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Xiaohe Song and Zhanao Deng

Gerbera (Gerbera hybrida) is an important floricultural crop in the United States and worldwide. Powdery mildew (PM) caused by Podosphaera xanthii is the most common and destructive disease in gerbera production and landscape use. Gerbera breeding line UFGE 31-19 is one of the few sources of resistance to PM in gerbera and has contributed its resistance to new gerbera cultivars. To determine the mode of inheritance for PM resistance in UFGE 31-19, one of its PM-resistant (PM-R) progeny, UFGE 4033, was crossed with PM-susceptible (PM-S) cultivar, Sunburst Snow White, and their progeny were evaluated for PM severity. Distribution of PM severity ratings among the progeny was continuous but with two peaks, suggesting that the PM resistance in UFGE 4033 and UFGE 31-19 is a quantitative trait, likely controlled by major genes. Bulked segregant analysis (BSA) identified 17 molecular markers present in UFGE 4033 and the PM-R bulk but absent in ‘Sunburst Snow White’ and the PM-S bulk. Eleven of the molecular markers were mapped to one genetic linkage group, and two regions on this linkage group together explained 71.1% of the phenotypic (PM severity rating) variance in the segregating population. It was proposed that the two regions be named Rpx1 and Rpx2 (resistance to P. xanthii). Conidia of P. xanthii inoculated on the leaf surface of UFGE 4033 germinated, formed secondary germ tubes, and formed appressoria at high percentages, similar to those on the leaf surface of ‘Sunburst Snow White’. However, P. xanthii hyphae branched significantly less, were significantly shorter, and produced substantially fewer conidia on the leaf surface of UFGE 4033 and its PM-R progeny than on the leaf surface of ‘Sunburst Snow White’. These results should provide a sound foundation for use of UFGE 31-19 and progeny UFGE 4033 in gerbera disease resistance breeding and facilitate further investigation and understanding of the genetic bases of PM resistance in gerbera.

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Zhanao Deng and Brent Harbaugh

Caladiums (Caladium×hortulanum) are ornamental aroids often forced in containers or grown in the landscape for their colorful leaves. The aesthetic value of caladium plants is largely determined by their leaf characteristics. Caladium breeding can be traced back to the mid-1800s when Gregor Mendel conducted his plant hybridization experiments, but information on the inheritance of caladium traits has been rather scant. To understand the mode of inheritance for three typical leaf shapes and three main vein colors in caladium, controlled crosses were made among commercial cultivars and breeding lines, and segregation of leaf shape and/or main vein color in the progeny was analyzed. The observed segregation ratios indicated that a single locus with three alleles seemed to determine the main vein color in caladium. The white vein allele was dominant over the green vein allele, but recessive to the red vein allele, which was dominant over both white and green vein alleles. The three leaf shapes (fancy, lance, and strap) in caladium seemed to be controlled by two co-dominant alleles at one locus. Leaf shape segregation was skewed in some crosses, which might imply the existence of other factors involved in caladium leaf shape development. Chi-square tests revealed that leaf shape and main vein color were inherited independently in caladium.

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Zhanao Deng and Brent K. Harbaugh

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Zhanao Deng and Brent K. Harbaugh

The sporadic nature of inflorescence production and flower protogyny in caladium (Caladium ×hortulanum Birdsey) makes it desirable to store pollen and to rapidly assess its viability for cross-pollinations in breeding programs. This study was conducted to develop a procedure to determine caladium pollen viability and to use that procedure to evaluate the effect of short-term storage conditions on pollen viability. The sucrose level in the culture medium was found to have a significant impact on the in vitro germination of caladium pollen; a concentration of 6.8% was determined to be optimal for pollen germination. Caladium pollen lost viability within 1 day under room (24 °C) or freezing (-20 °C) temperatures, but could be stored at 4 °C for 2 to 4 days. Pollen stored at 4 °C produced successful pollinations. Data obtained from large-scale greenhouse pollinations supported use of this in vitro germination assay as a convenient way to evaluate caladium pollen viability (and fertility).

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Zhanao Deng and Brent K. Harbaugh

Caladium (Caladium ×hortulanum) leaves can be injured at air temperatures below 15.5 °C. This chilling sensitivity restricts the geographical use of caladiums in the landscape, and leads to higher fuel costs in greenhouse production of pot plants because warmer conditions have to be maintained. This study was conducted to develop procedures to evaluate differences among caladium cultivars for chilling sensitivity and to identify cultivars that might be resistant to chilling injury. The effects of two chilling temperatures (12.1 and 7.2 °C) and three durations (1, 3, and 5 days) on the severity of chilling injury were compared for three cultivars known to differ in their sensitivity to low temperatures. Exposure of detached mature leaves to 7.2 °C for 3 days allowed differentiation of cultivars' chilling sensitivity. Chilling injury appeared as dark necrotic patches at or near leaf tips and along margins, as early as 1 day after chilling. Chilling injury became more widespread over a 13-day period, and the best window for evaluating cultivar differences was 9 to 13 days after chilling. Significant differences in chilling sensitivity existed among 16 cultivars. Three cultivars, `Florida Red Ruffles', `Marie Moir', and `Miss Muffet', were resistant to chilling injury. These cultivars could serve as parents for caladium cold-tolerance breeding, and this breeding effort could result in reduced chilling injury in greenhouse production of potted plants, or in new cultivars for regions where chilling occurs during the growing season.