593 HORTSCIENCE, VOL. 37(3), JUNE 2002 HORTSCIENCE 37(3):593–594. 2002. Screening of Strawberry Germplasm for Resistance to the Two-spotted Spider Mite S. Serçe and J.F. Hancock 1 Department of Horticulture, Michigan State University, East Lansing, MI 48824 Additional index words. Tetranychus urticae, germplasm evaluation, breeding, resistance, day-neutrality River-10) were studied. ‘Honeoye’, DHL 1336, and Montreal River-10 are short-day (SD) genotypes, whereas the rest are day-neutral (DN). More information about the F. virginiana genotypes can be found in Hancock et al. (2001a). Runners from these genotypes were rooted into pots under natural long days in a greenhouse at Michigan State Univ., East Lan- sing, in late Aug. 2000. Then, in late Oct. 2000, plants were moved to three growth chambers at 18 °C and 6-, 8-, and 10-h day lengths with ≈600, 450, and 360 μmol·s –1 ·m –2 photosyn- thetically active radiation (PAR), respectively. PAR was varied with day length so that the total energy received was equal in each cham- ber. Each treatment consisted of five plants of each genotype in a randomized complete-block design. The plants were held in the growth chambers for a total of 60 d; the infestation of TSM was noted ≈30 d after the plants were transferred to the growth chambers. When the experiment was terminated, geno- types were assigned a mite damage score (MDS) based on visual symptoms using a 0– 10 scale [0 = no visual symptoms, 10 = most mite damage (complete coverage of leaves with webbing)]. Three leaflets were randomly picked from each plant, and the number of mites (MNO) on their undersides was counted under a dissecting microscope. The surface area of these leaflets was measured and mite number per cm 2 leaf area (MNA) was calcu- lated. Analysis of variance tables were con- structed for all variables and least significant difference (LSD) was calculated for main ef- fects using the SAS program (SAS Institute, Cary, N.C.). Genotype was significant (P < 0.01) for all traits (Table 1). MDS was significantly corre- lated (P < 0.001) with both MNO (r = 0.40) and MNA (r = 0.35), and MNO and MNA were also positively correlated (r = 0.58). The means of MDS and MNA were pro- gressively reduced at 6-, 8-, and 10-h photope- riod treatments (Table 2). Mean MNO did not follow this trend exactly as mite numbers under 8 and 10 h were about the same, but numbers at 6 h were still highest. When Patterson et al. (1994) compared resistance of ‘Redchief’ and ‘Tribute’ to TSM under, 8- and 24-h day conditions, they also found the high- est mite numbers under the shorter day treat- ment (8 h) on both cultivars. Overall, the F. virginiana genotypes, Frederick-9 and Montreal River-10, proved most resistant to mite infestation (Table 2). Montreal River-10 was consistently in the lowest significance group for MDS, MNA and MNO, regardless of day length. Frederick-9 was in the lowest significance group for all the treatments except MDS under 8-h day lengths. The least resistance was generally found in the F. ×ananassa cultivars Ft. Laramie and Honeoye. The TSM resistance that we have identi- fied in F. virginiana may prove useful in strawberry breeding, as Montreal River-10 and Frederick-9 are considered among the most elite native genotypes and have already been incorporated into breeding populations (Hancock et al., 2001a, 2001b). Chaplin et al. (1968, 1970) and Barritt and Shanks (1981) have reported high heritability for TSM resis- tance, and a relatively high ratio of general to specific combining ability. We still need to test the durability of our resistance in the field; however, genotype resistance has often been shown to be stable across environments (Cary et al., 1995; Medina et al., 1999), and the laboratory rankings of resistance have in gen- eral agreed with field scores (Gimenez-Ferrer et al., 1993). Literature Cited Barritt, B.H. and C.H. Shanks. 1981. Parent selec- tion in breeding strawberries resistant to two- spotted spider mites. HortScience 16:323–234. Cary, N.C., C.H. Shanks, C.K. Chandler, E.D. Show, and P.P. Moore. 1995. Fragaria resistance to spider mites at three locations in the United States. HortScience 30:1068–1069. Chaplin, C.E., L.P. Stoltz, and J.G. Rodrigues. 1968. The inheritance of the resistance to the two- spotted mite Tetranychus urticae Koch in straw- berries. Proc. Amer. Soc. Hort. Sci. 92:376–380. Chaplin, C.E., L.P. Stoltz, and J.G. Rodrigues. 1970. Breeding behavior of mite-resistant strawber- ries. J. Amer. Soc. Hort. Sci. 95:330–333. Gimenez-Ferrer, R.M., J.C. Scheerens, and W.A. Erb. 1993. In-vitro screening of 76 strawberry cultivars for twospotted spider-mite resistance. HortScience 28:841–844. Received for publication for 30 Apr. 2001. Ac- cepted for publication 10 Sept. 2001. 1 To whom reprint requests should be addressed. Email: hancock@pilot.msu.edu Two-spotted spider mite (TSM; Tetranychus urticae Kock.) is a common strawberry pest that causes severe damage to strawberries. Mite populations in commercial fields reach a peak in late spring and decline rapidly in early summer after harvest (MacFarlane and Hepworth, 1994; Shanks and Doss, 1989). Mite damage occurs mainly in late spring and includes stomatal closure and reduced chloro- phyll content on the lower surface of leaflets (Sances et al., 1979), lowered photosynthesis, transpiration, and fruit size (Sances et al., 1981). Walsh et al. (1998) found an average of 25% yield reduction in TSM-treated plots. A number of studies have described ge- netic variation in TSM susceptibility (Hancock, 1999; Hancock et al., 1991). Most recently, Shanks and Moore (1995) studied mite resis- tance in genotypes of F. chiloensis (L.) Duch. and F. virginiana Duch., and F. ×ananassa L. cultivars and reported that while resistant F. ×ananassa exist, most are moderately to highly susceptible to TSM damage. Fragaria chiloensis genotypes had in general fewer mites than the F. ×ananassa cultivars, with CL-5 from California and a number of Chilean genotypes being most resistant. Some of the F. virginiana showed modest resistance, but none were superior to F. chiloensis genotypes. During growth chamber screens of the pho- toperiod requirements of five native F. virginiana genotypes and five F. ×ananassa genotypes, we were presented with an evenly spaced, natural infestation of TSM. As many of the genotypes had not been screened previ- ously for TSM resistance, we decided this was an opportunity to search for new sources of resistance to TSM. Herein, we report on the response of these genotypes to TSM infesta- tion as determined by both leaf damage symp- toms and mite population size under three day length treatments. Five native F. virginiana genotypes [(BT3, Frederick-9 (PI 612493), LH 50-4 (PI 612495), Montreal River-10 (PI 612497), RH 30 (PI 612499)], four F. ×ananassa cultivars (‘Fort Laramie’, ‘Ogallala’, ‘Tribute’, and ‘Honeoye’), and a F. ×ananassa x F. virginiana hybrid, DHL 1336 (‘Chandler’ x Montreal Table 1. Analysis of variance for two-spotted spider mite damage and population size on strawberries grown in growth chambers at 18 °C and 6-, 8-, and 10-h day lengths. Mite Mite Mite damage no./ no./ Source df score leaflet cm 2 leaf area Block (day length) 2 37.5 51.3 42.1 Genotype 27 18.2 * 63.3 * 42.1 * Error 120 1.5 11.8 7.3 * Significant at 0.01.