Screening <i>Prunus</i> genotypes with wild plum and almond parentage for resistance to Armillaria root disease

<i>Armillaria mellea</i> is the most virulent causal species of Armillaria root disease in California. Rootstocks widely planted in almond orchards, namely peach genotypes (e.g., 'Lovell'), are highly susceptible. Here we report on resistance of newly screened, diverse genotypes: plum hybrids '14-4', 'Magnus', and 'Rootpac R', complex hybrid 'Viking', and peach × almond hybrid 'SG 1'. The assay involved rooting clonal genotypes in semi-solid media, inoculating media with <i>A. mellea</i>, and assessing phenotypic traits (plant mortality, canopy necrosis, stem necrosis) weekly for 8 weeks. Effects of assay conditions on traits of non-inoculated plants were also monitored. A precise propagation schedule accommodated inoculations on different dates. Included were <i>Armillaria</i>-resistant controls 'Krymsk 86' and 'MP-29' (plum hybrids), susceptible control 'Lovell' (peach), and previously screened 'Hansen 536' (peach × almond hybrid) and 'Marianna 2624' (plum hybrid). Genotypes were compared at inflection points in trait development and when controls were significantly different (canopy and stem necroses at week 4, mortality at week 7). Based on consistent patterns among traits of inoculated plants, most resistant were '14-4', 'Hansen 536', 'Magnus', and 'SG 1' (statistically similar to 'Krymsk 86' and 'MP 29'). 'Rootpac R' and 'Viking' were most susceptible (statistically similar to 'Lovell'). However, high mortality, and canopy and stem necroses, of non-inoculated plants of 'Lovell', 'Rootpac R', and 'Viking' suggest these rootstocks were intolerant of the assay. 'Magnus', 'Marianna 2624', and 'SG 1' were statistically similar to resistant and/or susceptible controls, depending on the trait. Ongoing field trials will further evaluate performance of the rootstock genotypes.