Kentucky vegetable growers should be on the lookout for bacterial spot of pepper this week. As the most common disease of pepper in Kentucky, homeowners or growers not actively using preventative practices will likely experience at least some bacterial spot.
The disease can occur in transplants, and when those plants are set in the field, bacterial spot can proliferate under wet, humid conditions. Bacterial spot may also become apparent in the greenhouse or high tunnel, particularly near side walls or when plants are watered overhead. In addition to numerous types of peppers, tomatoes and pimento crops are also susceptible to bacterial spot. Prevention, early identification, and management will help reduce plant losses.
Here are the symptoms to look for, preventative tactics, and brief suggestions on how to treat crops once disease is confirmed.
Cause and Disease Development
Bacterial spot is caused by multiple species of Xanthomonas bacterial pathogens. These pathogens may originate from contaminated seed, unclean surfaces, or crop debris from the previous season. Cells of bacterial pathogens are spread via water, such as rain or irrigation. All above-ground plant parts, including fruit, may become infected at any stage throughout the season, but lush new growth is especially susceptible. Bacterial spot is favored by wind-driven rain, warm temperatures, and high humidity. Once established, the disease can spread rapidly in dense plantings; in severe cases, plants can be completely defoliated.
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 (Figures 2 & 3).
Managing Bacterial Spot
- Grow or purchase disease-free transplants. Only set healthy plants.
- Select varieties with disease resistance. Note that resistance is not available for all races of the pathogen.
- Practice crop rotation by avoiding fields where other solanaceous crops were planted the previous year.
- Remove plant debris and weeds from the growing area.
- Remove and destroy heavily infected plants.
- Avoid overhead watering to reduce leaf wetness.
- Use fertility at recommended rates. Excessive nitrogen can promote lush, disease-susceptible growth.
- Improve greenhouse ventilation to reduce humidity.
- Use recommended plant spacing to facilitate air movement and leaf drying.
- Clean greenhouse benches or other materials that come in contact with the crop using 10% bleach.
After setting transplants, weekly applications of copper+mancozeb are recommended to manage bacterial spot, as well as some of the common fungal diseases. An Actigard application is also recommended 4 weeks after transplanting. In addition to Actigard, Agriphage and other biocontrol materials have demonstrated efficacy in some fields with bacterial spot pressure.
As always, all label recommendations must be followed when applying chemicals to crops. In particular, pay close attention to pre-harvest intervals.
- Bacterial Spot of Pepper & Tomato (PPFS-VG-17)
- Greenhouse Sanitation (PPFS-GH-04)
- Home Vegetable Gardening (ID-128)
- IPM Scouting Guide for Common Pests of Solanaceous Crops in Kentucky (ID-172)
- Managing Greenhouse & High Tunnel Environments to Reduce Plant Diseases (PPFS-GH-01)
- Vegetable Production Guide for Commercial Growers (ID-36)
By: Kim Leonberger, Extension Associate and Emily Pfeufer, Extension Plant Pathologist