Phytophthora nicotianae and Rhizoctonia solani are the well-described soilborne pathogens of concern causing Phytophthora and Rhizoctonia root rot, respectively, of red maple plants (Acer rubrum L.), resulting in substantial economic losses to nursery growers. The management of root and crown rot disease of red maple is a big challenge. The objective of this study was to test the efficacy of several fungicide and biofungicide products to control Phytophthora and Rhizoctonia root rot on red maple plants in greenhouse conditions. Treatments, including fungicides and biofungicides, and nontreated and inoculated and nontreated and noninoculated as controls were arranged in a completely randomized design with six replications. Red maples planted in number 1 nursery containers were artificially inoculated with P. nicotianae or R. solani. Plant height, plant width, total fresh weight, and root fresh weight were measured and roots were assessed for root rot disease severity based on a scale of 0% to 100% root damaged. The pathogen recovery percentage of plant roots was determined by culturing ten randomly selected root pieces (≈1 cm long) cut from the root tips on Phytophthora selective medium (PARPH-V8) or Rhizoctonia semi-selective medium. All tested fungicides and biofungicides reduced Phytophthora and Rhizoctonia root rot on red maple plants compared with the nontreated and inoculated control. Likewise, pathogen recovery was lower for fungicide-treated and biofungicide-treated plants. Fungicides, such as mefenoxam, oxathiapiprolin, pyraclostrobin plus boscalid, and pyraclostrobin provided the most effective control of Phytophthora root rot. Pyraclostrobin plus boscalid and pyraclostrobin followed by biofungicides Bacillus amyloliquefaciens strain F727 and Trichoderma harzianum Rifai strain T-22 plus T. virens strain G-41 were most effective for suppressing Rhizoctonia root rot. There were no differences in plant height, plant width, plant fresh weight, and root fresh weight among the treatments. These findings will help nursery producers make decisions while formulating soilborne disease management strategies for red maple production.
Nursery growers rely on fungicides to manage root rot disease of boxwood caused by Phytophthora nicotianae and P. cinnamomi. Repeated use of the same fungicide may lead to the fungicide resistance. In this study, fungicides pyraclostrobin + cyazofamid (Empress + Segway), ametoctradin + dimethomorph (Orvego), ametoctradin + dimethomorph alternated with pyraclostrobin (Orvego alt Empress), ametoctradin + dimethomorph alternated with fluxapyroxad + pyraclostrobin (Orvego alt Orkestra), and oxathiapiprolin (Segovis) were evaluated for their efficacy in managing Phytophthora root rot under greenhouse conditions in Tennessee. One-year-old container-grown boxwood ‘Green Velvet’ plants were inoculated with P. nicotianae or P. cinnamomi. The first applications of fungicide treatments were made preventatively as a drench 48 h before pathogen inoculation. Following inoculation, four applications of fungicide treatments were performed on a 14-day intervals. Initial and final plant height and width were measured. Total plant fresh weight and root fresh weight were measured at the end of the trials, and plants were evaluated for root rot severity (0% to 100% roots affected) and pathogen recovery. All fungicides significantly reduced root rot severity and pathogen recovery of P. nicotianae and P. cinnamomi. Ametoctradin + dimethomorph alternated with pyraclostrobin (Orvego alt Empress) provided similar protection against P. cinnamomi to that of a single application of ametoctradin + dimethomorph (Orvego) or oxathiapiprolin (Segovis). For P. nicotianae, ametoctradin + dimethomorph alternated with pyraclostrobin + fluzapyroxad (Orvego alt Orkestra) was found to be as effective as a single application of either ametoctradin + dimethomorph (Orvego) or oxathiapiprolin (Segovis) in one of the two trials. Effects of fungicides on plant growth such as height, width, total, and root fresh weight were not significant. These findings will be useful to nursery growers in selecting the right fungicide program for the management of root rot disease of boxwood caused by P. nicotianae and P. cinnamomi.
