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Symptoms and Signs

White mold is easily identified by the characteristic white cottony mycelium of the pathogen that grows on the surfaces of infected aerial tissues (Fig. 1). The hyphae produce enzymes and oxalic acid, creating water soaked lesions, frequently with a distinct margin. Secondary symptoms such as wilting, bleaching, and shredding also can be observed on above-ground tissues including stems, leaves, petioles and reproductive organs.

At later stages of the disease, the cottony hyphae of the pathogen aggregate into (typically) pea-sized clumps of mycelium (Fig. 2). These clumps eventually mature into hard black sclerotia (Fig. 3), which are most commonly found on the outer surface of the diseased tissue, but sometimes inside of soft host tissues or cavities such as floral receptacles, fruits and the pith of stems.

Figure 1

Figure 2

Figure 3

Aerial infections
Initially, lesions caused by ascospores of Sclerotinia spp. appear as water-soaked spots that expand irregularly and indeterminately. As the lesions enlarge, affected stems may be girdled (Fig. 4), which causes distal portions of the plant to wilt and then become necrotic (Fig. 5). Affected tissues may develop a soft watery consistency, and a thick white mycelium forms, often both inside and outside the tissue. As the disease progresses, desiccated plant tissues appear bleached (Fig. 6) compared to those senescing normally, and frequently, diseased tissue develops a shredded appearance.

Figure 4

Figure 5

Figure 6

Crown rots and basal stem infections
These kinds of infections occur when S. minor, and occasionally S. sclerotiorum, infect plant roots and/or crowns by hyphal, or ‘eruptive’, germination of soil-borne sclerotia. In sunflower wilt disease, caused by S. sclerotiorum, tissues near the soil line typically show symptoms about the time of flowering; as the infection spreads, a girdling, basal stem canker is formed (Fig. 7) with bleached and shredded pith. In wet weather, fluffy white mycelium forms on the stem surface. The crown and basal stem cankers can cause plant turgor pressure to decrease rapidly, resulting in a sudden wilting of leaves. In ‘drop’ disease of lettuce, infection of roots by S. minor leads to crown infection and girdling of the stem; plants turn brown with a soft watery decay, followed by collapse of the leaves and finally of the whole plant (Fig. 8).

Figure 7

Figure 8

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