HORTSCIENCE 25(4):445-448. 1990. (W.R. Grace and Co., Allentown, Pa.). Magnesium sulfate was applied foliarly at Growing Conditions Influence 12.4 g·liter-1 on 9 Mar. Endosulfan (Thiodan 50W) and methomyl (Lannate 50W) were Damage on and Leaves applied as needed at 0.6 and 1.2 g a.i./liter, respectively, to control aphid and whitefly. Richard J. Campbell1, Kendrick N. Mobley1, and Carbaryl (Sevin 50W) was applied at 1.2 g Richard P. Marini2 a.i./liter to eliminate mite predators. A completely randomized design was used Department of Horticulture, Virginia Polytechnic Institute and State for each tree , with five trees per mite University, Blacksburg, VA 24061 treatment. One shoot per tree was allowed Additional index words. Malus domestica, Prunus persica, Tetranychus urticae, to develop, and a single leaf per tree was ulmi, net photosynthesis, transpiration tagged at unfolding to serve as a test leaf. On 11 Mar., each test leaf was randomly Abstract. The effect of growing conditions on the physiological damage caused by mite assigned as either a control or mite leaf, and feeding was evaluated. Control and twospotted (Tetranychus urticae Koch)- 50 adult twospotted spider (TSM) were infested ‘Imperial Delicious’ apple (Malus domestica Borkh.) and ‘Redhaven’ peach placed on each mite leaf. Mites were allowed [Prunus persica L. (Batsch.)] trees were grown under winter and summer greenhouse to develop naturally and were counted every conditions. Mite feeding reduced net photosynthesis (Pn) of apple in both winter and 3 days. Mite population densities were cal- summer, with equivalent regression models (Pn on mite days). Total chlorophyll con- culated as mite days per leaf (MD) by av- tent (TCHL) was decreased, and specific leaf weight (SLW) was increased by mite eraging the number of mites counted on two feeding in both winter and summer. In peach, Pn was also reduced by mite feeding, consecutive dates and multiplying by the with equivalent regression models in winter and summer. SLW was unaffected, and number of 24-hr periods elapsed between TCHL was reduced by mite feeding in both seasons. Transpiration (Tr) of apple and counts. MD quantifies both mite number and peach was inconsistently affected by mites in winter and summer. In another experi- feeding duration, where one mite feeding for ment, control and European red mite (ERM) [Panonychus ulmi (Koch)]-infested ‘Red- 10 days would be equivalent to 10 mites chief Delicious’ apple trees were grown in the summer under both greenhouse and field feeding for 1 day. conditions. ERM feeding negatively affected Pn in both the greenhouse and the field, Net photosynthesis (Pn) was measured in but reductions were less under field conditions. Mites reduced Tr 28% in the green- the laboratory with an Anarad model Ar-600 house and 12% in the field. TCHL, SLW, and leaf N content (LFN) were reduced by infrared gas analyzer at 0, 4, 8, and 16 days 16%, 24%, and 26%, respectively, in the greenhouse. In the field, TCHL was not after mite placement (DAMP) (Anarad, Santa significantly reduced, and SLW and LFN were reduced by 6% and 13%, respectively. Barbara, Calif.). The leaf chamber was a modification of the chamber described by Tetranychid mites are an important pest in haven’/’Halford’ seedling peach trees in 3.7- Syvertsen and Smith (1983), without the heat apple and peach orchards throughout the liter containers were removed from cold exchanger. Lighting was provided with 500R/ temperate zone. These mites reduce net pho- storage and placed in a white-washed (50% 3FL lamps (Westinghouse), and PPF was tosynthesis (Avery, 1964; Avery and Briggs, shade) fan- and pad-cooled glasshouse on 27 1050 µmol·s-1·m-2. The air flow rate into -1 1968; Hall and Ferree, 1975), transpiration Jan. 1988. Maximum temperature and pho- the chamber was 0.05 