Identification and Management of Phytophthora Aerial Blight Caused by Phytophthora nicotianae on Catharanthus roseus

Abstract

Phytophthora nicotianae is the most common pathogen in nurseries and gardens, infecting both woody and herbaceous ornamental plants. Phytophthora aerial blight symptoms such dull water-soaked lesions on shoot tips and leaf petioles, girdling on the main stem, necrosis, and wilting of annual vinca were observed in a commercial greenhouse in Warren Co., Tennessee, USA in May 2016. The objective of this study was to identify the causal agent of Phytophthora aerial blight and develop a fungicide management recommendation for ornamental producers. Attempts to isolate the pathogen from symptomatic leaf tissue were conducted and excised leaf pieces were embedded in the V8 agar medium. Morphological characterization, polymerase chain reaction (PCR), sequencing, and pathogenicity test of the isolate FBG2016_444 were conducted to confirm the pathogen identification. The sequence identity was 100% identical to Phytophthora nicotianae, and a combined phylogenetic tree (internal transcribed spacer [ITS]), the large subunit [LSU] of rDNA, and ras-related protein gene [Ypt1]) grouped isolate FBG2016_444 within the clade of P. nicotianae. In the pathogenecity study, all inoculated annual vinca plant showed the Phytophthora aerial blight symptoms and P. nicotianae was re-isolated whereas non-inoculated annual vinca plant remained symptomless. These findings confirmed P. nicotianae as the causal agent of Phytophthora aerial blight of annual vinca. In addition, two rates (0.078 and 0.156 mL·L-1) and three application intervals (7, 14 and 21 days before inoculation [DBI]) of oxathiapiprolin (Segovis®) were evaluated for their ability to reduce the Phytophthora aerial blight severity on annual vinca plants. The control groups were positive (non-treated inoculated) and negative (non-treated non-inoculated) plants. Both rates and application timings of oxathiapiprolin significantly reduced Phytophthora aerial blight severity and disease progress (area under disease progress curve [AUDPC]) on annual vinca plants compared to the positive control. However, 0.078 and 0.156 mL·L-1 of oxathiapiprolin applied at 7 or 14 DBI were the most effective treatments in reducing the disease severity and AUDPC on annual vinca plants. The plant growth parameters such as increase in height and width, total plant weight, and root weight were not influenced by the application of oxathiapiprolin. The finding reported in this study will help ornamental growers with better management of Phytophthora aerial blight of annual vinca.