Grapevine powdery mildew caused by the fungal pathogen Erysiphe necator is a persistent threat to wine grape production in California. While synthetic fungicides remain the primary management strategy, increasing concerns over pesticide resistance, environmental impact, and regulatory restrictions have driven interest in biofungicides as a sustainable alternative. However, their efficacy is inconsistent, requiring research into optimized application strategies, integration with synthetic fungicides, and their effects on disease suppression, grape yield, and fruit quality. This two-year study (2023–2024) was conducted at two vineyard sites, Trestle Vineyard in the San Luis Obispo Coast AVA and a commercial vineyard in the Arroyo Grande Valley AVA. In each experimental trial, 14 treatments were tested, which included three biofungicides applied alone at three different intervals, weekly, biweekly, and based on powdery mildew risk assessment index (RAI), and in rotation with synthetic fungicides, a grower standard, and a non-treated control. The treatments were assessed by disease incidence and severity, grape yield, juice chemistry (Brix, pH, titratable acidity), and economic profit. Field trials were conducted using a randomized complete block design, and the tested biofungicides contained the active ingredients extract of Reynoutria sachalinensis, Streptomyces lydicus Strain WYEC 108, and Bacillus subtilis Strain QST 713. Disease pressure was monitored using RAI, and an economic analysis was conducted to assess treatment viability based on input costs, application expenses, and revenue differences. Results demonstrated that treatments with biofungicide-synthetic rotations provided disease suppression comparable to synthetic-only programs, particularly when applied at optimized intervals. Standalone biofungicide treatments exhibited variable efficacy, with greater disease suppression under moderate disease pressure but reduced effectiveness under high-pressure conditions. At Trestle Vineyard, refinements in application timing and a typical disease progression led to improved control in 2024 compared to 2023. In Arroyo Grande Valley, biofungicides performed better under moderate disease pressure but declined in effectiveness when disease pressure intensified, emphasizing the role of environmental conditions in treatment success. Beyond disease suppression, biofungicide applications indirectly influenced grape chemistry, as improved disease control correlated with higher yield and balanced juice composition. Diseased clusters exhibited elevated °Brix and pH, suggesting stress-induced ripening, whereas healthier clusters maintained a more desirable sugar-acid balance, which is crucial for wine quality. Economic analysis confirmed that integrated biofungicide-synthetic programs consistently provided strong disease control and financial returns, reinforcing their potential as sustainable tools for powdery mildew management. This study highlighted the importance of strategic application timing, environmental conditions, and disease pressure in optimizing biofungicide efficacy. By demonstrating that biofungicides can effectively contribute to disease management when integrated with synthetic programs, these findings support efforts to reduce synthetic fungicide reliance while maintaining vineyard productivity and fruit quality.

<p>  </p> <p><em>Sclerotinia sclerotiorum </em>is a soilborne pathogen of soybean that causes Sclerotinia stem rot, alternatively called white mold. Sclerotinia stem rot can cause significant yield losses under cool and wet environmental conditions. Two biofungicides, <em>Coniothyrium minitans </em>and <em>Bacillus amyloliquefaciens, </em>are currently available and labeled to limit or suppress <em>S. sclerotiorum</em> in soybean. These biofungicides can be applied in place of synthetic foliar fungicides to provide an alternative mode of action for the control of Sclerotinia stem rot. However, limited information is available regarding the efficacy of <em>C. minitans </em>and <em>B. amyloliquefaciens </em>as biocontrol agents of <em>S. sclerotiorum </em>in soybean and the sensitivity of the biofungicides biological activity on <em>S. sclerotiorum </em>to pesticides commonly used in soybean production systems. This research aims to provide management recommendations for <em>S. sclerotiorum </em>in soybean using <em>C. minitans </em>and <em>B. amyloliquefaciens </em>and to develop guidelines for how to incorporate the biofungicides into an established soybean pest management program. To assess the effectiveness of <em>C. minitans </em>and <em>B. amyloliquefaciens </em>as biocontrol agents of <em>S. sclerotiorum </em>dual culture, amended media, and soil plate assays were conducted along with experiments in the growth chamber and field. The presence of a distinct inhibition zone surrounding the <em>B. amyloliquefaciens </em>colony in the dual culture assay and the absence of mycelial growth on the media plates amended with <em>B. amyloliquefaciens </em>confirmed that the bacteria can control the mycelial growth of <em>S. sclerotiorum </em>through antibiosis. The absence of an inhibition zone surrounding the <em>C. minitans </em>isolate in the dual culture assay along with the degradation of sclerotia following treatment with <em>C. minitans </em>in the soil plate assay indicates an inability to limit the mycelial growth of <em>S. sclerotiorum </em>and confirms that the primary mode of action is mycoparasitism. In the growth chamber, <em>B. amyloliquefaciens</em> at 14.03 L/ha applied using the dip method significantly reduced Sclerotinia stem rot lesion length when compared to the non-treated control and resulted in the lowest lesion area under the disease progress curve (lAUDPC). When <em>B. amyloliquefaciens </em>and <em>C. minitans </em>were applied in the field, no differences were observed between treatments for soybean moisture, test weight, or yield. To evaluate the sensitivity of <em>B. amyloliquefaciens </em>and <em>C. minitans</em> biological activity on <em>S. sclerotiorum </em>to pesticides commonly used in soybean production systems a poison plate assay as well as soil plate, growth chamber, and field experiments were conducted. In the poison plate assay <em>C. minitans </em>was most sensitive to the preemergence herbicide flumioxazin and the synthetic fungicides boscalid and fluazinam, while <em>B. amyloliquefaciens </em>was sensitive only to the synthetic fungicide fluazinam. In the soil plate assay the mycoparasitic activity of <em>C. minitans </em>on sclerotia of <em>S. sclerotiorum </em>was sensitive to flumioxazin, metribuzin, glyphosate, picoxystrobin, and boscalid. In the controlled environment experiments, none of the pesticides tested decreased the efficacy of <em>B. amyloliquefaciens</em>. There were no significant interactions between <em>C. minitans </em>and <em>B. amyloliquefaciens </em>with preemergence herbicides, postemergence herbicides, and synthetic fungicides for soybean moisture, test weight, and yield. This research demonstrates that <em>B. amyloliquefaciens </em>and <em>C. minitans </em>are effective biocontrol agents of <em>S. sclerotiorum </em>in soybean. However, antagonistic relationships exist between the biofungicides and certain preemergence, postemergence, and synthetic fungicides used in soybean production systems.</p>