Written collaboratively by Ruth Beck and Febina Mathew.
The wet growing season of 2019 resulted in some serious disease issues in sunflowers in some areas of South Dakota. Diseases caused by the fungus Sclerotinia sclerotiorum, include Sclerotinia wilt/basal stalk rot; mid-stem rot; and head rot. This fungus is already known to cause “white mold” in many crops such as soybean, dry edible beans, canola, and broad-leaf weeds.
Fungicides Not Effective
Sclerotinia diseases are a concern in sunflowers, because the fungus can infect the plant through the root, mid-stalk or the head. Fungicides containing active ingredients from the SDHI family (e.g., penthiopyrad) are labelled. However, the data from the fungicide efficacy trials are not consistent across years and/or locations. Additionally, due to multiple infection routes of the fungus and the difficulty of achieving satisfactory fungicide coverage to the target tissues, it is difficult to control Sclerotinia diseases in sunflower with fungicides.
Wet Conditions Increase Incidence
Wet weather and cool temperatures 2-3 weeks prior to and during flowering favor disease development. The fungus survives in the soil and crop debris as black particles called sclerotia. When the soil is wet, sclerotia germinate and produce mushroom-like structures that release fungal spores into the air. These spores are carried by wind and infect the sunflower heads to cause head rot. As the disease develops, the infection spreads down the stalk and the head looks like a straw broom (Figure 1). Also, on the head, large sclerotia develop and infected seeds fall out (Figure 2). Sunflowers are most susceptible to infection during bloom or the R5 growth stage.
Once the sunflowers reach the R6 growth stage when bloom is complete and ray and disk flowers are wilted, susceptibility of sunflowers to sclerotinia head rot drops sharply. Infections only occur at the R6 growth stage when conditions are very favorable for disease (very cool and wet). (Personal communication –Dr. Michael Wunsch, NDSU)
Sclerotinia head rot has a long latent phase, with the pathogen growing in the interior tissues of the sunflower head without showing outward signs of disease (personal communication-Dr. Michael Wunsch). Therefore, as we saw in South Dakota in 2019, symptoms usually do not appear until the R7 or R8 growth stage, but all infection events occur during bloom.
Crop rotation can be very important in managing diseases in sunflower, and it is recommended to have a three-year break between sunflower crops. However, crop rotation is not thought to be effective in managing sclerotinia because the sclerotia bodies can survive for three to seven years.
The sclerotia may deteriorate in cold open winters if left on the soil surface. However, tillage practices can bury sclerotia and reduce their number on the soil surface.
Foliar desiccants may be applied to reduce the rapid development of Sclerotinia head rot and disease damage at the end of the season.
Variety Selection Can Reduce Infection in Oilseed Types
No commercial hybrids have complete resistance to Sclerotinia head rot; however, hybrids are available with partial resistance or tolerance to the fungus. For information about partially-resistance or tolerant sunflower hybrids, please refer to seed company representatives.