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Impact of Pecan Leaf Blotch on Gas Exchange of Pecan Leaves PETER C. ANDERSEN, JAMES H. ALDRICH, and ANN B. GOULD, University of Florida, Institute of Food and Agricultural Sciences, Agricultural Research and Education Center, Monticello 32344 ABSTRACT Andersen, P. C., Aldrich, J. H., and Gould, A. B. 1990. Impact of pecan leaf blotch on gas exchange of pecan leaves. Plant Dis. 74:203-207. The influence of pecan leaf blotch infection (caused by Mycosphaerella dendroides) on net CO 2 assimilation rate, conductance, transpiration rate, intercellular CO 2 concentration, water use efficiency, and chlorophyll concentration of Cape Fear and Choctaw pecan (Carya illinoensis) leaves was assessed. Physiological variables were related to disease severity in a linear or curvilinear manner. A leaf blotch disease severity rating of 40% resulted in declines in net CO 2 assimilation rate of 63 and 47% for spring- and summer-flush Cape Fear leaves, respectively, and 72 and 79% for spring- and summer-flush Choctaw leaves, respectively. Smaller percentage reductions occurred in leaf conductance, transpiration rate, and leaf chlorophyll concentration. Leaf intercellular CO 2 concentration increased substantially with leaf blotch infection. Plots of net CO 2 assimilation rate versus leaf conductance to CO 2 and intercellular CO 2 concentration of leaf blotch-infected and control leaves revealed that net CO 2 assimilation approached zero before complete stomatal closure and that the degree of stomatal closure was not sufficient to prevent an increase in intercellular CO 2 concentration. These data indicate that leaf blotch may reduce leaf efficiency by adversely affecting the photosynthetic apparatus. Leaf fungal diseases contribute greatly to reductions in photosynthetic efficiency and often induce premature defoliation of deciduous fruit crops in the fall. Downy spot (caused by Mycosphaerella caryigena Demaree & J. R. Cole) (11), powdery mildew (caused by Micro- sphaerapenicillata (Wallr.:Fr.) Lev.) (8), and pecan scab (caused by Cladosporium caryigenum (Ellis & Langl.) Gottwald [= Fusicladium effusum G. Wint.]) (7) infections have been shown to reduce gas exchange of pecan (Carya illinoensis (F. A. Wagenheim) K. Koch) leaves. Net CO 2 assimilation of greenhouse-grown and greenhouse-inoculated pecan seedlings declined in a 1:1 ratio to the proportion of leaf surface infected with powdery mildew (8) or scab (7). By contrast, natural infection by downy spot in the field (11) resulted in reductions in net CO 2 assimilation rate of two or more times the percentage of infected leaf surface. The maintenance of high photo- synthetic rates in the fall can be impor- tant to the current and the subsequent season's nut production (12,16,24,25). Pecan leaves retain high levels of chlorophyll and net CO 2 assimilation until leaf abscission (1), which occurs after the first hard freeze if the trees have received proper insect and disease control management. Pecan nutlets are strong assimilate sinks during kernel development (August-October) (3,22). A Florida Agricultural Experiment Station Journal Series No. 9944. Accepted for publication 21 September 1989. single nutlet may require assimilates produced by eight to 20 leaves (2), although the leaf surface area required to manufacture sufficient photosynthates depends on the cultivar and on photo- synthesis (13,15). Late-season photo- synthetic capacity of pecan leaves during kernel development also influences the tendency of the trees to bear light and heavy crops in alternate