparison of the predicted and observed reductions in tuber growth dueto diseaseover a harvesting interval and over a season is shown in Figs 8 and 9, respectively. The total tuber loss over a season is not predicted very accurately, but the magnitude of the loss iscorrect.

Sensitivity to temperature changes

The sensitivity of the predicted time of disease outbreak and yield losstotemperaturechanges was studied with the models predicting the oubreak of the epidemic and the growth model for the30 year period 1961-1990at Jokioinen (60°49’ N, 23°30’ E). Climatic data were ob- tained from FMI. The model was run for ob- servedtemperatures and for adjustmentstothese at intervals of I°C from -1 to +3°C. Precipi- tation and radiation _werekept unchanged.

An epidemicwaspredicted^{to start}631 DDO Fig. 8. Comparisonof thepredictedand observed reduction intuber dry matter accumulation over ahar- vestinginterval. Pooled dataover 1993-1995and at both temperature levels. Also shown is thepredicted⁼ observed line.

Kaukoranta, T: Impact

of

^{globalwarming onpotato} late blight: risk,yield loss and control

after the dayon which the 16-day running mean temperature in spring exceeded O°C. Parame- tervalues estimated forcv.Pito(see above)were used in the growth model. LAI was kept atits maximum level until removal of thehaulm, ex- ceptwhen the growth of the infected cropswas computed. In the latter case,LAI was reduced linearly ^to zero in 14 days after the predicted outbreak of the epidemic. Haulm removal of the healthy cropwas assumed^totake place 10 days before the day ^on which the 30-year average daily temperature fell below 7°C. This rule re- fers tothe average date for the lifting of tubers atwhich growers aim by adjusting management practices (pers. comm. P. Kuisma). However, it ignores instances ofpremature haulm death due

tofrostordelayed harvest due^toalate growing season.

The growth model was run twice for each temperature level,first by assuming that the late blight epidemic ^starts as predicted by the dis- ease model and second, by assuming that the epidemic would start one week later than pre- dicted. The purpose of the secondrun was ^to demonstrate the effect of possible errorsin the date of blight outbreak on predicted yield losses.

The sensitivity test suggests that the period

from_potato emergencetohaulm removal length- ens by 10 days, on average, per I°C ofwarm- ing if^nofactors other thantemperature limit the length of the growing season. The accumulated

temperature during this period would increase by about 170^DDS, on average, and the poten- tial dry matteryield by about 2 t/ha per degree of warming (Fig. ^10a).

With each degree of warming themeandate of blight outbreak would shift earlier by 4-7 days. Since themeandate of haulm removal ina healthy crop is delayed by warming, these dates together determine the length of the period dur- ing which fungicidecover on potato needs^tobe maintainedto prevent leaves from being infect- ed, except during very dry spells. This period would lengthen by 10-20 days per degree of warming (Fig. 10b).

The simulated potential yield loss responds markedlytothe change intemperature (Fig. 10c).

On average, a I°C increase in temperature in- creased the yield loss ofcv. Pito by 1.7 t/ha of drymatter, if epidemics started aspredicted by the disease model. If epidemics startedoneweek later than predicted, the average yield losses would be0.5-1 t/ha less throughout the^temper- ature range, but the impact of warming would still be thesame.

Fig. 9. Comparisonof thepredicted and observed reduction intuber dry matter accumulation over a sea- son.Pooled dataover 1993-1995and at both temperature levels. Also shown is thepredicted⁼observed line.

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Discussion

The timing ofpotato emergence could be rela- tively well predicted by accumulating 631 DDO from the day when the 16-day running mean^tem- perature exceeds O°C. The data used for esti- mating accumulatedtemperature covers 18 years in the major potato-growing regions between lat- itudes 60°N and 65°N and longitudes 20°E and 28°E. Its validity is limitedtolight, sandy

soils, which warm rapidly after the melting of snow and frost. Only partof the variation in the

date of emergence is explained by the ^tempera-

turemodel, probably duetoits simplicity,asthe soil energy balance is ^not explicitly modelled.

However,part of the variation is also caused by delays in planting due_torain and _management practices whichcannotbe taken intoaccountby the model.

