Supporting litchi fruit growth and quality

S uppor ting litchi fruit growth and quality

Fruit abscission is a normal physiological process to produce a good crop with quality fruit. External factors can lead to excessive fruit drop and production loss. With orchard management, growers can to a great extent mitigate adverse events.

Stephan Schoeman1 and Regina Cronje2

1SOETKALMOES CONSULTATION 2ARC-TROPICAL AND SUBTROPICAL CROPS

Although litchi often flowers profusely, only 1-5% of the female flowers set fruit, depending on tree, cultivar, year and environment. Fruit abscission is a normal physiological process to ensure that limited resources are supplied to produce a good crop with good quality fruit.

This self-regulating mechanism is, however, greatly influenced by external factors such as adverse climatic conditions (extreme temperatures, low humidity, drought, strong winds, prolonged rain), lack of nutrients (especially boron and zinc) and competition with vegetative growth, which can lead to excessive fruit drop and production loss.

In addition, flowering and fruit growth are energy costly processes and require ample water, nutrients and carbohydrates. Growers can mitigate adverse events to a great extent and promote fruit quality and production by adequate orchard management.

SUPPORTING FLOWERING AND FRUIT SET (AUGUST/SEPTEMBER/OCTOBER) Pollination and fertilization are the preconditions for sufficient initial fruit set. Plenty of bee colonies (4 hives per hectare) should be introduced to the orchard before flowering. Sufficient initial fruit set is also the best way to suppress summer flush, which competes with the fruit for carbohydrates and can cause heavy fruit drop.

Irrigation Water and irrigation management plays a significant role to promote uniform flowering, good fruit set and early fruit development. Irrigation is particularly important during the sensitive first six weeks after female flowering. Heat stress and water deficits can lead to excessive flower and fruit drop.

During the cell division stage, these factors will limit potential fruit size and predispose fruit to cracking. If tensiometers are used, irrigation should be initiated when levels in the active root zone reach -30 kPa in heavier soils, and -25 kPa in lighter soils. In cooler areas, be careful to not over-irrigate at this time.

Fertilization Flowering and fruit development is a nutrient demanding phase. Fertilization with zinc, boron and calcium is important, because a significant amount of these elements are taken up by the plant during this phase. Foliar sprays of Kelpak or similar products (supplying cytokinins) applied at female flower, boron (maximum 0,5%) applied 15 days after fruit set, and calcium are utilised to support fruit set and cell division.

To support fruit set through to exponential fruit growth, 60-70% of the annual potassium requirement is soil applied between full bloom and after the second fruit drop, while 20% of the annual nitrogen requirement is applied at fruit set. Do not over-apply potassium in the early part of this phase as it could suppress sufficient calcium uptake, especially in cultivars prone to cracking. Sufficient calcium levels are critical for strong cell walls to limit fruit cracking, and promote fruit quality.

Generally, calcium should be maintained throughout the year, but particularly for fruit set, calcium nitrate is commonly applied as foliar spray. If leaf nitrogen levels are high, nitrogen-free calcium sources can also be considered (e.g. calcium chloride).

Girdling and plant growth regulators Girdling is an effective measure to suppress summer flush and root growth and significantly improves fruit set. Girdling for promoting fruit set is done during female flower bloom as a spiral around the trunk. Although very effective, girdling has not been widely used in South Africa.

Synthetic auxins have been proven to assist fruit retention. For the South African market, growers may apply Maxim® (3, 5, 6-TPA) for ‘Mauritius’ at a concentration of 20 ppm (2 tablets/100 L water) at the 2 g fruit mass stage (before the second fruit drop period).

PROMOTING FRUIT QUALITY (OCTOBER/NOVEMBER) Orchard practices to support fruit quality are aimed at pro-

Figure 1. Litchi crop cycle with suggested management actions to promote fruit quality and production (highlighted in blue). Adapted according to Huang and Chen (2014).

moting fruit size and colour, and preventing fruit cracking, sunburn and insect damage.

Irrigation Water management during the exponential fruit growth stage and maturation is important to promote fruit size and prevent sunburn. Trees that are under mild water stress during this stage often have smaller fruit. Moreover, under these stress conditions, fruit can swell excessively and crack after rainfall.

Optimal irrigation with micro sprinklers (depending on soil type) would be to initiate irrigation at 30-40% depletion of easily available soil moisture in the active root zone throughout flowering and fruit development. Trees should be irrigated early in the morning, especially during hot and windy periods, to ensure that the trees start the day with low stress levels. Soil moisture levels should be maintained close to field capacity, particularly when heavy fruit drop is expected.

Mid-canopy or overhead sprinkler irrigation can be used to increase humidity on expected hot days. However, this is only viable if sufficient water is available because atmospheric demand almost always exceed irrigation requirements. Low tree skirts and application of mulch can assist in maintaining good moisture levels in the root zone.

Fertilization Fertilizer applications suggested in the previous section will also promote fruit quality characteristics. A well-balanced nutrition is required for optimal fruit colouration as various elements play a role in fruit colour development (magnesium seems to be particularly important), as well as in fruit flavour and fruit quality.

Conventionally, leaf samples are taken during the fruit growth period (October/November) and can assist to determine immediate interventions required, e.g. foliar or soil applications of magnesium, manganese, copper, sulphur and potassium, and to plan the fertilizer requirements for the coming season. Too high nitrogen levels during this period can lead to reduced colour development.

Plant growth regulators and girdling The synthetic auxin Maxim® may also be used for improving litchi fruit size and should be applied at a concentration of 40 ppm (4 tablets/100 L water, applied at the 3-4 g fruit mass stage (after 2nd fruit drop period). Maxim should only be used once per season, either to reduce fruit drop or to improve fruit size.

Because girdling increases the fruit growth rate by increasing the carbohydrate availability to fruitlets, it can also be a valuable aid for increasing fruit size. For this purpose, girdling should be applied at the 2 g fruit mass stage. However, under South African climatic conditions, girdling is only recommended once per season.

Pest control Fruit flies, litchi moth and false codling moth can do considerable damage to the litchi crop if left uncontrolled in certain areas and years. Monitoring for these pests should start early in the season and recommended control measures should be applied to protect the crop.

References

HUANG, X.M. & CHEN, H.B. 2014. Studies on shoot, flower, and fruit development in litchi and strategies for improved litchi production.

Acta Hort. 1029: 127-136.