years (16,24,25). This study was initiated to determine the impact of pecan leaf blotch (caused by Mycosphaerella dendroides (Cooke) Demaree & J. R. Cole) on gas exchange and chlorophyll content of leaves of different ages. Specific variables mea- sured on fully expanded spring- and summer-flush leaves included net CO 2 assimilation rate, conductance to water vapor, transpiration rate, intercellular CO 2 concentration, total leaf chlorophyll concentration, and efficiency of water use. Our objectives were first to quantify the impact of leaf blotch on these physio- logical variables and second to determine whether changes in leaf gas exchange were mediated by effects on stomata or on the photosynthetic apparatus. MATERIALS AND METHODS Plant material. The planting was a 10- ha mixed block of Cape Fear and Choctaw pecan at the University of Florida Agricultural Research and Education Center in Monticello. Trees were in the second-leaf stage and received standard cultural and management practices, although fungicides were not applied. Leaf gas exchange measurements. CO 2 and H20 vapor exchanges were mea- sured in the field under ambient conditions of light, temperature, and relative humidity. Air temperature, leaf temperature, relative humidity, and photosynthetic photon flux varied between 19 and 27 C, 20 and 28 C, 16 and 49%, and 1,400 and 1,800 yrmol m-2 sec , respectively. Ambient and leaf chamber CO 2 and H 2 0 vapor concen- tration, ambient air and leaf chamber temperature, and photosynthetic photon flux were measured with an ADC Model LCA-2 infrared CO 2 analyzer, an air supply unit (flow rate maintained at 400 cm 3 min-'), and a Parkinson broadleaf leaf chamber (Analytical Development Corp., Hoddesdon, United Kingdom). Net CO 2 assimilation rate, leaf con- ductance to water vapor, transpiration rate, and intercellular CO 2 concentration were calculated with a DL2 Data-logger, GW-Basic software, and an IBM-com- patible personal computer. Boundary layer conductance was estimated according to Parkinson (14), and the infrared gas analyzer response to water vapor (Emax) was calculated according to von Caemmerer and Farquhar (18). Leaf conductance to CO 2 was calculated as leaf conductance to water vapor X 0.625, based on the ratio of water vapor and CO 2 diffusion coefficients in air. Water use efficiency was calculated by mole fraction (i.e., net CO 2 assimilation rate divided by transpiration rate and multiplied by 1,000). Leaf chlorophyll (a + b) was extracted by placing 6.25 cm 2 of leaf tissue in NN-dimethylformamide in darkness for 24 hr (9). Leaf chlorophyll (chl) was calculated from absorbance at 646 and 664 nm by the following equation: ,ug chl/ml = 17.90 A 6 46 + 8.08 A 664 . Leaves were categorized on the basis of age as expanded summer-flush leaves or expanded spring-flush leaves. Leaves were selected for measurement on the basis of age and the presence or absence of leaf blotch. Gas exchange was mea- sured on a 6.25-cm 2 area on two to 10 sun-exposed leaflets per tree. Approx- imately 10-20 trees were measured per day between 10 a.m. and 2 p.m. on 29 September and 1, 7, 8, 12, and 13 October 1987. All measurements were made with leaflets oriented perpendicular to sun- light. The portion of the leaflet used for gas exchange measurements was delineated with a felt-tip pen. Leaflets were then detached and transported to the laboratory in a cooler with ice for assessment of disease severity. Determination of disease severity. Disease severity, based on the percentage Plant Disease/March 1990 203 © 1990 The American Phytopathological Society

