UC Davis

Recent detections of branched broomrape (Phelipanche ramosa) in California tomato (Solanum lycopersicum) fields have led to increased interest in herbicide treatment programs to control this regulated noxious weed. Broomrapes (Phelipanche spp. and Orobanche spp.) are parasitic weeds that pose a significant risk to the processing tomato industry for several reasons that include California’s Mediterranean climate which is similar to that of branched broomrape’s native range, California agronomic practices (wide variety of host species cultivated, successive tomato crops, shared equipment) make the proliferation and spread of broomrape in and among fields highly likely, and broomrape’s phenological development makes it difficult to monitor and inaccessible to conventional weed control practices. In addition, California’s regulatory environment make soil fumigation difficult and costly and herbicides unavailable, while branched broomrape’s regulatory status as quarantine pest does not incentivize accurate reporting. A decision support system and herbicide treatment program, known as PICKIT, was developed over two decades of research in Israel, and has been proven to provide successful management of Egyptian broomrape (P. aegyptiaca) in tomato. The PICKIT system uses a thermal time model to forecast the belowground development of the parasite in order to precisely time the application of ALS inhibitor herbicides to target specific broomrape life stages. Research was conducted in 2019 and 2020 to determine if the PICKIT system could be adapted to manage branched broomrape in California processing tomatoes and to provide herbicide registration support data. Treatment programs based on the PICKIT system herbicides sulfosulfuron and imazapic were evaluated in 2019 and 2020 for crop safety on processing tomato. Treatments included several combinations of preplant incorporated (PPI) sulfosulfuron applications paired with different rates of imazapic either injected into the drip system (chemigation) or applied as foliar treatments. There were no significant differences in phytotoxicity or tomato yield among herbicide treatments in the three experiments. Additionally, a rotational crop study was conducted in which a tomato crop received PICKIT treatments in 2019 and several common rotational crops were planted and evaluated in 2020. Corn planted after the sulfosulfuron treatment suffered chlorosis and stunting, however, safflower, sunflower, melons, and beans were not injured by any of the treatments. After two field seasons, the PICKIT decision support system seems to have reasonable crop safety on processing tomato under California conditions. Rotational crop restrictions will need to be considered if branched broomrape becomes widespread in California and sulfosulfuron becomes part of a broomrape management program. An efficacy study was conducted in 2020 to evaluate the efficacy of a modified PICKIT system in California growing conditions. The study took place in an commercial field near Woodland, CA, reported to be infested with branched broomrape in 2019. This trial examined the efficacy of the PICKIT herbicides sulfosulfuron and imazapic as well as imazapyr, imazethapyr, and imazamox for control of branched broomrape. There were 12 treatments replicated four times, and 47 out of 48 plots (45 m2 ) had broomrape emergence. On average, non-PICKIT treatments had 38 broomrape clusters per plot while PICKIT treatments had 13 clusters per plot. There was a trend in which the PICKIT treatments had fewer broomrape shoots per plot than the non-PICKIT treatments, however, there were no significant differences in the number of broomrape shoots among PICKIT treatments. None of the treatments eliminated broomrape emergence; more studies evaluating PICKIT treatments should be conducted to improve the efficacy of individual treatments. Branched broomrape and Egyptian broomrape differ in phenological development and future research will investigate alternate chemigation times based on this difference. Futhermore, future research will evaluate and focus on imazamox, another imidazolinone herbicide, as this product is already registered in California on alfalfa and has a more promising registration pathway than imazapic.