Brassica tournefortii
(African mustard)

B. tournefortii is a widespread species of mustard, commonly known as African mustard or Sahara mustard. Native to Africa, Asia and Europe, it has spread globally and naturalised in North America, Australia and New Zealand. It is a highly invasive annual herb and is recorded as negatively affecting native species in some US states and Australia. Its fast growth rates enable it to monopolize soil moisture and light and mature before native wildflowers. B. tournefortii is often the dominant species in areas of usually diverse flora. CalEPPC (1999) lists the species as ‘regionally most invasive wildland pest plant’. Some factors may increase its invasive capability, for example in the western Sonoran Desert of California, USA, B. tournefortii quickly invaded areas of natural disturbance where soils were young while older geological surfaces were less vulnerable to invasion. Another study in New South Wales, Australia, found that rabbit mounds enhanced the germination of B. tournefortii seeds. 

B. tournefortii is a widespread mustard native to Africa, Asia and Europe. The species has become widely spread and naturalised in North America, Australia and New Zealand (USDA-ARS, 2015).

Specifically, B. tournefortii occurs in the Northern African countries of Algeria, Libya, Egypt, Tunisia and Morocco. It is widely spread across the Middle East and Western and Central Asia, including Oman, Saudi Arabia, Azerbaijan, Uzbekistan, Iraq, Syria and Turkey . It is also native to the European countries of Italy, Greece and Spain (USDA-ARS, 2015).

B. tournefortii has become naturalised in South Africa, Australasia (Australia and New Zealand), the UK, Mexico and the southern parts of the USA (USDA-ARS, 2015).

It has recently been recorded as a new alien species in Chile (Tellier et al., 2014).

B. tournefortii is a highly invasive species thus its spread and further introduction is of high concern. Curtis and Bradley (2015) modelled the current and future climatic suitability for establishment of B. tournefortii in the southwestern USA, concluding that the species presence and abundance may increase by 29% and 28% respectively.

Following spatial analysis and climatic modelling of B. tournefortii,Li et al. (2015) found that that the climate in the invaded range generally predicts the native distribution. 

B. tournefortii is commonly found in disturbed sites along roadsides, abandoned fields and waste grounds. Its native habitat is typically areas of wind blown, sandy arid, or semi-arid environments (Minnich and Sanders, 2000). It also inhabits sandy coastlines typically along the Mediterranean coastline (Thanos et al., 1991). During years of high rainfall it also has been found to inhabit areas of silty and rocky soils (Cal-IPC, 2015).

Berry et al. (2014) found that in the western Sonoran Desert of California, USA, B. tournefortii quickly invaded areas of natural disturbance where soils were young while older geological surfaces were less vulnerable to invasion. Another study in central-western New South Wales, Australia, found that rabbit mounds enhanced the germination of B. tournefortii seeds. The mounds were characterised as disturbed and degraded soil surfaces, suggesting that an increased abundance of B. tournefortii in the area can be attributed to the soil disturbance caused by rabbits (Eldridge and Simpson, 2002).

Australian Oilseeds Federation (2015) reported that B. tournefortii can contaminate canola oil crops, reducing yield through competition, and compromising oil quality. 

Genetics

The chromosome count for B. tournefortii was reported to be 2n=20 (Jepson Flora Project, 2014).

B. tournefortii has been hybridized with cultivated Brassica napus (rapeseed) to establish cytoplasmic male sterility B. napus (Liu et al., 1996). Phoma Lingam (black leg disease of Brassica crops) resistance has been shown to be transferred from B. tournefortii to the hybrid plants (Liu et al., 1995). In addition, due to B. tournefortii’s drought resistant genes it has been identified as a possible donor for hybridization with cultivated brassicas (Pratap and Gupta, 2009).

Reproductive Biology

B. tournefortii is an autogamous species with virtually one hundred percent fruit set. A well-developed plant will produce between 750 and 9000 seeds (Minnich and Sanders, 2000). Seeds may remain viable for up to three years (USDA, 2015). It has been reported that the seeds of B. tournefortii can also remain viable after extended submergence (Bangle et al., 2008).

In the Mediterranean region it is reported that seed germination is optimal between 15-25°C (Thanos et al., 1991), compared with 16-35°C in southwest USA (Bangle et al., 2008). Germination is light sensitive, and can be inhibited when exposed to light (Thanos et al., 1991). Malusa et al. (2003) found that seeds lying on the surface did not germinate due to photo-inhibition.

The gel like coating on the seeds waterproofs them and makes them sticky when wet. This enables the seed to survive and remain dormant for a number of months (Holman and Gardener, 2006).

Physiology and Phenology

B. tournefortii is a rapid growing annual, which germinates in the autumn, flowers and fruits in the winter and senesces in the spring. In California, seed germination occurs as a response to rains as small as 4 cm, the plant then grows vigorously within two or three months. The fast growth enables early establishment. In the Sonoran Desert it has been reported that the abundance of B. tournefortii increases with higher winter rainfall. It has been noted that hot dry spells cause B. tournefortii to prematurely flower and fruit (Minnich and Sanders, 2000).

Minnich and Sanders (2000) state that the plant population size can vary from year to year. In the Sonoran Desert it has been reported that population abundance is higher in winters of high rainfall compared with winters of low rainfall (Minnich and Sanders, 2000).

