Branched broomrape (Phelipanche ramosa) continues to challenge California’s processing tomato industry, particularly in the Sacramento Valley. As a parasitic plant that attaches to tomato roots, its presence threatens both yield and export market access. In a recently published study in Weed Technology, we evaluated the efficacy of chemigated rimsulfuron—a recently registered approach with potential to offer targeted, soil-based suppression of this invasive pest. In addition to chemigated rimsulfuron, preplant incorporated and chemigated sulfosulfuron and foliar maleic hydrazide were evaluated alone and in combination with chemigated rimsulfuron. 

Rimsulfuron via Chemigation: How and Why?

Rimsulfuron, an ALS-inhibiting herbicide, is labeled for postemergence use in tomatoes, but previous work suggested potential for suppression of broomrape when applied to the soil. In this study, we used drip chemigation to deliver rimsulfuron below the soil surface, directly into the root zone—where broomrape seeds germinate and attach.

Chemigation offers a practical way to target broomrape during its subterranean attachment phase. Our work refined previously developed protocols to identify a sweet spot: early-season applications of rimsulfuron, synchronized with broomrape germination cues, provided the most consistent suppression.

We tested a range of application timings (single and sequential) and rates across multiple field trials with multiple combinations of rimsulfuron, sulfosulfuron, and maleic hydrazide. Treatments were timed to match key broomrape developmental windows based on growing degree-day accumulation. Results showed that three sequential applications of 1.33 oz/a of chemigated rimsulfuron significantly reduced broomrape emergence and did not impact crop safety. Preplant incorporated sulfosulfuron may offer additive performance when combined with chemigated rimsulfuron, while chemigated sulfosulfuron had similar levels of reduction to chemigated sulfosulfuron. Foliar maleic hydrazide was also fairly successful in reducing broomrape emergence versus control. 

Practical Takeaways for Growers and PCAs

• Sequential rimsulfuron chemigation can reduce broomrape emergence by 80% or more.
• Timing is critical: GDD-based scheduling targets early parasitic stages before emergence. Earlier applications (~20, 30, 40 days after transplant) tended to have slightly better efficacy than later applications.
• No visual phytotoxicity or negative yield impacts were observed in treated tomato crops.

This method provides an in-season management tool for broomrape, which has traditionally been difficult to control once established. While not a standalone solution, chemigated rimsulfuron could become an important part of an integrated broomrape suppression strategy. Sulfosulfuron and maleic hydrazide are not currently registered for this use but ongoing research may support future registration of these products

For more information, check out the full article in Weed Technology:
https://doi.org/10.1017/wet.2024.31