Epidemiology and Control of Ranunculus Bacterial Blight Progress Reports — 1998 June
PROJECT PROGRESS REPORT Epidemiology and Control of Ranunculus Bacterial Blight
Donald A. Cooksey, University of California, Riverside, May 29, 1998
Date:
5/29/98
Title of Project:
Epidemiology and Control of Ranunculus Bacterial Blight
Institution where work is being conducted:
University of California, Riverside
Amount of Endowment Grant:
$23,162
Covering Period:
7/97 to 6/98
Anticipated Date of Project Completion /Final Report:
Individual(s) Conducting Project:
(List Project Leader First)
Donald A. Cooksey
Telephone Number: 909-787-4115
This project follows our recently published description of a new Ranunculus disease caused by the bacterium Xanthomonas campestris. In addition to causing symptoms of blight on leaves and stems, the bacterium was shown to move systemically into root tubers and to infest seed. It is likely that the bacterium is spread in the industry in infested seed and tubers, and disinfestation of these materials should be key to preventing the disease during Ranunculus selection and propagation. All seed lots tested were infected with this pathogen, which has become widespread and severe in the major Ranunculus-growing region of Southern California. Infested seed lots have a lower percentage of viable seeds than noninfested seed lots, and infested tubers also show signs of necrosis. Infested tubers have resulted in diseased plants appearing in homeowners gardens. This proposal is to test seed and tuber treatment methods for eliminating the bacterial pathogen and to follow disease progression during two growing seasons at a commercial Ranunculus production site. The first season is nearing completion under funding from the American Floral Endowment.
Objectives:
- Test the efficacy of seed and tuber treatments for removing Xanthomonas campestris, and measure effects on Ranunculus seed and tuber viability.
- Quantify the severity and systemic spread of Xanthomonas campestris in field plots established with treated and nontreated Ranunculus seed and tubers.
- Assess the role of weeds or other plant hosts as reservoirs of inoculum.
Progress to date:
Objective 1.
Our original proposal was to conduct our experiments during two seasons beginning with a fall planting in 1997. That planting was made, and sampling and disease assessments have been conducted at regular intervals through the growing season. The final harvest is scheduled in June of 1998. In addition, we conducted a preliminary field trial at the commercial Ranunculus production site in Carlsbad, California, during the 1996-1997 growing season. That preliminary trial confirmed the appropriateness of our general plot design and of its placement in the commercial production field. Reviewers of the original proposal expressed concern that bacterial inoculum from the non-treated Ranunculus plants at the commercial site might contaminate our plots. However, the extensive use of border rows of clean plants surrounding the treated and untreated plots prevented contamination between plots and from the surrounding infested fields in the preliminary trial.
In the current 1997-98 trial, early season samplings indicated again that our seed treatments with chlorine bleach were successful in reducing or eliminating bacterial inoculum and establishment of the bacterium in field plots. However, as this season progressed, we had unusually severe rains and wind-driven moisture across the plots. Our later sampling dates indicated that the pathogen had spread across border rows to infest our treated plots. All plots are now diseased, in spite of our early data that the seed treatments were successful. While disappointing, our hypothesis is still that if an entire field was planted with treated seed, then the overall disease incidence should be reduced. In practice, untreated and infested control plots would not be present in a production field to provide the inoculum source that our plots apparently did. In our second season that will start in fall 1998, we will consider modifying our field plot design to separate untreated controls spatially along the large Carlsbad field site. The grower will be incorporating seed treatments into their commercial plantings, so that the majority of the site should be more disease free. We will also be recommending greater attention to weed control, based on results described below.
Objective 2.
The severe disease incidence during this high rainfall season was actually a benefit for our second objective to follow disease spread over the season. After our last sampling, the data will be analyzed to gain insight on the rate of spread from inoculum sources, and the extent to which late-season infections result in the production of infested seed and tubers.
Objective 3.
Within our field plots, 19 different species of weeds were detected during the growing season, and all were assayed for the presence of Xanthomonas campestris by washing plant parts and plating on selective media. No disease symptoms were observed on weeds, but epiphytic populations were detected on five species overall. Two grasses were the most common weeds harboring significant epiphytic populations of Xanthomonas campestris. The bacterium was also recovered infrequently on three other species, including two clovers and common cheeseweed. The strains of X campestris recovered are still being characterized to determine if they are actually the Ranunculus pathovar, but our preliminary work suggests that weeds can be a significant source of residual inoculum.
Next steps and anticipated industry benefit:
We plan to conduct a second season of field trials starting in fall 1998, with appropriate modifications of plot design, in the event of another season of severe weather. In the intervening months, we will continue to refine detection methodology, using modified selective media and PCR techniques.
The information gained from this project will be disseminated through a peer-reviewed journal and Cooperative Extension publications directed toward both the floriculture industry and homeowners. The results are expected to provide practical solutions to this disease problem during Ranunculus selection and propagation, as well as improve the quality of Ranunculus in commercial and home gardens. Our recent finding that weeds may harbor significant epiphytic populations of X campestris will be confirmed and incorporated into disease management recommendations.
