Datura innoxia
(downy thorn apple)

D. innoxia is invasive in the Galapagos islands, New Caledonia, mainland China and Taiwan, and in Ethiopia, Kenya, Namibia and South Africa. It is widely naturalized in the tropics and increasingly in temperate Europe and North America. It is one of several invasive Datura species including D. ferox, D. metel and D. stramonium, that achieve high densities and are difficult to control. It is toxic to animals and humans, with all plant parts and seeds containing toxic alkaloids. Several species of Datura have been spread around the world as contaminants of agricultural seed. Seed inspection and certification agencies should be especially vigilant when batches of seed for planting are imported from areas where Datura species are known to occur. D. innoxia also readily escapes from gardens where it is being cultivated. However, timely cultivation or use of herbicides can easily control the species. D. innoxia has been listed as a Declared Weed in South Africa which is prohibited and must be controlled (AGIS, 2007).

D. innoxia is native to the tropical and subtropical Americas, from southwestern USA and Mexico, through Central America to northern and western South America, to Peru, and east to Bolivia and Paraguay (USDA-ARS, 2015). Whereas USDA-ARS (2015) noted records from the Caribbean as being possibly naturalized, other sources accept that the species is also native to Caribbean islands.

D. innoxia has been widely introduced and is found throughout the tropics. It is invasive in Namibia (Boyer and Boyer, 1989) and also in temperate areas where introduced as an ornamental but has since naturalized. D. innoxia has spread over Croatia since the early 1980s, occurring in large numbers regenerating from seeds and roots, with past and new localities described by Franjic and Trinajstic (1996). Vukovic et al. (2010) recorded it as invasive but of low occurrence. In the USA it is native only to Texas (USDA-ARS, 2014), but is also recorded in the southwestern states, Arkansas and in the northeast USA as far as Ontario and Quebec, Canada (USDA-NRCS, 2014).

D. innoxia was noted as an invasive alien species in Namibia by Boyer and Boyer (1989) but they observed that it offered less of a threat to native vegetation as compared to Prosopis spp., Nicotiana glauca and Ricinus communis, and that control of these latter species should take priority. Two decades later the situation did not appear to have altered greatly, as Shapaka et al. (2008) identified D. innoxia as one of eight invasive species and active management was recommended, but it was of lower priority as compared to the others. See also Katjirua (2008).

Several species of Datura have been spread around the world as contaminants of agricultural seed. Seed inspection and certification agencies should be especially vigilant when batches of seed for planting are imported from areas where Datura species are known to occur. D. stramonium was introduced into parts of Victoria, Australia, in contaminated safflower seed, and contaminated soyabeans have resulted in new infestations in Norway on several occasions. It is particularly difficult to separate from sorghum seed which is of a similar size (Parsons and Cuthbertson, 1992). Legislation in Queensland, Australia, prohibits the sale of crop seed which is contaminated by D. ferox or D. stramonium (Genn, 1987). D. innoxia was also one of five species recommended for eradication from Isabella Island in the Galapagos archipelago along with the closely-related D. metal (Guézou et al., 2007). It is declared as category 1, prohibited weed in South Africa because of its aggressive growth habit (AGIS, 2007). PIER (2014) gives a very high risk assessment score of 21 for this species in the Pacific region.

D. innoxia grows naturally in disturbed areas such as eroded areas, uncultivated fields, vacant lots, overgrazed pastures and rangeland, roadsides, abandoned roadbeds, and fencerows. Disturbance and reduced competition appear to be required for D. innoxia to become established (PIER, 2014). In China it is found growing near villages and on roadsides but is also cultivated (PIER, 2014).

Genetics

A single ploidy level of 2n = 24 for D. innoxia in Mexico was reported by Palominl et al. (1988) and many others. However, colchicine-induced tetraploids have been produced which exceeded the diploids in atropine and scopolamine content and showed better adaptability (Dzhurmanski and Yankulov, 1981). Different levels of ploidy (2n, 4n, 6n) was reported by Badea and Raicu (1983), and haploid, diploid, triploid, tetraploid and hexaploid cytotypes by Gupta et al. (1984).

Reproductive Biology

Reproductive biology of D. innoxia is described in Henderson (2001) and Parsons and Cuthbertson (1992). Datura species, including D. innoxia, may flower overnight and be pollinated by moths. Hummingbirds sometimes also visit the flowers but are affected by the alkaloids in the nectar and must limit their consumption, whereas honeybees are apparently unaffected.

