Branched broomrape (Phelipanche ramosa) is of growing concern to California tomato and specialty crop and seed producers. It is an obligate root holoparasite that gains all of its nutrients from the host crop and its unique biology means it spends the majority of its lifecycle below the soil surface, making it inaccessible to conventional weed control strategies like cultivation or foliar herbicides. Its regulatory status as an “A-listed” pest in California requiring crop destruct and a hold order on any infested field creates a huge economic burden for affected growers, even if the weed itself is not yet at levels that cause yield reduction.

Chemigation, also known as herbigation in other parts of the world, is simply defined as the application of pesticides through irrigation water. This is NOT a conventional spray application and our previous research showed foliar-applied Matrix did not provide much broomrape control. The 24c Matrix chemigation label calls for 1.33 oz of rimsulfuron to be applied through drip irrigation three times throughout the growing season, at approximately 30, 50 and 70 days after transplant (Figs. 1,2).  Note: in our broomrape management studies, results suggest slightly earlier applications provided improved efficacy of chemigated rimsulfuron (~20, 30, 40 DAT).

The goal with chemigation is to use the irrigation water to uniformly apply the herbicide across the field and distribute it horizontally and vertically in the soil around the application point to fully treat the crop's root zone. In this case, the Matrix is applied three times to account for the relatively short persistence of rimsulfuron in the soil due to chemical and microbial degradation and ensure control of broomrape over an extended germination period. Chemigation best practices described by irrigation equipment manufacturer Netafim generally call for application starting between ½ and 2/3 of way through the irrigation set so that the soil is initially wetted, the herbicide is applied over a period of time to facilitate distribution in the soil, and the application is followed by enough time fully clear the pesticide solution from the lines.

Chemigation of herbicides and other pesticides can be done with an existing fertigation systems commonly used in our production system. One common fertigation system utilized for chemigated Matrix could include a cone tank and Venturi-injection system (Fig. 3). Simply mix the appropriate amount of Matrix for the treated area into the cone tank with water and begin application via Venturi-injection or an electric-pump. Matrix is formulated as a Water-dispersible Granule (WG), so agitation of the solution during mixing and throughout the injection is necessary. Inject the solution throughout the last 1/3 of the irrigation set and once finished, run the irrigation for another 0.5 to 1 hour to clear the distribution and irrigation lines of any residual chemigation solution.

After three seasons of research, chemigated Matrix has been shown to reduce branched broomrape emergence in California processing tomato fields. In small plot and large-scale demonstration trials, three applications of 1.33 oz/A of chemigated rimsulfuron reduced broomrape emergence by 85% and had no effect on tomato yield versus the non treated control plots (Fig. 2). Utilizing the approved rimsulfuron treatments applied through the subsurface drip irrigation system is a relatively inexpensive way to reduce your risk of broomrape in fields known to have the pest or at risk of having had it introduced via seed movement.