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 Lansing, 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·m photosynthetically active radiation (PAR), respectively. PAR was varied with day length so that the total energy received was equal in each chamber. 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, genotypes 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 leaf area (MNA) was calculated. Analysis of variance tables were constructed for all variables and least significant difference (LSD) was calculated for main effects using the SAS program (SAS Institute, Cary, N.C.). Genotype was significant (P < 0.01) for all traits (Table 1). MDS was significantly correlated (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 progressively reduced at 6-, 8-, and 10-h photoperiod 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, 8and 24-h day conditions, they also found the highest mite numbers under the shorter day treatment (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 identified 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 resistance, 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 general agreed with field scores (Gimenez-Ferrer et al., 1993).
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Serçe et al. (2002) studied this question.