First report of Macrophomina phaseolina causing charcoal root rot of Hebe (Veronica cupressoides, V. ochracea, and V. pinguifolia) in Oregon, U.S.A.

Authors: Weiland, Gerald - Jerry

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/23/2021
Publication Date: 1/3/2022
Citation: Weiland, G.E. 2022. First report of Macrophomina phaseolina causing charcoal root rot of Hebe (Veronica cupressoides, V. ochracea, and V. pinguifolia) in Oregon, U.S.A. Plant Disease. 106(7):1984. https://doi.org/10.1094/PDIS-09-21-2036-PDN.
DOI: https://doi.org/10.1094/PDIS-09-21-2036-PDN

Interpretive Summary: A new pathogen was discovered causing disease on two ornamental Hebe plant species in landscape plantings of western Oregon. Hebes are grown for their flowers and evergreen leaves. Branches on infected plants turned brown and died. Eventually the entire plant was killed. We consistently isolated the soilborne fungus Macrophomina phaseolina from symptomatic plants at four locations. We tested three isolates of the fungus for their ability to cause disease on potted Hebe plants in the greenhouse. Plants began to die within three months after inoculation and the same fungus was recovered, thus proving that M. phaseolina was the cause of disease. Macrophomina has been previously reported causing disease in strawberry in southern Oregon, but has never been reported from the north part of the state. These results are important because they show an expanded host and geographic range for this pathogen.

Technical Abstract: Hebe is a genus of ornamental perennials and shrubs grown in the Pacific Northwest for their flowers and symmetric, evergreen leaves. From 2016 to 2021, branches on plants in four landscape plantings (2 to 37 km apart) of up to 20, 3 to 5-year-old Hebe ochracea and Hebe pinguifolia plants in Benton County, OR began to wilt, turn brown and die. The soilborne fungus Macrophomina phaseolina was consistently isolated from all infected plants and identified by morphology and by 99 to 100% homology to the internal transcribed spacer sequence from the type specimen (KF766195). Three isolates were used to complete Koch's postulates by inoculating 6-month-old plants of Hebe ochracea 'James Stirling' grown in a greenhouse. The experiment was repeated three times. One to three months later, inoculated plants begin to turn yellow, wilt, and die whereas control plants remained healthy. The same pathogen was reisolated from 24/27 inoculated plants, but never from the negative controls. M. phaseolina was reported on strawberry in southern Oregon in 2014 (Pscheidt and Ocamb 2021), but has not previously been reported from landscape ornamentals located further north in the state where soil temperatures are cooler. There is some concern that the pathogen was spread by contaminated plants produced in the nursery industry, where the warmer temperatures in greenhouses would provide a more conducive environment for the pathogen's growth and spread.