Think of citrus and you probably think of freshly squeezed orange juice, and a grapefruit for breakfast, or later, a slice of lemon in your G and T. But have you heard of the many other citrus fruit that make-up this vast family which collectively are the number one fruit in international trade? Try calamondins, citrons, pomelos or ugli fruit sometime.
One third of global citrus production is for juice and this is the area of increasing consumption rather than fresh fruit. More than 80% of citrus going for juicing are oranges, the rest are mostly grapefruit. However, the conveniently small (‘lunch-box size’) and easy-peel, often seedless, tangerines, mandarins, clementines and satsumas have risen in popularity in recent years.
Paraquat is an essential tool in citrus
Paraquat is a broad spectrum herbicide. Its mode of action is to inhibit photosynthesis. This process is essential to plants and means that paraquat destroys all green tissue.
Although it is termed ‘non-selective’, paraquat is safe to citrus trees for several reasons. First, paraquat is immobilized on contact with the soil meaning that it cannot move to roots and be taken up into plants.
You can read more about paraquat’s unique soil properties here.
Second, it is sprayed around the citrus trees trunks which are protected by bark that paraquat cannot penetrate. Third, even if small amounts of paraquat land on citrus leaves there is little or no damage to the tree because paraquat does not move through plants systemically like the alternative non-selective herbicide glyphosate.
Unlike glyphosate, paraquat is very fast acting and rainfast.
A video showing paraquat’s unsurpassed speed of action can be viewed here.
Weeds sprayed with paraquat in the morning will often show symptoms (browning) by the afternoon, making it easy for spray operators to see which areas have already been sprayed. This holds true even if rain falls within 15-30 minutes of spraying, making it possible to spray for longer before rain is expected.
In tree crops such as citrus, emphasis is on the management of weeds rather than their permanent removal. This is because maintaining a particular balance of weeds in the citrus grove is important for sustainability by providing habitats for predators of insect pests and minimising soil erosion through the anchoring effect of plant roots.
In recent years, intensive use of glyphosate has caused new weed problems as less well controlled species have ‘shifted’ to become more dominant and some species have evolved biotypes which are resistant to glyphosate. Using paraquat as an alternative non-selective herbicide with a different mode of action in integrated weed management systems is helping to avoid problems of weed shifts and resistance.
Paraquat has a very robust environmental profile. It does not leach because it is extremely tightly bound to soil particles immediately on contact, so it cannot move into groundwater, or surface waters by run-off.
Further details of paraquat’s safety to the environment, spray operators and consumers can be found by visiting the different sections of the Paraquat Information Center or referring to the Paraquat Fact Sheet.
You can read more about the benefits of using paraquat here
Where do citrus fruit come from?
What are citrus?
Citrus is the collective name for many related species of fruit trees. They are all evergreen shrubs or small trees growing 5-15 m tall, often with thorny branches.
Recent genetic evidence points to only three core species: Citrus maxima (pomelo), Citrus medica (citron) and Citrus reticulata (tangerines and mandarins). Citrus species readily hybridize and varieties of species such as orange (Citrus sinensis), grapefruit (Citrus paradisi) and lemon (Citrus limon) have been developed since ancient times.
Interestingly, the peel, the part of citrus we discard, is botanically the same as the flesh we eat on most other fruit! However, the peel is also a valuable source of essential (volatile) oils. These are used in pharmaceuticals, soaps, perfumes and cosmetics.
One orange supplies all the vitamin C an adult needs in a day. Vitamin C is a powerful anti-oxidant, a class of chemicals, which help combat damage to the body by toxins. Oranges and other citrus fruit are also good sources of folic acid, fibre and potassium, as well as other vitamins.
Where are citrus fruit grown?
Citrus groves are found in Mediterranean climates. Temperatures more than a few degrees below freezing will destroy most citrus species.
Oranges are the most widely grown type of citrus fruit, by far. They account for around 43% of the citrus area and over 52% of production. Taken together, tangerines, madarin oranges and clementines are next most widely grown, followed by lemons and limes, with a smaller area of grapefruit.
