SummaryYoung almond (Prunus dulcis, cvs Carmel, Peerless and Price) orchards established on the plum rootstock Marianna 2624 (P. cerasifera×P. munsoniana) contained trees that exhibited poor terminal shoot growth and wilted, chlorotic leaves. The scion/rootstock graft union showed an external splitting of the bark and an internal line of necrotic bark tissues that extended into the woody cylinder of the union, which was deeply pitted. Affected trees declined. The disease was named almond brown line and decline (ABLD). Incidence of ABLD ranged up to 55% per cultivar in some orchards. Numerous attempts to graft‐transmit orchard collections of ABLD to healthy almond/Marianna 2624 indicators failed. Also, ABLD does not appear to be soil‐borne. However, ABLD was serendipitiously determined to be bud‐perpetuated when infected scion buds from an apparently healthy appearing Peerless almond/peach tree located in a foundation orchard were grafted onto healthy rooted cuttings of Marianna 2624 to produce yearling trees. Also, graft‐inoculations on the almond scion portion of healthy trees, but not the plum rootstock portion, with the peach yellow leafroll mycoplasma‐like organism (PYLR‐MLO) caused symptoms resembling ABLD. Laboratory and glasshouse assays of several symptomatic trees did not detect tomato ringspot virus and two ilarviruses. These results suggest that an MLO, possibly PYLR‐MLO, may be the causal agent of ABLD and that Marianna 2624 is probably resistant t