liters·sec . Air tem- (Ferree and Hall, 1981), total chlorophyll tosynthetic photon flux (PPF) during the perature in the chamber was maintained at content (Chapman et al., 1952; Zwick et al., winter experiment were 32C and 900 28 ± 1C and dew point was 16 ± 3C. Tran- 1976), and N content of leaves (Herbert and µmol·s-1·m-2, respectively. The medium spiration (Tr) was measured simultaneously Butler, 1973; Klopfenstein and Holdsworth, consisted of equal parts, by volume, of peat, with Pn with a General Eastern model 1100 1978). Much of the research on mite feeding perlite, and vermiculite. All trees were pruned dew point hygrometer (General Eastern In- damage has been conducted in greenhouses, to »5 cm above the bud union. Trees were struments, Watertown, Mass.). in both the winter and summer. To develop fertilized every 2 weeks with 200 mg N/liter At the conclusion of the experiment (6 meaningful recommendations for mite con- from a Peters 20 N-8P–15K soluble fertilizer Apr.), leaves were harvested, weighed, and trol in the field, the relationship between growing condition and mite feeding must be assessed. The objectives of this study were to compare mite feeding damage on green- house-grown apple and peach trees in the winter and summer and on field vs. green- house-grown apple trees. Winter (Expt. 1). Ten 1-year-old ‘Imperial Delicious’/MM.111 apple trees and 10 ‘Red-

Received for publication 24 May 1989. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regu- lations, this paper therefore must be hereby marked advertisement solely to indicate this fact. lGraduate Student. 2Associate Professor.

HORTSCIENCE, VOL. 25(4), APRIL 1990 445 consisting of a control and a mite treatment with 20 replications per treatment. On 3 July, ‘Delicious’ apple spurs infested with ERM were placed in the mite-treated trees. Control trees received cyhexatin applications every 2 weeks (beginning on 3 July) at 225 mg a.i./liter to control mites. Mite populations on the other trees were allowed to develop naturally. Mites (on infested leaves) were counted weekly, and mite populations were expressed as MD. Mites were killed with a cyhexatin application on 18 Aug. Pn and Tr were measured on sunny days (PPF > 1000 µmol·s-1·m-2) with an ADC portable in- frared gas analyzer and LI-COR steady state porometer. Pn and Tr were measured four times at weekly intervals beginning on 18 Aug. (after mite removal). Leaves were har- vested on 16 Sept. and LFN, fresh and dry weight, TCHL, and SLW were determined. Statistical analysis. Pn was regressed on MD for each experiment and species, and slopes and intercepts of the regression models were compared using indicator variables in multiple regression analysis (Montgomery and Peck, 1982). To account for day-to-day fluc- tuations in Pn during the experiment, Pn of each mite-treated leaf was expressed as a percentage of the mean Pn rate of the con- trols on each measurement date. TCHL, SLW, and LFN data for each experiment were ana- lyzed by analysis of variance. Experiment 1. Mite feeding reduced Pn of apple in both the winter and summer exper- iment, with statistically equivalent linear regression models (Fig. 1). Accumulation of 1200 MD reduced apple Pn by »40% for both winter- and summer-grown trees. Mite feeding also reduced Pn of peach in both the winter and summer experiment (equivalent their areas measured with a LI-3000 portable were counted every 3 days and population models), but the slopes of these models dif- leaf area meter (LI-COR, Lincoln, Neb.). densities were expressed as MD. Pn was fered significantly from the slopes for apple. Two leaf disks (1.2 cm2 total area) were measured with an ADC Model LCA-2 port- Accumulation of 1200 MD reduced peach Pn sampled from each side of the midrib, and able infrared gas analyzer (Analytical De- by »25%. Mite