Since 1980, the world area of oranges has increased from about 2.3 to nearly 4 million hectares. Brazil has maintained its lead position in area grown and production, but cropping in China and India has increased substantially. In terms of production, Brazil grows twice as many oranges (18.1 million tonnes in 2012) as its nearest rival, the US.
Sustainable citrus production
Commercial citrus trees are grafts of the desired fruiting species onto a rootstock of some other citrus species developed specifically for that purpose. Rootstocks are selected for resistance to diseases and nematode attack, and hardiness. Choice of rootstock will affect the vigor of the tree and the size of fruit.
Citrus trees generally require infrequent and only light pruning once established. Sometimes they are grown as a hedge system.
It takes 3-5 years from transplanting saplings until they start to bear fruit and an additional 5 or so years to reach peak bearing. Different species and varieties of citrus have fruit ready for harvest over many months.
The yellow and orange colours of citrus develop in cooler weather. Citrus grown in tropical climates remain green like limes, and if not picked, ripe oranges can revert to green in spring. Citrus fruit do not continue to mature after picking, so are harvested ripe. Readiness for harvest is determined in more intensive production systems by degrees of ‘brix’. This is noted by an assessment of the sweetness (dissolved sugars) by measuring the specific gravity of juice with a hand-held refractometer.
Pests and Diseases
Paying attention to pest and disease control is essential, particularly in fresh citrus markets where appearance as well as yield is very important. Key insect pests in European citrus include the Mediterranean fruit fly (Ceratitis capitata), the cotton melon aphid (Aphis gossypii), the citrus flower moth (Prays citri), the citrus leaf miner (Phyllocnistis citrella) and various mites and scale insects, which can be particularly damaging. Aonidiella auranti (California Red Scale), and Tetranichus spp. and Panonichus spp. mites are often serious pests.
Diseases include ‘Mal Secco’ caused by the fungus Phoma tracheiphila, root and fruit rots (‘aguado’) caused by Phytophthora species and Alternaria alternata. Phytophthora and Alternaria can cause considerable economic losses.
Important viral diseases are Citrus Chlorotic Dwarf (CDD) and Citrus Leaf Blotch. Citrus Tristeza Virus (CTV) has caused devastation in Spanish citrus in the past. Root stock are certified ‘virus-free’ as a phytosanitary measure.
Many annual and perennial weeds infest citrus groves. Weeds may compete with crops for space, water and nutrients, and shade the crop plants, especially when they are young.
Traditionally, and in poorer farming communities, citrus groves are hoed by hand to remove weeds. This is labor-intensive and time-consuming, and therefore can limit opportunities for other activities, including education. Effective use of herbicides can very significantly reduce the resources needed to control weeds. The most commonly used herbicides include paraquat and glyphosate which have no activity in the soil, and the class of herbicides known as ‘residuals’ which remain active in the soil and prevent the germination of weed seeds.
However, the inappropriate use of herbicides can lead to adverse changes in the weed flora (‘weed shifts’) and contribute to soil erosion caused by excessive cultivations especially on sloping ground.
Intensive use of the systemic non-selective herbicide glyphosate and residuals has led to changes in weed flora (‘weed shifts’) as species more tolerant to their particular modes of action become more dominant. ‘Soft’ weeds, typically prostrate annual grasses that are easily controlled, are replaced by re-invasion of cleared land by more aggressive, pernicious weeds, which reduce crop yields. These compete with citrus trees to reduce yield and quality and may make harvesting difficult.
Using paraquat, however, to manage the weed flora rather than eliminate it, can help to maintain a balanced flora, which precludes the dominance of aggressive species. Paraquat only removes the top growth of well-established weeds, and does not affect the germination of new seedlings allowing vegetation to re-establish after 1-2 months. A controlled presence of soft weeds maintains the balance of the weed flora and prevents weed shifts to more competitive species simply because bare ground for them to colonize is less available. The presence of non-competitive vegetative cover also provides habitats to encourage biodiversity. The wildlife encouraged will include predators of insect pests, which would otherwise have to be controlled chemically.
