Bacterial leaf spot of begonia
These practices will help limit the risk of bacterial leaf spot outbreaks in your greenhouse begonias.
Bacterial leaf spot of begonia is caused by Xanthomonas axonopodis pv.begonia (formerly known as X. campestris pv. begonia) Symptoms include water-soaked foliar lesions and wilting. Foliar lesions are especially visible from the underside of the foliage. Severely affected leaves may fall off the plant. This bacterium can also become systemic, meaning it colonizes the vascular tissues of the plant, causing wilting symptoms.
When I mention Xanthomonas many growers automatically think of geraniums. The good news is that this is a different “kind” or pathovar of Xanthomonas. Bacterial leaf spot on begonia is caused by a Xanthomonas that can only infect plants in the genus Begonia. There are many different types of begonias, most of which can be affected, although they vary somewhat in susceptibility.
Disease management begins with healthy plant material, because the disease is frequently spread via contaminated plant material. Inspect incoming begonias for signs of disease. Plants with any suspicious symptoms should be tested at a diagnostic lab before they are put in close proximity to other begonias. Please note that diagnostic test kits for X. axonopodis pv. pelargonii (bacterial wilt of geranium) cannot be used to detect X. axonopodis pv. begoniae.
Within the greenhouse, spread occurs when irrigation water splashes from an infected plant to the foliage of nearby hosts. Overhead irrigation should be carefully timed to minimize the amount of time that the foliage will remain wet. Longer durations of leaf wetness can promote further disease spread.
The bacterium can also be spread mechanically from plant to plant. Greenhouse staff can readily spread the bacterium by handling plants during pruning of blossoms, removing dead leaves, etc. While some handling may be required periodically throughout production, be sure to avoid handling the plant material when the foliage is wet. Frequent hand washing should be encouraged when working with susceptible begonias.
Diseased plant material should be removed and destroyed. It is also important to note that plants adjacent to symptomatic plants may be harboring the bacterium, although they may not yet be showing symptoms. This is especially true if plants are overhead watered. Sanitation is also a critical component of disease control.
Applications of copper-based-fungicides can be used to reduce disease spread. However, note that plants that are systemically infected cannot be successfully treated. Copper-based products vary in their formulations. When choosing a particular copper-based product, note the percent of active ingredient contained in the formulation as well as the REI specified on each product label. These factors may influence your choice of the particular product that you choose to apply.