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James R. McKenna and Lynn Epstein

Crown gall, caused by the common soil-borne bacterium Agrobacterium tumefaciens, can be an economic problem in walnut nurseries and production orchards in California. The principal rootstocks used for commercial walnut production in California are the native Northern California black walnut, Juglans hindsii, and “Paradox,” which are interspecific hybrids between a black walnut, primarily J. hindsii, as the maternal parent, and J. regia, the English walnut, as the paternal parent. Recent evidence has shown that some commercial black walnut trees producing Paradox hybrid seedlings are actually hybrids between J. hindsii and two other North American black walnut species, J. major and J. nigra. Here, we document that there was a higher incidence of crown gall on Paradox (J. hindsii ×J. regia) than on J. hindsii in three sites with natural soil inoculum. Paradox seedlings (with a female parent that was primarily J. hindsii with some J. nigra) inoculated with A. tumefaciens on the roots during transplanting had a higher incidence of crown gall than either J. hindsii or J. regia. When stems were inoculated with A. tumefaciens, J. hindsii ×J. regia populations had significantly larger galls than either J. hindsii or J. regia. Similarly, in stem inoculations on four out of six Paradox genotypes with a hybrid black walnut maternal parent, the progeny produced significantly larger galls than either J. hindsii or J. regia. However, two Paradox populations from black walnut hybrids that contained J. major, J. nigra, and J. hindsii produced galls that were no different in size than in the black walnut species and J. regia. Results suggest that J. regia and black walnut species are less susceptible to crown gall than most Paradox populations.

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Oleg Daugovish, Mark Bolda, Sukhwinder Kaur, Maren J. Mochizuki, Daniel Marcum, and Lynn Epstein

Strawberry anthracnose caused by Colletotrichum acutatum is often asymptomatic on plants in sprinkler-irrigated nurseries but destructive after transplantation into fruiting fields. This study evaluated the impact of strawberry nursery sprinkler and drip irrigation in the presence or absence of C. acutatum on post-transplantation plant growth, mortality, and fruit yield in fruiting fields in California. In a 2005 nursery at Tulelake, CA, dip infestation of mother plants with C. acutatum reduced early-season mother plant canopy size by 21% and delayed runner production in mother plants but otherwise had no obvious disease symptoms. In comparison with sprinkler-irrigated treatments in the nursery, drip irrigation of infested nursery plots reduced plant losses in fruit production fields by 86% at Watsonville, CA, in 2005 and 50% and 75% at Oxnard, CA, in 2005 and 2008. Transplants from infested nursery treatments had a 33% to 60% smaller canopy and 11% to 42% lower yield than transplants from uninfested nursery treatments. However, transplant canopy size and yield from the infested and then drip-irrigated nursery treatment were similar to the sprinkler-irrigated, non-infested nursery treatment. Quantitative polymerase chain reaction measurements of C. acutatum in crown tissue of fruiting field plants seven weeks after transplanting showed significantly more C. acutatum (≈11×) in their crowns in sprinkler-irrigated than drip-irrigated, infested nursery transplant treatments. During the course of fruit production, the amount of C. acutatum in crown tissue increased in all treatments. However, at the end of the fruit season, there was still significantly more (≈8×) C. acutatum in the crowns of the plants produced by sprinkler irrigation than by drip irrigation in the nursery. These data suggest that if C. acutatum is present in the nursery, drip irrigation can reduce subsequent plant stunting and yield losses in strawberry production fields.