Etiology of almond shriveled kernel disease

Abstract

In a commercial orchard, mature trees of the almond (Prunus dulcis (Mill.) D. Webb) cultivar ‘Butte’ propagated on peach (P. persica (L.) Batsch) seedlings exhibited delayed budbreak in spring, sparse foliage, poor shoot growth, and produced shriveled kernels. Extracts of leaf petioles from diseased, but not healthy, ‘Butte’ trees were positive by dot-blot hybridisation assays for the presence of a phytoplasma. In addition, diseased almond buds were graft-inoculated onto the ‘Peerless’ almond propagated on ‘Marianna 2624’ plum rootstock and induced brown line and pitting at the rootstock/ cultivar union, a condition associated with peach yellow leaf roll phytoplasma (PYLR-phytoplasma) infection in this indicator host. Furthermore, buds of peach infected with PYLR-phytoplasma induced similar canopy and kernel symptoms when bud-inoculated onto previously healthy trees of the almond cultivars ‘Peerless’ and ‘Nonpareil’ propagated on peach seedlings. However, a ‘Peerless’ tree infected with X-disease phytoplasma appeared unaffected. Ilarvirus and dsRNA analyses of diseased ‘Butte’ trees were negative. Diseased trees treated with oxy-tetracycline caused remission of symptoms. These results indicate that infections by a phytoplasma, specifically PYLR-phytoplasma, are responsible for kernel shrivel symptoms in almond trees grown on peach seedlings. © 1999 Taylor & Francis Group, LLC.