White Mold
White Mold (diseases caused by Sclerotinia sclerotiorum)
The fungal pathogen Sclerotinia sclerotiorum is the most damaging of the ‘Sclerotinia diseases’ that impact sunflower in the United States. The disease, commonly called ‘white mold’, affects most broadleaf crops and weeds. Sclerotinia sclerotiorum can be devastating to sunflower, but is highly dependent on weather conditions.  The disease is particularly damaging to sunflower, in part because it can infect the plant in three different ways; 1) root infection originating from sclerotia which results in wilt or stalk rot; 2) airborne ascospore-initiated mid-stalk infection that results in mid-stem rot; and 3) airborne ascospore-initiated infection that results in head rot. 

Sclerotinia wilt/Basal stalk/stem rot, Sclerotinia mid-stem rot and Sclerotinia head rot are often treated as three different diseases, even though they are caused by the same fungus. Sclerotinia head rot is often considered the most prevalent and economically important among the sunflower diseases in the Northern Great Plains, but Sclerotinia wilt and Sclerotinia mid-stem rot frequently occur and can cause yield loss. In the Central High Plains and Southern Great Plains, Sclerotinia wilt is the most common and economically important of the three diseases, but mid-stem rot and head rot can occur in prolonged periods of cool and wet weather.

The fungus survives as irregularly-shaped, very hard and black structures called sclerotia (Figure 1). Sclerotia develop on and in Sclerotinia-infected plant tissue, are soil-borne, and can persist for many years.  The size and shape of sclerotia produced on sunflower vary greatly, ranging from ‘rat dropping’ size to as large as the face of a sunflower (Figure 2). Sclerotia produced from Sclerotinia sclerotiorum are much larger than those produced by S. minor or A. rolfsii, which is helpful when differentiating the pathogens and diseases. 
sclerotia
Figure 1. Sclerotia removed from an infected sunflower stem
sclerotia
Figure 2. Sclerotia formed over the face of an infected sunflower
Sclerotinia wilt/Basal stalk rot.  The disease cycle begins when the soil-borne sclerotia germinate to form mycelium (filamentous fungal strands), which can directly infect growing root tissue of the sunflower. Symptoms manifest weeks later and are usually observed as a whole plant wilt, that can occur singly (Figure 3), in a row (Figure 4) or in a cluster of plants (Figure 5). At the base of a wilted plant, a cream, tan or light brown colored lesion will be observed at the soil line girdling the stem (Figure 6). In humid conditions, dense white fungal growth may be apparent in and/or on the lesion (Figure 7). Hard black sclerotia may be visible in and/or on the lesion, often accompanied by dense white fungal growth (Figures 8 and 9). Infected plants may lodge and the basal lesion may shred (Figure 10).
 
basal stalk rot
Figure 3. Single sunflower plant wilting from Sclerotina basal stalk rot
row of sf plants with basal stalk rot
Figure 4. A row of sunflower plants wilted from Sclerotina basal stalk rot
 
cluster of sf with basal stalk rot
Figure 5. A cluster of sunflower plants wilted from Sclerotina basal stalk rot
 
basal stalk rot lesion
Figure 6. Sclerotina basal stalk rot lesion
basal lesion with white fungal growth
Figure 7. Basal lesion with white fungal growth of S. sclerotiorum
 
basal lesion with hard black sclerotinia
Figure 8. Basal lesion with hard black sclerotia and white fungal growth of S. sclerotiorum
basal lesion with hard black sclerotinia
Figure 9. Basal lesion with hard black sclerotia and white fungal growth of S. sclerotiorum
 