feeding had an inconsistent total leaf chlorophyll content (TCHL) was velopment Corp., Hoddesdon, U. K.) at 0, effect on Tr of both apple and peach (data determined by acetone extraction following 11, and 33 days after mite placement. Tran- not presented). the method of Arnon (1949), as modified by spiration was measured with a LI-COR Model Direct comparison of the apple and peach Marini (1986). Leaves were dried at 60C and LI-1600 steady state porometer. TCHL and SLW data between the winter and dry weights recorded. Specific leaf weight On 3 Sept., the leaves were harvested and summer conditions is made with caution due (SLW) was calculated by dividing dry weight fresh weight, TCHL, and dry weight were to the discrepancy in accumulated MD (Ta- by leaf area (corrected for disk removal). determined as previously described. Leaf N ble 1). Under winter conditions, TCHL of Summer (Expt. 1). Ten ‘Imperial Deli- (LFN) was determined by a modified Kjel- apple was reduced, and SLW increased with cious’/MM.111 apple trees and 10 ‘Redha- dahl analysis (Bremner and Breitenbeck, 1782 MD. In the summer experiment, TCHL ven’/’Halford’ seedling peach trees were 1983). was reduced, and SLW increased nonsig- removed from cold storage and placed in the Field Expt. 2). Forty 2-year-old ‘Red- nificantly with 2170 MD. Peach data under same glasshouse on 15 Apr. 1988. The air chief Delicious’/MM.111 trees growing on winter conditions were similar to that found maximum was 35C and PPF was 1000 the Virginia Tech Horticulture Farm, with apple, although differences were not µmol·s -1·m-2. The materials and methods Blacksburg, Va., were selected. Trees were significant. Under summer conditions, TCHL were identical to those described for the win- spaced 2 × 3 m. On 25 Feb. 1988, the trees was reduced, and SLW increased nonsig- ter experiment. The experiment was termi- were headed to »30 cm above the bud union. nificantly following 2500 MD. nated on 24 Aug. 1988. All but two uniform shoots were removed. Experiment 2. In the greenhouse, a neg- Greenhouse (Expt. 2). Eighteen 1-year-old On 15 May, one shoot leaf per tree was tagged ative linear relationship was found between ‘Redchief Delicious’/MM.111 trees were at unfolding to serve as a test leaf. Pn of apple leaves (percent of control) and planted as in Expt. 1 on 17 June 1987. The Endosulfan (Thiodan 50W) was applied as ERM MD (Pn = 100 – 0.024 MD, R2 = trees were pruned, fertilized, and maintained needed at 0.6 g a.i./liter to control aphids. 0.82). This model was statistically equiva- as previously described. The experimental Carbaryl (Sevin 50W) was applied as needed lent to the peach regression models with TSM, design was completely randomized with nine at 1.2 g a.i./liter to control Japanese beetle but not the apple regression models with TSM trees per mite treatment. and eliminate mite predators. On 23 May, in Expt. 1. Pn was reduced 50% and Tr 28% On 29 July, one leaf per tree was ran- cyhexatin (Plictran 50W) was applied at 225 following 2330 MD (Table 2). Since results domly assigned as either a control or mite mg a.i./liter to all trees to control mites until were similar for all measurement dates from leaf, and 40 adult European red mites (ERM) mite treatments commenced. the field experiment, only data for the final were placed on each mite test leaf. Mites A completely randomized design was used, measurement date are presented. In the field,

