Comparative Performance of Chemical and Biological-based Products in Management of Algal Leaf Spot on Magnolia

Abstract

Magnolia trees (Magnolia sp.) are a popular choice for consumers when choosing flowering woody plants for landscapes. Magnolia species grow in a wide variety of both temperate and tropical locations. Southern magnolia (Magnolia grandiflora) is one of the more popular magnolias due to its pleasing aesthetics: large showy flowers in a range of colors and evergreen foliage. However, magnolias can be affected by algal leaf spot. Algal leaf spot is caused by Cephaleuros virescens, which is a widespread plant parasitic green alga. There has been little research on how to treat algal leaf spot on magnolia plants. This study focuses on identifying effective biological- and chemical-based fungicides for the management of algal leaf spot disease of magnolia plants. Two experiments were conducted in a randomized complete block design with six replications per treatment and a total of 12 treatments, including a nontreated control. The first experiment (Expt. 1) was conducted in a shade house (56% shade) at McMinnville, TN, using southern magnolia plants. The second experiment (Expt. 2) was conducted at a commercial nursery in McMinnvillle, TN, in a field plot planted with ‘Jane’ magnolia (Magnolia liliiflora ‘Nigra’ × Magnolia stellata ‘Rosea’). The algal leaf spot disease severity, disease progression, plant marketability and growth parameters were evaluated. In both experiments, all treatments reduced algal leaf spot disease severity and disease progress in comparison with the nontreated control. In Expt. 1, copper octanoate, copper oxychloride, chlorothalonil water-dispersible granules, chlorothalonil suspension concentrate, didecyl dimethyl ammonium chloride, azoxystrobin + benzovindiflupyr, hydrogen peroxide + peroxyacetic acid, and mono- and di-potassium salts of phosphorus acid + hydrogen peroxide reduced the disease severity and disease progress the most and were not statistically different from one another. In Expt. 2, azoxystrobin + benzovindiflupyr, didecyl dimethyl ammonium chloride, and copper oxychloride significantly reduced disease severity and disease progress (area under disease progress curve). Treatments had no deleterious effect on plant growth parameters such as height and width, and no phytotoxicity of applied treatments or defoliation was observed. Treated magnolia plants had better plant marketability compared with the nontreated control plants. The findings of this study will help growers to achieve better management of algal leaf spot disease on magnolia trees.