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Impact of Pecan Leaf Blotch on Gas Exchange of Pecan Leaves

PETER C. ANDERSEN, JAMES H. ALDRICH, and ANN B. GOULD, University of Florida, Institute of Foodand Agricultural Sciences, Agricultural Research and Education Center, Monticello 32344

ABSTRACTAndersen, P. C., Aldrich, J. H., and Gould, A. B. 1990. Impact of pecan leaf blotch on gasexchange of pecan leaves. Plant Dis. 74:203-207.

The influence of pecan leaf blotch infection (caused by Mycosphaerella dendroides) on netCO2 assimilation rate, conductance, transpiration rate, intercellular CO 2 concentration, wateruse efficiency, and chlorophyll concentration of Cape Fear and Choctaw pecan (Carya illinoensis)leaves was assessed. Physiological variables were related to disease severity in a linear orcurvilinear manner. A leaf blotch disease severity rating of 40% resulted in declines in netCO 2 assimilation rate of 63 and 47% for spring- and summer-flush Cape Fear leaves, respectively,and 72 and 79% for spring- and summer-flush Choctaw leaves, respectively. Smaller percentagereductions occurred in leaf conductance, transpiration rate, and leaf chlorophyll concentration.Leaf intercellular CO2 concentration increased substantially with leaf blotch infection. Plotsof net CO 2 assimilation rate versus leaf conductance to CO 2 and intercellular CO2 concentrationof leaf blotch-infected and control leaves revealed that net CO2 assimilation approached zerobefore complete stomatal closure and that the degree of stomatal closure was not sufficientto prevent an increase in intercellular CO2 concentration. These data indicate that leaf blotchmay reduce leaf efficiency by adversely affecting the photosynthetic apparatus.

Leaf fungal diseases contribute greatlyto reductions in photosynthetic efficiencyand often induce premature defoliationof deciduous fruit crops in the fall.Downy spot (caused by Mycosphaerellacaryigena Demaree & J. R. Cole) (11),powdery mildew (caused by Micro-sphaerapenicillata (Wallr.:Fr.) Lev.) (8),and pecan scab (caused by Cladosporiumcaryigenum (Ellis & Langl.) Gottwald [=Fusicladium effusum G. Wint.]) (7)infections have been shown to reduce gasexchange of pecan (Carya illinoensis (F.A. Wagenheim) K. Koch) leaves. NetCO2 assimilation of greenhouse-grownand greenhouse-inoculated pecanseedlings declined in a 1:1 ratio to theproportion of leaf surface infected withpowdery mildew (8) or scab (7). Bycontrast, natural infection by downy spotin the field (11) resulted in reductionsin net CO2 assimilation rate of two ormore times the percentage of infected leafsurface.

The maintenance of high photo-synthetic rates in the fall can be impor-tant to the current and the subsequentseason's nut production (12,16,24,25).Pecan leaves retain high levels ofchlorophyll and net CO2 assimilationuntil leaf abscission (1), which occursafter the first hard freeze if the trees havereceived proper insect and diseasecontrol management. Pecan nutlets arestrong assimilate sinks during kerneldevelopment (August-October) (3,22). A

Florida Agricultural Experiment Station JournalSeries No. 9944.

Accepted for publication 21 September 1989.

single nutlet may require assimilatesproduced by eight to 20 leaves (2),although the leaf surface area requiredto manufacture sufficient photosynthatesdepends on the cultivar and on photo-synthesis (13,15). Late-season photo-synthetic capacity of pecan leaves duringkernel development also influences thetendency of the trees to bear light andheavy crops in alternate years (16,24,25).

This study was initiated to determinethe impact of pecan leaf blotch (causedby Mycosphaerella dendroides (Cooke)Demaree & J. R. Cole) on gas exchangeand chlorophyll content of leaves ofdifferent ages. Specific variables mea-sured on fully expanded spring- andsummer-flush leaves included net CO2assimilation rate, conductance to watervapor, transpiration rate, intercellularCO2 concentration, total leaf chlorophyllconcentration, and efficiency of wateruse. Our objectives were first to quantifythe impact of leaf blotch on these physio-logical variables and second to determinewhether changes in leaf gas exchangewere mediated by effects on stomata oron the photosynthetic apparatus.

MATERIALS AND METHODSPlant material. The planting was a 10-

ha mixed block of Cape Fear andChoctaw pecan at the University ofFlorida Agricultural Research andEducation Center in Monticello. Treeswere in the second-leaf stage and receivedstandard cultural and managementpractices, although fungicides were notapplied.

