2017 ASHS Annual Conference

Palo verde broom, or witches broom, is a disorder affecting primarily blue palo verde (Parkinsonia florida) trees. Broom symptoms are manifest as dense clusters of short, flexible, thorn-less branches, and stunted leaves. Trees of any age, starting in the nursery, can be affected, with a single, small sized broom or broom comprising the entire canopy. Landscape trees exhibiting witches broom are damaged by broom dieback, breakage of branches, and sunburn on exposed branches after the broom or damaged branches are removed. The cause of palo verde witches broom disease has not been determined, despite the appearance of disease symptoms in trees over 50 years ago in Arizona. The disease has spread widely in nurseries during the last decade, as blue palo verde are in high demand in the Southwest. Newly developing leaves, flowers, and seeds were collected from symptomatic and asymptomatic blue palo verde trees, and arthropods associated with branches during 2015 and 2016. A comparison of similar-length broom and asymptomatic branches indicated that brooms, compared to asymptomatic branches, have up to four times as much biomass in stems and leaves. Brooms produced no flowers in 2015, but some flowers during 2016. Broom-trees had fewer seedpods than asymptomatic trees, with broom-pods being shorter and having fewer, smaller seeds per pod. Identification of arthropods collected in sweeps revealed the presence of a number of homopteran insects, and eriophyid mites, groups previously reported as vectors of plant viral and fastidious bacterial pathogens. To identify the suspect, biotic causal agent of witches broom disease, total RNA was isolated from nitrogen-powdered plant tissue. Transcripts and small RNAs were isolated, fractionated, quantified, quality-checked, and used to construct RNAseq and TrueSeq libraries, respectively, for Illumina Hi-Seq 2500 sequencing. Reads were processed, template- or de novo-assembled, and annotated. Four RNAs, 1-4, at 7015, 2098, 1356, and 1481 nucleotides (nt), respectively, were recovered that shared 50-60% nt sequence identity with the genus, Emaravirus (Fimoviridae), a group of negative-sense, single-stranded RNA (-ssRNA) plant viruses infecting perennial, deciduous shrubs or trees, and transmitted by eriophyid mite vectors. The palo verde mite Aculus cercidii was identified in tree sweeps, regardless of broom presence or absence. Although viral etiology has not been proven (Koch’s Postulates), the presence of a complete Emaravirus-like genome, present in high titer in symptomatic but not asymptomatic leaves, provides the first robust association between a previously undescribed -ssRNA virus and witches broom disease of palo verde.