446 HORTSCIENCE, VOL. 25(4), APRIL 1990 3056 MD reduced Pn only 17% and Tr by peach leaves are more tolerant of mite feed- tion thresholds, experimental conditions 12%. ing than apple. Peach and apple leaves gen- should be considered and, ideally, based on TCHL, SLW, and, LFN were reduced by erally responded similarly to mite feeding, field experiments. 16%, 24%, and 26%, respectively, follow- but peach leaves were affected to a lesser ing 2330 MD in the greenhouse (Table 2). extent by similar white densities. Mobley Literature Cited In the field, there was a 6% and a 13% de- (1989) found that 900 TSM MD reduced ap- Arnon, D.I. 1949. Copper enzymes in isolated crease in SLW and LFN, respectively, fol- ple Pn 45%, whereas 3000 MD were re- chloroplasts. Polyphenoloxidase in Beta vul- lowing 3056 MD. TCHL was not significantly quired for an equivalent reduction in peach garis. Plant Physiol, 24:1-15. affected by mite feeding in the field. Pn. Accumulations of 8900 ERM MD on Avery, D.J. 1964. Carbon dioxide exchange by ERM is the most important mite pest in ‘Cresthaven’ peach trees did not affect shoot plum and apple leaves damaged by red Virginia apple and peach orchards. How- growth, and 3600 MD did not influence fruit spider mite. Rpt. East Mailing Res. Sta., Maid- ever, it was necessary to use TSM in the first quality (color, flesh firmness, soluble solids stone, England 1964:94–97. experiment due to the constraints of the ERM concentration, or size) (McClernan and Ma- Avery D.J. and J.B. Briggs. 1968. Damage to leaves caused by the fruit tree red spider mite, life cycle. ERM overwinter as eggs and rarely rini, 1986). Bailey (1979) found that TSM Panonychus ulmi (Koch). J. Hort. Sci. 43:463- hatch before early April, even in a warm populations of 40 to 50 mites/leaf had no 473. greenhouse. TSM have been used in many effect on yield of ‘Wright’ peach, but lesser Bailey, P. 1979. Effect of late season populations previous experiments on leaf physiological populations of ERM decreased ‘Delicious’ of two-spotted mite on yield of peach trees. J. damage and they reproduce year-round in the fruit weight and number (Klopfenstein and Econ. Entomol. 72:8-10. greenhouse. Holdsworth, 1978). Blair, C.A. 1951. Damage to apple leaves by the The reduced Pn of apple leaves due to Youngman et al. (1986) found Tetrany- fruit tree red spider mite, Metatetranychus ulmi mite feeding was similar to the results of chus spp. (TSM) more damaging than Pan- (Koch). Rpt. East Malling Res. Sta., Maid- previous studies. Hall and Ferree (1975) re- onychus spp. (ERM) on leaves, and stone, England 1950:152–154. Bremner, J.M. and G.A. Breitenbeck. 1983. A ported similar percentage decreases in Pn for Mobley (1989) found similar results with ap- simple method for the determination of am- ‘Delicious’ leaves with TSM. Ferree et al. ple. ERM feeding was generally less dam- monium in semimicro-Kjeldahl analysis of soils (1986) measured no clear Tr effect following aging to leaf physiology than TSM feeding and plant materials using a block digester. TSM feeding on winter-grown, ‘Delicious’ in our studies. ERM densities were greater Commun. Soil. Sci. Plant Anal. 14:905-913. leaves. In a related study, Tr of ‘Golden De- in the field than in the greenhouse. Physio- Campbell, R.J. and R.P. Marini. 1990. Nitrogen licious’ leaves was reduced by 135 TSM MD, logical responses were significantly reduced fertilization influences the physiology of apple but not at greater populations (Ferree and in the field, but to a lesser extent than in the leaves subjected to European red mite feeding. Hall, 1981). Tr of apple was generally re- greenhouse. Morphological differences be- J. Amer. Soc. Hort. Sci. 115:89-93. tween the greenhouse and field leaves may Chapman, P. J., S.E. Lienk, and O.F. Curtis, Jr. duced in our study by ERM feeding, but the 1952. Responses of apple trees to mite infes- results with TSM on apple and peach were explain these differences. Compared to tations: I. J. Econ. Entomol. 