Extension briefs AUGUST and SEPTEMBER 2025

Crop and fruit quality management

(P.J.R. Cronjé)

Keep managing and monitoring fruit pickers throughout the harvest season to reduce culling of export fruit through picking injuries. Pruning of early- and mid-season cultivars should commence soonest after harvest, to allow ample time for flower induction.

Maturity indexing helps predict the rate of change in fruit maturity, to harvest fruit at a maturity that would maintain an optimal commercial shelf life. The aim is to define changes or the rate of change in acids and sugars and to build up a database over several years, for comparisons. Random sampling of fruit every week, from each of 10 representative trees, should be four to six weeks before the expected harvest date. Titratable acidity is determined by titration with sodium hydroxide. Sugar content (Brix) is determined using a refractometer, and the sugar to acid ratio calculated. Fruit colour should be read from a colour chart. This data should be plotted on a graph to determine the optimal picking window. Growers should adhere to the time and temperature protocols for each citrus type, to ensure optimal shelf life of the fruit (Cutting Edge No. 99). Maintain good records of the maturity indicators over several years, to identify and possibly manipulate potential problems associated with internal and external quality parameters.

Degreening and postharvest rind disorders The two publications Common Defects Associated with Degreening of Citrus by Andy Krajewski and Tim Pittaway, and Postharvest Rind Disorders of Citrus Fruit by Paul J.R. Cronjé are must-reads for growers. Both are available from the CRI online shop at www.citrusres.com.

Pruning For early and late cultivars, pruning should be done soonest after harvest. The following should be removed during pruning: old, broken and dead shoots/twigs; weak and entangled shoots crossing each other, and rootstock regrowth (water shoots). Removal of all dead wood is important to reduce fruit blemishes and the inoculum of latent pathogens, which cause postharvest decay. A light intensity level of at least 30% of full sunlight is necessary for optimal photosynthesis, and sufficient light intensity levels improve fruit colour development. In dense and old trees, light intensity inside the tree canopy can drop to below 30% and adversely affect fruit set and size. At least one “window” cut should be made to allow for adequate light distribution to improve bearing wood within the tree canopy. An increase in photosynthesis and light distribution will promote increased fruit size and internal fruit quality, better fruit colour, increased rind condition, and less variation in fruit quality within the canopy. Pruning should be used as a thinning technique. Prune more heavily after a light crop (if a heavy crop is expected in the subsequent season), and if the orchard has a history of alternate bearing. A follow-up of regrowth management in the summer is critically important to maintain light management throughout the season. Proper pruning also improves spray penetration, leading to effective control of target pests and diseases. This is vital for the effective control of phytosanitary pests and diseases. Pruning tools must be sanitised regularly with a 10% Jik solution, at least after each row and when moving from one orchard to another, to prevent the spread of viral diseases.

Flowering One or two pre-bloom foliar urea applications (low biuret urea at 1%) should be applied for uniform flowering and fruit set, especially when leaf nitrogen (N) levels are low, and a light blossom is expected. If leaf nitrogen (N) levels are sufficient, consider replacing the foliar urea application with a 1.5% potassium nitrate (KNO3) application, only if leaf K levels are below optimum.

Fruit set Treatments need to be applied according to cultivar requirements. A general guideline cannot be given, as fruit set treatments differ by cultivar and – in many cases – by orchards, depending on the previous crop load. Specific treatments include the application of gibberellic acid (GA3) and trunk or branch girdling, especially for parthenocarpic cultivars with a poor set. Girdling during full bloom improves set, as a cut through the bark temporarily restricts carbohydrate allocation to roots and allows for utilisation by flowers. Don’t girdle too deep into the trunk, or remove a strip of bark. Focus on girdling scaffolding branches, for example three of the five scaffolding branches rather than the main stem. If girdling is not conducted properly, losing a scaffold branch is better than losing a tree. Moisture stress should be avoided at all costs during full bloom, fruit set and early fruit growth – periods are characterised by the cell division stage of fruit development, during which water supply is of critical importance. Stress during this period has an irreversible effect on fruit size.

Geïntegreerde bemesting

(P. Raath)

Stikstofbemesting van sitrus aan die einde van die groeiseisoen

1. Blaar- en grondmonsters moes aan die einde van die groeiseisoen geneem gewees het (Februarie tot Mei).
2. Resultate van die blaar- en grondontledings tesame met boord-inligting (ouderdom van bome, onderstam, kultivar, verwagte opbrengs, groeikrag, blaarkleur, ens.) word gebruik vir bemestingsaanbevelings.
3. Bestudeer die “Handleiding vir bemesting van sitrus in Suid-Afrika” om die analises te vergelyk met die norme in die handboek.

