Southern Blight
Sclerotium diseases caused by Athelia rolfsii (Sclerotium rolfsii) include: base rot, crown rot, southern blight, collar blight

Athelia rolfsii is a soil-borne pathogen responsible for large economic losses across the world due to a broad host range, prolific growth rate and sclerotia as highly effective survival structures. More than 500 species from over 100 plant families are recorded as hosts world-wide including sunflower, legumes, maize, sorghum, cucurbits, wheat, multiple weeds and ornamental flowers. Although often a minor pathogen of sunflower, A. rolfsii is particularly damaging in tropical and sub-tropical regions where seasonal crops and weeds of the rotation may also become infected. A. rolfsii is heavily favored by hot temperatures, and distribution and severity of A. rolfsii has been increasing in countries such as Australia as temperatures increase.

The pathogen survives as small, pale to caramel brown sclerotia, resembling mustard seeds (Figure 1).
sclerotia and fungal growth
Figure 1. Sclerotia and fungal growth (hyphae) of Athelia rolfsii and sunflower seeds in a petri plate

They survive in soil and crop (and weed) residues for many years and are the source of inoculum (Figure 2).
small round caramel-colored sclerotia on soil
Figure 2. Small, round and caramel-colored sclerotia of Athelia rolfsii on soil
High soil moisture, high temperatures (77F to 95F), dense planting and frequent rain or irrigation promote germination of sclerotia, although infection can occur at lower temperatures under wet conditions. Notably, winter temperature is the major limiting factor in the range of A. rolfsii, with few reports of occurrence where winter temperatures fall below freezing. Acidic soils (pH 2-5) favor germination of sclerotia; germination is inhibited at pH 7+, calcareous soils.
 
Symptoms develop rapidly once infection occurs. In some cases, seedling infection can cause a soft, water-soaked appearance at the base of the plant and result in damping off. More commonly, a rot develops at the stem base at ground level when plants approach the flowering growth stage. Lesions appear to emerge from the soil line and spread into the stem (Figure 3).
basal lesion on sunflower
Figure 3. Basal lesion on sunflower infected with Athelia rolfsii

Lesions are round to elliptical, may appear water-soaked and may darken to a tan or dark brown or orange color as they age. Light and dark zone lines will develop across the lesion if environmental conditions alternate between wet and dry (Figure 4).
basal lesion with light and dark zone lines
Figure 4. Basal lesion with light and dark zone lines on sunflower infected with Athelia rolfsii

Unless dry conditions follow infection, white fungal (hyphae) threads colonize around the base of the plant as well as the surrounding soil surface in a fan shaped formation (Figure 5).
fungal mat and sclerotia growing on stem
Figure 5. Fungal mat (hyphae) and sclerotia of Athelia rolfsii growing on infected sunflower stem
Small pale fuzzy sclerotia can become visible on the soil line and within the mat of fungal threads within 1-2 days and darken to brown after a further 1-2 days (Figure 6).
profuse fungal growth and sclerotia
Figure 6. Profuse fungal (hyphae) growth and sclerotia of Athelia rolfsii growing on infected sunflower stem

The upper roots may also become infected and rot.  When examining a plant for suspected infection, it is very important to dig out the root ball and surrounding soil; sclerotia may remain buried if a plant is pulled out from the soil (Figures 7a and 7b).
tan colored basal lesion
Figure 7a. Tan-colored basal lesion on sunflower, without visible fungal growth or sclerotia
dark-caramel colored sclerotia on roots and soil
Figure 7b. Dark-caramel colored sclerotia on sunflower roots and soil are visible after plant was carefully dug (not pulled) and examined

 
Stem symptoms closely resemble those caused by Sclerotinia sclerotiorum wilt (basal stem rot) and Sclerotinia minor stem rot. However, the presence of profuse white fungal growth on the lower stem and across the soil surface, and the mature caramel-tan colored sclerotia help differentiate the disease.  Sclerotia produced by Sclerotinia sclerotiorum are large and black, while sclerotia produce by S. minor are small, black and angular.
 
Leaves wilt but may initially remain green (sometimes yellowing). Leaves become necrotic and wilt as plants dry out, although the plant remains erect. Yield losses will depend on the incidence of plant infection as well as the speed of symptom progression. Crops under irrigation can suffer high levels of disease whereas in dryland crops, single plants are more likely infected.
 
Active prevention and/or management may be necessary in areas where the disease is an economic problem. Using cultural practices that limit pathogen spread (such as cleaning equipment) will help the pathogen from spreading within a field and/or being introduced to new fields. Deep tillage decreases survival as S. rolfsii requires aerobic conditions. As the pathogen infects many hosts, informed crop rotation and excellent weed management may limit pathogen increases. In some areas of the world, adjusting planting dates to avoid high soil temperatures in wet seasons is recommended. Selection of a hybrid with tolerance or resistance, if available, is recommended.
Images
Figure 1. Sclerotia and fungal growth (hyphae) of Athelia rolfsii and sunflower seeds in a petri plate (Sue Thompson, University of Southern Queensland).
Figure 2. Small, round and caramel-colored sclerotia of Athelia rolfsii on soil (Sam Markell, NDSU).
Figure 3. Basal lesion on sunflower infected with Athelia rolfsii (Sue Thompson, University of Southern Queensland).
Figure 4. Basal lesion with light and dark zone lines on sunflower infected with Athelia rolfsii (Sue Thompson, University of Southern Queensland).
Figure 5. Fungal mat (hyphae) and sclerotia of Athelia rolfsii growing on infected sunflower stem (Sue Thompson, University of Southern Queensland).
Figure 6. Profuse fungal (hyphae) growth and sclerotia of Athelia rolfsii growing on infected sunflower stem (Sue Thompson, University of Southern Queensland).
Figure 7a. Tan-colored basal lesion on sunflower, without visible fungal growth or sclerotia (Sam Markell, NDSU).
Figure 7b. Dark-caramel colored sclerotia on sunflower roots and soil are visible after plant was carefully dug (not pulled) and examined (Sam Markell, NDSU).
Additional Resources
CABI. 2019. Invasive Species Compendium. Athelia (Sclerotium) rolfsii.

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). 
return to top of page
   More about Sunflower ►