Leaf gas exchange measurements. CO2and H20 vapor exchanges were mea-sured in the field under ambientconditions of light, temperature, and

relative humidity. Air temperature, leaftemperature, relative humidity, andphotosynthetic photon flux variedbetween 19 and 27 C, 20 and 28 C, 16and 49%, and 1,400 and 1,800 yrmol m-2sec , respectively. Ambient and leafchamber CO2 and H2 0 vapor concen-tration, ambient air and leaf chambertemperature, and photosynthetic photonflux were measured with an ADC ModelLCA-2 infrared CO2 analyzer, an airsupply unit (flow rate maintained at 400cm3 min-'), and a Parkinson broadleafleaf chamber (Analytical DevelopmentCorp., Hoddesdon, United Kingdom).Net CO2 assimilation rate, leaf con-ductance to water vapor, transpirationrate, and intercellular CO2 concentrationwere calculated with a DL2 Data-logger,GW-Basic software, and an IBM-com-patible personal computer. Boundarylayer conductance was estimatedaccording to Parkinson (14), and theinfrared gas analyzer response to watervapor (Emax) was calculated according tovon Caemmerer and Farquhar (18). Leafconductance to CO2 was calculated asleaf conductance to water vapor X 0.625,based on the ratio of water vapor andCO2 diffusion coefficients in air. Wateruse efficiency was calculated by molefraction (i.e., net CO2 assimilation ratedivided by transpiration rate andmultiplied by 1,000). Leaf chlorophyll (a+ b) was extracted by placing 6.25 cm 2

of leaf tissue in NN-dimethylformamidein darkness for 24 hr (9). Leaf chlorophyll(chl) was calculated from absorbance at646 and 664 nm by the followingequation: ,ug chl/ml = 17.90 A6 46 + 8.08A6 6 4 .

Leaves were categorized on the basisof age as expanded summer-flush leavesor expanded spring-flush leaves. Leaveswere selected for measurement on thebasis of age and the presence or absenceof leaf blotch. Gas exchange was mea-sured on a 6.25-cm2 area on two to 10sun-exposed leaflets per tree. Approx-imately 10-20 trees were measured perday between 10 a.m. and 2 p.m. on 29September and 1, 7, 8, 12, and 13 October1987. All measurements were made withleaflets oriented perpendicular to sun-light. The portion of the leaflet used forgas exchange measurements wasdelineated with a felt-tip pen. Leafletswere then detached and transported tothe laboratory in a cooler with ice forassessment of disease severity.

Determination of disease severity.Disease severity, based on the percentage

Plant Disease/March 1990 203

© 1990 The American Phytopathological Society

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Fig. 1. Influence of leaf blotch disease severity on (A) net CO2 assimilation, (B) leaf conductance to water vapor, (C) transpiration rate, (D)intercellular CO 2 concentration. (E) leaf chlorophyll concentration, and (F) water use efficiency of expanded summer-flush (n = 37) and expandedspring-flush (n = 17) Cape Fear pecan leaves.

204 Plant Disease/Vol. 74 No. 3

of leaf surface area occupied by a lesion,was determined on the portion of eachleaflet previously measured for gasexchange. Care was taken to eliminateleaflets that exhibited symptoms ofdiseases other than leaf blotch. Leafletsshowing uniform distribution of thedisease were selected whenever possible,although no attempt was made toquantify disease incidence outside this6.25-cm2 sector. Tissue affected by leafblotch was irregular in shape, and theamount of disease varied between theabaxial and adaxial leaflet surfaces. Toaccount for this disparity, two evaluatorsindependently estimated the percentageof disease on both sides of each leaflet,and the estimates for both surfaces wereaveraged. The difference in the diseaseestimates of the two evaluators wasgenerally within 5%.

