Encyclopedia
Please note this is an old version of this entry, which may differ significantly from the current revision.
Subjects:
Horticulture
European hazelnut (Corylus avellana L.) is a shrub native to temperate zones of the northern hemisphere, and it is the most important species among the Corylus genus, mainly due to its high kernel demand from the confectionery industry.
- Corylus avellana L.
- suckers’ management
- herbicides
1. Introduction
European hazelnut (Corylus avellana L.) is one of the most important species among the nut trees [1], and the steadily increasing demand for hazelnuts by the confectionery industry at global level has led to continuous expansion of hazelnut cultivated areas in recent years, both in historically suitable areas (Turkey, Italy, Spain, Georgia, and the USA) and in new countries such as Australia, South Africa, and Chile [2,3]. To date, hazelnut cultivation covers over 700,000 harvested hectares worldwide, and the yearly average in-shell nut production is mainly concentrated in the following countries: Turkey (665,000 t), Italy (140,560 t), the USA (64,410 t), Azerbaijan (49,465 t), Chile (33,939 t), Georgia (32,700 t), and China (24,263 t) [4].
The continuous formation of new herbaceous suckers at the base of the collar of woody stems or plant stumps during the growing season is the consequence of its typical bushy development [5]. To date, the bush training system is still widespread in hazelnut cultivation, especially in the Mediterranean basin and in Turkey [2].
European hazelnut is a species with high suckering emission aptitude, and the number of suckers emitted varies among cultivars [6], such that the main hazelnut varieties have been classified according to their sucker’s emission aptitude [7,8]. However, suckering aptitude is not exclusively determined by genotype; it can also be influenced by additional factors, such as training system, planting layout, seasonal weather conditions, and biennial bearing aptitude of the species.
In the past, the sucker emission was considered of economic value as it was functional to the gradual renewal of the orchard and it allowed on-farm supply self-rooted plants to be used to plant new hazelnut orchards.
Furthermore, this idea was consolidated, as hazelnut plants trained as multi-stemmed bush play a preventive role against soil erosion phenomena [1].
Despite these advantages, in the new-generation hazelnut orchards with high levels of mechanization, the presence of suckers is detrimental for several reasons, including competing with the plant canopy for nutrients and water [9], causing a reduction in plant growth, extending the juvenile phase, and reducing nut yield [10]. In addition, suckers can decrease the ventilation capacity of the plant, promoting environmental conditions conducive to disease development [11], and they may hinder the orchard management (chopping, fruit picking, etc.).
For these reasons, from the second year after planting, sucker removal is an essential practice. During the juvenile stage of the plants, suckers are removed manually, taking special care to remove only those in excess and with poor growth and orientation, thus promoting the development of the future bush, leaving 4–5 vigorous and well-oriented suckers to develop.
From the fourth year, control strategies can be carried out manually or mechanically, although the time-consuming nature and high cost of this operation [12,13], increasingly accompanied by a lack of skilled labor, makes chemical sucker control using authorized herbicides the most widespread practice [1].
Chemical suckers’ management can be carried out during their herbaceous stage, and usually at least a couple of interventions during the growing season are needed due to the sucker regrowth, or later in the season when suckers are lignified mainly using physical techniques [11].
2. Manual Control
In traditional Italian and Turkish hazelnut districts, manual sucker control is still largely applied, even if this cultural operation requires high labor force influencing up to one-fifth of the annual orchard management costs [9,13]. Manual sucker removal performed with proper tools such as shears or small hoes may be considered highly suitable for small-scale and organic farms.
This practice is usually carried out in a single summer operation, as this is a time when seasonal suckers have slowed down their growth rate and tend to lignify.
The main advantages related to manual sucker control include its status as an ecological method and promotion of the gradual rejuvenation of the plant stump in the orchards grown on a multi-stemmed bush, a feature particularly appreciated by the grower [1].
Contrariwise, the main disadvantages associated with this technique, in addition to those already mentioned and the opening of wounds in the collar area that may favor the penetration of pathogens, are also related to manual removal of suckers being ergonomically dangerous and tiring work, since the repetitive pruning movements and prolonged bending could cause occupational injuries [14].
3. Mechanical Control
Managing suckers in organic hazelnut orchards farming, in addition to manual control, can be achieved using mechanical approaches.
This practice can be applied both to plants trained as single-trunk and to shrubs, taking care during removal operations to avoid damages on the main branches.
This type of control is mainly carried out using appropriate shoulder-mounted brush cutters equipped with cutting nylon line or metal disk, a flail mower with a lateral rotary cutter on a swing-arm linkage or using an orchard-vineyard de-suckering machine equipped with a horizontally rotating drum with a whipping brush of nylon strings [15].
Mechanical control is a quick practice that can be replicated with multiple seasonal interventions on suckers at the herbaceous stage.
When applied on plants trained as multi-stemmed bushes, it does not allow all suckers to be removed, especially those inserted in the inner portion of the bush [15].
The application of mechanical sucker control determines a significative reduction in labor (up to 55%) and costs (up to about 20%) compared to manual control [13]. Moreover, mechanical control can be used as a complement to chemical control if some suckers have resisted the phytotoxic activity of the suckering herbicide.
