Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 18, 2001
Publication Date: N/A
Interpretive Summary: Diverse Phytoplasma strains were reported to be associated with diseases and cause significant loss of production in apricot, plum, peach, and cherry in Europe, Asia, and North America. An unknown disease associated with almond, another member of stone fruit family, has led to rapid decline of almond trees in three major almond production regions in Lebanon during the last decade. The symptoms, including stunted growth, leaf rosetting, and witches'-broom, were suggestive of phytoplasmal infection. We conducted a survey to investigate the extent of disease spread and to identify the putative phytoplasma associated with the disease. A diagnostic procedure developed earlier in our laboratory for detection and identification of phytoplasmas was used in this study. The results reveled that the almond witches'-broom disease was associated with a previously undescribed phytoplasma that is closely related to members of pigeon pea witches'-broom (PPWB) phytoplasma group (16SrIX). This new phytoplasma has never been reported to be associated with stone fruits elsewhere, and could pose a major threat to almond production in other major almond growing countries, including USA, if introduced. The information will be useful to extension workers and diagnosticians and to regulatory agencies for implementation of new quarantine regulations.
An epidemic of almond witches'-broom disease has devastated almond production in Lebanon. Thousands of almond trees have died over the past ten years due to the rapid spread of the disease. The symptoms, which include early flowering, stunted growth, leaf rosetting, dieback, off-season growth, proliferation of slender shoots and witches'-brooms arising mainly from the main trunk and roots, resemble those caused by phytoplasmal infections. For the detection of the putative causal agent, nested PCR was performed using universal primers (P1/P7, R16mF2/R16mR1, and R16F2n/R16R2) commonly used for the specific diagnosis of plant pathogenic phytoplasmas. Phytoplasmas were readily detected in trees with witches'-broom symptoms from three major almond growing regions in Lebanon. RFLP analysis of PCR products amplified using primer pair R16F2n/R16R2 revealed that the phytoplasma associated with infected almonds is similar to, but distinct from, members of pigeon pea witches'-broom phytoplasma group (16SrIX),and represented a new subgroup, designated 16SrIX-B. Sequencing of the phytoplasma 16S rRNA gene indicated that almond witches'-broom (AlmWB) phytoplasma is most closely related to members of the pigeon pea witches'-broom phytoplasma group (with sequence homology ranging from 98.4% to 99.0%). Phylogenetic analysis of 16S rDNA sequences from AlmWB phytoplasma and from representative phytoplasmas from GenBank showed that the AlmWB phytoplasma represents a distinct lineage within the pigeon pea witches'-broom subclade. The same phytoplasma appears also to infect peach and nectarine seedlings.