Click on any
image for a more
detailed view

Symptoms and signs

Sclerotium rolfsii infects seedlings, herbaceous plants, woody plants, and fleshy roots, bulbs or fruits. Most frequently, this fungus infects lower stems near or at the soil surface, but it also may infect any part of a susceptible plant as long as favorable environmental conditions exist. The first symptom usually noticed by the homeowner or grower is wilt. Wilted plants often decline and die rapidly as a result of an extensive lower stem rot.

Crown and Lower 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 (Figure 1). The lesion spreads rapidly to girdle the stem (Figure 2). On many herbaceous plants and seedlings, the girdling lesion will quickly cause the plant to wilt (Figure 3) 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 (Figure 4).


Figure 1	Figure 2

Figure 3	Figure 4

On peanut, one or more branches may wilt and leaves on these branches appear slightly faded and then turn brown (Figure 5) . 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.


Figure 5

As lower stems of herbaceous plants decay, a white mat of mycelium develops at the lesion site (Figure 6). 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 (Figure 7). These mustard-seed-sized structures, called sclerotia, soon become smooth and light tan, brown or black in color (Figures 8,9). 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.


Figure 6	Figure 7

Figure 8	Figure 9

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 (Figures 10, 11). Foliage wilting and dieback develop as a consequence of the rotting of the lower trunk or crown tissues (Figure 12).


Figure 10	Figure 11

Figure 12

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 (Figure 13). The mycelium eventually spreads over the base of grass blades where sclerotia develop (Figure 14). 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 as Dichondra spp. when conditions are warm (above 24ºC / 75ºF) and moisture is abundant.


Figure 13	Figure 14

Root Decay
On some plants, such as tomato, pepper, and sweet potato, root infection may follow crown infection. On apples, roots are the primary infection site and crown rot develops subsequently. Usually the characteristic white mycelial mat and sclerotia develop near and on infected crown tissues or in and around roots close to the soil surface (Figures 10, 11, 15 ). The leaves eventually die, and branch dieback develops (Figure 4). The lower stem (trunk) above the girdling lesion usually appears normal for a long time and will be the last part of the plant to die.


Figure 15	Figure 16

Tuber, Bulb, and Fleshy Organ Rot
Sclerotium rolfsii causes a soft rot of fleshy organs. On potato tubers, small slightly sunken lesions (2-3 mm) at lenticels will develop into yellow-tan colored lesions. Tissues become soft and collapsed. Mycelium and sclerotia develop abundantly on rotting tissues (Figure 16). Sometimes bacteria enter the previously damaged tissues, and a sour odor is noted. When narcissus bulbs become infected, several layers of scales become reddish brown with white streaks. Rotted tissues are initially moist, but later tissues dry and smell of dried, rotted wood. Sclerotia develop in the rotted areas.

Fruit
Tomato fruit and other fruit at or near the soil surface may become infected with S. rolfsii. Soft, water-soaked, sunken, slightly yellowish lesions develop. These lesions quickly spread throughout most or all of the fruit, which will eventually collapse. Coarse white mycelium develops with sclerotia (Figure 17).


Figure 17	Figure 18

Leaves
On low growing plants such as strawberries, statice, and peanut, leaves touching the soil may be infected with S. rolfsii. Necrotic leaf spots have been described on many plants (Figure 18). Usually the high humidity near the soil surface is an important factor in disease development. There have been some reports of leaf spots at locations considerably above the soil. For example, S. rolfsii has been reported to cause a leaf spot on tung oil trees on leaves 2-3 m (6-9 ft) above ground level. Also, an unusual leaf spot has been described on soybean in North Carolina. The spots are circular, medium brown to light brown or straw-colored with narrow borders and are usually one cm or less in diameter. Many of the lesions have concentric rings, and small clumps of mycelium on them often develop into sclerotia.

RETURN TO TOP