Evaluating Paradox Walnut Rootstocks for Resistance to Armillaria Root Disease

Abstract

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) 3 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.

Comparative genomics of six Juglans species reveals disease‐associated gene family contractions

“…In California, the cultivation of J. regia is most commonly facilitated using Paradox rootstock ( J. hindsii ♀ × J. regia ♂), which is valued for its resistance to soil‐borne pathogens (Potter et al , ; Browne et al , ). Despite improved resistance to several diseases, Paradox rootstock remain susceptible to Armillaria root rot, which is caused by a basidiomycete, A. mellea (Baumgartner et al , ). The impact of this disease is worsened by the lack of post‐infection controls.…”

The good, the bad and the tasty: The many roles of mushrooms

“…), apple ( Malus spp.) and highbush blueberry ( Vaccinium corymbosum ) (Baumgartner and Rizzo, 2001, Prodorutti et al., 2009, Thomidis and Exadaktylou, 2012, Baumgartner et al., 2013, Elías-Román et al., 2013). Losses can be substantial: 10 % in infected pear ( Pyrus spp.)…”

A faster inoculation assay forArmillariausing herbaceous plants

“…have used herbaceous plants that are quick to establish and easy to manipulate as an alternative to woody species in long-term trials or use of in vitro approaches that require laborious tissue culture methods. This assay can be completed in 3 months a substantial reduction in experimental duration in comparison to traditional inoculation assays which may last for several years (Cleary et al, 2013;Mallett & Hiratsuka, 1988;Shaw et al, 1981;Tsopelas & Tjamos, 1997) and comparable with in vitro infection assays (Baumgartner et al, 2010(Baumgartner et al, , 2013. Moreover, our assay is conducted in a controlled environment, which will avoid fluctuating environmental conditions that can introduce further variability to experiments and will also allow the use of transgenic strainssomething that would be problematic in an open field trial.…”

“…Another recent study has also endeavoured to use agar as Armillaria inocula without success (Sitienei, Wagara, Kariuki, & Jefwa, 2015). Agar plugs have been used previously in Armillaria inoculation assays, but only in vitro (Baumgartner et al, 2010(Baumgartner et al, , 2013 or in the inoculation of cassava tubers where mycelial agar plugs were inserted into wounds that were subsequently sealed to prevent desiccation (Mwenje et al, 1998). Since desiccation of inocula is common in Armillaria inoculation assays, and smaller sized inocula is more prone to drying out than larger inocula (Baumgartner et al, 2010;Perez-Sierra & Gorton, 2005), agar plugs seem inappropriate for use as soil inocula unless protected from desiccation.…”

“…Furthermore, the conventional assays are extremely slow: preparation of woody inocula usually takes 3 to 6 months, the host plants require years to establish prior to inoculation, and then assessment of disease often lasts several years (Cleary et al, 2013;Morrison, 2004;Shaw, MacKenzie, Toes, & Hood, 1981;Solla et al, 2011). Inoculation assays using alternative hosts or other types of inocula such as rhizomorphs or wood chips (Holdenrieder, 1987;Pellegrini et al, 2014;Perez-Sierra & Gorton, 2005) are not well established and whilst the available in vitro assays are useful in that they are faster (Baumgartner, Bhat, & Fujiyoshi, 2010;Baumgartner, Fujiyoshi, Browne, Leslie, & Kluepfel, 2013;Nogales, Camprubí, Estaún, Marfà, & Calvet, 2010), they involve the preparation of tissue cultured material and substantial microscopy and molecular work. For these reasons, a faster inoculation assay that could be conducted under controlled conditions requires limited preparation and permits a simple assessment of infection would be desirable.…”