Physiology and Phenology

In the tropics, D. innoxia is a perennial species, but it will behave as an annual in temperate regions. D. innoxia can flower and fruit throughout the year in some environments but only from July to September in India (Parrotta 2001) and September through November in Nicaragua (Stevens et al., 2001). Germination in D. innoxia is slow, poor and variable due to the presence of an inhibitor in the seed coat (Singh, 1974).

Environmental Requirements

D. innoxia grows in a wide variety of tropic and temperate climates. It is found on a wide variety of soils but prefers well-drained soils, on both igneous and sedimentary parent materials. In Puerto Rico, the species grows naturally in areas with 750-1000 mm mean annual precipitation, and from near sea level up to 900 m altitude (Stevens et al., 2001). An after-ripening period of 5-11 months may be required (Parsons and Cuthbertson, 1992) In a study of the effects of temperature and light on germination, maximum germination was obtained under continuous light at alternating temperatures of 20 and 30°C. Germination was reduced under continuous darkness (Barakat et al., 2013).

A number of diseases have been reported on D. innoxia, including Sclerotium root rot (Thakur and Sastri, 1970), P. nicotianae (reported for the first time in India) (Sattar et al., 1982), Alternaria crassa leaf spot (Grzybowska and Kapala, 1976) and other Alternaria blight (Janardhanan and Husain, 1973). Various viruses have been reported (Garga, 1958), including leaf curl disease (Marwal et al., 2012), virus wilt (Kovachevski, 1974), figwort mosaic virus (Shepherd et al., 1987), and cucumber mosaic virus (Raj et al., 1999). D. innoxia is also a host of the nematode, Xanthomonas vesicatoria (Raj and Moniz, 1975).

Drugs, stimulants, social uses

• Hallucinogen
• Narcotic
• Psychoactive

General

• Ornamental

Medicinal, pharmaceutical

• Source of medicine/pharmaceutical
• Traditional/folklore

Datura species are annual, whereas members of the closely related genus Brugmansia are perennials. However, D. innoxia can be confused with species of both genera, bearing similarly distinctive flowers and fruit. Species of both genera have a trumpet shaped flowers, but those of Brugmansia point downwards with a sweet fragrance, with Datura flowers often pointing upward with spicy or lemony fragrance.

D. innoxia is especially misidentified with other invasive Datura species, including D. ferox, common thornapple (D. stramonium L.), downy thornapple (D. inoxia Mill. = D. innoxia Mill.), hoary thornapple (D. metel L.) and hairy thornapple (D. wrightii Rogel). D.stramonium and D.ferox produce erect fruit capsules on straight stalks with black to greyish seeds, whereas fruit capsules of D. leichhardtii, D. metel, D. inoxia, D. wrightii appear on curved stalks with brown to yellow seeds.

Morphological comparisons between D. innoxia and D. metel showed that D. innoxia could be distinguished by much greater plant height, deeper leaf indentations, larger capsules, a wider corolla (when slit on one edge and rolled out flat), a wider calyx, and heavier and sturdier roots (Kosova and Chladek, 1957). D. innoxia is also reported to be similar to D. metel in habit, but D. innoxia is differentiated by its pubescent whereas D. metel is glabrous.

Parsons and Cuthbertson (1992) distinguish D. innoxia from most other weedy Datura species by the combination of curved seed-stalk, brown to yellow seeds, and hairy leaves; and from D. wrightii by its erect, glandular hairs and pure whiter flowers (lavender-tinged in D. wrightii).

Full descriptions are given by Haegi (1976) of the wild species D. inoxia, D. stramonium, D. ferox, D. leichhardtii, and D. wrightii, and of the cultivated species, D. metel.

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.

Control

Physical/mechanical control

Isolated plants should be hand-pulled before they set seed, whereas larger areas of infestation are controlled by cultivation when weeds are in the seedling stage. Cultivation becomes less effective as plants mature – stems become woodier and roots may not become completely severed. Seedlings emerge over a long period of time so repeated cultivations may be necessary to reduce the level of infestation (Parsons and Cuthbertson, 1992). However, tillage may promote seed survival as seeds decay more rapidly on the soil surface than when buried, and there is greater loss of seed to predators under no-till than conventional tillage systems.

Chemical control

D. innoxia can be controlled with 2,4-D in the seedling stage and young growth stages, but become resistant as they mature (Parsons and Cuthbertson, 2001). Other non-selective herbicides include atrazine, diquat, paraquat and glyphosate. The following herbicides are used in specific crops: acifluorfen and bentazone in soybeans and peanuts; 2,4-DB in certain varieties of peanuts; dicamba in grain sorghum and maize; metolachlor in maize; and picloram + 2,4-D in summer cereals (Parsons and Cuthbertson, 2001).

12/11/2014 Original text by:

Nick Pasiecznik, Consultant, France