What Causes Begonia Leaf Spot: Treating Leaf Spots On Begonia Plants

What Causes Begonia Leaf Spot: Treating Leaf Spots On Begonia Plants

By: Tonya Barnett, (Author of FRESHCUTKY)

Begonia plants are a popular choice for garden borders and hanging baskets. Readily available at garden centers and plant nurseries, begonias are often among the first flowers added to newly revitalized flower beds. Highly praised for their varying colors and textures, both tuberous and seed grown begonias offer growers a plethora of colorful flowers and waxy multi-colored foliage.

With these attributes in mind, it is easy to see why many growers may have cause for alarm when their previously healthy begonia plants begin to show signs of distress, such as leaf spots on begonia.

What Causes Begonia Leaf Spot?

Leaf spots of begonia are caused by a pathogen calledXanthomonas. Among the first signs and symptoms that growers may notice whendealing with leaf spot on begonia is the appearance of darkened spots or “watersoaked” leaves. As the disease progresses, leaf spot can continue to spreadthroughout the host plant and to other begonia plants near it. If severe, thebegonia plant will eventually die.

Leaf spot on begonias is a disease which is most commonlyspread by infected plant matter. Begonias with leaf spot are often introducedinto the existing flower bed, thus causing problems in the garden.

Treating Begonia Bacterial Leaf Spot

The best way to maintain a healthy planting of begonias isto monitor and check the overall health of the flowers before planting theminto the garden. Closely examine the leaves of the begonia plants. The firstsigns of begonia leaf spot may often be present on the underside of the plants’foliage.

Buying from a reputable source will help to decrease thelikelihood that the begonia plants have come into contact with this bacterialissue.

In some cases, presence of the bacteria may not beimmediately apparent. If begonia leaf spot does become an issue in the flowerbed, growers can help combat it by removing and destroying infected plants.

Always make certain to thoroughly clean any garden toolswhich have been used to handle the begonias with leaf spot, as these can spreadthe disease too. As with many plants, it is best to avoid overhead watering, asthis process can also encourage the transport of the disease to other begoniaplantings.

This article was last updated on

MSU Extension Floriculture & Greenhouse Crop Production

Jeremy Jubenville, and Heidi Lindberg, Michigan State University Extension, and Jan Byrne, MSU Diagnostic Services - April 27, 2018

Cultural practices and chemical treatment of Xanthomonas in begonia.

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.


  • 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.

Other articles in series

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).

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Botrytis is a fungal disease that causes splotches of the angel winged begonia to become soggy and brown. If left untreated, these soggy brown areas will spread and eventually kill the begonia. Botrytis is a sign that you need to prune the interior of your plant to improve air circulation and stop overhead watering. To control the spread of the disease, prune the brown area out of the plant or cut off the affected cane, then sterilize the cut with methylated spirits. Finally, remove any dropped leaves or flowers--they may harbor fungal spores.


Begonias that are properly cared for have far less risk of contracting one of the begonia diseases. Begonia plants should be planted a minimum of 9 inches from other plants so they receive adequate ventilation. Begonias should only be watered when the top soil is dry to the touch. Plant owners should avoid splashing the begonia’s foliage with water and should not use overhead irrigation systems. Begonia plants should never be placed in direct sunlight because they can’t tolerate high temperatures very well.