The time of epidemic outbreak is largelyex- plained by the daily temperatures in spring, which determine the time of the planting and emergence ofpotato,and by the number ofrainy days insummerand the averagetemperatures of the rainy days. The outbreak of late blight is pre- Fig. 10.Effect ofchanges inannual meantemperatureby -I to 3°C on(a) tuberpotential dry matteryield, (b) the lengthof the periodfor whichfungicideapplications are required,and (c) thepotentialloss of tuberdry weightcaused bylate blight. Results of annual simulationsover 1961- 1990at Jokioinen. In 10aand 10b the average effect is shown by the continuousline, 0.05and0.95percentiles by the dotted lines. In 10cthe bold lines show the average, 0.05 and0.95 percentilesof the lossiflateblightstarts as predicted.The thin lines indicate therespective lossesif the onset ofepidemicisdelayed by 7 days from that pre- dicted.

dieted by accumulating 156 DDB over the days withat least 0.1 mm of rain after potato emer- gence. The maximum accumulation oftempera- tureon anyone day is 10 degree days. The mod- els forpotatoemergence andblight outbreakare abletodiscriminate between early and late^out- breaks of late blight, but much of the variation in the dates of outbreak remains unexplained.

The long timestep of the blight model is one reason forthat, asthe time coefficients of spore production, germination and infectionareshorter thanoneday (Crosier 1934. Wallin 1953).How- ever, asignificantpart of the unexplained varia- tion is probably not caused by inadequacies in

the models, but by variation in theamount of primary inoculum and weather. The appearance ofsymptoms and precipitationwere sampled at only_onepoint in_aregion. Values for both symp-

tom appearance andprecipitation tend to vary locally, significant differences being found over adistance of 1-5 km. The transformation of pre- cipitation toadiscrete scale hides any quantita- tive effect of the precipitation, but it also pre- ventsoccasional local heavy showers fromover- influencing the prediction.

The dependence of the disease on weather is described only for the period preceding the^out- break of the epidemic even though the weather also influences thecourse of the epidemic after the outbreak. This simplification is justified as slow destruction of the haulm does ^not neces- sarily cause less tuber loss than rapid destruc- tion. It is commonly found that the more pro- longed the destruction, the higher is the proba- bility of tuber infections and later tuber damage, because the probability of spores being washed down^totubers increases with time(A. O. Han- nukkala, pers.^comm.,Croxall and Smith 1976).

The plot experiments conducted for estimat- ing parameters for the growth model were set up in a large greenhouse. Because of the large space,mostof the problems commonlyencoun- tered in closed or open-top chambers could be avoided. Chambers oftencausevariations in^tem-

perature, humidity and radiation betweeen plots (Ashenden etal. 1992, Hakala etal. 1996).

The conditions in the plot experimentsrep

resented ^currentclimatic conditions in southern Finland and those under which the annualmean temperature had risen by 3°C. Thus the growth model, which predicted the reduction in tuber growth reasonably well, canbe used for condi- tions under whichtemperatures range from the

currenttoa levelatleast 3°C higher. The yields measured in the experiments are quite low, be- cause the radiation levels in the experiments were lower than under natural conditions and the fertilization and final harvesting datesweresub- optimal atelevated temperatures.As the_rate of fertilization _was the _same under bothtempera- tureconditions, it_{was not}meaningful _toextend growingseasonsatelevatedtemperaturestotheir maximum possible length.

The values of RUE determined here are a function of the extinction coefficient. This _was not determined in the experiments; instead, the value0.55 reported by Firman and Allen(1989) was used. Slightly loweror much higher values have been reported by Khurana and McLaren (1982), Spitters(1987), Jefferies and Heilbronn (1991) and Haverkort^etal. (1991). RUE deter- mined by regression analysis(cf.Fig. 7)ranged from 3.41 to 3.67 g/MJ. Efficiencies tended to be higher ^at elevated than^atambient tempera- tures, but because of experimental error in the dry matter estimates and a potential sampling error in the LAI estimates, it cannotbe estab- lished thattemperature affectedRUE. RUE has been reported tovary quite widely dueto envi- ronmentalfactors, but the values obtained here arewithin the range reported by Manrique^etal.

(1991), Jefferies and McKerron (1989) and Haverkort and Harris(1986).

In this work the only climatic controls con- sidered were precipitation and temperature for late blight epidemics, and radiation and_temper-

ature forpotatoyield accumulation. The increas- ing concentration of atmospheric carbon diox- ide is expectedtoenhance growth and potential tuber yields(Wheeler etal. 1991,80wes1993).

