Fire blight, caused by the bacterium Erwinia amylovora (Burrill) Winslow et al., is one of the most destructive diseases of plants in the Rosaceae subfamily Maloideae. Artificial inoculations, using E. amylovora strain E2002a, were conducted to determine levels of resistance to fire blight among taxa of flowering pears (Pyrus L. spp.) and quince (Chaenomeles Lindl. spp.). The level of resistance was measured as the length of the fire blight lesion as a percentage of overall shoot length. Considerable variation in resistance was observed among both pears and quince. Pyrus ussuriensis Maxim. `Prairie Gem' was highly resistant with a lesion length of 1% of the total shoot length. Pyrus calleryana Decne. `Bradford' was intermediate with a 50% lesion length while P. calleryana `Chanticleer' was significantly more resistant with a lesion length of 31%. Nine pear taxa were highly susceptible and did not differ significantly from 100% disease severity (total shoot death). Chaenomeles speciosa (Sweet) Nak. `Contorta' was highly resistant with a lesion length of 15%. Six quince taxa, including C. × superba (Frahm) Rehd. `Cameo', `Texas Scarlet', and `Jet Trail' were highly susceptible while nine other taxa showed intermediate resistance.
Andrew C. Bell, Thomas G. Ranney, Thomas A. Eaker, and Turner B. Sutton
Whitney D. Phillips, Thomas G. Ranney, Darren H. Touchell, and Thomas A. Eaker
Flowering pears are popular landscape plants due to a combination of desirable traits including broad adaptability, pest resistance, and attractive ornamental features. However, in some areas, flowering pears readily reseed and naturalize. Considering the value and utility of these trees, the development of infertile cultivars would be desirable. Breeding of triploid plants is one of the approaches that has been successfully used to develop seedless cultivars of many crops. The objective of this study was to evaluate female fertility and reproductive pathways of triploid flowering pear hybrids. Female fertility was characterized by evaluating fruit set, seeds per fruit, seed germination, seedlings per flower, and percent relative fertility [(seedlings per flower for triploid/seedlings per flower for diploid control) × 100]. Flow cytometry was used to determine relative genome sizes and ploidy levels of female parents, seedlings, and seeds (both embryo and endosperm) and to make inferences regarding reproductive pathways. Mean holoploid genome sizes were confirmed for the diploid [1.25 ± 0.05 (se) pg] and triploid [1.88 ± 0.12 (se) pg] female parents. Relative female fertility was significantly reduced in triploids, but varied considerably among accessions and ranged from 0.0% to 33.6%. Of the 13 triploids used in this study, five accessions had a relative fertility of <2%. One accession had no measurable female fertility. Cytometric analysis of seeds and seedlings from triploid maternal parents showed that they were predominantly abnormal aneuploids, which typically results in seedlings with reduced fitness and fertility. Fertilization with unreduced gametes, apomixis, and pseudogamy were documented in triploid-derived embryos/offspring, but were relatively uncommon. The considerable reduction in female fertility of some triploid selections, coupled with the limited production of primarily aneuploid progeny, provides desirable options for new infertile flowering pears to prevent or reduce reseeding and naturalizing.
Thomas G. Ranney, Thomas A. Eaker, Paul R. Fantz, and Clifford R. Parks
Franklinia alatamaha Bartr. ex Marshall represents a monotypic genus that was originally discovered in Georgia, but is now considered extinct in the wild and is maintained only in cultivation. Although Franklinia is very ornamental, with showy flowers and crimson/maroon fall foliage color, it tends to be short lived when grown as a landscape tree and is known to be susceptible to a variety of root pathogens. Schima argentea Pritz is an evergreen tree that is native to Asia and is valued for its glossy foliage, late-summer flowers, and broad adaptability in mild climates. Hybridization between these genera could potentially combine the cold hardiness and desirable ornamental characteristics of F. alatamaha with the greater adaptability, utility, and genetic diversity of S. argentea. Controlled crosses between F. alatamaha and S. argentea resulted in new intergeneric hybrid progeny. A morphological comparison of parents and the progeny is presented. ×Schimlinia floribunda Ranney and Fantz (mountain schimlinia) is proposed as the name for these hybrids and is validated with a Latin diagnosis.
Elisabeth M. Meyer, Thomas G. Ranney, Thomas A. Eaker, and Kelly Ivors
Trees in the Theaceae tribe Gordonieae are valuable nursery crops, but some of these taxa are known to be highly susceptible to root rot caused by Phytophthora cinnamomi Rands. The objective of this study was to evaluate a collection of Gordonieae taxa for resistance to this pathogen. These taxa included Franklinia alatamaha Bart. Ex Marshall, Gordonia lasianthus (L.) Ellis, Schima wallichii Choisy, S. khasiana Dyer, ×Schimlinia floribunda Ranney & Fantz, and ×Gordlinia grandiflora Ranney & Fantz. Abies fraseri (Pursh) Poir. was also included in the study as a positive control. Container-grown trees were inoculated with three isolates of P. cinnamomi and symptoms were rated over an 84-day period during the summer of 2008. Disease symptom ratings from 1 (healthy) to 4 (dead) were collected twice weekly and area under the disease progress curve (AUDPC) values were calculated. None of the S. khasiana or S. wallichii exhibited any root rot symptoms or mortality, whereas the remaining species showed symptoms of infection at varying levels over time. Symptoms in F. alatamaha and A. fraseri were apparent before other taxa, and mortality for both species reached 100% by the end of the experiment. Comparison of AUDPC values indicated that F. alatamaha was the most susceptible followed by A. fraseri. There was no significant difference in AUDPC among the more resistant taxa, including G. lasianthus, both Schima species, and the intergeneric hybrids. Values for AUDPC in the hybrid taxa were similar to their more resistant parental genus, indicating that resistance to P. cinnamomi is a partially dominant trait in these plants. These results further suggest the potential to breed improved hybrids of Gordonieae trees with substantial resistance to P. cinnamomi.