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Title: First report of downy mildew caused by Hyaloperonospora sp. on Iberis sempervirens in the United States

item SALGADO-SALAZAR, CATALINA - Rutgers University
item RANE, KAREN - University Of Maryland
item Crouch, Jo Anne

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/17/2017
Publication Date: 1/24/2017
Citation: Salgado-Salazar, C., Rane, K.K., Crouch, J. 2017. First report of downy mildew caused by Hyaloperonospora sp. on Iberis sempervirens in the United States. Plant Disease. 101(6):1058. doi: 10.1094/PDIS-08-16-1158-PDN.

Interpretive Summary: Downy mildew diseases cause substantial damage to many flowering and vegetable-producing crops worldwide. This research describes a novel downy mildew disease outbreak affecting candytuft, a popular herbaceous perennial plant that is prized for its low-growing, evergreen foliage and fragrant flowers. The candytuft disease is referred to as a downy mildew due to the fluffy white appearance of the parasitic water mold that causes the disease coating the undersurface of infected leaves. This candytuft downy mildew was found in a commercial nursery in Maryland, where all infected plants were destroyed by the disease. This is the first time that downy mildew has ever been officially reported from candytuft in the United States. Knowledge of this disease will be useful to growers, extension personnel, plant pathologists, and quarantine officials working to control downy mildew diseases in the United States, and provides new information about the identity of the water mold parasite that causes the disease.

Technical Abstract: Iberis sempervirens (candytuft) is a herbaceous perennial in the Brassicaceae. It has evergreen foliage and fragrant, pure white flowers, and is a popular groundcover. During March 2016, downy mildew-like symptoms were observed on potted plants in a commercial greenhouse located in Baltimore Co. Maryland. Initially plants exhibited mild, uneven chlorosis and slight leaf twisting. As the disease progressed, leaves turned pale brown, white mycelia formed on the abaxial leaf surface, and plants were rapidly defoliated. Microscopic observations of the diseased specimen (BPI 910164) revealed straight hyaline sporangiophores monopodially branched, with colorless branchlets slightly curved or recurved, 157.5 to 409.5 µm long (mean 245.5 µm, n = 25). Sporangia were ellipsoid, light to pale brown with smooth surfaces measuring 21 to 31 × 16 to 20 µm (mean 26.1 × 18.4 µm, n = 34). Morphological characteristics were most similar to those described for the genus Hyaloperonospora (Constantinescu and Fatehi 2002) and matched the description of Peronospora iberidis (Gäum 1927). PCR amplification and bidirectional sequencing of the rDNA ITS and mtDNA cox2 was performed using primers ITS-O/LR-O and Cox2-F/Cox2-RC4 (Choi et al. 2015). Using GenBank BLASTn, the ITS sequence (KX702312) showed 99% similarity to an unnamed species of Hyaloperonospora infecting I. sempervirens collected from Germany (AY531461; Göker et al 2009), but only 88% similarity to a voucher specimen of H. parasitica (AY210987). The cox2 sequence (KX702313) showed greatest similarity (94%) to a specimen of H. malyi (KC494982). Cox2 sequences from Peronosporaceae infecting Iberis were absent from GenBank. These data indicated that the I. sempervirens pathogen reported herein is a member of the genus Hyaloperonospora, but there were insufficient taxonomic resources available to provide a species-level identification. Based on morphology, previous reports of downy mildew associated with Iberis sp. have attributed the disease to H. parasitica (syn. P. parasitica) or P. iberidis (Farr and Rossman 2016). However, phylogenetic studies show that H. parasitica is limited to Capsella bursa-pastoris (Göker et al. 2009), and the characteristics of that species do not match the organism described in the present report. Based on morphological similarities and association with the host genus Iberis, it is possible that the Hyaloperonospora sp. reported here and by Göker et al. from I. sempervirens might be identical to P. iberidis described from I. amara, but the inclusion and transfer of that species into the genus Hyaloperonospora has not been formally addressed in any taxonomic treatment. To our knowledge, this is the first report of a downy mildew affecting I. sempervirens in the U.S. (Farr and Rossman 2016). Colloquially, this disease is well known in greenhouses and nurseries. However, in the absence of documentation and with scant information about the identity of the causal agent, it is difficult to accurately assess the potential impact of this destructive downy mildew on candytuft production.