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Esmaeil Fallahi, William S. Conway, K.D. Hickey and Carl E. Sams

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J.M. Sherman, K.S. Reddy, S.E. Newman and J.A. Spencer

The objective of this study was to determine whether tissue culture can be used for studying the blackspot resistance found in some roses. Callus was initiated from leaves, petioles, and stems of resistant and susceptible genotypes. Good callus formation for susceptible roses (hybrid teas) was obtained on a medium containing MS basal salts, vitamins, sucrose, and 8 g/L agar supplemented with 2 mg/L 2,4-D, 1 mg/L NAA, and 0.2 mg/L BA. Callus formation for resistant roses (species roses) was best when the concentrate ions of growth regulators in the medium were halved. Browning in species rose cultures, was decreased with the addition of 0.05 g/L ascorbic acid to the medium followed by incubation in the dark. The subculture calli were inoculated with the fungal conidia and were analyzed for proteins by SDS-PAGE. These protein profiles were compared to those of whole leaf samples. The results are discussed in terms of similarities and differences in the biochemical responses of callus cultures versus whole leaves to the blackspot infection.

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Eliezer S. Louzada, Jude W. Grosseti, Frederick G. Gmitter Jr., Beatriz Nielsen, J.L. Chandler, Xiu Xin Deng and Nicasio Tusa

Protoplast culture following polyethylene glycol-induced fusion resulted in the regeneration of vigorous tetraploid somatic hybrid plants from eight complementary parental rootstock combinations: Citrus reticulata Blanco (Cleopatra mandarin) + C. aurantium L. (sour orange), C. reticulata (Cleopatra mandarin) + C. jambhiri Lush (rough lemon), C. reticulata (Cleopatra mandarin) + C. volkameriana Ten. & Pasq. (Volkamer lemon), C. reticulata (Cleopatra mandarin) + C. limonia Osb. (Rang-pur), C. sinensis (L.) Osb. (Hamlin sweet orange) + C. limonia (Rangpur), C. aurantium (sour orange) + C. volkameriana (Volkamer lemon) zygotic seedling, C. auruntium hybrid (Smooth Flat Seville) + C. jambhiri (rough lemon), and C. sinensis (Valencia sweet orange) + Carrizo citrange [C. paradisi Macf. × Poncirus trifoliata (L.) Raf.]. Diploid plants were regenerated from nonfused callus-derived protoplasts of Valencia sweet orange and Smooth Flat Seville and from nonfused leaf protoplasts of sour orange, Rangpur, rough lemon, and Volkamer lemon. Regenerated plants were classified according to leaf morphology, chromosome number, and leaf isozyme profiles. All somatic hybrid plants were tetraploid (2n = 4× = 36). One autotetraploid plant of the Volkamer lemon zygotic was recovered, apparently resulting from a homokaryotic fusion. These eight new citrus somatic hybrids have been propagated and entered into field trials.

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Zeynel Dalkilic, L.W. Timmer and Frederick G. Gmitter Jr.

The inheritance of resistance to a host-specific isolate (Shinn) of Alternaria alternata (Fr.:Fr.) Keissl. from `Minneola' tangelo (a cross between Citrus paradisi Macf. `Duncan' and C. reticulata Blanco `Dancy') was shown to be controlled by a single recessive allele, aaM1, within the citrus genome. A backcross between resistant `Clementine' mandarin (C. reticulata) and susceptible LB#8-10 (a hybrid of `Clementine' mandarin and `Minneola' tangelo) resulted in 61 resistant (R) and 58 susceptible (S) plants (χ2 = 0.0756, P ≥ 0.05), but the reciprocal cross deviated from the expected 1R:1S ratio (87 R and 36 S plants (χ2 = 21.1463, P ≥ 0.05). A dominant allele, AaM1, of this resistance gene was found in a loose coupling phase linkage with two RAPD markers, P12850 (15.3 cM) and AL31250 (36.7 cM), after JOINMAP computer analysis.

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Esmaeil Fallahi, William S. Conway, Kenneth D. Hickey and Carl E. Sams

In several experiments, strong negative correlations were found between fruit and leaf N vs. fruit color and fruit N vs. firmness, but a positive correlation existed between fruit Ca vs. firmness in apples. Based on these relationships, several models were developed to predict postharvest quality using preharvest nutrient status. Quantity and timing of N application to produce optimum-quality fruit in `Delicious', `Fuji', and `Gala' apples have also been investigated. High levels of nitrogen adversely affected fruit quality and increased endogenous ethylene and respiration. In separate experiments, the effects of seven post-bloom CaCl2 applications on various postharvest pathogens were studied in four apple cultivars. Calcium applications did not increase fruit Ca sufficiently enough to reduce colonization or maintain firmness after 4 months of 0C storage, but did slightly reduce infection by these pathogens during the growing season.

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K.S. Reddy, S.E. Newman, J.A. Spencer and R.N. Paul

Blackspot disease, caused by Diplocarpon rosae, is a devastating disease of garden roses. Most hybrid teas and floribundas are susceptible to this disease in contrast to many species roses, which are resistant. The basis of this resistance is not known. The first barrier to invasion by the pathogen is the outer surface of the leaf. The physical nature of this surface may influence the attempted infection, landing, germination and penetration by the fungal spore and may cause a failure of infection. The leaf surfaces of susceptible and resistant genotypes were observed using SEM that allowed examination of the fine structure of the leaf surface. The characteristics of the leaf surface topography including wax structures were pictorially compared and visual concepts developed in relation to the dynamic nature of the leaf surface in space and time as leaf is infected by the pathogen.