Associations

Sanchez-Flores et al. (2008) report that B. tournefortii shows association with microphyllous desert scrub, grassland, and sarcocaulescent desert scrub through land cover characterization.    

Environmental Requirements

B. tournefortii is suited to arid and semi-arid sandy desert conditions; however, it inhabits a wide variety of disturbed environments (USDA, 2015). Minnich and Sanders (2000) found that it is becoming more common in the semi-arid coastal areas of California. They noted that it is particularly common in areas of wind blown sediments.

B. tournefortii favours sandy disturbed soils, and can tolerate soil salinity (ASDM, 2015). It requires very low soil nutrient levels, and is able to grow in extremely poor soils and sand dunes (Dremann, 2005). Plants for a Future (2015) report that B. tournefortii prefers well-drained yet moist soils and that it cannot grow in the shade. It can be found at elevations as high as 1000 m though is abundant below 305 m (Minnich and Sanders, 2000). Malusa et al. (2003) found that B. tournefortii in the Mohawk dunes, Arizona, preferred areas of greatest moisture. In the Coachella Valley, California, B. tournefortii favours years of higher precipitation, biomass abundance drops in years of low precipitation (Barrows et al., 2009).

Following spatial analysis and climatic modelling of B. tournefortii,Li et al. (2015) found that that the climate in the invaded range generally predicts the native distribution. Therefore, suggesting that either there has been little local adaptation to climate occurring since introduction or the biological interaction experienced in the invaded range has not driven the species to occupy climatic conditions much different from its native range.

There is little information on natural enemies of B. tournefortii. Suazo et al. (2012) concluded that harvester ants in the Mojave Desert, Arizona, might act as seed predators. Harvester ants carried B. tournefortii seeds to their nests and as plant density was lower near the ant’s nest it was concluded that the ants were seed predators rather than seed dispersers (Suazo et al., 2012). 

Human food and beverage

• Oil/fat
• Vegetable

B. tournefortii can be confused with other yellow flowering mustards in the field, including Hirschfeldia icana (shortpod mustard), Brassica nigra (black mustard), B. juncea (Indian mustard) and B. napus (rapeseed) (BCC, 2015). B. tournefortii can be distinguished from other yellow flowering mustards by its pale, dull in comparison, small yellow flowers and the stiff white bristles on the herbs lower stems. Also the fruit of B. tournefortii is relatively elongated with a long beak (Di Tomaso and Healy, 2007).

In the USA, the Sahara Mustard and Volutaria Eradication Task Force (2015) have noted a few native desert plants that co-occur and are similar in appearance to B. tournefortii. These include Descurainiapinnata (tansy mustard), Guillenia lasiophylla (California mustard) and Camissonia californica (false mustard). Identification and differentiation is important for the control and eradication of B. tournefortii in these native habitats.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

B. tournefortii can not be effectively controlled in one year. Multiple management methods need to be applied over multiple years in order to effectively control a B. tournefortii invasion. Effectiveness of control increases by using a combination of methods (USDA, 2015).

A study on seed production by Trader et al. (2006) found that seed production of individual plants can be higher in plots with lower densities. Therefore, control measures that reduce density but do not clear the entire area of plants may result in an increase in net seed production from those individuals that have not been removed.   

Cultural Control and Sanitary Measures                      

Early detection and removal is vital to inhibiting the spread of B. tournefortii. Education and public awareness regarding B. tournefortii’s invasiveness, and identification at different life stages, will lead to the public aiding prevention and control by reporting new infestations. In addition land users should be discouraged to use vehicles and machinery, and graze livestock in infested areas in order to mitigate the spread of seeds (USDA, 2015).

Physical/Mechanical Control

Hand weeding is the most common method of control (Holt and Barrows, 2014). However it is only effective where an infestation has been detected in the early stages, it is not a feasible option for large scale infestations of B. tournefortii (Barrows, 2010). The entire plant should be removed, bagged and disposed of off site (USDA, 2015). Abella et al. (2013) showed that physical treatment worked best in the early stages of B. tournefortii growth and seed development.

Biological Control

The use of biological control may not be suitable due to B. tournefortii being closely genetically related to a number of important agricultural crops such as broccoli (Brassica oleracea), canola (Brassica spp.) and cabbage (Brassica oleracea). Finding a bio-control that would not attack the cultivated species may be difficult (Holt and Barrows, 2014).

Chemical Control

Chemical control, such as herbicides, are an effective control against B. tournefortii. Unlike physical control it can be effective once the plant has reached the seed developmental stages of its lifecycle (Abella et al., 2013). Herbicide treatment when used in ecosystems such as Mojave Desert or Coachella Valley may cause a negative impact on the surrounding native wildflowers. However, the early phenology of B. tournefortii provides a window for selective control using herbicides without affecting the native annual wildflowers. Early treatment of B. tournefortii would reduce its abundance and allow native wildflowers to germinate and establish (Holt and Barrows, 2014). Chemical control may be best suited when B. tournefortii infests agricultural crops.

Monitoring and Surveillance

Recent studies, e.g. Sankey et al. (2014), have used high spatial resolution to detect presence, cover and biomass of B. tournefortii.

27/01/2016 Original text by:

Madeleine Florin, Consultant, UK