In order to write about Limes, it is necessary to restrict the subject somewhat. I keep unearthing ever more interesting facts and I’ll still be researching on Christmas Day otherwise. The most interesting stories seem to relate to Mexican Limes, so that will be this Advent Botany blog’s focus.
The Lime is not native to Mexico. Europeans took Lemons and Limes to North and South America. Researchers have found that the Mexican Lime, Citrus aurantifolia, is a direct hybrid between Citrus medica and Citrus micrantha.
Citrus aurantifolia is the most popular Lime species in Mexico. It is small, green-yellowish in colour with a thin peel. Limes and Lemons are key ingredients in Mexican cuisine. Mexico is the second largest consumer of Limes and Lemons. Limes are also crucial to the Mexican economy for exports, not only for consumption, but also as Citrus oils for the perfumery industry. Mexico without Limes today is unthinkable. They are considered irreplaceable.
Mexico also grows Persian or Tahiti Limes, Citrus latifolia. These Limes are preferred by the US market, as they are larger, juicier and relatively seedless. The US market demand is changing the traditional balance of Mexican Lime production. It has increased in favour of Persian Limes over the local preference for Mexican Limes. The US state of Florida is traditionally the largest producer of Citrus fruit. It has had to turn to other suppliers due to poorly timed and managed, endemic disease in its Citrus orchards.
A bacterial infection is ravaging the world’s Lime crops. It is described by several different names: Citrus Greening, Huanglongbing or HLB for short. Trees produce green, misshapen and bitter fruit. Infected tree leaves turn blotchy and yellow. The disease is caused by the bacterium Candidatus Liberibacter asiaticus. It is spread by the Asian Citrus Psyllid, a small insect that is related to aphids.
Citrus Greening is now found in all regions of cultivation except Australia and the Mediterranean basin. Lime oil is mainly produced in two Mexican states: Michoacán and Colima. Citrus Greening has resulted in significant shortfalls in the Lime crop in Colima over the past 5 years. The disease is not regarded as so severe in Michoacán, where the temperature is often over 40ºC.
Unexpected changes in the seasonal weather have impacted the 2019 crop in Colima. Torrential rains normally occur in July. They arrived in mid-August instead. This delay resulted in reductions in lime oil production for the perfumery industry. At the end of September 2019, Colima was struck by a Category 1 hurricane (wind speeds of 74 to 95 mph). The long-term effects of this weather event are unknown. They nonetheless highlight the potential harmful effects of climate change on Mexican Lime production combined with the disease concerns already mentioned.
Mexico’s Lime crop situation illustrates the huge challenges of modern-day agriculture in the world. Intensive agriculture boosts crop yields. At the same time, it increases crop susceptibility to pests and diseases. As scientists seek new solutions, growers and policymakers need to reflect on both the positive and negative effects of pursuing a monoculture. It places a country’s economy, local people, livelihoods, food, culture and way of life at risk.
Citrus Greening Cure?
There is currently no known cure for Citrus Greening. The USA is funding research in a 4 -pronged attack:
- Creation of disease-resistant varieties – i.e. genetically-engineered citrus trees despite consumer misgivings about such crops
- Creation of a chemical barricade
- By seeking to harness naturally occurring soil bacteria to control the Citrus Psyllid
- Chemotherapy treatments.
In the meantime, other solutions are sought by experimenting with new tree varieties, new bactericides and the release of parasitic wasps.
European and Asian Research Alternatives
Luro et al.’s 2017 observations about the nature of Lime diversification might have some bearing on the best way forward. The fruit has spread widely across the world from its original roots. The Mexican Lime has become geographically restricted and therefore vulnerable with limited gene flow.
Citrus species appear to remain sexually compatible despite adapting to different regions of the world. Asian Citrus species’ diversity is high with wider genetic resources. According to Kew’s Plants of the World, the Citrus genus in the Rutaceae family contains some 30 wild species and around 145 clone and cultivated species. Velasco and Licciardello (2014) suggest new interspecies combinations or a return to wild species. Might these species exhibit greater resistance to climate, pest and disease stresses? Such an approach will be more readily understood and accepted by the general public. Plant selection and cultivation of the fittest has been our way for centuries.
© Karen Netto (Andrews)
References and Further Reading
Culham, Alastair (2019): #AdventBotany 2019, Day 1: Clementine, Satsuma, Tangerine; what’s the difference? 1 December 2019. Last accessed 20 December 2019.
Davidson, Alan (2006): The Oxford Companion to Food. Second Edition. Edited by Tom Jaine. Oxford University Press.
Gro Intelligence (2017): Mexico Battles Against Citrus Greening. 15 February 2017. Last accessed 19 December 2019.
Luro, François & Curk, Franck & Froelicher, Yann & Ollitrault, Patrick (2017): Recents insights on Citrus phylogeny and diversity. AGRUMED: Archaeology and History of Citrus Fruit in the Mediterranean. 2017. 15 January 2018. Publications du Centre Jean Bérard. Last accessed 20 December 2019.
Pisano, Richard Jr. (2019): The State of Citrus Oils, 2019-2020. The long-awaited return of a buyer’s market. Perfumer & Flavorist. Volume 44. December 2019. (Citrus & Allied). Last accessed 18 December 2019. (p.44 Lime oil).
Velasco, Riccardo & Licciardello, Concetta (2014). A genealogy of the citrus family. Nature Biotechnology. Volume 32. Number 7. July 2014 pp.640-642. Last accessed 20 December 2019.
Waterworth, Kristi (2017): What is Citrus Greening Disease: Saving Plants Affected by Citrus Greening. Gardening Know How. 15 February 2017. Last accessed 19 December 2019.