Statistical analysis. Data wereanalyzed by regression, following generallinear model procedures. For eachcultivar, regression analysis was per-formed by treating disease severity as theindependent variable and measured phy-siological variables as dependentvariables. Dependent variables wereanalyzed in terms of a first-, second-, orthird-degree polynomial as appropriate.Cape Fear and Choctaw were analyzedseparately because the slopes of regres-sion lines for the two cultivars were oftensignificantly different (from analysis ofthe heterogeneity of slopes). Relation-ships between net CO2 assimilation andleaf conductance to CO2 or intercellularCO2 concentration were plotted bydisease severity and by leaf age; in thiscase, data for both cultivars wereanalyzed collectively because slopes didnot differ significantly (P < 0.05).

RESULTSLeaf tissue infected by M. dendroides

was delineated by olive green to darkbrown tufts of fungal hyphae in variousstages of coalescence on summer-flushleaves. Chlorotic patches were alsoapparent. Infected spring-flush leaveshad extensive dark brown necrotic areas.

Leaf blotch affected all measuredvariables of expanded spring- andsummer-flush leaves in a linear orcurvilinear manner (Fig. 1 and Table 1).Net CO2 assimilation rate was less than5 /.mol m-2 sect at a disease severityrating of 40% or greater (Fig. IA). Leafconductance to water vapor and tran-spiration rate were reduced considerablyless than net CO2 assimilation rate, whichresulted in low water use efficiency (Fig.1 and Table 2). A 40% disease severityrating of leaf blotch was associated with55 and 76% decreases in net CO2assimilation rate, 33 and 56% reductionsin leaf conductance to water vapor, 17and 38% reductions in transpiration rate,and 20 and 36% reductions in leafchlorophyll concentration for Cape Fearand Choctaw leaves, respectively, when

values for both leaf ages were averaged(Table 2).

Plots of net CO2 assimilation versusleaf conductance to CO2 showed that netCO2 assimilation rate was reducedslightly more than leaf conductance forboth leaf ages (i.e., the ratio of thedecrease in net CO2 assimilation rate tothe decrease in leaf conductance to CO2was 1.2:1) (Fig. 2A). Data from leafblotch-infected leaves are generallyrepresented by net CO2 assimilation ratesless than 12 and 7 ,tumol m-2 sect forexpanded summer-flush and expandedspring-flush leaves, respectively; inter-cellular CO2 concentration of infectedsummer- and spring-flush leaves wasgenerally greater than 210 and 240 ,molmold-, respectively. The negative slopeof equations of net CO2 assimilation rateversus intercellular CO2 for expandedsummer-flush and expanded spring-flushleaves (Fig. 2B) shows that leaf blotchinfection was associated with an increasein intercellular CO2 concentration.

DISCUSSIONPecan leaves affected by leaf blotch

often had a ratio of percentage ofdiseased surface area to percentagedecrease in net CO2 assimilation rategreater than 1:2. Low disease severityoften resulted in greatly reduced leaf gasexchange (Fig. IA and Table 1).

Expanded spring-flush Choctaw

leaves were particularly affected by leafblotch, as evidenced by the great declinein net CO2 assimilation rate and leafchlorophyll concentration (Tables 1 and2). Leaf blotch-infected leaves contri-buted little to whole-tree carbon gain,although they continued to use water ata moderate rate. For example, a 40% leafblotch disease severity rating translatedinto 55 and 76% reductions in net CO2assimilation rate for the two cultivars(Table 2). Other physiological variableswere affected much less (Fig. 1 andTables 1 and 2), and consequently wateruse efficiency was extremely low.

Stomatal aperture (i.e., proportionalto leaf conductance to water vapor orC0 2) and the tendency of mesophyll cellsto fix CO2 (net CO2 assimilation rate)were closely linked in this study. Thisphenomenon has been well documentedfor many plant species (5,17,19-21).Abscisic acid treatment (4), alteration innitrogen or phosphorus nutrition (20),and short-term changes in photon fluxdensity (21) often result in concomitantand similar percentage reductions in netCO2 assimilation rate and leaf con-ductance to CO2 such that intercellularCO2 concentration remains essentiallyconstant. Similarly, at moderate levelsof plant moisture stress, intercellular CO2concentration remains unaffected; how-ever, under severe moisture stressconditions, the mesophyll capacity to fix