lodging caused by sclerotinia wilt
Figure 10. Lodging caused by Sclerotinia wilt
Sclerotinia mid-stem rot. The disease cycle begins when sclerotia germinate to form a small (1/4 inch in diameter) mushroom-like structure called an apothecium (Figure 11). Sclerotia are environmentally sensitive, and may only germinate when they have been previously exposed to cold conditions (winter), are in the top 1 to 2 inches of the soil profile, and when soil moisture is near saturation for 10 to 14 days. Ascospores are produced in apothecia, which are liberated and aerially dispersed.  Infected begins when ascospores colonize cast flower petals or other decaying plant material on the leaf tissue; ascospores do not infect healthy green tissue directly. Infection is favored by prolonged periods of free moisture and cool temperatures (75 to 78 F). Once established, fine strands of the fungus then grow from this infected tissue into healthy plant tissue, often causing a soft tan-brown lesion with concentric growth rings apparent (Figure 12). The pathogen will spread through the leaf and petiole into the stem, causing a cream to tan-colored lesion around the petiole. The lesion will enlarge (> 6 inches) and will girdle, shred and break the stem, lodging the plant (Figures 13 and 14). In humid conditions, white fungal growth may be visible on and/or inside the lesion. Towards the end of the season, black sclerotia will be visible on and/or inside the stem (Figure 15 and 16). Once a plant is infected plant to plant spread can occur; commonly, when healthy green tissue comes into contact with infected tissue of a neighboring plant (Figure 17).
apothecia in cereal residue
Figure 11. Apothecia of S. sclerotiorum in cereal residue
sclerotinia spreading from colonized SF floret
Figure 12. Sclerotinia sclerotiorum spreading from a colonized sunflower floret into healthy green tissue of a sunflower leaf
mid-stem lesion
Figure 13. Mid-stem lesion of Sclerotinia mid-stem rot
shredding of sf stem
Figure 14. Shredding of sunflower stem caused by Sclerotinia mid-stem rot
sclerotia in shredded midstem lesion
Figure 15. Sclerotia of S. sclerotiorum in shredded mid-stem lesion
stem breakage and white fungal growth
Figure 16. Stem breakage and white fungal growth of S. sclerotiorum in mid-stem lesion
plant to plant spread of sclerotinia
Figure 17. Plant to plant spread of S. sclerotiorum
Sclerotinia head rot. The disease cycle is the same as in Sclerotinia mid-stalk rot, but infection begins when ascospores land on flower petals still attached to the sunflower head. Symptoms are first observed as a soft, mushy, brown lesion on the back of the head (Figure 18 and 19). The lesion will enlarge to cover a quadrant or the entire head (Figure 20 and 21). As the disease progresses, the head will become skeletonized and may be decapitated (Figure 22, 23 and 24). Throughout the infection process, white fungal growth may appear on the face of the head (Figure 25). Abundant sclerotia are produced in and on the sunflower head (Figure 26, 27 and 28). Unlike sclerotia produced in the stem, they vary heavily in shape and size and a single sclerotia may be large enough to cover the entire face of the head.
soft mushy brown lesion on SF head
Figure 18. Soft, mushy and brown lesion on the edge of a sunflower head, caused by Sclerotinia head rot
soft mushy brown lesion caused by sclerotinia
Figure 19. Soft, mushy and brown lesion caused by Sclerotinia head rot
head rot lesion
Figure 20. Sclerotinia head rot lesion on the quadrant of an infected sunflower head
head rot lesion around SF neck and head
Figure 21. Sclerotinia head rot lesion progressing around the sunflower neck and head
sf head and neck colonized by sclerotiorum
Figure 22. Sunflower head and neck completely colonized by S. sclerotiorum
shredded sf head due to sclerotinia head rot
Figure 23. Shredded sunflower head due to Sclerotinia head rot
decapitated sf head as a result of sclerotinia head rot
Figure 24. Decapitated sunflower head as a result of Sclerotinia head rot
white fungal growth
Figure 25. While fungal growth (mycelium) of S. sclerotiorum on sunflower head
colonized sf head with profuse sclerotia
Figure 26. Colonized sunflower head with profuse sclerotia of S. sclerotiorum
abundant sclerotia in shredded sf head
Figure 27. Abundant S. sclerotiorum sclerotia in shredded sunflower head
 
