Reports of southern blight have been coming in over the last several weeks, and 2016 looks to be a tough year for the disease. Southern blight, or basal stem rot as it is sometimes called, most commonly affects tomato, pepper, cucumbers, beans, and cabbage, but it has a very wide host range and can infect many other crops. The disease occurs in any system where plants are grown in natural soil. Prevention, early identification, and management will help reduce plant and yield losses.
Here are the symptoms to look for, preventative tactics, and brief suggestions on how to treat crops once disease is confirmed.
Cause & Disease Development
Southern blight is caused by the fungal pathogen, Sclerotium rolfsii. This pathogen overwinters in crop residues or in soil as sclerotia (small, round fungal structures, white when immature then transitioning to orange or tan). These sclerotia are smaller than the dark brown to black sclerotia developed by the timber rot/ white mold pathogen. Sclerotia enable the fungus to survive for many years in soils, even through adverse conditions. Southern blight is favored by high temperatures and humidity. Disease severity increases when undecomposed organic matter is left on and in the soil.
The first symptom of southern blight is a sudden wilting of foliage, which is followed by yellowing of leaves and stems/branches that turn brown (Figure 1). Plant death occurs as a result of stem or crown decay at the soil level. Infected plant parts may be covered by a white, thread-like mycelium (fungal body) (Figure 2). Over time, small, round structures, called sclerotia, appear within the mycelium and on the stem (Figure 2). Sclerotia first appear white, but then darken to a brown or orange color. When fully developed, each sclerotium is about the size of a mustard seed.
- Remove and destroy infected plants.
- Do not plant in fields with a history of southern blight.
- In gardens or on a small scale, apply a physical barrier, such as wrapping aluminum foil around the lower stem and crown of the plant.
- Bury sclerotia by deep tilling fields, and then maintain as fallow or plant non-host cover crops for several seasons.
- Soil solarization for at least 6 weeks may reduce disease incidence in the following growing season.
Southern blight is extremely difficult to manage once it becomes a problem in field or greenhouse soils. The pathogen has nearly 1,900 hosts, and can even infect common rotational crops like corn, soybean, sorghum, and rye (Farr and Rossman, USDA-ARS). Following deep tillage of an affected crop, it is important to maintain plant-free fallow periods using herbicide applications to keep the pathogen from increasing on common weeds. Ideally this would occur over multiple seasons. For growers under space constraints and/or in high-value production areas, soil fumigation by a licensed company may need to be considered.
If transplanting into a field that has had southern blight issues in the more distant past, an at-transplant treatment of Blocker 4F may be used in certain crops’ transplant water. Post-transplant, soil-directed applications of select labeled fungicides in FRAC groups 7 or 11 are recommended to continue to suppress disease through the season. For organic growers, several biological products are labeled for southern blight management.
As always, all label recommendations must be followed when applying chemicals to crops. In particular, pay close attention to pre-harvest intervals.
- Farr, D.F., & Rossman, A.Y. Fungal Databases (Systematic Mycology and Microbiology Laboratory, ARS, USDA, Retrieved July 18, 2016).
- Home Vegetable Gardening (ID-128)
- IPM Scouting Guide for Common Problems of Cucurbit Crops in Kentucky (ID-91)
- IPM Scouting Guide for Common Problems of Solanaceous Crops in Kentucky (ID-172)
- Soil Solarization for High Tunnels (Hort Fact – 7003)
- Southern Blight (PPFS-VG-03)
- Sustainable Disease Management of Cucurbit Crops in the Home Garden (PPFS-VG-19)
- Sustainable Disease Management of Solanaceous Crops in the Home Garden (PPFS-VG-21)
- Vegetable Production Guide for Commercial Growers (ID-36)
By Kim Leonberger, Extension Associate and Emily Pfeufer, Extension Plant Pathologist