ISHS Acta Horticulturae 705: V International Walnut Symposium
EVALUATION OF RESISTANCE TO PHYTOPHTHORA CITRICOLA AMONG DIVERSE CLONES OF PARADOX HYBRID ROOTSTOCKS |
| Authors: | G.T. Browne, S.T. McLaughlin, W.P. Hacket, G.H. McGranahan, C.A. Leslie |
| Keywords: | J. california, J. hindsii, J. major, J. microcarpa, J. nigra, Juglans regia × J. hindsii, Phytophthora crown rot, Phytophthora citricola, resistance, rootstock |
| Abstract:
In two 2004 experiments, 10 clones of diverse walnut hybrids from black (maternal, M) and English (paternal) parents were evaluated for resistance to Phytophthora citricola, a virulent and prevalent crown and root pathogen affecting California walnuts. The clones were selected from seed families for one or more traits, including putative tolerance to P. citricola, Cherry Leaf Roll Virus, or Pratylenchus vulnus; or unique parentage. The selections were propagated as microshoots, rooted, acclimatized and grown in a greenhouse in 4 x 20 cm cones (3 months), subjected to dormant chilling (6 °C, 3 to 5 months), and allowed to resume growth in a greenhouse (2 to 3 months) before inoculation with P. citricola. The plants were transferred from the cones into 2-liter pots filled with non-infested soil (6 plants per clone) or soil artificially infested with P. citricola (12 per clone). Starting 2 weeks after transplanting, all plants received biweekly 48-h periods of soil flooding. Three months after transplanting, each plant was washed free from soil and rated for severity of root and crown rot. Severity of crown rot was affected by interactions of inoculation × clone (P<0.0001) and experiment × inoculation × clone (P=0.05). Across experiments, non-inoculated controls developed little disease (means 0 to 5% crown length rotted), but inoculated clones developed moderate to severe crown rot. Highly susceptible clones (means 52 to 90% of crown length rotted) included AX-1 (M=J. californica); GZ-1 and PX-1 (M=J. hindsii); and WIP3 (M=J. hindsii × regia). Moderately tolerant clones (20 to 43% crown length rotted) included AZ-2, AZ-3, and NZ-1 (M=[J. major × hindsii] × nigra); JX-2 and VX-211 (M=J. hindsii); and RX-1 (M=J. microcarpa). The results, obtained under conditions highly conducive to the disease, suggest that the moderately tolerant clones have potential as improved Paradox rootstocks in orchards infested with P. citricola. | |
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