Peronospora belbahrii
(basil downy mildew)

Basil downy mildew was first identified from Uganda during 1932 and 1937, resulting in significant crop losses (Hansford, 1933; Hansford, 1939). Following these original outbreaks, the disease was reported sporadically in Africa during the twentieth century: in Tanzania in 1960 (Riley, 1960), then again in Benin during 1998 (Gumedzoe et al., 1998). The disease was first identified outside of Africa in 2001, when it was reported from Switzerland (Belbahri et al., 2005). Unlike the intermittent African outbreaks of the twentieth century, the twenty-first century outbreaks of basil downy mildew are persistent, and the geographic range of P. belbahrii continues to expand. Since 2001, P. belbahrii has spread throughout Europe, North America, Asia, and parts of Africa, Central and South America, and the Caribbean. Losses incurred due to basil downy mildew in the USA alone are estimated to reach tens of millions of dollars (Wyenandt et al., 2015).

Basil downy mildew was first identified from Uganda during 1932 and 1937, resulting in significant crop losses (Hansford, 1933; Hansford, 1939). Following the original outbreaks, the disease was reported sporadically in Africa during the twentieth century: in Tanzania in 1960 (Riley, 1960), then again in Benin during 1998 (Gumedzoe et al., 1998). The disease was first identified outside of Africa in 2001, when it was reported from Switzerland (Belbahri et al., 2005). Unlike the intermittent African outbreaks of the twentieth century, the twenty first century outbreaks are persistent, and the geographic range of P. belbahrii continues to expand. Since 2001, P. belbahrii has spread throughout Europe, North America, Asia, and parts of Africa, Central and South America, and the Caribbean.

In the USA, after the disease was first identified from Florida in 2007, scientists began monitoring the spread of P. belbahrii using the online reporting tool Basil Ag Pest Monitor (https://basil.agpestmonitor.org/), as described in Wyenandt et al. (2015). Between 2009 and 2019, 1386 reports of basil downy mildew have been made from 44 states and the District of Columbia (http://blogs.cornell.edu/livegpath/extension/basil-downy-mildew/where-in-the-usa-is-basil-downy-mildew/).

The origin of P. belbahrii is unknown. Thines et al. (2009) speculated that P. belbahrii might be of African origin, given the fact that the biodiversity centre of Ocimum is Africa (Patton, 1992). This interpretation is consistent with the fact that the pathogen was only known from Africa for almost 70 years.

There is a risk of intentional introduction of this pathogen. There is evidence that the pathogen is transmitted through seeds, providing a means for long-distance transport (Cohen et al., 2017). The pathogen may also be present in asymptomatic stems and leaves, facilitating unintentional transport on host cuttings (Cohen et al., 2017).

There is a risk of intentional introduction of this pathogen. There is evidence that the pathogen is transmitted through seeds, providing a means for long-distance transport (Cohen et al., 2017). The pathogen may also be present in asymptomatic stems and leaves, facilitating unintentional transport on host cuttings (Cohen et al., 2017).

Prior to 2020, the name P. belbahrii was sometimes used to refer to Peronospora causing newly emergent downy mildew diseases affecting two other herbaceous plants in the Lamiaceae: namely, agastache (Agastache spp.) and coleus (Plectranthus scuttarioides). However, as early as 2009, many researchers recognized that the pathogens associated with these three different hosts were diverse, and exhibited variation between one another in DNA marker sequences. The publication originally describing P. belbahrii first introduced the idea that the coleus pathogen was most likely distinct from P. belharii on basil (Thines et al., 2009). As a result, while the literature shows some authors referring to the causal pathogens of coleus and agastache as P. belbahrii (Henricot et al., 2010; Denton et al., 2015; Ito et al., 2015), many others identified these organisms as Peronospora sp. or P. belbahrii sensu lato (Daughtrey et al., 2006; Palmateer et al., 2008; Thines et al., 2009; Rivera et al., 2016a; Rivera et al., 2016b; Gorayeb et al., 2020). In 2020, the pathogen of coleus was described as the new species, P. choii (Hoffmeister et al., 2020), while the pathogen of agastache was described as the new species, P. monardae (Salgado-Salazar et al., 2020). Peronospora belbahrii, P. choi and P. monardae are reliably distinguished from one another on the basis of molecular and morphological criteria (Hoffmeister et al., 2020; Salgado-Salazar et al., 2020).

There is some evidence that P. balbahrii isolates from basil can infect other plants in the Lamiaceae under experimental conditions (Micromeria fruticosa, Nepeta curviflora, Rosmarinus officinalis, Salvia pinnata and S. fruticosa; Cohen et al., 2017) but it is unknown whether any of these plants play any role in natural populations.

P. belbahrii is an obligate plant pathogen, requiring a living host to survive. The pathogen has been shown to colonize basil leaves, stems and seeds (Farahani-Kofoet et al., 2012). However, it is not clear to what extent P. belbahrii colonizes seeds or systemically infects host plants during natural infections (Cohen et al., 2017).

Although environmental conditions influence the timing of specific events in the life-cycle of P. belbahrii, there is a general understanding of how the pathogen colonizes and reproduces in the host plant. Cohen et al. (2017) produces a comprehensive account of the infection cycle. Briefly, P. belbahrii reproduces asexually, producing dark-brown, purplish conidia that germinate to form germ tubes (Thines et al., 2009; Koroch et al., 2013; Zhang et al., 2020) between 3-6 days after coming into contact with a susceptible host (Zhang et al., 2020). At least 2 hours of exposure to water and temperatures of 15-20^°C is required for germination (Cohen and Ben Naim, 2016), although some reports show 6 h leaf wetness is required (Garibaldi et al., 2007). Host tissue is penetrated directly (Cohen and Ben-Naim, 2016; Cohen et al., 2017; Zhang et al., 2020) or the pathogen may enter via stomata reported (Koroch et al., 2013; Cohen et al., 2017). A latent period can last for 5-10 days and is influenced by light and temperature (Cohen et al., 2017). Between 6 and 14 days after infection, the pathogen produces conidiophores that emerge through stomata on the abaxial leaf surface, or 1-2 days later through stomata on the adaxial surface (Zhang et al., 2020). New conidia can be produced as soon as 10 days after the initial infection (Zhang et al., 2020). Sporulation is inhibited by light (López-López et al., 2014) and may be inhibited by the lack of water although not all studies agree on this point (Garibaldi et al., 2007; Cohen and Ben-Naim, 2016; Cohen et al., 2017).

Although downy mildew pathogens overwinter by producing sexual reproductive structures called oospores, to date these structures are only known for P. belbahrii from Israel (Cohen et al., 2013; Elad et al., 2016; Cohen et al., 2017). No information is available about whether P. belbahrii oospores are produced through a homothallic or heterothallic mating system.

​There is evidence that the pathogen is transmitted through seeds (Cohen et al., 2017).

12/05/20 Original text by:

Jo Anne Crouch, USDA-ARS, 10300 Baltimore Avenue, Beltsville, MD 20715, USA.