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Pathogen Biology

Sclerotinia sclerotiorum (Lib.) DeBary, S. minor (Jagger), and S. trifoliorum (Eriks) are fungal pathogens in the Ascomycetes, Order Helotiales. All three species produce fluffy white mycelium on and in infected plant parts. This mycelium aggregates itself into sclerotia (Fig. 9, 10), which are the structures that allow Sclerotinia species to survive in soil in the absence of a plant host.

Sclerotia have a hard, black exterior rind with a white to light beige interior. They are irregularly shaped, and typically measure 2 to 5 mm in diameter and up to 25 mm in length for S. sclerotiorum and S. trifoliorum, and smaller (0.5 to 3 mm) for S. minor. Sclerotia buried in the plow layer of soil can survive and remain infective for up to 5 years.

Carpogenic (apothecial) germination of sclerotia
Sclerotia of Sclerotinia spp. germinate by one of two methods. Sclerotia of S. sclerotiorum and S. trifoliorum most commonly produce a mushroom-like fruiting body termed an apothecium (Fig. 11). At soil depths of up to 2 cm, apothecia can extend from the sclerotia to reach the soil surface. One or several apothecia can emerge from a single sclerotium (Fig. 12). Apothecia are fleshy-colored discs measuring 4 to 8 mm in diameter. They produce ascospores (Fig. 13) as a result of a sexual process. Each apothecium may release in excess of 10 million ascospores (Fig. 14) over a period of several days, which are blown by wind to the aerial portions of plants. Ascospores are hyaline (clear or non-pigmented), unicellular and thin-walled, and consequently, they survive for only a few days after release.

Figure 9	Figure 10	Figure 11
Figure 12	Figure 13	Figure 14

No asexual spores (conidia) are produced by Sclerotinia species.

Myceliogenic (hyphal) germination of sclerotia
S. minor sclerotia rarely form apothecia, germinating instead by the direct emergence of hyphae (termed ‘myceliogenic’ or ‘eruptive’ germination). White hyphal strands extend from sclerotia that have been stimulated to germinate by host plant exudates. These hyphae infect roots, and crowns and other plant parts that are touching the ground. Infection then spreads to aboveground plant parts.

Occasionally, S. sclerotiorum germinates myceliogenically rather than carpogenically. In a sunflower crop, this type of germination can result in a serious disease called sunflower wilt (or basal stem rot) (Fig. 7).

Figure 7

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