First Report of Phytophthora parsiana Causing Crown and Root Rot on Guayule in the United States.

Abstract

Guayule (Parthenium argentatum A. Gray) is a perennial shrub plant (approximately 50 cm in height) cultivated in the southwestern United States. It produces natural low-allergenic latex, resins and high-energy biofuel feedstock. During August 2021, a crown and root rot disease was observed on 2-year-old plants of direct-seeded guayule cultivar 'Az 2' in research plots located in Pinal county, Arizona, where a record 36 cm of rainfall fell during monsoon season. Symptoms included yellowing of leaves, wilting, and plant death. Average disease incidence was 16%. Isolation from necrotic crown and root tissues on 10% clarified V8-PARP (Jeffers and Martin 1986) yielded Phytophthora-like colonies. Three isolates were subcultured on V8 agar and chlamydospores and hyphal swellings were abundant in 2-week-old cultures. All three isolates produced abundant noncaducous and nonpapillate sporangia ranging from 33 to 54 μm × 20 to 39 μm (average 45.5 × 28.5 μm, n = 20) in soil water extract solution. Isolates did not produce oospores after 2 weeks on carrot agar at 20°C in the dark. Isolates had optimum vegetative growth at 30 oC and grew well at 35 oC. There was no growth at 5 and 40 oC. Genomic DNA was extracted from the mycelia of three isolates using DNeasy Plant Pro Kit (Qiagen Inc., Valencia, CA) according to the manufacturer's instructions. The internal transcribed spacer (ITS) region of rDNA, mitochondrially encoded cytochrome c oxidase 1 (cox 1) gene, and beta-tubulin (β-tub) gene were amplified with primers ITS1/ITS4 (White et al., 1990), COXF4N/COXR4N and TUBUF2/TUBUR1 (Kroon et al., 2004) and the resulting 3 amplicons were sequenced (GenBank Accession No. OK438221, OK484426, and OK484427). A BLASTn search of 811-bp amplicon (OK438221) revealed 99% match (762/766) with ITS sequences MG865562 which was Phytophthora parsiana Ex-type CPHST BL 47 from Iran. BLAST analysis of the 867-bp amplicon (OK484427) showed 99% identity (866/867) with the COX 1 sequence of P. parsiana (KC733455) from Virginia. BLAST analysis of the 941-bp amplicon (OK484426) showed 99% identity (928/938) with the β-tub sequence of P. parsiana (AY659746). To fulfill Koch's postulates, pathogenicity tests were conducted twice on 2-week-old 'Az 2' guayule seedlings grown in 10 plants per 1.9-liter pot filled with a steam-disinfested potting mix. Pots were placed in a plastic container and watered three times a week by flooding, to create waterlogged conditions. Plants were maintained in a greenhouse with 12 h day/12 h night (15-28 oC) and fertilized weekly with a 20-20-20 fertilizer at 1mg/ml. Fifty plants in 5 pots were challenged with a P. parsiana isolate by drenching each pot with 50 ml of a 1×105 zoospore/ml suspension. Fifty plants in 5 pots, serving as a control, received each 50 ml of distilled water. Symptoms of wilting, root rot, and plant death were observed 1 week afterward in inoculated plants, whereas control plants remained asymptomatic. P. parsiana was reisolated from necrotic roots of inoculated plants but not from control plants. To our knowledge, this is the first report of crown and root rot in guayule caused by P. parsiana in Arizona. P. parsiana is a species known for causing root rot on woody plants such as pistachio in California (Fichtner et al., 2016) and Iran (Mostowfizadeh-Ghalamfarsa et al., 2008). Arizona is home of desert woody guayule plant. P. parsiana may represent a significant barrier to commercialization of guayule for rubber in low desert areas of Arizona. The origin, distribution, and virulence of the pathogen on Arizona guayule is currently unknown. Disease resistance evaluation may help identify resistance in guayule germplasm that are useful in breeding for resistant cultivars.