Sclerotinia sclerotiorum deBary is a
major soilborne plant pathogen. Sclerotinia sclerotiorum infects
nearly 400 plant species and causes economic damage to a wide range of
crops (7). Sclerotinia sclerotiorum is responsible for causing root,
crown, and stem rots on various plant hosts.
Host Range and Distribution
Sclerotinia sclerotiorum has a world-wide
distribution, however it is most prevalent in cool moist regions (2). The
fungus causes disease on such crops as tobacco, vegetables, and ornamentals.
Sclerotinia sclerotiorum can be isolated from host tissue and soil. In order to isolate the fungus from the host, sclerotia are collected from infected plant parts and air-dried. Isolation from soil requires soil sieving to collect sclerotia. Sieves with openings of 0.43-2.00mm are used to collect sclerotia of S. sclerotiorum (5). Sclerotia can be germinated by placing on moist filter paper, surface-sterilized with NaOCl or CaOCl, sectioned aseptically, and plated on PDA. To isolate S. sclerotiorum from diseased tissue, sections are incubated in a black plastic bag with moist filter paper at 20 C (5). After several days hyphae can then be transferred to PDA and incubated at room temperature.
A medium for isolating airborne ascospores
of S. sclerotiorum was developed by Steadman, et al. (7). This medium
consists of PDA amended with pentachloronitrobenzene, penicillin, streptomycin,
and bromophenol blue. The medium will change from blue to yellow as colonies
of S. sclerotiorum develop. The yellow halo is a result of oxalic
acid production by the fungus. Gutierrez and Shew modified the medium by
adding calcium chloride and etridiazole. Sclerotinia was detected
by the production of small round colonies with a raised mycelial mass and
a yellow halo. (3). The raised mycelial mass was not observed in the medium
developed by Steadman et al..
Sclerotinia sclerotiorum is best identified by observation of the ascoma.
The ascoma is apothecioid in shape and yellow-brown to tan in color. Apothecia
are not formed on a stroma, rather they arise from a sclerotium produced
by mature hyphae. Apothecia produce asci that will stain blue when exposed
to iodine. Sclerotia formed by this fungus are large, irregular in shape,
and dark-brown to black in color. This fungus can be partially identified
on plant material by the production of a white fluffy mycelium. (4)
Signs and sypmtoms caused by S. sclerotiorum
vary depending on the host. However, the most obvious sign is the appearance
of white fluffy mycelial growth that will later produce sclerotia (1).
The fungus infects and produces mycelium at the base of the plant that
will eventually move up the stem causing it to rot.
Sclerotinia sclerotiorum on tobacco seedlings (collar rot) produces small black-brown lesions at the base of the stem. These will eventually cause rapid destruction of the seedlings as the fungus moves up the stem and into the leaves (6). Fruits such as cucumber, squash, and eggplant are also susceptible to S. sclerotiorum. In this case the fungus causes a wet rot that will eventually engulf the entire fruit. White mycelium can be seen both externally and within the infected fruit (1).
Ecology and Life Cycle
Mycelium of S. sclerotiorum will give rise to sclerotia that later germinate to produce apothecia. After apothecia have formed, they mature to produce asci containing ascospores. These ascospores will discharge from the asci, land on host tissue, and germinate to form hyphae. The hyphae will infect lower stems and roots eventually invading plant tissues, which will result in collapse. Mycelium will continue invading tissues while spreading throughout the rest of the plant and eventually form sclerotia.
There are essentially three methods of controlling Sclerotinia sclerotiorum; chemical, cultural, and biological control. Chemical and cultural control is most widely used, while biological control is used to a lesser degree. The only chemical control that is employed to control S. sclerotiorum is fumigation. The soil is fumigated to reduce the amount of inoculum present. Cultural practices are often used to prevent the spread of the pathogen from one host to another by killing the pathogen or by providing unfavorable conditions for the pathogen to develop. Sanitation, proper ventilation, and adequate spacing of plants are all effective cultural practices used by growers in controlling S. sclerotiorum. Although biological control is used less extensively than chemical and cultural methods, some biological control agents have been labeled. Many antagonistic fungi such as Coniothyrium minitans, Gliocladium roseum, Gliocladium virens, Sporodesmium sclerotivorum, and Trichoderma viride have been shown to provide effective control of S. sclerotiorum (1). These mycoparasitic fungi inhibit the growth of S. sclerotiorum by destroying existing sclerotia while inhibiting the formation of new sclerotia (1). All three methods for the control of S. sclerotiorum are important to long term health and viability of the plants affected by this pathogen.