Sweet basil is the most commercially important, annual, culinary herb crop grown in the United States (Wyenandt et al., 2010). Culinary basil is grown for both fresh and dry consumption. Basil is a source of essential oil and oleoresin for manufacturing perfumes, food flavors, and aromatherapy products (Simon et al., 1990). Basil downy mildew, caused by Peronospora belbahrii, is an economically important disease of basil species in the United States (Roberts et al., 2009; Wyenandt et al., 2015). Downy mildew has been reported in many countries and several continents (Belbahri et al., 2005; Garibaldi et al., 2004a, 2005; Hansford, 1933; Khateri et al., 2007; McLeod et al., 2006; Ronco et al., 2008). In the United States, the pathogen was first discovered in Fall 2007 in Florida (Roberts et al., 2009). Since then, basil downy mildew has been found throughout most of the United States, including all major production areas (McGrath et al., 2010; Wick and Brazee, 2009; Wyenandt et al., 2015). The symptoms of basil downy mildew on an infected plant can resemble a nutrient deficiency (Fig. 1). Like other downy mildew pathogens, P. belbahrii will only produce sporangia on the abaxial surface of infected leaves. Numerous dark, purplish-brown sporangia are produced when environmental conditions are favorable for disease development (Fig. 2). Periods of high humidity, warm temperatures, poor air circulation, and extended durations of leaf wetness are favorable for P. belbahrii and can result in extensive development of basil downy mildew throughout plantings (Cohen et al., 2017; Garibaldi et al., 2005, 2007; McGrath et al., 2010; Spencer, 1981). In northern areas, such as the mid-Atlantic and northeast regions of the United States, there is no evidence that this obligate pathogen can survive outdoors during winter months between field-grown basil crops. However, oospore production in basil leaves has been reported in Israel (Cohen et al., 2013). The pathogen has been shown to survive on contaminated seed (Garibaldi et al., 2004b). Since 2010, the epidemiology of basil downy mildew has been studied extensively (Cohen et al., 2017). However, making accurate predictions about its development and dissemination across broad geographic regions remains problematic. In 2009, 49 visual reports of basil downy mildew were logged from 17 U.S. states and Canada (McGrath et al., 2010). These reports confirmed the first major basil downy mildew outbreak in the eastern U.S. and helped establish that, under ideal weather conditions, basil downy mildew can disseminate over a wide geographic region in a single growing season across the United States (McGrath et al., 2010). Since 2009, basil downy mildew has been reported in 42 U.S. states, including Hawaii and Alaska (Wyenandt et al., 2015). Since its arrival, 100% losses have occurred on many farms in New Jersey and across the mid-Atlantic and northeast regions as well as the rest of the United States. Once basil plants become infected and develop symptoms on leaves, they are no longer marketable as a fresh product (Fig. 1).
It has been widely suggested that the rapid and widespread distribution of basil downy mildew in recent years has been attributed to the distribution of the pathogen via infested seed (Cohen et al., 2017; Garibaldi et al., 2004b; Wyenandt et al., 2015). In Italy, Garibaldi et al. (2004a) collected sweet basil seed from 17 commercial sources and in greenhouse and growth chamber experiments were able to detect P. belbahrii in 4 of 17 plant samples. However, the seed contamination observed in their study appeared to be high enough to allow for rapid spread of the disease. Belbahri et al. (2005) were able to detect P. belbahrii using a newly develop PCR technique but were not able to detect the pathogen in or on infected seed. Farahani-Kofoet et al. (2012) were able to detect P. belbahrii on artificially infested seed using a PCR assay to the level of a single spore per seed. They also detected P. belbahrii in 80% to 90% of the commercial seed lots tested and demonstrated that seed collected from infected plants could result in infected plants, with or without symptom development. In Israel, Cohen et al. (2017) could not demonstrate the transmission of P. belbahrii in seeds collected from heavily infected plants. Microscopic examination of the seed did not show mycelium or sporangia of the fungus in the embryo (Cohen et al., 2017). However, R.L. Wick and A.M. Madeiras (unpublished data), using seed that tested positive for P. belbahrii by PCR assay, found downy mildew sporangiophores after centrifuging the water that was used to rinse infested basil seed (Fig. 3).
Commercial seed companies currently are seeking to develop seed treatment methods for controlling P. belbahrii in basil seed. Currently, there are conventional and organic fungicide options for basil downy mildew control; however, commercial fungicides only work well under low disease pressure and excellent management practices, and no organic options have proven effective (Homa et al., 2014; Wyenandt et al., 2015). Importantly, the lack of genetic resistance to the pathogen in widely grown sweet basil cultivars continues to make control difficult (Homa et al., 2014; Pyne et al., 2015, 2017; Wyenandt et al., 2015). Efforts have been made in recent years at breeding downy mildew resistance into commercially acceptable sweet basil lines (Ben-Naim et al., 2018; Pyne et al., 2015, 2017, 2018), but this has focused on using a few strains of the pathogen and, as with all disease-resistant plants, growers still need to use the improved germplasm within an integrated pest management program that still requires fungicide applications. Should additional strains (e.g., races) of basil downy mildew develop, such as for other downy mildews on other hosts, continued breeding efforts will be required. The development of seed treatment methods and rapid tests to determine the presence of the pathogen on seed also will remain relevant to the global basil industry and stakeholders.
The objective of this study was to determine whether P. belbahrii could be detected on seed of different basil cultivars and species from field-grown plants following downy mildew epidemics in southern New Jersey.
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