EFFECTS OF UNION MILD ETCH, A NEWLY RECOGNIZED DISORDER, ON ALMOND SCIONS GROWING ON MARIANNA-2624 ROOTSTOCK

Symptoms of union mild etch (UME) disease appeared on young trees of a lmond (Prunus dulcis) and consisted of light green to yellow drooped l eaves that detached prematurely from current-season shoots otherwise e xhibiting normal growth and development. Concomitantly, a mild etching was observed in the woody cylinder at the junction of the almond scio ns and the plum rootstock, Marianna 2624 (P. cerasifera x P. munsonian a). Affected trees were diagnosed in several orchards; one orchard had a 75% incidence and some trees died the same year they were diagnosed . However, many trees appeared to recover, with partial or complete la ck of symptoms. Transmission attempts by grafting bud chips and bark p atches to healthy almond/Marianna 2624 trees proved unsuccessful. Simi larly, assays for viral, phytoplasma, and root rot pathogens were nega tive. The causal agent of UME is currently unknown. Measurements of tr ee trunks and nut harvests over four seasons showed that UME-affected Mission trees had significantly lower yields and less vegetative growt h than healthy trees.