California Agriculture journal, April-June 2021, volume 75, issue 2

Branched broomrape (Phelipanche ramosa), a parasitic weed that was the focus of a $1.5 million eradication effort four decades ago in California, has recently re-emerged in tomato fields in several Central Valley counties. Processing tomatoes are important to the California agricultural economy; the state produced over 90% of the 12 million tons of tomatoes grown in the United States in 2018. Branched broomrape is listed as an “A” noxious weed by the California Department of Food and Agriculture (CDFA); discovery of broomrape in California tomato fields leads to quarantine and crop destruction without harvest, resulting in significant economic loss to growers. In countries where broomrape is common, yield reductions caused by this parasitic weed can range from moderate to 80%, depending upon the infestation level, host and environmental conditions. Developing a detailed understanding of the biology of this weed under local conditions is an important step towards developing effective management plans for California. In this review, we discuss branched broomrape in the context of California production systems, particularly of tomato. We also discuss the potential management practices that could help to prevent or reduce the impacts of branched broomrape in tomatoes and other host crops. Processing tomatoes are important to the California agricultural economy; in 2018, California accounted for over 90% of the 12 million tons of tomatoes grown in the United States (USDA NASS 2019). Some of the most potentially damaging pests of tomato include the weedy broomrapes (Orobanche and Phelipanche spp.), which have recently made an appearance in several California tomato fields after a 40-year hiatus. While broomrape is not currently at levels that can impact yield, presence in a field causes a large economic loss to growers because of the weed’s status as a quarantine pest. The establishment and spread of broomrape in California tomato production regions could cause severe consequences for individual growers and the entire tomato industry. Broomrapes are obligate root parasitic plants that can cause devastating damage to tomatoes and many other economically important broadleaf crops. These weeds use a modified root, called a haustorium, to fuse into a host plant root and extract nutrients and water. This greatly reduces productivity and sometimes kills the host. Globally, seven broomrape species have been identified that can cause damage to crops. Of UC Davis graduate student researcher Matthew Fatino and Emeritus UC Cooperative Extension Farm Advisor Gene Miyao conduct early season scouting for branched broomrape in a field trial at a commercial processing tomato field site. Photo: Bradley Hanson. 64 CALIFORNIA AGRICULTURE • VOLUME 75, NUMBER 2 these, small broomrape (Orobanche minor), Louisiana broomrape (Orobanche ludoviciana), Egyptian broomrape (Phelipanche aegyptiaca) and branched broomrape (Phelipanche ramosa) are known to be economically important pests in the United States (Jain and Foy 1989; Miyao 2016). Tomato is highly susceptible to both branched broomrape and Egyptian broomrape. (A comparison of these broomrapes is shown in table 1.) Branched broomrape is currently classified in California as an “A” pest. An “A” pest is an organism of known economic importance subject to state-enforced action involving “eradication, quarantine regulation, containment, rejection, or other holding action” (CDFA 2020). The discovery of branched broomrape in a commercial tomato field leads to quarantine and crop destruction without harvest; processers will not accept a load of tomatoes from an infested field. Egyptian broomrape, which, like branched broomrape, has been detected in some California tomato fields (Miyao 2016), is listed as a “Q” species. “Q” species have a temporary “A” classification pending determination of permanent rating by the state. Though Egyptian broomrape is currently considered less of a threat to California tomato crops than branched broomrape, Egyptian broomrape is also highly destructive. Studies in Israel showed that at high infestation levels (~100 shoots per square meter [m2]), Egyptian broomrape can cause processing tomato yield losses as high as 70% (fig. 1). In Chile and Israel, annual economic losses in tomato due to Egyptian broomrapes have been estimated at $5 and $200 million, respectively (Hershenhorn et al. 2009). Globally, branched broomrape is one of the most damaging and widespread of the weedy broomrape species, infesting nearly 6 million acres (about 2.6 million hectares) of broadleaf crops across Asia, the Mediterranean basin and North Africa (Mauromicale et al. 2008) (fig. 2). Branched broomrape infests a wide range of crops including tomato, cabbage, potato, eggplant, carrot, pepper, beans, celery, peanut and sunflower (table 2). A broomrape-parasitized plant suffers growth and yield reduction, and death can result in cases of severe infestation. Yield reduction can be significant depending on the level of infestation, susceptibility of the host and environmental conditions (Bernhard et al. 1998; Kogan 1994). Growers have reported up to 80% tomato crop loss due to branched broomrape in Chile (Kogan 1994). This is highly concerning given the similarity in tomato production systems and broomrape species with California. The spread of branched broomrape Branched broomrape was first documented in Europe in the 17th century (GBIF 2019), and is now present in 24 countries in Europe, North and South America, Africa and Asia (GBIF 2019; Mohamed et al. 2006). Most of the countries or locations where branched broomrape is reported have a Mediterranean climate, with warm-dry summers and rainy winters (fig. 2). In the United States, branched broomrape was first reported in 1890 and, since then, over 150 occurrences have been documented (GBIF 2019; Musselman 1996). Reports of branched broomrape in the United States have been increasing, from seven occurrences in 2015 to 65 in 2019 (GBIF 2019), and it has been documented in Texas, Virginia, South Carolina, Illinois, New Jersey, Tennessee, Kentucky, Alabama and California (GBIF 2019; USDA-APHIS 2019). In California, branched broomrape was first seen in Butte County (1903) and later in Alameda County (1929) (Hrusa 2008). 0 20 40 60 80 100 140