4. Physical Control
Along with manual and mechanical control, physical management is another agronomic practice useful for suckers’ removal, especially in organic farms. Water steam and fire (open flame) administered by means of proper machines attached to the tractor are the two thermal procedures used for physical control of suckers. These techniques have been extensively studied in Italy by [11,15], with the aim of comparing both treatments and establishing their effectiveness, economic feasibility, and potential damage to the main branches or plant stumps.
The application of these methods consists of a short-lasting jet of intense heat sprayed directly onto the herbaceous suckers, causing them to wither and die a few days after treatment [16].
This control method can be carried out on suckers both in herbaceous and lignified stage. If applied early (during the herbaceous stage), at least two to three interventions are necessary during the growing season due to the fast regrowth of new suckers after treatment. On the other hand, if the treatment is carried out on lignified suckers, manual removal of wilted suckers will be necessary. Further, this practice is considered very attractive, as it is environmentally friendly.
Results obtained by [11] showed that both methods have enough efficiency in suckering without damaging the plants.
Despite the effectiveness of both practices, the water-vapor method is more complicated due to a very low working speed, the high amount of water and fuel needed, and very expensive equipment.
In contrast, flaming is easier, since both equipment cost and fuel consumption per year and hectare are quite low. Furthermore, when using flaming it is recommended to carry out the operation in the early morning and early spring, when the grass is still green, to avoid potential fires [15].
In conclusion, these techniques, if applied in automatic mode using tractors, can only be carried out on flat or slightly sloped surfaces, while if applied by operators, high specialization and experience is requested. Lastly, applying physical control of suckers does not allow plant rejuvenation, as all treated suckers will have the same course (wilting and then death).
5. Chemical Control
The use of herbicides for suckering (Figure 1) is the most widely used control technique in conventional agriculture due to its main advantages, such as quick implementation and lower cost compared with other sucker control methods [1].
Figure 1. Comparison of manual and chemical hazelnut suckering (on the left: sucker canopy in multi-stemmed bush before manual suckering; in the middle: same multi-stemmed bush after manual suckering carried out in early summer; to the right: hazelnut suckers in wilted stage after chemical treatment).
Despite these benefits, chemical control of suckers highlights problems associated with the environmental pollution, induces depression of soil microbial activity, and may lead to phytotoxicity on the crop [17]. In addition, the use of herbicides for both weed control and suckering promotes soil erosion, especially in areas characterized by slopes [11].
Generally, the number of seasonal herbicide applications for sucker control varies from two to four, depending on the climatic condition, the age of the orchard, the plant training system, the sucker emission aptitude of the cultivars [6], and the persistence of suckering herbicide. Treatments must be carried out quickly when the suckers are in the herbaceous phase and reach about 15–20 cm high, as their development is not uniform (they show high variability in height).
If suckers are allowed to grow during the season until they lignify, many more seasonal interventions will be required to control them, often assisted by other control methods [18].
Further, [19] points out that the effectiveness of herbicides in sucker control is not only related to their development and to the number of herbicide interventions, but also to the size of spray droplets. In support, several studies shows that a reduction in droplet diameter is positively correlated with an increase in herbicide efficacy [20].
The first trials on the application of active ingredients for sucker control were conducted in Italy and Oregon (USA) in 1960 [1], although the first attempts to eliminate suckers by chemical control were carried out directly by farmers in Oregon in the first half of the 1950s [21,22].
In the early experimental years, many researchers conducted trials in which the degree of phytotoxicity of active ingredients used for sucker control was assessed, including aminotriazole, bromacil, chlorthiamid, dichlobenil, paraquat, dinoseb, diquat, cypromid, cacodilic acid, 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), dicamba, 2,4-dichlorophenoxyacetic acid (2,4-D), and picloram [23,24].
Later, some studies carried out in Italy, and revised by [17,25], focused on the evaluation of additional herbicides as 1-naphtaleneacetic acid (NAA) esters and glufosinate-ammonium.
Given their high environmental impact, some active ingredients listed above are no longer authorized, such as Paraquat (N,N′-dimethyl-4,4′-bipyridinium dichloride), which was banned in Europe in January 2015 due to its harmful effects on avifauna and insects. Nevertheless, its use is not forbidden in the United States, where it is still used for suckering. Similar to Paraquat, glufosinate-ammonium was also authorized as herbicide in the EU until 2018, after which it was banned due to its assumed reprotoxic effects.
Despite the high efficacy of 2,4-D in sucker removal [26], during the summer, when treatments are often carried out under conditions of high temperatures and low air humidity, the risk of active ingredient volatilization increases significantly with a high probability of damaging the plants [27,28].
This issue could be overcome thanks to the use of NAA, which is a non-volatile compound [29]. NAA is a synthetic plant growth regulator within the auxin family that acts as an herbicide by promoting the production of abscisic acid and hydrogen peroxide, causing growth inhibition or wilting, tissue necrosis, and, consequently, death of the affected plant tissue [30].
Recently, [17,25] demonstrated the effectiveness of different derivatives of NAA in suckering without highlighting plant phytotoxicity issues or yield decline. Furthermore, other updates supporting their reports were obtained by [28], confirming that NAA performs satisfying suckering without causing direct damage to the plant. In addition, application of herbicides in solution with NAA could improve suckering when compared with treatments made individually with NAA or herbicides, with the effect of reducing the number of treatments to be performed.
This entry is adapted from the peer-reviewed paper 10.3390/plants11243416
This entry is offline, you can click here to edit this entry!
