Resistance to Phytophthora and Graft Compatibility with Persian Walnut among Selections of Chinese Wingnut

Seedlings from seven open-pollinated selections of Chinese wingnut (Pterocarya stenoptera) (WN) representing collections of the USDA-ARS National Clonal Germplasm Repository at Davis, CA, and the University of California at Davis were evaluated as rootstocks for resistance to Phytophthora cinnamomi and P. citricola and graft compatibility with scions of five cultivars of Persian walnut (Juglans regia). Seedlings of Northern California black walnut (NCB) (J. hindsii) and Paradox hybrid (PH) (typically J. hindsii × J. regia) were used as standards. In greenhouse experiments, potted plants of the rootstocks were subjected to intermittent flooding in soil artificially infested with the pathogens. All WN seedlings were relatively resistant to the pathogens (means of 0% to 36% of root and crown length rotted) compared with NCB (44% to 100%) and PH seedlings (11% to 100%). Negligible disease occurred in flooded control soil without the pathogens. In 9-year graft compatibility trials in an orchard, NCB and PH rootstocks supported relatively good survival and growth of all tested scion cultivars (‘Chandler’, ‘Hartley’, ‘Serr’, ‘Tulare’, and ‘Vina’; final scion survival 80% to 100%, mean scion circumference increase 292 to 541 mm), whereas results with WN were mixed. Wingnut rootstocks from all sources were incompatible with ‘Chandler’ (final scion survival 20% to 60%, scion circumference increase 17 to 168 mm). Conversely, all WN rootstocks from all sources were compatible with ‘Tulare’ and ‘Vina’ (final scion survival 80% to 100%, scion circumference increase 274 to 556 mm). Use of the WN rootstocks produced variable results in ‘Hartley’ and ‘Serr’ (final scion survival 10% to 100%, mean scion circumference increase 69 to 542 mm). There was a tendency for more rootstock sprouts on WN selections than on NCB or PH. In a commercial walnut orchard infested with P. cinnamomi, ‘Hartley’ survived and grew markedly better on WN selections than on PH. High resistance to P. cinnamomi and P. citricola was common to all of the WN selections. The results indicate that WN selections may be useful rootstocks for cultivars Tulare and Vina in soils infested with P. cinnamomi or P. citricola and that WN selections may contribute valuable resistance to these pathogens in walnut rootstock breeding efforts.

First Report of Witches'-Broom Disease of Chinese Wingnut in China and its Association with a Phytoplasma of Aster Yellows Group (16SrI)

Pterocarya stenoptera C. DC., commonly known as Chinese wingnut, is a fast-growing deciduous tree with tough bark and attractive foliage. Because of its tolerance of compact and nutritionally poor soil, drought, and heat, Chinese wingnut is an important component of the biological diversity in natural ecosystems and is a favorable shade tree in China. Chinese wingnut has also been used as a rootstock for walnuts because of its high resistance to soilborne Phytophthora spp. In the spring of 2004, a disease characterized by witches'-broom symptoms was observed affecting Chinese wingnut trees growing in suburban Taian, Shandong, China. The diseased trees developed dense clusters of highly proliferating branches with shortened internodes, leaves on the affected branches were significantly smaller, and some branches and twigs suffered dieback. Phytoplasma infection was suspected as the cause of this Chinese wingnut witches'-broom (CWWB) disease because the disease occurred in an area where phytoplasmal diseases, such as paulownia witches'-broom (PaWB) and jujube witches'-broom (JWB), are common (3). Nested polymerase chain reactions (PCR) were performed on DNA samples extracted from leaves of six diseased trees using phytoplasma-universal 16S rDNA primers (R16mF2/R16mR1 and R16F2n/ R16R2) (1,2). Results revealed that all diseased trees examined were infected by phytoplasma, whereas PCR assays of leaf samples from two nearby symptomless Chinese wingnut trees were negative. Subsequent restriction fragment length polymorphism analysis of the PCR-amplified 16S rDNA indicated that all diseased trees contained the same phytoplasma and that the CWWB phytoplasma belongs to subgroup B of the “Candidatus Phytoplasma asteris” (AY) group (16SrI). Nucleotide sequence analysis of a 16S rRNA gene cloned from CWWB phytoplasma (GenBank Accession No. AY831966) suggested that this phytoplasma is closely related to, but distinct from, PaWB phytoplasma, another member of group16SrI. To our knowledge, this is the first report of Chinese wingnut witches'-broom disease and of its association with a phytoplasma. Further work is being undertaken to examine the ecological and evolutionary relationship between CWWB phytoplasma and other phytoplasmas in the region and to assess the impact of CWWB on walnut rootstock selection. References: (1) D. E. Gundersen and I.-M. Lee. Phytopathol. Mediterr. 35:144, 1996. (2) C. D. Smart et al. Appl. Environ. Microbiol. 62:2988, 1996. (3) S. Zhu et al. Acta Hortic. 472:701, 1998.