Table 1. The relationship of physiological variables to severitya of leaf blotch for Choctawpecan

Dependent Leafvariable ageb Equation r2 pC

Net CO2 assimilation 1 13.29 - 0.64X + 0.0 13X 2 - 0.000075X3 0.77 0.00012 8.44 - 0.25X + 0.0021X 2 0.77 0.0001

Leaf conductance 1 295.95 - 12.97X + 0.29X 2 - 0.0018X3 0.67 0.00012 224.37 - 5.08X+ 0.051X2 0.63 0.0006

Transpiration rate 1 4.66 -0.036X 0.40 0.00012 4.40 - 0.092X+ 0.0011X

2 0.51 0.0041Intercellular CO2 1 215.03 + 2.08X- 0.09X2 0.59 0.0001

2 233.72 + l.90X- 0.014X2 0.79 0.0001Leaf chlorophyll 1 51.43 - 0.26X 0.24 0.0004

2 55.05 - l.OlX + O.0079X2 0.87 0.0001

Water use efficiency 1 2.79 -0.066X + 0.00065X 2 0.56 0.00012 1.90 - 0.023X 0.68 0.0001

aPercentage of infected leaf area.bl = Expanded summer-flush leaves (n = 48); 2 = expanded spring-flush leaves (n = 18).cProbability of significant slope.

Table 2. Percentage change in physiological variables associated with a pecan leaf blotch diseaseseverity rating of 40% on leaves of pecan cultivars Cape Fear and Choctawa

Cape Fear Choctaw

Expanded Expanded Expanded Expandedsummer-flush spring-flush summer-flush spring-flush

leaves leaves leaves leavesVariable (%) (%) (%) (%)

Net CO2 assimilation -63 -47 -72 -79Leaf conductance -43 -23 -58 -54Transpiration rate -20 -15 -31 -44Intercellular CO2 19 14 25 23Leaf chlorophyll -17 -23 -20 -51Water use efficiency -45 -43 -57 -48

aPercentage change was determined from the regression equations in Table 1 and Fig. 1. Thedisease severity rating of 40% is about half the maximum disease severity rating.

Plant Disease/March 1990 205

CO2 (i.e., mesophyll conductance) maybe reduced more than stomatal con-ductance, and hence intercellular CO2would tend to rise (10). The negative y-intercept of the plot of net CO2 assim-ilation rate versus leaf conductance toCO2 (Fig. 2A) indicates that net CO2assimilation rate was reduced to zerobefore complete stomatal closure.Intercellular CO2 concentration wasoften elevated to ambient CO2 partialpressure (Fig. 2B) with leaf blotchinfection. Our data show that a reductionin leaf conductance to CO2 cannotcompletely account for the observedreductions in net CO2 assimilation rate.Because intercellular CO2 was elevateddespite partial stomatal closure (Fig. IDand Table 2) and because leaf chlorophyllconcentration was substantially reduced(Fig. IE and Table 2), our data supportthe hypothesis that the photosyntheticapparatus was damaged in leaf blotch-infected leaves. We must invoke aresidual, nonstomatal component to beresponsible for the disproportionatedecrease in net CO2 assimilation rate.Interestingly, our previous work withscab-infected leaves showed that zero netCO2 assimilation and complete stomatalclosure occurred concomitantly (data notreported).

Direct damage to the photosyntheticapparatus may only partially explain thedisproportionate decline in net CO2assimilation rate. Leaf blotch tended tocover leaf surfaces to the extent that lightpenetration into the leaf interior wascertainly reduced. Net CO2 assimilationof pecan leaves approaches a maximumat relatively high photon flux densities(i.e., 1,200-1,500 gImol m-2 sec-') (P. C.

