Suitable rootstocks with density make citrus farming save land

Joint studies between Embrapa and the Citrus Defense Fund (Fundecitrus) show that the association of orchard density, with a greater quantity of plants per hectare, with rootstocks – varieties that correspond to the root part of the citrus plant – are more productive It has great potential to promote land saving in citrus farming.

Work published in Agronomy, a scientific magazine published by the Multidisciplinary Digital Publishing Institute (MDPI) platform, points out that, at the highest level of density used in the experiments (thousand plants per hectare), a vigorous rootstock (more productive) and a semi-dwarfing rootstock (which reduces canopy size by up to 40%) increased land efficiency by 64% and 21%, respectively, up to eight years of orchard age, when compared to the current average productivity of the state of São Paulo. Even with intermediate density (696 plants per hectare), the efficiency gain can reach 45%. However, the technique requires a lot of care from the producer.

The study is part of the objective of the two institutions to develop more intensive orange production systems, aiming at the sustainability of the sector and increasing competitiveness in the long term. The experiment highlighted in the article was set up in 2012 by Fundecitrus and was designed by researcher Leandro Peña, now at the Superior Council for Scientific Investigations (CSIC), Spain. As of 2014, the team from Embrapa Mandioca e Fruticultura (BA) that operates in the advanced field in the interior of São Paulo joined the project, which is still ongoing in the area of ​​Terral Agricultura e Pecuária, a partner company located in the municipality of Matão.

“This article tells the story of the orchard’s first eight years, which is equivalent to about half of the orchard’s life. It shows the advantages and disadvantages and the role of the rootstock in this densification process, serving as a guide for the producer on how to produce better and indicating what should be improved through research to increasingly advance the efficiency of land use. We are now telling the first chapter. We need to wait another eight years to close the second chapter, with long-term performance results, after the impact of more frequent use of pruning on adult plants. Anyway, the work continues”, informs one of the authors, Embrapa researcher Eduardo Girardi, coordinator of the Mixed Research and Technology Transfer Unit (UMIPTT) Cinturão Citrícola, which includes Embrapa, Fundecitrus and the Coopercitrus-Credicitrus Foundation. .

Higher density works better with more productive rootstocks

The results so far have indicated that density reduced plant size (the same fertilization per linear meter was used for all treatments). This happens because they compete more with each other in the area for fertilizer, water and sun. “When comparing the density of a thousand plants per hectare with that of 513 plants per hectare, the production per plant was 35% lower. However, as there is 54% more total volume of plants occupying the same area, more plants end up being produced. So, in the initial seven harvests [2014 to 2020], the accumulated production per hectare was, on average, 27% higher due to the greater density adopted”, informs Girardi.

The researcher emphasizes that the performance of the dense system changes depending on the rootstock. For example, IAC 1710, which is the most vigorous, had an accumulated production per plant 23% greater than the commercial one (Swingle) and two and a half times greater than the tetraploid Swingle (runt), on average for the densities studied. So, despite being a large plant, which needs pruning and makes phytosanitary spraying difficult during densification, as it is very productive, it was much superior to the dwarf plant. “We observed that the dwarf plant resulted in lower productivity than the other rootstocks at any density up to eight years. Compared to the commercial Swingle, tetraploid selection reduced both canopy size and production per plant by around 50%: the common Swingle produced 70 kg per plant in traditional spacing, while the tetraploid 31 kg”, he explains.

As Girardi points out, productivity is generally the first factor that the producer will take into consideration when deciding what to do. The work shows that the rootstock is a more important factor than density for this indicator. Although the density of a thousand plants per hectare increased the potential of 27% gain in relation to the orchard of 513 plants per hectare for up to eight years, the rootstock managed to be more decisive for productivity in the same period, even without changing other aspects of crop management. “The more vigorous rootstocks had much higher productivity than the dwarf stock; So, in the short term, it is more advantageous for the producer to work with the vigorous varieties already available today, even if he decides not to expand his orchard too much”, he highlights.

Density x rootstock combination increases land use efficiency by 21% to 64%

As for the indicator related to land use efficiency, the research showed that the density of a thousand plants per hectare improved land use by 22%, producing more fruits per area compared to the average productivity in the state of São Paulo between 2014 and 2020 ( around 32 tons per hectare), up to the eighth year of the orchard's life, regardless of the rootstock used. But, as already highlighted by the researcher, the rootstock proves to be more important than density: IAC 1710, very productive, increased land efficiency by 46% in the same period, regardless of density, compared to the state average . If used at the highest density, this potential gain reached 64%. The semi-dwarfing citrandarin IAC 1697 resulted in a 21% gain, while the dwarfing only matched the state average if used at the highest density. 

