Essential concepts for managing Xanthomonas in begonia — Part 2

Cultural practices and chemical treatment of Xanthomonas in begonia.

Foliar symptoms of Xanthomonas in Rieger begonia. Photo by Jeremy Jubenville, MSU Extension.
Foliar symptoms of Xanthomonas in Rieger begonia. Photo by Jeremy Jubenville, MSU Extension.

In Part 1 of this article, we presented an overview of the Xanthomonas bacterial group, typical pathways of bacterial infection and how to recognize the symptoms of Xanthomonas in begonias. In Part 2, we will cover the cultural practices that reduce disease pressure and chemical treatments to prevent the spread of the pathogen.

Xanthomonas can be difficult to control once it's inside the facility, so the primary focus should be on prevention. Ideally, we should try and keep it from coming into the greenhouse. That's not always possible though because plants may not display symptoms until months after the initial infection, allowing the disease to go undetected.

If Xanthomonas is already in the greenhouse, then management efforts should include heightened sanitation and cultural practices as well. Chemical treatments can help hinder the spread in certain circumstances, but options are limited.

Cultural disease management

Scouting and roguing

  • Scout begonias at least once per week for symptoms of bacterial leaf spot.
  • While wearing disposable gloves, place infected plants in a garbage bag and remove them from the greenhouse.

Moisture management

  • Minimize the time leaves are wet.
  • Avoid watering late in the day or at night.
  • When possible, avoid overhead irrigation and apply water directly to the growing media (e.g., drip tape).
  • Reduce humidity and ventilate the greenhouse more frequently.
  • Limit the number of pesticide applications as much as possible.

Isolation and prevention

  • Separate groups of different cultivars with a little extra space.
  • Space plants in a way that encourages air flow.

Sanitation

  • Wash hands frequently or use disposable gloves.
  • Disinfect tools between varieties and crops.
  • Remove any plant debris. Bacteria can reside on dead leaves long enough to find a new host.
  • Use disinfectant products to sanitize any areas where infected plants have been residing (floors, benches, walls, etc.).
  • Consider replacing the black weed mat after a disease outbreak.

Chemical disease management

There are products available that can help protect healthy plants from infection, many of which are included in the 2018 Greenhouse Disease Recommendations by Mary Hausbeck, Michigan State University plant pathologist. You may notice that all the products listed for bacterial disease control contain copper as an active ingredient. It's an indication that we do not have a lot of chemical tools to manage bacterial pathogens. Bacteria can become resistant to copper, which is more likely to happen with multiple treatments, so be judicious when considering repeated applications.

Copper-based products vary in their formulations. When choosing a product, take note of the amount of active ingredient, the restricted-entry interval (REI), personal protective equipment (PPE) requirements and any crop tolerance information included on each product label. Sensitivity to copper can vary between plant species and cultivars. Test any unfamiliar products on a small number of plants and check them one to two weeks later for phytotoxicity and visible residue that may affect crop marketability.

In Part 3 of this article series, we will present some scenarios in a question-and-answer format and help you rogue through the crop to minimize losses.

As always, contact a member of the MSU Extension floriculture team for additional information and technical assistance.

Note: Reference to commercial products or trade names does not imply endorsement by Michigan State University Extension or bias against those not mentioned.

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