Waar mikro-spuite of enige ander stelsel, behalwe druppers, gebruik word

1. Begin met stikstoftoedienings in Julie, met die uitsondering van die Wes- en Noord-Kaap wat in Augustus begin of sodra die grondtemperatuur in die wortelsone vir ’n paar dae bo 13°C is.
2. Afhangende van die klei-inhoud van die grond, word die stikstof (N) in een tot vier toedienings verdeel.

Waar druppers gebruik word

1. Begin in Julie met die stikstoftoedienings, behalwe in die Wes- en Noord-Kaap wat in Augustus begin of sodra die grondtemperatuur vir ’n paar dae bo 13°C in die wortelsone is.
2. Verdeel die volumes wat per maand aanbe- veel is in ten minste weeklikse toedienings.
3. Moenie dat die besproeiingswater/kunsmismengsel dieper as die boonste deel van die wortelsone (30 tot 40 cm) indring nie. Stikstoftoediening geskied volgens die fenologiese stadium van die boord. Verwys na die riglyne wat volg.

Blaarbespuitings met stikstof

1. Dien LB ureum as blaarbespuiting in Julie toe om blom en vrugset te bevorder, of ná Oktober vir stikstof-aanvulling.
2. Die kritiese vereistes vir suksesvolle blaarvoeding is die kontaktyd waartydens die blare nat bly, druppelgrootte, asook die konsentrasie van die voedingselement in die spuit-oplossing.

Beperk só die loging van stikstof

1. Dien stikstof in die middel, of aan die einde van die besproeiingsiklus toe.
2. Voorkom oormaat van stikstof in die grond-oplossing. Stikstofkonsentrasies wat hoër is as 150–200 mg/ het geen addisionele voordeel nie – dit sal net lei tot loging van die stikstof en/of moontlike probleme met vrugkwaliteit.
3. Staak verkieslik stikstoftoediening aan die einde van die groeiseisoen, sodat die stikstof-inhoud in die grond gedurende die rypwordingsperiode kan afneem.
4. Die regte hoeveelheid water moet tydens besproeiing toegedien word. Loging van ‘n sekere hoeveelheid stikstof tydens besproeiing is onvermydelik. Maar oorbesproeiing versnel die proses en groot hoeveelheid stikstof kan dan verby die wortelsone geloog word. Neem ook reënval tydens besproeiingskedulering in ag.

Nitrogen fertilisation of citrus At the end of the growing season

1. Leaf and soil samples should have been taken between February and May.
2. Results from the soil and leaf analyses, combined with additional orchard information (tree age, tree vigour, tree colour, expected yield and rootstock), are used to compile a fertiliser programme.
3. Use the “Handbook for fertilization of citrus in South Arica” to compare the analysis with the norms provided.

When micro-jets or any other system – except drippers – are used

1. Start with nitrogen fertilisation in July (in the Western and Northern Cape, start in August or when the soil temperature in the root zone is above 13°C for a few days).
2. Split the nitrogen application into one to four instalments, depending on the clay content of the soil.

When drip irrigation is used

1. Start with nitrogen fertilisation in (in the Western and Northern Cape, start in August or when the soil temperature in the root zone is above 13°C for a few days).
2. Split the volumes recommended per month into weekly applications, at least. Ensure that the water and fertilisers do not penetrate deeper than the upper root zone (30 to 40 cm).
3. Application of nitrogen should be done according to the phenological stages. Refer to the guidelines.

Foliar spray of nitrogen

1. Spray the LB urea in July to improve flowering and fruit set, and after October to supplement nitrogen.
2. Critical requirements for successful foliar sprays are contact time, droplet size, and concentration of the nutrient element in the spray solution.

Four measures can be used to prevent or minimise the leaching of nitrogen

1. Inject nitrogen in the middle of, or late in an irrigation event.
2. Avoid excessive concentrations of nitrate in the soil solution. Nitrogen concentrations greater than 150–200 mg/ do not provide any additional benefit – they increase the risk of leaching by successive irrigation or rainfall events. Oversupplying nitrogen can also lead to fruit quality issues.
3. Complete the nitrogen fertilisation programme by the end of the growing season to allow depletion of nitrate in the soil by winter.
4. Some movement of nitrate may be inevitable with each irrigation cycle, but overwatering is likely to speed up that process and move nitrate beyond the root zone. Allow for expected rainfall when calculating the depth of water to apply – this will also help reduce the likelihood of leaching.

