Submitted to: Trade Journal Publication
Publication Type: Trade Journal
Publication Acceptance Date: 5/1/2011
Publication Date: 5/1/2011
Citation: Sudarshana, M.R. 2011. Almond Brownline Disease. Trade Journal Publication. 17:26-35.
Technical Abstract: A recent outbreak of almond brownline that occurred in Sutter County in 2008, provided an opportunity to investigate on the nature of the pathogen associated with the disease. A grant support was provided by the California Almond Research Board and the Tree Fruits & Nuts and Grapevines Advisory Board in the California Department of Food and Agriculture to conduct these investigations and develop a molecular diagnostic assay for adoption by routine testing by private and public laboratories. Amplifi cation of ribosomal DNA sequences from nucleic acid extracts from diseased almond leaves and subsequent sequence analysis established association of a phytoplasma in almond trees with brown line disease. Genetically this region is of almond brown line phytoplasma is almost indistinguishable from the ribosomal DNA sequences of phytoplasmas causingpeach yellow leafroll and pear decline diseases. However, examination of the chromosomal region distal to the ribosomal DNA region has not supported this notion and further work is in progress to establish the true identity of the phytoplasma which is essential to develop specific and sensitive molecular tools to identify bud wood sources free from brown line phytoplasma. Currently, a real-time quantitative PCR assay has been optimized for the detection of almond brown line phytoplasma at the USDA-ARS laboratory in Davis. In nature, phytoplasmas are generally transmitted when bud wood is obtained from infected trees. The trees can also become infected when insect vectors carrying phytoplasma make a transient visit to almond trees or peach rootstocks and hence is generally isolated. In the event of a disease incidence, because of the lethal nature of disease in almonds on plum rootstock, the disease is self eliminating in trees on plum rootstock. Nurseries in California generally maintain almond mother trees on plum rootstock to recognize brown line phytoplasma and thus prevent distribution of trees with phytoplasma infections. Almond brown line disease is unlikely to become a major problem for almond production in California. If trees in young orchards happened to show brown line disease, it is best to remove the diseased trees and replant immediately. Horizontal spread of the disease has not been seen so far in diseased orchards and this is likely due to almond trees not being a good host for the insect vector that transmits the phytoplasma.