Reports of bacterial spot of pepper have been rolling in like the raindrops lately. Bacterial spot is one of the most common diseases on pepper, and it is caused by multiple species of Xanthomonas. These bacterial pathogens can infect many different kinds of pepper, in addition to tomato.
What are the symptoms of bacterial spot?
Lesions are initially small, brown, and circular, but may become more angular as they expand and begin to grow together. Lesion centers may whiten and eventually fall out, and leaf tissue can become distorted (Figure 1). Brown lesions can also develop on stems, fruit, and petioles, with petiole infections resulting in defoliation (Figure 2).
Older leaves fall off first in heavily infected plants, with more leaves falling off as the disease progresses. Spots on fruit may be raised or sunken, depending on the variety of pepper grown (Figure 3).
Where did bacterial spot come from?
The bacterial spot pathogen can be transmitted on seed or crop debris left over from the previous season. Infected transplants grown from infested seed, may be another source of bacterial spot. Recent weather conditions in Kentucky have been optimal for the spread of bacterial spot: wind-driven rain, warm temperatures, and high humidity. The pathogen can also enter plant tissue through wounds or hydathodes (natural openings along leaf edges for gas exchange).
How can bacterial spot be managed?
- Growing or purchasing disease-free transplants is the first line of defense against bacterial spot of pepper.
- There are varieties with resistance to bacterial spot; however, resistance is not available for all races of the pathogen.
- Pepper plants should be transplanted in an area that was not planted to any solanaceous crop (pepper, tomato, potato, etc.) the previous year.
- Wider plant spacing can help lower humidity and prevent the spread of the pathogen from plant to plant.
- If caught early in home gardens, a few infected leaves may be pruned, bagged, and removed from the area; however, this is not recommended in commercial fields due to the extensive labor required.
- Copper sprays may be used to reduce the incidence of bacterial spot, yet these sprays only protect plant tissue from new infections, rather than “curing” existing infections. Other commercial products that have been effective in some cases are plant defense-inducing products and phage-based products.
- After completion of the season, it is important to remove infected crop debris from the field. In addition, greenhouse bench space and any metal stakes or cages used should be cleaned with 10% bleach.
- If uncertified or untreated seed is purchased, it may be heat-treated prior to planting to reduce the incidence of bacterial spot. For seed treatment recommendations, see page 128 in the Appendices of Vegetable Production Guide for Commercial Growers (ID-36).
Additional Information on bacterial spot
By Emily Pfeufer, Extension Plant Pathologist
The author thanks Julie Beale, Plant Disease Diagnostician, for her review.