Sclerotinia Minor
Sclerotinia minor
Sclerotinia minor is one of two Sclerotinia species which cause a basal rot and lesions on sunflower. The other more commonly identified species, Sclerotinia sclerotiorum causes Wilt, Stalk Rot or ‘White Mold’ at the stem base, as well as mid-stem lesions or head infection. Both species may be present in a single field, and both species can cause devastating disease outbreaks and survive on 500+ hosts, both crops and weeds. Although Sclerotinia sclerotiorum is the most dominant species on sunflower in the US, S. minor is present in some areas and yield losses have been recorded on other crops (for example peanuts).

The primary survival structures of S. minor are small, black, angular sclerotia. Size and color of sclerotia can be used to differentiate S. minor (small black) from S. sclerotiorum (large black) and also from a third basal rot pathogen, Athelia (Sclerotium) rolfsii (small brown).

Sclerotia can survive in the soil or on the soil surface in sunflower stubble or other plant residues for at least five years, depending on environmental conditions. Infection usually occurs during the vegetative and budding growth stages, and begins when fungal threads (hyphae) erupting from sclerotia directly penetrate the host tissues. High humidity (greater than 95%) and active root growth initiates infection, particularly lateral root proliferation. Infection is favored by moist soil conditions and temperatures of 18-23˚C.

Symptoms are often first observed as a pale grey to light brown basal lesions which may appear water-soaked (wet). Lesions are first seen near the soil line, and can be more readily observed by scraping soil back from the stem base. Lesions can extend upwards for up to eight inches under favorable conditions and often develop a dark brown edge and diurnal striations (Figure 1).
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Figure 1. Basal lesion on sunflower infected with Sclerotinia minor
Generally, the first symptoms appear between budding and flowering when either single plants or clumps rapidly wilt, heads droop, leaves turn necrotic and hang down along the stem (Figure 2).
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Figure 2. A cluster of sunflowers infected with Sclerotinia minor.  Note basal lesions, plant wilting, leaf drooping and shrunken heads
Premature death, smaller seeds and heads (if any) result in yield loss, which is exacerbated under hot conditions. Plants may remain standing with early senescence or lodge from the base as the stems dry and plants approach physiological maturity. Numerous small black microsclerotia may be visible inside infected plants (Figures 3 and 4).
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Figure 3. Sclerotia of Sclerotinia minor inside infected sunflower
 
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Figure 4. Sclerotia of Sclerotinia minor inside infected sunflower
An Integrated Pest Management (IPM) approach is recommended for Sclerotinia minor. Ideally, all possible measures should be utilized to minimize the threat of introduction and spread of the pathogen by ensuring cultivation and harvesting equipment are free of plant residues. Crop rotations away from broadleaf crops to corn, sorghum or cereals can help reduce inoculum and disease potential. Excellent weed control is important during rotation years, as weeds and volunteers also aid pathogen survival. Use of less susceptible or tolerant sunflower hybrids (if available) will minimize disease severity, however little information is available for varietal tolerance to S. minor in sunflower.

Tillage is of limited value, and it will reduce the inoculum load near the surface initially, but in following years further tillage may bring surviving sclerotia to the surface. Fungicide applications for management of S. minor have not been recommended (or are available) on sunflower in the US or in other countries (such as Australia). Investigation of biocontrol agents is ongoing.
Images
Figure 1. Basal lesion on sunflower infected with Sclerotinia minor (Sue Thompson, University of Southern Queensland).
Figure 2. A cluster of sunflowers infected with Sclerotinia minor.  Note basal lesions, plant wilting, leaf drooping and shrunken heads (Sue Thompson, University of Southern Queensland).
Figure 3. Sclerotia of Sclerotinia minor inside infected sunflower (Sue Thompson, University of Southern Queensland).
Figure 4. Sclerotia of Sclerotinia minor inside infected sunflower (Sue Thompson, University of Southern Queensland).
 
Additional Resources
CABI Invasive Species Compendium. Sclerotinia minor.

Thompson S (2017). Sunflower, Section 9, Diseases, GRDC Grownotes. Grains Research & Development Corporation, Barton ACT, Australia.
 
Other NSA Resources
Disclaimer statements
Information based in part on and reproduced from Kandel, H., Endres, G. and Buetow, R. 2020. Sunflower Production Guide. North Dakota Extension Publication A1995. Informational updates made possible by the Sunflower Pathology Working Group, and is/was supported by the USDA National Institute of Food and Agriculture, Crop Protection and Pest Management Program through the North Central IPM Center (2018-70006-28883).
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