Southern stem rot

On tomato, peanut, pepper, and many other herbaceous and woody hosts, disease begins with a small, water-soaked lesion on the lower stem at or near the soil surface. The lesion spreads rapidly to girdle the stem. On many herbaceous plants and seedlings, the girdling lesion will quickly cause the plant to wilt and fall over. With container Hosta, older and lower foliage wilts, becomes yellow and then brown. Lower stems (crowns) often become rotted, but rotting may not extend completely throughout the crown tissues. Hosta plants may or may not completely die as a result of infection. On mature pepper and tomato, the stem cortex several centimeters (inches) above and below the soil surface will decay, but the stem central cylinder does not decay. As the lower stem decay develops, plants usually remain erect and foliage wilts. On many host plants, wilted leaves gradually become brown and remain hanging on the plant.
On peanut, one or more branches may wilt and leaves on these branches appear slightly faded and then turn brown. A few branches often survive on each plant. Peanuts infected in dry weather will have 2.0 - 2.5 cm (~1 in.) cankers located 2 - 3 cm (1-2 in.) below the soil line. Only one canker is usually seen per plant. When stems are partly girdled, leaves are often small with a mild brown coloration, but wilt does not occur. When peanuts are infected in wet, hot weather, stems become totally rotted except for the xylem.
As lower stems of herbaceous plants decay, a white mat of mycelium develops at the lesion site. This white mat will often spread out onto the nearby soil surface. Shortly after the mycelial mat develops, small (0.5-1 mm), white, round, fuzzy mycelial bodies begin to appear. These mustard-seed-sized structures, called sclerotia, soon become smooth and light tan, brown or black in color. Sclerotia serve as overwintering bodies and may be seen in the mycelium, on diseased tissues above or below ground, on soil surfaces, or in soil crevices.
Sclerotium rolfsii infections of woody plants usually begin as a crown rot. Characteristic white mycelial mats and sclerotia also develop at the crown infection sites when conditions are favorable. Foliage wilting and dieback develop as a consequence of the rotting of the lower trunk or crown tissues.
When monocots such as wheat and some other grasses are infected, symptoms and signs of the disease are different from those described above for dicots. Brown lesions occur at the crown and lower parts of the culm. Lesions are often small, but they may extend into the hollow part of the culm. Strands of mycelium grow inside the lower internodes. Seedheads may appear normal, but they are devoid of grain. Premature ripening also may occur. On millet, grain sorghum, and tall fescue, the leaf sheath becomes water-soaked, and the brown discoloration turns to dark brown or black. Mats of white mycelium develop and spread upward inside and outside the leaf sheaths. The mycelium eventually spreads over the base of grass blades where sclerotia develop. Southern blight is not common on wheat and small grains, but it does occur on cool season turf species such as bentgrass, fescue, perennial ryegrass, bluegrass, and broad leaf turf species such asDichondra spp. when conditions are warm (above 24 C / 75 F) and moisture is abundant.