Crapemyrtle (Lagerstroemia sp.) is a top-selling deciduous flowering tree in the United States, and its salability is often compromised by cercospora (Cercospora lythracearum Heald & F. A. Wolf) leaf spot. To compare cercospora leaf spot resistance, 32 crapemyrtle cultivars belonging to Lagerstroemia indica, Lagerstroemia fauriei, L. indica × L. fauriei, and L. indica × L. fauriei × Lagerstroemia limii and 12 cultivars or unnamed selections belonging to L. indica, L. indica × L. fauriei, L indica × L. fauriei × L. limii, L. limii, and Lagerstroemia subcostata were planted in field plots in 2004 and 2011, respectively. The experiment was a completely randomized block design with three and four replications in the 2004 and 2011 plantings, respectively. Plants were evaluated for cercospora leaf spot disease severity and defoliation using a scale of 0% to 100% foliage affected from August to October of 2015, 2016, and 2017. Area under the disease progress curve (AUDPC) was calculated for the evaluation period of each year. L. fauriei cultivars Fantasy, Kiowa, Townhouse, and Woodlander’s Chocolate Soldier and L. indica × L. fauriei Apalachee from the 2004 planting, and the L. subcostata and L. limii selections from the 2011 planting had lowest cercospora leaf spot disease severity ratings, AUDPC, and defoliation. L. indica × L. fauriei cultivars Choctaw, Miami, Natchez, Osage, Sarah’s Favorite, Tonto, Tuscarora, and Tuskegee, and L. indica × L. fauriei × L. limii Arapaho were moderately resistant to cercospora leaf spot, whereas cultivars belonging to L. indica and L. indica × L. fauriei × L. limii Cheyenne were highly susceptible to cercospora leaf spot. Results from this research may aid breeders, nursery producers, and landscapers in selecting crapemyrtle species and cultivars with cercospora leaf spot resistance.
Magnolias (Magnolia sp.) are economically important woody ornamental plants; however, plant damage associated with frost and powdery mildew (Microsphaera alni and Phyllactinia corylea) disease is a major production challenge. To understand the tree architecture and powdery mildew resistance, 26 yellow-flowering magnolias (Magnolia sp.) were evaluated in McMinnville, TN, USA (USDA Plant Hardiness Zone 6b). One-year-old containerized trees were planted in a field plot during Mar 2006, with 4.6-m plant-to-plant and 3.7-m row-to-row spacing. The experiment was a completely randomized block design with three single-plant replications. Plant height and canopy diameter were measured on Dec 2016 and Feb 2018, and the apical dominance ratio (i.e., indicating tree architecture) was calculated by dividing the height by the canopy diameter. Plants were evaluated for powdery mildew severity and defoliation using a scale of 0% to 100% of the total plant canopy affected or defoliated, respectively, from Aug to Oct of 2016 and 2017. The area under the disease progress curve (AUDPC) was calculated for the evaluation period of each year. The plant apical dominance ratio ranged from 1 to 3, with ‘Gold Cup’ and ‘Sun Spire’ having the highest value, and ‘Lois’, ‘Gold Star’, ‘Golden Gala’, ‘Solar Flair’, ‘Stellar Acclaim’, ‘Sun Ray’, ‘Sunburst’, and ‘Sundance’ having the lowest value (i.e., relatively round shape). Of the 26 cultivars, Sundance, Sun Spire, Sun Ray, and Gold Cup had the lowest powdery mildew severity (10% to 33% in 2016 and 40% to 60% in 2017), AUDPC, and defoliation. Cultivars Anilou, Gold Star, Golden Pond, Golden Rain, Golden Sun, Green Bee, Honey Liz, Judy Zuk, Koban Dori, Lois, Solar Flair, Stellar Acclaim, and Yellow Bird were highly susceptible to powdery mildew (>80% disease severity) and had the highest AUDPC values. Results of this research may aid breeders, nursery producers, and landscapers when selecting yellow-flowering magnolia cultivars with desirable tree architecture and resistance to powdery mildew.