Andersen, unpublished). Becausephotosynthetic photon flux was 1,400-1,800 /Imol m-2 sect during leaf gas ex-change measurements, it is likely that leafblotch on the cuticle reduced lightpenetration into the chloroplasts ofmesophyll and palisade cells to levelsbelow light saturation. Similarly, Woodet al (23) reported that sooty mold(caused by Capnodium sp.) on pecanleaves inhibited net CO2 assimilation byup to 70% by preventing light penetrationinto the leaf. Respiration of the leafblotch pathogen itself would also con-tribute to a decrease in the reduction innet CO2 assimilation relative to thereduction in leaf conductance to watervapor.

Pecan trees have an exceptionally highlate-season energy demand. Maintaininga high level of photosynthetic activity inthe fall is important to facilitate kernelfilling and to provide assimilate reservesnecessary for future nut production. Apremature decline in photosyntheticefficiency or leaf defoliation is alsoknown to enhance the process of alter-nate bearing (16,24,25). We have shownthat leaf blotch has a profound effect onall physiological variables investigatedwhen measured under field conditions.Leaf blotch frequently covered morethan 50% of the total leaf surface, andphotosynthetic efficiency declinedgreatly. Retention of leaf blotch-infectedpecan leaves with a disease severity ratingabove about 40% may be undesirablegiven that they contribute little to whole-tree carbon gain yet continue to use waterat a moderate rate.

An abbreviated pecan disease controlprogram limited to 10 wk after nut set

(6) may be a viable management optionunder normal conditions but is definitelynot advisable when there is a prevalentsummer growth flush or when late-summer disease incidence is high. Asummer growth flush is most commonon young or rapidly growing trees or onbearing trees in an off-year in thealternate bearing cycle. The photo-synthetic capacity of healthy summer-flush leaves remains high until leafabscission (1). Thus, under theseconditions, it is desirable to suppress leafblotch and other pecan diseases duringlate summer.

ACKNOWLEDGMENTSWe extend our appreciation to Paul Bertrand

(University of Georgia, Tifton), the Division of PlantIndustry, Gainesville, FL, and the University ofFlorida, Cooperative Extension Service PlantDisease Clinic, Gainesville, FL, for assistance in theidentification of pecan diseases and to BrentBrodbeck for technical assistance.

LITERATURE CITED1. Andersen, P. C., and Brodbeck, B. V. 1988. Net

CO 2 assimilation and plant water relationscharacteristics of pecan growth flushes. J. Am.Soc. Hortic. Sci. 113:444-450.

2. Crane, H. L., Hardy, M. B., Loomis, N. H.,and Dodge, F. N. 1934. Effect of nut thinningon size, degree of filling, and annual yields ofpecan. Proc. Am. Soc. Hortic. Sci. 32:29-32.

3. Davis, J. T., and Sparks, D. 1974. Assimilationand translocation patterns of carbon-14 in theshoot of fruiting pecan trees, Carya illinoensisKoch. J. Am. Soc. Hortic. Sci. 99:468-480.

4. Dubbe, D. R., Farquhar, G. D., and Raschke,K. 1978. Effect of abscisic acid on the gain ofthe feedback loop involving carbon dioxide andstomata. Plant Physiol. 62:413-417.

5. Farquhar, G. D., Dubbe, D. R., and Raschke,K. 1978. Gain of the feedback loop involvingcarbon dioxide and stomata: Theory andmeasurement. Plant Physiol. 62:406-412.

6. Gottwald, T. R., and Bertrand, P. F. 1988.Effects of an abbreviated pecan disease controlprogram on pecan scab disease increase and crop

so, I

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Fig. 2. The relationship of net CO2 assimilation rate to (A) leaf conductance to CO2 and (B) intercellular CO2 concentration for expandedsummer-flush (n = 85) and expanded spring-flush (n = 35) pecan (Cape Fear and Choctaw) leaves infected with leaf blotch.

206 Plant Disease/Vol. 74 No. 3

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