Bacteria are normally present on plant surfaces and will only cause problems when conditions are favorable for their growth and multiplication. Many foliage plants are susceptible to bacterial diseases, typically during gloomy winter months. Bacterial diseases are most prevalent on foliage plants during the winter months because that is when light intensity and duration are reduced. During this time, plants are not growing actively and are easily stressed. Too much, too little, or irregular watering can put plants under stress and may predispose them to bacterial infection. Other conditions that produce stress include low light intensity, fluctuating temperatures, over-crowded plants, poor soil drainage and improper soil nutrients. Improper nutrients can mean that there is a deficiency or an excess, which can lead to an increase in bacteria.
Bacterial Leaf Blight, also known as Bacterial Leaf Spot, actually grows best in warm-to-hot, wet, and humid environments. This causes poor air circulation around plants and provides a film of water on the leaves where bacteria can multiply. During the summer months, especially if plants are watered by overhead sprinklers, sufficient moisture may be present for infection when the bacteria are splashed or blown on to leaves. Wind and rain transmit the bacteria to plants as well.
Common symptoms of plants infected with Bacterial Leaf Spot and Bacterial Leaf Blight include:
- Leaf spots, blights, and wilting
- Brown or black water-soaked spots on the foliage, sometimes with a yellow halo, usually uniform in size
- The spots enlarge and will run together under wet conditions
- The spots have a speckled appearance under dry conditions
- As spots become more numerous, entire leaves may yellow, wither and drop
Prevention and Control
Prevention should be the aim! Provide plants with light conditions that are optimum for their growth. Avoid placing plants where there are conditions of high humidity, crowding, or poor air circulation. Do not mist plants and avoid wetting the foliage when watering, as bacteria need water to multiply and spread to healthy leaves. Water plants according to recommendations, being careful not to overwater them. Proper watering, repotting every 6 months to 1 year in fresh sterile soil, fertilizing every 8-12 weeks during the spring and summer, and controlling insect infestations will keep plants growing in healthy condition and lessen the likelihood of infestation by bacteria or other disease organisms.
Bacterial diseases restricted to the leaves can often be controlled because bacteria cannot penetrate directly into plant tissue. It must enter through wounds or natural openings such as stomata (pores for air exchange) in leaves. However, if the bacteria attack systemically or internally, the infection then causes the yellowing of new leaves, wilting, and a mushy, foul-smelling stem rot which often results in the death of foliage plants.
Should a plant become infected by bacteria, these suggestions may help to halt the spread of infection.
- Provide conditions that are optimum for the plant’s growth, as described above.
- Isolate the diseased plant and prune infected leaves, but avoid excessive handling of diseased plants. If more than one third of the plant is involved, prune infected leaves over a period of time, since removing too many leaves at one time will put the plant under further stress.
- Disinfect scissors before each cut by dipping them into a freshly made solution of 1 part Chlorox or Hilex bleach and 9 parts water.
- There are chemical sprays available for bacterial blight but are often not feasible from a homeowner’s perspective. Most chemicals require a full coverage spray before the disease appears, at an interval of 7-10 days and especially after each rain.
- If the disease is systemic and has spread throughout the plant internally, affecting the stems as well as the leaves, the plant cannot recover. Removing the plant to prevent spread of the bacteria to healthy plants is recommended.