shredded sf head with seed dropped and skeletonized head
Figure 28. Shredded sunflower head with most seed dropped, leaving skeletonized head with sclerotia present
Management of Sclerotinia wilt, Sclerotinia mid-stem rot and Sclerotinia head rot is challenging, and an Integrated Pest Management (IPM) approach is needed. Lengthening crop rotations between sunflower is very important but only partially effective, as sclerotia survive for many years in the soil and the pathogen attacks all broadleaf crops. Management of broadleaf weeds and volunteers increases the effectiveness of crop rotation. Production practices that increase canopy density (excessive nitrogen or seeding rate) should be avoided. Selection of hybrids that are less susceptible are critical in areas prone to Sclerotinia-associated diseases. Extensive research on use of foliar fungicides (including application timing, product efficacy, application technology and more) has been conducted. However, due to insufficient management of the diseases with fungicides, they are currently not recommended.  Soil-incorporation of mycoparasites for biological control of S. sclerotiorum may be available, but data is currently lacking.
Images
Figure 1. Sclerotia removed from an infected sunflower stem (Sam Markell, NDSU).
Figure 2. Sclerotia formed over the face of an infected sunflower (Tom Gulya, USDA).
Figure 3. Single sunflower plant wilting from Sclerotina basal stalk rot (Sam Markell, NDSU).
Figure 4. A row of sunflower plants wilted from Sclerotina basal stalk rot (Sam Markell, NDSU).
Figure 5. A cluster of sunflower plants wilted from Sclerotina basal stalk rot (Sam Markell, NDSU).
Figure 6. Sclerotina basal stalk rot lesion (Sam Markell, NDSU).
Figure 7. Basal lesion with white fungal growth of S. sclerotiorum (Sam Markell, NDSU).
Figure 8. Basal lesion with hard black sclerotia and white fungal growth of S. sclerotiorum (Sam Markell, NDSU).
Figure 9. Basal lesion with hard black sclerotia and white fungal growth of S. sclerotiorum (Sam Markell, NDSU).
Figure 10. Lodging caused by Sclerotinia wilt (Sam Markell, NDSU).
Figure 11. Apothecia of S. sclerotiorum in cereal residue (Sam Markell, NDSU).
Figure 12. Sclerotinia sclerotiorum spreading from a colonized sunflower floret into healthy green tissue of a sunflower leaf (Sam Markell, NDSU).
Figure 13. Mid-stem lesion of Sclerotinia mid-stem rot (Sam Markell, NDSU).
Figure 14. Shredding of sunflower stem caused by Sclerotinia mid-stem rot (Sam Markell, NDSU).
Figure 15. Sclerotia of S. sclerotiorum in shredded mid-stem lesion (Sam Markell, NDSU).
Figure 16. Stem breakage and white fungal growth of S. sclerotiorum in mid-stem lesion (Sam Markell, NDSU).
Figure 17. Plant to plant spread of S. sclerotiorum (Sam Markell, NDSU).
Figure 18. Soft, mushy and brown lesion on the edge of a sunflower head, caused by Sclerotinia head rot (Sam Markell, NDSU).
Figure 19. Soft, mushy and brown lesion caused by Sclerotinia head rot (Sam Markell, NDSU).
Figure 20. Sclerotinia head rot lesion on the quadrant of an infected sunflower head (Sam Markell, NDSU).
Figure 21. Sclerotinia head rot lesion progressing around the sunflower neck and head (Sam Markell, NDSU).
Figure 22. Sunflower head and neck completely colonized by S. sclerotiorum (Sam Markell, NDSU).
Figure 23. Shredded sunflower head due to Sclerotinia head rot (Sam Markell, NDSU).
Figure 24. Decapitated sunflower head as a result of Sclerotinia head rot (Sam Markell, NDSU).
Figure 25. While fungal growth (mycelium) of S. sclerotiorum on sunflower head (Sam Markell, NDSU).
Figure 26. Colonized sunflower head with profuse sclerotia of S. sclerotiorum (Sam Markell, NDSU).
Figure 27. Abundant S. sclerotiorum sclerotia in shredded sunflower head (Sam Markell, NDSU).
Figure 28. Shredded sunflower head with most seed dropped, leaving skeletonized head with sclerotia present (Sam Markell, NDSU).
Additional Resources
Prospects for using drop nozzles to improve fungicide coverage and control of Sclerotinia head rot. (NSA Research Forum paper)

Sclerotinia head rot of sunflower: Improving the methods used to screen sunflowers for resistance and prospects for management with fungicides. (NSA Research Forum paper)

Evaluation of fungicides for Management of Sclerotinia Head Rot on Sunflower, 2011. (NSA Research Forum paper)
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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