Essential concepts for managing Xanthomonas in begonia — Part 1
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EmailXanthomonas in begonia: What it is, what it does and how to know if your plants have it.
Michigan State University’s Plant & Pest Diagnostics recently received samples of begonia with Xanthomonas axonopodis pv. begoniae. Many greenhouse growers are asking questions about the biology of this bacterial pathogen, how it spreads in the crop, how to minimize losses, and how to sanitize after disposing of diseased plants. In this three-part series, we will cover these questions.
Xanthomonas is the name of a group of bacterial species which are often separated into strains (called pathovars or "pv.") that indicates the plants each one attacks. As the name suggests, Xanthomonas axonopodis pv. begoniae is the pathovar that infects begonias. This bacterial pathogen does not infect other crops such as geraniums.
How infection happens
Xanthomonas bacteria usually enter a plant through a wound or natural opening (e.g., stomata). They are motile and move through water on the leaf surface and splash droplets. When the leaves are wet, the bacteria can move into the plant via stomates or hydathodes. Once inside the plant, the bacterial cells reproduce and eventually enter vascular tissue and become systemic.
In a commercial production setting, splashing water from overhead irrigation, misting and sprinklers can spread the pathogen from infected plants to those nearby. In addition, water dripping off Xanthomonas-infected begonias in hanging baskets can spread the pathogen to begonias growing below. Therefore, growers should avoid placing hanging baskets with begonias above those on benches.
While water is the primary means of dispersal, Xanthomonas can also be carried to healthy plants by pruning tools and handling.
Impact
Xanthomonas causes leaf spots and blight (Photos 1 and 2). Symptoms and their severity vary depending on the type of begonia. Symptoms may not be present until months after the initial infection.
- Leaf spots are often small and looked water-soaked. Eventually the spots coalesce into a larger blighted area.
- Blighted areas may have an undefined edge with a yellow halo and spotting.
- Blight lesions are often V-shaped (Photos 1 and 2).
These unsightly lesions often result in the plant being unsalable. In some types of begonias, the symptoms become severe with the leaves falling off and the plant wilting and dying (Photo 3).
Diagnosis
It's important to obtain an accurate identification of the pathogen from a plant diagnostics laboratory as management differs among diseases. Also, a lab diagnosis may be necessary for records and accounting. Depending on the lab, the testing process will employ culturing of the pathogen, ELISA or PCR-based methods. Commercially available tests used to detect Xanthomonas hortorum pv. pelargonii in geraniums cannot be used to detect the Xanthomonas that infects begonias.
Please note: If the situation involves crop insurance claims or the legal system, you will need documentation from a reputable lab. Many, if not all, land-grant universities provide plant diagnostic services to growers. Don’t hesitate to contact your local Extension office for more details—we’re ready to help!
Continue to Part 2 of this Michigan State University Extension article series for suggestions on cultural practices and chemical treatments.
