Location: Horticultural Crops Disease and Pest Management Research Unit
Project Number: 2072-22000-045-061-S
Project Type: Non-Assistance Cooperative Agreement
Start Date: Sep 1, 2025
End Date: Aug 31, 2028
Objective:
Develop improved methods for detecting Phytophthora species and managing Phytophthora diseases affecting the ornamental horticulture industry.
Objective 1. Characterize and assess potential host specialization among isolates of Phytophthora nicotianae from three economically important ornamental plants: lavender, annual vinca, and petunia.
Objective 2. Examine the plasticity of mefenoxam resistance in Phytophthora nicotianae from floriculture crops.
Objective 3. Evaluate the potential of anerobic soil disinfestation (ASD) to reduce populations of Phytophthora nicotianae in lavender field sites.
Objective 4. Etiology of Phytophthora root rot on nursery-grown hemlocks.
Approach:
1. Identify new host-pathogen associations for Phytophthora species on ornamental crops and trees.
2. Maintain a permanent collection of isolates of these species as a resource for future investigations.
3. Improve detection methods of Phytophthora species on nursery-grown plants.
4. Evaluate management strategies for Phytophthora species on ornamental plants.
Evaluate and compare morphological and physiological characteristics of isolates from the different host plants, we will use 15 isolates of P. nicotianae recovered from each ornamental plant – lavender, annual vinca, and petunia. All isolates will be invigorated by inoculating in vitro plant tissue from their original hosts and then isolating actively growing cultures. We will use three replicates for each isolate for laboratory experiments, and all experiments will be conducted at least twice. We will evaluate colony morphology on several media, compare growth rate on 5% cV8A, and quantify sporangium and zoospore production. We then will evaluate isolates for virulence to each of the different host plants. Replicate plants of each species will be inoculated with a composite mixture of five representative isolates of P. nicotianae from each of the three host plants. Plants will be maintained in the greenhouse under standard grower conditions for at least 5 weeks and assessed weekly for disease severity by evaluating the proportion of foliage showing symptoms of wilting and necrosis. At the end of the experiment or as plants die, we will isolate from representative plants to confirm infection by the pathogen. We also will evaluate the sensitivities of all isolates to five commonly used fungicide active ingredients contained in registered brand name products that target oomycetes: mefenoxam (Subdue Maxx), fluopicolide (Adorn), phosphorous acid (Reliant), ametoctradin + dimethomorph (Orvego), and cyazofamid (Segway). Isolates from each host will be grown on 5% cV8A amended with the recommended label rate of one of the fungicides, and cultures will be incubated at 25°C for 3 days. Growth (i.e., colony diameters) will be measured and compared to growth on non-amended 5% cV8A (i.e., control). Growth inhibition by each fungicide will be calculated for each isolate.