Grondgedraagde siektes

(MC Pretorius en J. van Niekerk)

Aalwurms Grond- en wortelmonsters kan in die lente getrek word en vir ontleding na die Diagnostiese Sentrum in Mbombela (Nelspruit) gestuur word, sodat die aalwurmpopulasie in die wortels bepaal kan word. Die resultaat sal dien as bestuurshulpmiddel om kostedoeltreffende aalwurmbeheerstrategie daar te stel. Die gebruik van chemiese aalwurmdoders vir die beheer van die sitrusaalwurm word nie aanbeveel voor ten minste 30 mm reën geval het nie. Elke aalwurmdodertoediening behoort met behoorlike besproeiing opgevolg te word om te verseker dat die middels deeglik deur die grondprofiel gewas word. Toedienings behoort slegs volgens etiket-aanbevelings toegedien te word. Afwyking van die geregistreerde dosisse, om kostes te bespaar, lei tot oneffektiwiteit. Dit is belangrik om ’n program van twee tot drie toedienings te volg met twee maande intervalle (om toksiese vlakke vir ’n langer periode hoog genoeg in die grond te hou) – behalwe vir fluopyram. Volg etiket-aanwysings van die betrokke produk. Afwyking van hierdie aanwysings kan lei tot oneffektiewe beheer en dus ’n mors van geld.

Phytophthora-wortelvrot Die gebruik van fosfonaatprodukte is ’n uiters effektiewe en bekostigbare beheermaatreël wat suksesvol deur produsente gebruik word. Dit is van uiterste belang dat die etiket deeglik bestudeer en dat daar op die waarskuwings gelet word voordat die produk gebruik word. Dit verseker effektiwiteit en voorkom fitotoksisiteit. Om die doeltreffendheid van die fosfonate te verseker behoort ten minste twee, maar verkieslik drie opvolgtoedienings agt weke uit mekaar toegedien te word. Indien kraagvrotletsels voorkom kan ’n stamverf of blaarbespuiting aangewend word (drie aanwendings per seisoen met agt weke intervalle). Vir wortelvrotbeheer word ook drie blaarbespuitings (met agt weke intervalle) aanbeveel. Bespuiting tydens blom kan die moontlikheid van blomblaarval verhoog. Doen dus die eerste bespuiting direk na blomblaarval. Dit word sterk aanbeveel om nuwe aanplantings en nie-draende bome op ’n fosfonaatprogram van drie aanwendings per jaar, twee maande uitmekaar te hou om gesonde wortel-ontwikkeling te verseker. Produsente word gemaan om seker te maak dat bome nie oor- of onderbesproei word nie.

Fruit and foliar diseases

(J. van der Waals)

Alternaria brown spot is caused by the fungus Alternaria alternata and occurs in all citrus-production areas of SA. The most susceptible cultivars are those with Dancy parentage, e.g. tangerines and tangelos. ABS results in leaf, twig and fruit symptoms, which can be a limiting factor in production areas with high humidity or rainfall. Spore release, from lesions (spots) on mature leaves, is triggered by rain and sharp drops in humidity. The optimal temperature range for infection is 23–27°C, but infection can occur at temperatures as low as 17°C and as high as 32°C, with extended wetness periods. Low levels of infection can occur with leaf wetness duration of four to eight hours, but usually 10–12 hours of wetness is needed for substantial infection. Spread of the pathogen from one tree or orchard to another is by airborne spores produced on leaf, stem and fruit lesions.

Alternaria alternata damage to young vegetative shoots varies from small circular leaf spots to large necrotic blighted lesions that cover a major portion of the leaf. Lesions tend to extend out along the veins as the toxin produced by the fungus spreads in vascular tissues. Infected twigs provide the most continuous source of infection of lower-hanging fruit. Leaves become resistant to further attack once they are fully expanded. When young fruitlets become infected, they usually drop. Older infected fruit may be retained, as it forms secondary tissues in a series of radial tissue layers that isolate the pathogen and give rise to corky protrusions on the rind. These can easily be dislodged, leaving pockmarks. The healing reaction of the outer cell layers can be enough to completely inactivate the pathogen. However, some pustules, though apparently completely healed, may become active again and give rise to slowly expanding necrotic brown spots approximately 5 mm in diameter. Management is aimed at reducing inoculum, increasing airflow through the canopy and protecting uninfected fruit in a preventive approach. Chemical management of Alternaria brown spot can be integrated with fruit protection programmes aimed at citrus black spot (CBS) control. However, chemical programmes aimed at Alternaria brown spot (ABS) control should commence as soon as the first spring flush emerges. For CBS management, it’s earlier than needed and should continue for longer, as fruit stays susceptible until harvest. Copper, mancozeb, dodine (guanidine) and azoxystrobin are registered against ABS on citrus in SA.

Alternaria core rot is also known as navel-end rot or black rot. It’s caused by the fungus, Alternaria alternata, and occurs in all areas of Southern Africa. The disease is most prevalent on citrus cultivars like navels and Clementines, which are characterised by the presence of a secondary fruitlet – the navel. The navel develops at the stylar end of the fruit and varies in size. The formation of the navel-end opening, and its size are influenced by climatic conditions during fruit set. Under cool weather conditions, the secondary fruit style successfully fuses with the style of the primary fruit. Consequently, both the secondary and primary style abscise during petal fall, producing a closed navel end. However, under extreme weather conditions (e.g. warm, dry and windy conditions), the primary fruit style abscises prior to fusion with the secondary fruit style resulting in the formation of a cavity between the primary and the secondary fruit. Such cavities provide entry points for fungi, such as A. alternata, to penetrate and form infections. These remain quiescent until favourable conditions stimulate further growth of the fungus. Alternaria core rot is linked to fruits with large or malformed navel ends.

