First report of Orobanche cumana Wallr. (sunflower broomrape) in America

Orobanche cumana Wallr. is a root parasitic plant lacking chlorophyll. The only crop it attacks is sunflower (Helianthus annuus L.), causing broomrape, that is a major disease of sunflower in Europe and Asia. The most effective control alternative is genetic resistance to races A to G of the parasite, which have increasing virulence (Molinero-Ruiz et al., 2015). Broomrape causes worldwide annual crop losses of more than 2 billion € (Cvejic et al., 2020). In 2023 important attacks of broomrape were observed for the first time in American sunflower fields, particularly in Bolivia (region of Santa Cruz de la Sierra). Parasite attacks were again observed in the same region, in the areas of El Puente and San Julian, in 2024. In El Puente, three fields of 15, 35 and 40 hectares, were MG-360 hybrid was grown, had 40, 20 and 25%, respectively, of plants affected. In San Julián, two 4-ha fields (hybrids SYN and MG-360, respectively) showed incidences of affected sunflowers of 25 and 30%. Heavily infested patches had 100% broomrape incidence. Average infestation levels varied between 2 and 65 broomrape stems per sunflower plant, and average yield losses in the fields were between 10 and 60%. In each of the five fields dry broomrape stems were uprooted and parasite seeds recovered with sieves of mesh size between 0.8 and 0.25 mm. The parasite was morphologically identified as O. cumana (Pujadas-Salvà and Velasco, 2000) and molecularly confirmed by PCR amplification of the rbcL1 region resulting in the 1,000 bp diagnostic band of the species (Aly et al., 2019). Its pathogenicity was confirmed in a greenhouse experiment (23 to 27°C and photoperiod of 14 h light) from March to May 2025. Populations from El Puente (BEP-1 to BEP-3) and San Julian (BSJ-1 and BSJ-2) were independently inoculated to 4-day-old seedlings of the susceptible cv. B117 (Molinero-Ruiz et al., 2008) and of lines NR5 (susceptible to O. cumana race F and resistant to races <F), L86 (susceptible to race E and resistant to races F and <E), and P96 (resistant to races F and <F) (Molinero-Ruiz et al., 2015). Individual sunflower seedlings were transplanted into 42 ml pots with sand/silt (1:1), uniformly infested with 0.103 mg of parasite seed per g of soil, and grown in the greenhouse for two weeks. Then, they were transplanted to 2-liter pots filled with sand/silt/peat moss (2:1:3) and kept in the same conditions. Six replications (pots) were established for each population × genotype combination. Four weeks after inoculation, the first broomrape stems were observed. Two weeks later, the final degree of attack (FDA) was assessed as number of nodules and broomrape stems per sunflower plant. The infection of B117 confirmed the Koch’s postulates, with average FDA values from 4.5 (BEP-2) to 9.7 (BSJ-2). Besides, NR5, L86 and P96 remained resistant. These results show that populations of O. cumana collected in Bolivia and included in this work are a race <E. Therefore, they could be controlled by growing sunflower hybrids with genetic resistance to races E and higher. The most important outcome of this work is that it reports the infection of sunflower by O. cumana for the first time in America. Urgent containment measures must be implemented in order to manage sunflower broomrape in Bolivia, first because of the threat to sunflower production that it poses in the country, and second to prevent its spread to Argentina, which is the third worldwide producer after the Russian Federation and Ukraine.