The general manager of Fundecitrus, Juliano Ayres, one of the authors of the work, states that this research is important for the sector. As he points out, after a few years of increasing planting density, more recently producers have resisted adopting density, due to management difficulties. “Densification in phase 1 of the orchard [up to 7-8 years] really increases productivity, but in phase 2 [after 8-10 years], performance in the orchard should reduce productivity, because there is more competition between adult orange trees . If there is too much reduction between rows, the orchard closes quickly and requires early pruning, which creates difficulties. 

Field results indicate that the producer can maintain a good planting density with more space between rows and reducing the spacing between plants, for example, adopting a balanced density of 7,0 m x 2,5 m, greatly facilitating operations in the orchard and with excellent productivity. With dwarfing rootstocks and smaller canopy varieties, such as Folha Murcha, you can think about reducing the spacing further”, emphasizes Ayres. But, according to him, correct pruning management and the use of irrigation are fundamental for those who choose to use greater density.

An estimate made by the team indicates that, if this orchard model were used, of a thousand plants per hectare with a more productive rootstock than those currently used, the area of ​​the state of São Paulo with citrus trees could be reduced practically by half, in an ideal world. "This highlights for us the challenge of enabling a higher planting density associated with rootstocks that are very productive, but less vigorous, due to the issue of pruning, spraying, etc. A model like this would have the potential to increase efficiency of use very large land”, adds Girardi.

Now it is necessary to prove whether the benefit of increased productivity in the first years of these densely populated orchards will be the same in the long term. Right after the first pruning, at eight years of age, the volume of the Valencia orange crown on IAC 1710 reduced by 20% in the densest orchard, demonstrating the impact of this practice on the most vigorous rootstocks. In the runts, there was practically no loss due to pruning.

Another point in favor of densification is the shorter lifespan of the orchard today. Until a few years ago, an orange orchard lived for 25, 30 years. Today, with HLB, growers plan their orchards to produce for less than 20 years. “So densification became interesting because it is a way of producing more in less time”, says Girardi. However, several aspects must be considered before adopting it, such as climate, type of soil, ripening time for the fruits of the copa variety, irrigation and, of course, the rootstock. In other ongoing experiments, the relationship between some of these factors and the pruning season has been studied to provide citrus growers with useful information for decision-making. Density, for example, can be prioritized in plots on the edge of properties in order to mitigate the impact of the spread of HLB.

Research aims to improve the performance of dwarf eaters 

With this result, which shows vigorous rootstocks to be more productive than dwarf ones in the dense system, but limited due to plant size management, the research points to ways to improve the productivity of small rootstocks. In the short term, either one tries to work with larger populations, from a thousand to 1,5 thousand plants per hectare, that is, to further densify the dwarf plant in search of increased productivity per area, or one invests in studies to optimize cultural practices to make these dwarf plants produce more, for example, via differentiated irrigation or fertilization. Another line of work in this sense refers to the use of larger seedlings in planting, which can increase production by up to 30% in the first harvests. Girardi explains that, despite being a more expensive seedling, it is earlier, which anticipates the plant's cycle. “This research is still in the validation stage, but the results so far indicate that this would be one of the ways to improve the management of pineapple”, he assesses. 

In the medium and long term, the objective is to obtain genetically improved runts. “We have put a lot of effort into this. Today, commercially, we only rely on the Flying Dragon trifoliate as a dwarfing agent, previously recommended by Embrapa for irrigated orchards. The Embrapa and IAC [Agronomic Institute] Genetic Improvement Programs have been generating new hybrids that will soon be launched with excellent productive potential. The tetraploid citrumelo and other similar ones also continue to be selected alongside Fundecitrus. Having a very small plant, much more suitable for dense management, but with greater production capacity, would be a 'win'", highlights Girardi. According to him, in the meantime, semi-dwarfing rootstocks are a more viable option for those who want to practice higher planting density, with important gains in land use and easier plant management. “The dwarfing rootstock will be fundamental to approach the mechanized harvesting of the fruit”, also says Peña.

Rootstocks are decisive for fruit quality

When monitoring four harvests, it was found that, in general, there was little influence of density on the quality of the fruit. Although there is a small reduction in fruit size, the biggest concern is the amount of sugar. “This quantity was not affected by densification; we even saw an increasing trend. So the quality hasn't changed much. But, logically, when you produce more fruit in dense areas, you also produce more sugar per hectare, which is important for industries. In short, the sugar per fruit did not change much, but, per hectare, the denser fruit produced more sugar”, explains the researcher.

