PSI: The case of the spotty bacteria

Identification of the suspect

Bacteria are single celled living organisms, some of which are plant pathogens. There are several genera of bacteria and hundreds of species responsible for vascular wilts, blights, soft rots, and leaf spots of a wide variety of ornamental plants. Species in the genera Pseudomonas and Xanthomonas can cause leaf spots on chrysanthemum, geranium, impatiens, Dieffenbachia, Philodendron, Syngonium, Aglaonema and other foliage and flowering plants. Most of the leaf spotting bacteria are not specific to a particular plant. That is, most leaf-spotting bacteria can cause spotting on many different crops. Splashing water or wind-driven rain are the main ways that leaf-spotting bacteria are spread from leaf-to-leaf and plant-to-plant. However, if work is being done among plants whose leaves are wet, bacteria can be moved about on workers hands, clothing and tools.
 

When bacterial leaf spot is most dangerous

Bacterial leaf spots are most damaging at any time during production that water is on the foliage. During propagation when cuttings are being misted or whenever sprinkler irrigation is being used, water is applied directly to the foliage. These plant pathogenic bacteria require water in order to invade the plant tissue.
 

Be on the lookout

It would be wise to search the Internet for ‘bacterial leaf spot of....’, and fill in the name of the plant of concern. You will likely find several fact sheets with good photographs. Then, print off and post those fact sheets in a location where staff can refer to the photographs readily. A specific person should be designated to scout plants regularly but particularly at times during the production cycle when irrigation water is being applied directly to foliage or the plants are receiving rainfall. During warm, wet weather, bacterial leaf spots can develop and spread extremely rapidly. Everyone in the operation should be familiar with the symptoms of bacterial leaf spots and report their presence immediately. The spots are generally water-soaked (wet looking) and dark brown to black. Depending on the plant infected, the leaf spots may have a yellow halo.
 

Preventing damages

Where possible, pathogen-free plants should be purchased. In the case of vegetatively propagated crops, elite propagators go to extraordinary lengths to try to eliminate bacteria from their stock plants and thus from the cutting you purchase. In the case of crops grown from seed, seed producers may grow the plants in areas where weather and climate generally suppress bacterial diseases (dry climates). In some cases, seeds are treated to reduce the presence of bacteria but often seeds are not treated.

The most important and effective way to keep bacterial leaf spots from developing is to keep water off the foliage. Irrigate in a manner that puts the water on the potting mix, not on the leaves.

Copper, mancozeb, combinations of copper and mancozeb, and the phosphite materials can suppress bacterial leaf spot initial infections. But once a plant is infected, these chemicals cannot cure a plant.
 

Recovery and treatment

Once bacterial leaf spot has been diagnosed, it is best to discard plants exhibiting symptoms. Alternatively, carefully remove leaves that have symptoms and immediately stop watering in a manner that puts water on the foliage and protect the plants from receiving rainfall on the foliage or drip from plants hanging above them. But, never apply water to the foliage of those plants again. The bacteria can survive on the leaf tissue and resume activity as soon as they have the necessary water. Scout affected crops intensively and repeatedly and discard plants exhibiting symptoms.

Gary W. Moorman is a professor of Plant Pathology, Dept. of Plant Pathology and Environmental Microbiology, The Pennsylvania State University, (gmoorman@psu.edu)