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Xiaomei Cheng and Kendra Baumgartner

Indigenous arbuscular mycorrhizal (AM) fungal communities were characterized by examining spores in five fumigated and five nonfumigated vineyards in Northern California. None of the vineyards surveyed lacked spores, but species composition differed among the vineyards. Most of the fungi were in the genus Glomus; Paraglomus occultum Morton & Redecker, G. etunicatum Becker & Gerd., and G. aggregatum Schenck & Smith emend. Koske were the most common species identified. Fungal diversity was greater in nonfumigated than in fumigated vineyards. Field-propagated grapevine nursery stock was examined as a potential source of AM fungi for fumigated vineyards. We quantified fungal colonization of new roots initiated from field-grown benchgrafts and potted benchgrafts of Cabernet Sauvignon on three rootstocks (101-14, 110R, and St. George). After 7 months of growth in the greenhouse, new roots initiated from dormant roots of field-grown and potted benchgrafts were colonized by AM fungi. Mycorrhizal colonization of new roots of field-grown benchgrafts was significantly higher than that of potted benchgrafts. Our results suggest that field-propagated nursery stock can serve as a source of AM fungi and may be better suited for fumigated and/or low phosphorus soils than potted benchgrafts.

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Kendra Baumgartner, Phillip Fujiyoshi, Greg T. Browne, Chuck Leslie and Daniel A. Kluepfel

The most common rootstock for Juglans regia (Persian or “English” walnut) in California is Paradox, typically a hybrid of J. hindsii (Northern California black walnut) × J. regia. Unfortunately, Paradox is very susceptible to Armillaria root disease. The relative resistance to Armillaria mellea of six clonally propagated Paradox rootstocks (AX1, Px1, RR4 11A, RX1, Vlach, VX211) was evaluated and compared with that of clonally propagated J. hindsii rootstock selection W17, J. regia scion cultivar Chandler, and Pterocarya stenoptera (Chinese wingnut). In a growth-chamber assay, plants were micropropagated and rooted in vitro before inoculating the culture medium with A. mellea. At two months post-inoculation, the most resistant and susceptible Paradox rootstocks were AX1 and VX211, respectively, with 9% vs. 70% mortality, and this finding was consistent across three isolates of A. mellea and three replicate experiments. This broad range of resistance within Paradox is consistent with past field trials that tested other genotypes. Our finding of similarly high susceptibility of ‘Chandler’ and W17 (61% vs. 69% mortality) is in contrast to two field trials, in which other J. regia genotypes were more susceptible than those of J. hindsii. A third trial, however, identified some J. regia genotypes as more resistant than those of J. hindsii. Therefore, it is possible that W17, which was not previously tested, is an Armillaria-susceptible genotype of J. hindsii. Based on our findings of repeatable mortality levels across three isolates of A. mellea and three replicate experiments, the growth-chamber assay has promise, albeit with confirmed resistant and susceptible controls, for identifying putative resistant rootstocks (e.g., AX1) in preparation for a field trial with controlled inoculations.

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Kendra Baumgartner, Phillip Fujiyoshi, Craig Ledbetter, Roger Duncan and Daniel A. Kluepfel

Prunus dulcis (almond) is one of the most susceptible horticultural crops to Armillaria root disease. Resistance to Armillaria mellea and Armillaria tabescens, the geographically isolated causal fungi that attack almond and closely related Prunus persica (peach), has been evaluated in studies of almond, peach, and other Prunus rootstocks, but not in one comprehensive study. We evaluated the relative resistance to A. mellea and A. tabescens of six clonally propagated almond and peach rootstocks (Bright’s 5, Empyrean 1, Hansen 536, Krymsk 1, Krymsk 86, and Lovell) in comparison with that of clonally propagated Marianna 2624 rootstock (resistant control) and clonally propagated Nemaguard rootstock (susceptible control). Replicate clones used in the growth chamber assay were micropropagated and rooted in vitro before inoculating the culture medium with Armillaria spp. At 2 months, the most resistant and susceptible rootstocks were Krymsk 86 and Hansen 536, respectively, with 27% vs. 89% mortality. This finding was consistent among two isolates of A. mellea and one isolate of A. tabescens in three replicate experiments. Our finding of low mortality among Krymsk 86, Krymsk 1, and Marianna 2624, which all share Prunus cerasifera (Myrobalan plum) parentage, is consistent with past reports of resistance in the field to A. mellea, but conflicts with reports of susceptibility to A. tabescens. Resistance to A. tabescens of genotypes with Myrobalan plum parentage in our assay may reflect the simplified rooting environment of tissue culture medium, which does not perfectly mimic a field trial, in which biotic and abiotic factors may affect host resistance. Nonetheless, our growth chamber assay may provide a more rapid alternative to identify sources of resistance for breeding and to screen progeny of such crosses.