Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
An astute grower noticed a scattering of wilted pepper plants in an otherwise very nice-looking pepper field (Fig. 1). Upon closer inspection she noticed that at the base of a wilted plant the stem had a white mycelial growth (Fig. 2). When pulled out of the soil and examined the white mass was much more extensive and dark streaks could be seen along the stem (Fig. 3). And if you look closely at the base of the plant just above the soil line you can see brown/tan round sclerotia on the stem. This last observation is conclusive for the problem being Southern blight.
Southern blight is caused by the fungus Athelia rolfsii (syn Sclerotium rolfsii). This fungus likes very hot (>86oF day and >68oF night) and usually rainy weather. Unlike other diseases that cause lower stem and crown rot, southern blight is actually more prevalent in well drained soils. Southern blight has a wide host range, affecting over 500 plant species. Vegetable and fruit hosts include tomato, pepper, onion, strawberry, carrot, lettuce, cucumber, melon and asparagus.
The fungus survives on plants (including weeds), and in plant debris as well as in the soil as sclerotia, which can persist for several years. The fungus can be spread through movement of infested soil and plant debris, on infected plants, in contaminated irrigation water, and through contaminated equipment. Fruit that touches infested soil or that has soil splashed up on it can become infected, resulting in a wet rot and rapid decay of fruit. The fungus is generally restricted to the upper 2 to 3 inches of soil and will not survive at deeper depths. Which would indicate it would be possible to use steam treatment on the soil for management.
Cultural Controls: Use well-composted mulch. Avoid overwatering. Time irrigation lengths and frequencies by taking rainfall into account. During the season, remove and destroy symptomatic plants. Deep plow or bury crop debris at the end of the growing season. Avoid movement of infested soil to clean fields. Because acid soils favor disease development and germination of sclerotia is inhibited at a pH of 7 or greater, try to keep soils from becoming acidic. Rotate host crops with corn, wheat, barley, or other non-host crops for 2-3 years. Inoculum levels can be reduced by burying infected plant debris and sclerotia by deep plowing. Be sure the previous crop is decomposed prior to planting, which may require disking the field several times. Tomato varieties grafted onto tomato rootstocks ‘Maxifort’, ‘Big Power’, and ‘Beaufort’ have been shown to maintain yields in southern blight infested fields.
Chemical controls include using azoxystrobin 2.08F or Terraclor 75WP applied to transplant water. Check label before using. Infected soils can be steamed to a depth of 3-4 inches.
Figure 1. Single pepper plant wilting with other nearby plants looking good.
Figure 2. Wilted pepper plant with white mycelial mass at bottom of stem with a canker.
Figure 3. Much larger mycelial mass at bottom of wilted pepper plant with brown streaking of stem