Abstract
Armillaria and Desarmillaria spp. are causal agents of a devastating root-borne disease of peach. Breeding resistant rootstock requires a reliable screening tool. An in vitro co-culture screen designed for almond was modified by replacing agar-gelled medium with a more aerated phenolic foam and combining resistant and susceptible rootstocks (i.e., common garden experiment) and minimizes variation in inoculum pressure or rooting substrate among replicate vessels. Eight Prunus rootstocks tested (peach, plum, peach × plum, and choke cherry) were rooted and had no decline in health. Susceptible peach rootstock, ‘GF 305’, was cultured for 15 wk in phenolic foam in the same vessel with a resistant peach × plum hybrid, ‘MP-29’, inoculated with Armillaria mellea at week 5, that led to more severe shoot symptoms in the former after an additional 8 wk. This method accommodated peach genotypes that were difficult to root in agar medium. The difference during a uniform challenge with the A. mellea fungus recapitulates resistant/susceptible reactions. The phenolic foam-based co-culture method will work on many Prunus spp. of potential use in rootstock breeding.
Highlights
Armillaria root rot (ARR) is currently the greatest threat to peach production in the southeastern USA, with devastating impacts on cherries in Michigan, and sporadic occurrences on peaches and almond in California
This project had two experiments: first, a preliminary experiment observed rooting of a range of rootstocks on the Oasis® IVE in a common environment, and second was the inoculation experiment of two rootstocks, resistant MP-29 and susceptible GF305 in a common garden experiment with A. mellea
The amount of rooting was described on a qualitative scale because quantitatively removing roots from the phenolic foam would cause excessive root loss
Summary
Armillaria root rot (ARR) is currently the greatest threat to peach production in the southeastern USA, with devastating impacts on cherries in Michigan, and sporadic occurrences on peaches and almond in California. This study included rooting of peach and several other Prunus selections that are relevant to ARR resistance for the southeastern USA in a common rooting medium. The goal of this experiment was to advance the methodology suggested by Baumgartner et al (2018) by (1) eliminating rouging of large numbers of plants prior to inoculation, (2) allowing root growth in a more aerated medium for better root development, (3) growing resistant and susceptible plants in the same vessel exposed to similar fungal pressure, and (4) confirming symptom development in known susceptible rootstock in a time course to establish a correct timing for assessment
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