A report commissioned by the European Commission’s DG Environment on ICM Systems in the EU includes a case study on a plan in the Valencia region of Spain, which includes 10,000 ha of citrus. Cultivation equipment that destroys soil structure is banned. A green cover must be allowed to develop from mid-autumn to the end of winter to anchor the soil, thus reducing the risk of erosion. Another advantage of weed covers is that they help to prevent Phytophthora diseases in fruit by minimising the impact of rain drops splashing soil particles on to the lower fruits as can happen with a bare soil.
Paraquat is not systemic like glyphosate so it only controls weed shoots, retaining roots and allowing regrowth of many species. Additionally, because paraquat has no soil activity, it does not prevent the germination of weed seeds as residual herbicides do.
Alternatively, a cover crop can be sown between the lines of trees. Studies in olives in Spain have shown that this type of system has reduced erosion by more than 80%.
Oxalis spp. covers are commonly used as inter-row cover crops in citrus. Paraquat can be used to manage the cover, eg to reduce competition for water in summer without killing the bulbils by which the plants spread.
Paraquat can be safely sprayed to manage the weed flora along the crop rows between the grass or legume strips without fear of damaging the citrus trees. Paraquat is immobile in soil and cannot move to the roots and up into the shoots. Tree bark cannot be penetrated by paraquat meaning that it can be sprayed right up to the base of the trees. Even if paraquat drifts onto citrus leaves there is little or no damage because paraquat does not move through plants like glyphosate does.
CASE STUDY: Paraquat saves soil in China
Professor Shui Jian-guo described how using paraquat to control weeds in citrus orchards reduced soil erosion at the International Soil Conservation Organisation’s conference in Brisbane, July 2004.
In China’s Zhejiang province, the typical 1600 mm of rain makes soil erosion a serious problem in citrus orchards growing on hillsides. The red soils there are fertile, but need careful management to avoid compaction, erosion and reductions in pH. The Soil and Fertilizer Institute of Zhejiang Academy of Agriculture Sciences, Hangzhou has investigated techniques to minimize soil erosion.
Working on three farms, scientists found that three applications of paraquat each year gave much better results than cultivating the ground to control weeds. Not only was soil erosion reduced, but yields improved by 8% and one-third less labor was needed.
The chart shows the average soil loss over the three sites between 2001 and 2003. Over this period there were 33,000 litres per hectare of surface water run-off on average for cultivated ground. This was reduced by 21% by glyphosate, but using paraquat resulted in 48% less run-off.
This good performance was explained by paraquat’s effect of only killing weed shoots, then allowing regrowth. Competition with the crop is removed at critical times, while weed roots remain to support the soil structure. Glyphosate was less effective because both weed shoots and roots were killed. After spraying glyphosate, the weed flora comprised more broadleaved species, which provide less support to the soil than the fibrous roots of grasses, which regrow after spraying paraquat.
References & Resources
Markets, production, trade
United Nations Food and Agriculture Organisation FAOSTAT: http://faostat.fao.org/faostat/
European Commission DG Environment ICM Systems in the EU
UN Conference On Trade & Development (UNCTAD): http://www.unctad.info/en/Infocomm/Agricultural_Products/Citrus-fruit/market/
CIRAD (Centre de coopération internationale en recherche agronomique pour le développement) conference: The quality of fresh and processed citrus fruits. Montpellier, France, 10-11 October 2002: http://citrus2002.cirad.fr/index_en.html#
Integrated Crop Management Systems in the EU. A report by Agra CEAS Consulting for EC DG Environment (2002).
Soil and water protection
Shui Jian-guo, Liao Gen-qing, Au Jeff, Zhou Quan-kang, Allard Jean-Louis (2004). Effects of different natural vegetation management measures on red soil erosion in hilly orchards. ISCO 2004 – 13th International Soil Conservation Organisation Conference – Brisbane, July 2004 Conserving Soil and Water for Society: Sharing Solutions.