45:815-821. inconsistent. greenhouse-grown leaves, field-grown leaves Ferree, D.C. and F.R. Hall. 1981. Influence of Reductions in TCHL and LFN in apple were thicker, with nearly twice the SLW and physical stress on photosynthesis and transpir- leaves due to mite feeding are well docu- TCHL. Blair (1951) found that visible dam- ation of apple leaves. J. Amer. Soc. Hort. Sci. mented (Chapman et al., 1952; Klopfenstein age and nutrient depletion were less severe 106:348-351. and Holdsworth, 1978), and, in our study, in ERM-infested leaves with a thick palisade Ferree, D.C., F.R. Hall, and M.A. Ellis. 1986. reductions were measured under greenhouse parenchyma. Avery and Briggs (1968) found Influence of mites and diseases on net photo- conditions in both winter and summer. Mites that mites generally penetrate less than one synthesis of apple leaves, p. 56-62. In: A.N. remove cellular contents when feeding (Av- half of the thickness of the leaf during feed- Lakso and F. Lenz (eds.). The regulation of photosynthesis in fruit trees. Symp. Proc. Publ. ery and Briggs, 1968); therefore, decreases ing. Therefore, due to the physical limita- New York State Agr. Expt. Sta., Geneva. in SLW following ERM feeding in both the tions of mite stylet penetration, thicker leaves Hall, F.R. and D.C. Ferree. 1975. Influence of greenhouse- and field-grown leaves were not should sustain less damage. twospotted spider mite populations on photo- unexpected (Expt. 2). However, TSM feed- Our greenhouse results indicate that apple synthesis of apple leaves. J. Econ. Entomol. ing resulted in higher SLW (Expt. 1). This and peach trees respond similarly to mite 68:517-520. discrepancy may be partly due to differences feeding in winter or summer. However, there Herbert, H.J. and K.P. Butler. 1973. The effect in feeding patterns between the two mite were large differences in the physiological of European red mite, Panonychus ulmi (Acar- species, but SLW of apple leaves increased response to mite feeding in greenhouse- and ina: Tetranychidae), infestations on N, P, and upon ERM feeding in a previous greenhouse field-grown ‘Delicious’ leaves. These results K concentrations in apple foliage throughout the season. Can. Entomol. 105:263–269. study (Campbell and Marini, 1990). In- suggest that the physiological function (gas- Klopfenstein, W.G. and R.P. Holdsworth. 1978. creased SLW may be due to a wound re- exchange, LFN, and TCHL) of greenhouse- The effects of European red mite feeding on the sponse by the leaf induced by mite feeding. grown leaves is more susceptible to mite growth and yield of spur-type Delicious apple. The effect of mite feeding on SLW needs feeding than that of field-grown leaves, lead- Res. Circ. Ohio Agr. Res. Dev. Ctr. 239:35- more study. ing to the adoption of conservative control 37. Our results support the indications that levels for mites. In the development of ac- Marini, R.P. 1986. Do net gas exchange rates of

HORTSCIENCE, VOL. 25(4), APRIL 1990 447 green and red peach leaves differ? HortScience Univ., Blacksburg, Va. M.M. Barnes. 1986. Comparison of feeding 21:118-120. Montgomery, D.C. and E.A. Peck. 1982. Intro- damage caused by four tetranychid mite species McClernan, W.A. and R.P. Marini. 1986. Euro- duction to linear regression analysis. Wiley, New on gas-exchange rates of almond leaves. Envi. pean red mite on yield, fruit quality, and growth York. ron. Entomol. 15:190-193. of peach trees. HortScience 21:244-246. Zwick, R.W., G.J. Fields, and W.M. Mellenthin. Mobley, K.N. 1989. Gas exchange characteristics Syvertsen, J.P. and M.L. Smith. 1983. An inex- 1976. Effects of mite population density on of apple and peach leaves as influenced by Eu- pensive leaf chamber for measuring net gas ex- ‘Newtown’ and ‘Golden Delicious’ apple tree ropean red mite and twospotted spider mite. MS change. HortScience 18:700-701. performance. J. Amer. Soc. Hort. Sci. 101:123- Thesis, Virginia Polytechnic Inst. and State Youngman, R. R., V.P. Jones, S.C. Welter, and 125.

448 HORTSCIENCE, VOL. 25(4), APRIL 1990