First report of downy mildew caused by a Plasmopara halstedii on Ageratum houstonianum in the United States

Authors: Pisani, Cristina; Patel, Pragna; Rosskopf, Erin; ABBASI, MEHRDAD - Purdue University; AIME, MARY - Purdue University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/12/2019
Publication Date: 9/9/2019
Citation: Pisani, C., Patel, P.C., Rosskopf, E.N., Abbasi, Mehrdad, Aime, M.C. 2019. First report of downy mildew caused by a Plasmopara halstedii on Ageratum houstonianum in the United States. Plant Disease. https://doi.org/10.1094/PDIS-06-19-1331-PDN.
DOI: https://doi.org/10.1094/PDIS-06-19-1331-PDN

Interpretive Summary: Ageratum houstonianum, or floss flower, is an ornamental crop that is commonly cultivated for the cut flower industry and landscape horticulture in Florida and around the world. Unfortunately, downy mildew, an obligate parasite in the family Peronosporaceae, was observed on floss flower cultivars Blue Horizon and Blue Planet on a commercial cut flower farm in Martin County, Florida. Downy mildew symptoms included foliar lesions, circular to irregular, accompanied by a gray growth on the underside. Brown lesions were observed on stems, leading to necrosis. The downy mildew was morphologically characterized as Plasmopara halstedii and identification was confirmed by sequencing. Studies were conducted to confirm its pathogenicity. To our knowledge, this is the first report of infection by downy mildew, Plasmopara halstedii, on Ageratum houstonianum in the United States, although it has been reported on Black-eyed Susan in Florida and on Ageratum in Brazil. These findings will assist flower growers and the ornamental industry work on effective control measures to manage the disease on a popular cut flower and landscape species.

Technical Abstract: Ageratum houstonianum, commonly known as floss flower, belongs to the family Asteraceae and is native to Central and South America. It is commonly cultivated as a specialty ornamental crop for both the cut flower industry as well as landscape horticulture in Florida and worldwide. Ageratum is easy to grow and can produce continuous blooms for most of the year and is one of the few annuals that can tolerate light shade, adding to its versatility and making it an excellent, colorful choice for planting beds in Florida. In 2016 and again in 2018, signs typical of downy mildew were observed on A. houstonianum cultivars Blue Horizon and Blue Planet on a commercial cut flower farm in Martin County, Florida. Symptoms included foliar lesions, circular to irregular, accompanied by a gray mycelial growth on the underside. Brown lesions were observed on stems, leading to necrosis. The morphological characteristics observed using microscopy led to identification as Plasmopara halstedii. To confirm pathogenicity, leaves were detached from field-collected plants and cut pieces were placed between the undersides of two leaves of clean ‘Blue Horizon’ plants, which were then gently wrapped with parafilm. Five plants were inoculated in this manner and placed in plastic bags misted with sterile deionized water. After 48 hours, parafilm was removed and plants were placed on the greenhouse bench. Five bagged but non-inoculated plants served as controls. Symptoms and signs of the fungus were visible within two weeks of inoculation. Diagnosis was conducted based on hypophyllous sporangiophores, emerging through stomata, cylindrical, up to 550 µm long and 8.5 to 14.4 µm wide, hyaline, aseptate, straight, branching 3 times and terminating in a group of sporangium-bearing denticles. Primary branches were alternate, arising 70–90° to the main axis, up to 144 µm long. Secondary branches, up to 85 µm long, alternate or opposite. Tertiary branches, up to 24 µm long, with three or four denticles with truncate tips. Sporangia were oblong, more or less cuneiform, sometimes ellipsoid, from 16 to 24 µm long and 12.8 to 18 µm wide, hyaline, smooth, and mucronulate. The morphological identification was confirmed by sequencing. The 28S LSU region was amplified using primer pairs LROR and LR7. The consensus sequence was deposited in GenBank (Accession No. KX096708.1) and a representative specimen at the Kriebel Herbarium (PUL F2909). BLAST search sequence data revealed 99% (1241/1244 bp) sequence identity with a Plasmopara halstedii isolate on Helianthus annus (Asteraceae) (Accession No. EF553469.1) (Voglmayr and Constantinescu 2008). To our knowledge, this is the first report of infection by downy mildew, Plasmopara halstedii, on Ageratum houstonianum in the United States, although it has been reported on Black-eyed Susan in Florida and on Ageratum in Brazil. These findings will assist flower growers and the ornamental industry work on effective control measures to manage the disease on a popular cut flower and landscape species.