2012 Annual Report
Sclerotinia head rot, caused by the fungal pathogen Sclerotiniasclerotiorum, continues to be a major yield-limiting factor in sunflower production in the United States. Recent improvements in fungicide options for white molds management in other crops and a high market price for sunflowers has resulted in a revival of interest of fungicides as a potential management option for disease. Additionally, with some improvements in genetics for resistance the possibility of fungicide application in an IPM strategy may be viable. Fungicide efficacy evaluations were conducted at the North Dakota State University (NDSU) Carrington Research Extension Center in Carrington, ND (CREC), the NDSU Langdon Research Extension Center in Langdon, ND (LREC) and the University of Nebraska Panhandle Research Extension Center in Scottsbluff, NE (UNSB). At all locations, the trials were designed as RCBD with four replications and planted in two-row plots with row width 30 inches. Two-row buffer plots were included between treatment plots. In Carrington and Scottsbluff, harvested plot lengthswere approx. 19 feet; in Langdon, harvested plot lengths were approx. 11 ft. The confection hybrid ‘Jaguar’was planted at a high seeding rate (approx. 60,000 seeds/ac) and manually thinned to 16,000 or 18,000 plants/ac at the V4 to V8 growth stage (4- to 8 true-leaves). Agronomic management of the trials, including fertility and weed control, were conducted in accordance to recommended university production practices.Sunflower heads were inoculated with a suspension of Sclerotiniasclerotiorumascospores multiple times during the R5.0 to R5.9 growth stages. To facilitate disease development, supplemental irrigation was applied for 3 to 5 days after each inoculation. In Carrington and Langdon, microsprinkler misting systems were used, with irrigation applied 1 to 5 minutes every 30 minutes; in Scottsbluff, overhead pivot irrigation was used once daily. Fungicides evaluated in this study represented a wide range of FRAC groups, and all major chemistries that have shown efficacy against Sclerotinia on other crops were included. Fungicides were applied overhead in 10 or 15 gallons of water/ac at early bloom (approx. R5.2 to R5.6 growth stages) and again approximately 7-10 days later. Disease was evaluated as incidence (total heads infected), severity on a 0-5 scale where: 0 = no symptoms, 1 = 1 to 25% of the head diseased; 2 = 26 to 50% of the head diseased; 3 = 51 to 75% of the head diseased; 4 = 76 to 99% of the head diseased; 5 = 100% of the head diseased. A disease severity index (DSI) was determined using both incidence and severity data. Yield was determined at harvest. Data was analyzed with SAS v.9.2. in CREC and LREC and ARM 8 (UNSB).
Sclerotinia head rot developed at LREC and CREC, often at high levels. The disease failed to develop at UNSB. A combination of high temperatures and the lack of microsprinkler misting capabilities were likely responsible for the lack of disease development in Scottsbluff. A severe hail storm (2 – 3 cm stones) on 24 July in CREC severely damaged sunflowers in that location and yield could not be obtained from the trial but yield assessments were successfully obtained from Langdon and Scottsbluff.No differences in disease control were observed between the non-treated controls and any of the fungicide treatments at either Carrington or Langdon. A trend towards lower disease levels was observed in several of the treatments in Langdon, but small plot sizes (11 ft by 5 ft) combined with poor sunflower emergence associated with a cool, wet spring resulted in extremely small sample sizes and highly variable results across experimental replicates in Langdon. The highly variable results across replicates made it very difficult to discern statistically significant differences across treatments in Langdon. In Carrington, levels of disease were extremely high (approaching 100% incidence at physiological maturity), and the high disease pressure likely overwhelmed the fungicides.No differences in yield were observed between the non-treated control and any of the fungicide treatments at any location. However, the small plot sizes in Langdon, hail damage and extremely high disease pressure in Carrington, and lack of disease pressure in Scottsbluff make it difficult to discern whether fungicides applied for Sclerotinia head rot control may or may not have an influence on sunflower yield.
In sum, experiments conducted in 2011 were inadequate for developing rigorous recommendations with respect to using foliar fungicides to control Sclerotinia head rot on sunflower. To ensure better results in future research, the following modifications will be made to the methods: (1) Misting protocols will be standardized to ensure that disease pressure is not excessive, (2) trials will exclusively be conducted under micorprinkler misting systems and never under pivot irrigation so as to ensure that disease establishment is successful, and (3) plot sizes will be increased to a minimum harvested length of 26 feet and width of 5 feet so as to ensure that plot-level sample sizes are large and thuswithin-treatment variability across replicates is minimal.