Tebuconazole, difenoconazole and 2,4-D are registered for control of this disease. Difenoconazole is applied at 50% and 100% petal fall. Tebuconazole and 2,4-D can be applied together, once at 80–100% petal fall.

Botrytis on lemons has a role that still cannot be clearly defined regarding lemon fruit drop and the formation of ridging of the rind. Damage can be caused during blossom on lemon petals when prolonged wetting and cool weather occur simultaneously. Pyrimethanil, azoxystrobin and a pyrimethanil+fludioxonil combination product are registered against Botrytis cinerea on citrus, as is a Trichoderma asperellum biological control agent. The fungicides are applied once at full blossom, while the biological control agent is recommended as a foliar spray every 10–14 days, depending on disease pressure. It can also be applied monthly as a year-round soil drench.

Postharvest pathology – waste prevention

(W. du Plooy, L. Mamba, N. Jackson and J. Landman)

By this time of the citrus season, fruit volumes are quite high and packhouses are straining under the demand for processing. This will take its toll on management and may lead to compromised diligence in the management of critical control points.

Critical control points for improved postharvest disease management

• Communicate with the pre-harvest managers and ensure an up-to-date exchange of information by:
• Monitoring the incidence of insect activity in the orchards and implementing appropriate measures to reduce their populations, especially fruit fly and FCM numbers that could increase with hotter weather.
• Monitoring orchard practices and sanitation:
• Advise the orchard managers and producers if sanitation is being improperly managed or neglected. This can also play an overbridging role for FCM from one season to the next. It is important to follow the FMS correctly. o Remove rotten and fallen fruit. This is crucial to keep spore loads down and to reduce the risk of postharvest decay.
• Monitor injuries to fruit during picking, handling and transport to the packhouse. Advise orchard managers and/or producers accordingly.
• Keep the time from harvest to the first fungicide treatment to a minimum. It is strongly recommended to treat within six hours from picking, but within 24 hours after picking.
• Ensure that all fungicide applications are replaced frequently as per protocol and kept clean.
• Keep the fruit dumping site cleanest where fruit will enters the packhouse.
• Ensure proper removal of any rotten fruit before sanitising and treating it.
• Have a sanitation action between fruit sorting and the first fungicide application.
• Ensure that fruit is dry before entering the fungicide treatment.
• Manage the concentration of imazalil in the fungicide treatment by doing regular titrations.
• Do not wax wet fruit; this will put fruit quality at severe risk.
• Apply the correct amount of wax onto the fruit (i.e. 1.0–1.2 per tonne of fruit) and ensure even spread of the wax over the whole fruit.
• Reduce the time from harvest to cold chain storage.
• Packhouse sanitation should be a continuous process, and not just a once-daily or weekly activity.
• Store fruit destined for the juicing factory far away from the packhouse and have it promptly removed.

Checklist for chemical application

• Chlorine application o Solution pH: 6.5–7.5 o Chlorine concentration: 75–100 ppm (free active chlorine)
• RP: 800 mV. • Fungicide dip tank
• Imazalil concentration: 500 ppm o Solution pH: not higher than six.
• Exposure time: one to three minutes in a solution at pH3 or not longer than 45 seconds in a solution at pH6.
• Maintain the concentration by following the advised imazalil top-up protocol or according to regular titrations.
•  Wax application o Brushes must be kept clean and maintained to a very high standard, to achieve the best results.
• Fruit should be dry when they reach the wax applicator, as film formation on wet fruit is not ideal and will cause disintegration during transport.
• Wax load should be between 1.0 and 1.2 per tonne of fruit depending on manufacturers’ recommendations:
• Over-application can lead to MRL exceedance.
• Under-application will lead to poor fruit quality and poor disease control.
• When using an impeller or venturi type of wax blending tanks, the wax solution should be agitated continuously (24 hours per day, seven days per week):
• Thiabendazole tends to precipitate to the bottom of the wax drum and cannot be fully re-suspended in these types of tanks.
• The funnel-shaped tanks, and horizontal tanks with a scraper paddle, can be switched off during non-operational hours, but need to be switched back on an hour before the packline starts in the morning.
• Drying tunnels after the wax applicator should not be overheated:
• Obtain the optimal temperature for the specific wax from the wax manufacturer.
• The correct drying of the wax is crucial to gain the desired effect.