This alone would increase the yield losses caused by late blight if relative losses remainedattheir currentlevel. Adaptingtoclimatic change would require changes infertilization, which is implic-

AGRICULTURAL AND FOOD SCIENCE IN FINLAND

Kaukoranta^,T:Impactofglobal warming on^potato lateblight:risk,yieldloss and control

Vol.5(1996):311-327.

itly taken into account inthe growth model by assuming that the duration of leafareacould be extended by applying more nutrients over the growing season as temperatures rise. Adjust-

mentsin fertilizer use and the increase in the carbon dioxide contrentration could change the carbon/nitrogen ratio in plants, thus possibly affecting the general level of susceptibility of potato to late blight.

The results of the sensitivity study made at one location in southern Finland ^support the hypothesis that climate warming would consid- erably increase the impact of late blight. The length of the period during which the disease needstobe controlled by fungicide applications would increase by roughly 10 days per I°C of warming from thecurrentaverage of 40days^at Jokioinen. The average increase in yield losses dueto late blight in unprotected crops would be of thesamemagnitude_asthe increase inpoten-

tial yield, whichwas estimated^tobe about 2 t/

ha of drymatterperdegree of warming. Tocoun- terthe increasedrisk,fungicide usewould have

tobe intensifiedor potato varieties would have tobe bred that are significantly moreresistant tolate blight than they aretoday.

Acknowledgements.This work isapart of theFinnish Re- searchProgramme for ClimateChange^(SILMU)and was supported, inpart,bytheAcademyof Finland. Iacknowl- edge^thePotato^ResearchInsitute, Lammi,Finland for al- lowingmetousethevarietytrial data and H. Talvitie, A.

Kangas,A. Hannukkala, P. Simojoki, H. Hakkola,J.-P. Palo- huhta, H. Kahiluoto, E. Seppänen (Agricultural Research Centre of Finland),A.Rahkonen (Potato Research Insti- tute) and^J,Vehmas(Päijät-Häme AgriculturalInstitute) for supplying the lateblightobservations.Ithank J. Poikulai- nenand J. Naatula for theirhelpatall stages of the field experimentsand A.O. Hannukkala for adviceonconduct- ing the fieldexperimentsand forproviding the inoculum for them.

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SELOSTUS

Ilmaston lämpenemisen vaikutus perunaruttoon

Timo Kaukoranta

Maatalouden tutkimuskeskus

Ilmaston lämpenemisen vaikutusta perunaruton ai- heuttamaan satotappioon jarutontorjuntatarpeeseen tutkittiin rakentamalla malli ruttoepidemian ajoitta- miseen ja malli perunan kasvusta rutonrajoittama- na. Perunan istutus- jataimettumisaika ennustetaan tehoisan lämpösumman kertymisellä ja ruttoepide- mian alkamisaika tehoisanlämpösumman kertymisel- läsadepäivinä. Kasvumallin parametritarvioitiin pe- runan kasvun mittauksista,jotkatehtiin normaalissa jakohotetussa(+3 °C) lämpötilassa 1993-1995.Pe- runan ennustetaantaimettuvan,kun yli 0^°Claskettu lämpösumma saavuttaa631 astepäivää. Lämpösum- man laskeminen aloitetaan keväällä 16 vrk:nkeski- lämpötilannoustessayli 0°C.Ruttoepidemian ennus-

telaanalkavan, kun taimettumisesta alkaen laskettu sadepäivien (sade^>0,1 mm) lämpösummasaavuttaa 156 astepäivää. Lämpösumman päivittäinen kertymä ruton ennustamisessa onkorkeintaan 10°C. Jokiois-

tensäähavaintoja käyttäen tehtiin alustavaherkkyys- koe 1-3°Csuuruisenlämpötilan^nousunvaikutukses- ta ruttoon japerunan kasvuun. Aika, jolloin peruna täytyysuojatarutolta fungisidiruiskutuksella, pitenee 10-20 päivää ^I°C lämpenemistäkohti. Ilmaston läm- penemisen aiheuttama satotappioitten kasvu rutolta suojaamattomissa kasvustoissa on samaa suuruus- luokkaa kuinsatopotentiaalinkasvu: noin2t/ha kui- va-ainetta l°C lämpenemistäkohti.