Effects of Union Mild Etch, a Newly Recognized Disorder, on Almond Scions Growing on Marianna 2624 Rootstock. J. K. Uyemoto, USDA-ARS, University of California, Davis 95616. J. H. Connell, University of California Cooperative Extension, Oroville 95965; and C. A. Greer, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 80:1406-1408. Accepted for publication 17 September 1996. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1996. DOI: 10.1094/PD-80-1406.

Symptoms of union mild etch (UME) disease appeared on young trees of almond (Prunus dulcis) and consisted of light green to yellow drooped leaves that detached prematurely from current-season shoots otherwise exhibiting normal growth and development. Concomitantly, a mild etching was observed in the woody cylinder at the junction of the almond scions and the plum rootstock, Marianna 2624 (P. cerasifera x P. munsoniana). 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 lack of symptoms. Transmission attempts by grafting bud chips and bark patches to healthy almond/Marianna 2624 trees proved unsuccessful. Similarly, assays for viral, phytoplasma, and root rot pathogens were negative. The causal agent of UME is currently unknown. Measurements of tree trunks and nut harvests over four seasons showed that UME-affected Mission trees had significantly lower yields and less vegetative growth than healthy trees.

Keyword(s): tree size, union disorder, yield efficiency