Girardi highlights that, once again, the rootstock proved to be the most important aspect, influencing all the fruit quality variables analyzed (fruit size, sugar, acidity, harvest time, juice content, etc.). Each of the rootstocks in the experiments achieved something different. Therefore, when the producer chooses the rootstock and is concerned about quality, he must pay much more attention to the rootstock than to the density. “For example, IAC 1710, despite being much more productive, had a slightly lower sugar content. This is normal, because very vigorous rootstocks generally translocate a lot of water, diluting the sugar. Smaller rootstocks had more sugar in general. Despite the nanico inducing more sugar in the orange, it was still more advantageous for the producer to use the vigorous, as it produces much more orange. Now, for factories that make juice, the story is a little different. In this case, the priority is better quality juice; They will probably prefer to diversify, using rootstocks that result in more sugar, as is the case with IAC 1697. The decision for the rootstock will then depend on the producer's priority: whether it is quality or productivity.”

Harvesting and spraying can benefit from dense orchards that use dwarf rootstocks

Two aspects highlighted in the study indicate why it is important for research to insist on dwarfing rootstock: the issue of harvesting and spraying. 

The work showed that manual harvesting of the nanico, as it is a smaller plant, was three times faster per tree than that of a large plant. The harvest per kilogram of fruit proved to be 17% more efficient in the dwarf plant, regardless of density. “There is less use of ladders, it is easier to walk between the rows, the plant is small, the fruits are closer to the hand... This all made the process faster and, therefore, more efficient because kilo collected. In other words, it is cheaper to harvest the dwarf plant. This operation is one of the highest costs of orange production, in addition to the fact that labor is increasingly less available in the field. Furthermore, the rootstock will need to be small to enable fully mechanized harvesting in the future,” says Girardi (photo above and right). For him, the work shows, once again, that improving dwarfs via a more productive dwarfing variety or through cultural management is strategic for citrus farming, as harvesting will increasingly be a critical activity.

The goal is, according to the researcher, for the dwarf rootstock to have a maximum size of 2,5 m to 3 m in height and produce, on average, 40 kg to 60 kg per plant. “Some of the new hybrids showed potential for this in selection work. The expectation is that these new varieties will allow this scenario of ultra-dense orchards, from a thousand to 1,5 thousand plants per hectare, with compact and more productive plants, to actually happen, resulting in productivity similar to or above the current production system. ”

Regarding spraying, the most important aspect is the issue of HLB. In 2021, after nine years of planting and controlling the vector with insecticides, the accumulated incidence of HLB in the most vigorous plants in the experiment (on the IAC 1710 rootstock) was 27%, practically double the incidence in dwarf plants ( 14%). This difference is explained by three factors: the more vigorous rootstock induces more and larger shoots to the scion, being the organ where the psyllid feeds and reproduces; the vector finds large plants more easily than small ones when this insect migrates from external areas to the orchard; the psyllid is more present in the upper third of adult orange trees; Therefore, in taller orange trees, it is more difficult to spray the tops of the plants. As it is very dense between the lines, the machine has difficulty traveling and covering the entire plant. According to Girardi, this indicates the need to carry out frequent pruning to control the size of the plant or adjustments to spraying equipment, or even not to overcrowd vigorous varieties.

He points out that other studies have already demonstrated that the accumulated incidence of HLB is lower in dwarfing rootstocks under strict control of the insect vector. Despite not resisting the bacteria associated with HLB, this type of rootstock induces less sprouting in the scion and, possibly, less exposure to the psyllid. The volume of spray solution is smaller and it is easier to cover the plants completely because they are smaller. “Obtaining smaller and more productive rootstocks has the potential to significantly change the way citrus is grown, because it can bring together several benefits. We will, of course, launch vigorous varieties, because they are productive and, mainly, because they are generally more tolerant to drought, but the future points in another direction. Compact plants are the way to go. We have to improve the instrumentation and operational aspects, both for the dense orchard that already exists with the vigorous plant and for the future as well.” And Girardi highlights an important message for the citrus grower: “these more intensive systems that we anticipate here are interesting and can save even more land, but the producer cannot neglect HLB management; otherwise, you won’t be able to make your orchard viable.”

Fundecitrus researcher Renato Bassanezi, one of the authors of the work, recalls that, regardless of the scion/rootstock combination or planting density, as all scion and rootstock varieties are susceptible to HLB and will suffer its damage in production and fruit quality, it is essential that the citrus grower continues to rigorously control HLB through the preventive application of insecticides and the elimination of diseased plants, both inside and around commercial properties. “The sooner a plant is infected, the faster the bacteria takes over the plant and the lower its productive potential will be, and the faster a greater number of plants are infected in the orchard, the shorter the productive longevity of that orchard will be”, he adds. Bassanezi.