Extension briefs for August and September 2023

Mealybug (S.D. Moore and T.G. Grout)

Mealybug infestation levels continue to be problematic, particularly under nets. This can be exacerbated by high thrips pressure, leading to disruptive applications of thripicides late in the season, and by the disappearance of buprofezin as a corrective treatment for mealybug. Early preventative treatment of mealybug is almost twice as effective as a late corrective treatment. However, as mealybug has such an effective biocontrol complex, spraying for mealybug in a good integrated pest management (IPM) situation is often superfluous. The need for an early spring spray can be determined based on the following criteria:

1. if ≥10% of fruit were infested when a pre-harvest infestation analysis was conducted the previous season,
2. if a late corrective spray for mealybug was justified the previous season,
3. if there were any rejections for mealybug on inspection in the packhouse or port,
4. if any infestation of the wood was noted during winter or of the new growth in spring and
5. if spraying of any broad spectrum thripicides is planned during summer. Spraying at or just before petal drop has been shown to be more effective than spraying pre-blossom. This can be done using an organophosphate, buprofezin, Closer or Tivoli.

If mealybug was problematic in the last season, two early sprays four to six weeks apart might be justified. Treatments must be applied as high volume, full cover film sprays, which can only be achieved if trees have been pruned and the spray machine is properly calibrated. Where a parasitoid augmentation programme is being followed for mealybug, the earlier in the season releases are initiated, the more effective they are. This might be difficult where early thripicides are detrimental to the parasitoids. Good ant control will be required where parasitoids are being released, to prevent disruption of the natural enemies. River Bioscience is now selling an ant bait for both the pugnacious ant and the brown house ant, called Saga.

Mealybug infestation under nets is often more severe than in open orchards. As with open orchards, biocontrol of mealybug under nets can also be excellent. However, due to the environment, mealybug tends to build up to a higher level under nets and biocontrol agents tend to take longer before an impact is seen. Consequently, one should act more conservatively under nets when deciding whether to spray early maturing orchards or not.

False codling moth (S.D. Moore)

It may be very tempting to neglect orchard sanitation in winter, as false codling moth (FCM) levels are usually low and Valencias – being harvested at this time – are generally less susceptible to FCM than some of the earlier season cultivars. However, this would be a big mistake. It has been shown that Valencias can fulfil an overbridging role for FCM from one season to the next. The most effective way in which to ensure the lowest FCM inoculum possible at the start of a season, is to diligently continue sanitising orchards until the previous season is truly over. Most importantly, growers must ensure that no fruit, whatsoever, remains on trees or on the orchard floor after harvesting is completed.

Sanitation must also be done in lemon orchards. Although lemons at the commercially ripe stage for harvesting for export are not a host for FCM, overripe lemons can be. Therefore, they must be removed and destroyed. In the warm northern areas of the country, FCM activity starts a lot earlier than in the Cape regions. This could present a risk for late maturing Valencias.

Bollworm (S.D. Moore)

Depending on the region of the country, which influences temperature and blossom phenology, bollworm may begin invading orchards as early as September, or even in August. Hence, growers should begin weekly blossom inspections for bollworm eggs and larvae in early September (no later). Particularly if a biological product such as DiPel, Helicovir or Bolldex is going to be used, sprays should be applied as soon as eggs begin to hatch.

This can only be determined through vigilant and regular scouting. Honey bee populations will benefit if sprays need to be applied during bloom, because these products are harmless to bees.

Citrus flower moth (S.D. Moore)

Moths of the citrus flower moth (also known as the lemon borer moth), Prays citri, are attracted to lemon blossoms. Growers should inspect these blossoms in spring to determine if they are infested with larvae or pupae. These can be identified by their colouration, which is usually greenish, as well as by the association of webbing with pupation. Even if the damage to and loss of blossom are not considered sufficiently severe to justify control measures, no intervention may allow the development of a second generation. It is the moths of this second generation that lay their eggs on the lemon fruitlets. Hatching larvae can potentially cause severe damage. Therefore, it is essential to control the first generation if one wants to prevent damage to the fruit.

An experimental threshold for intervention based on pheromone delta trap catches has been set at 150 moths per trap per week. However, there is no trapping system registered yet. This equates to an infestation level of 5% blossom clusters with larvae and pupae, which will lead to approximately 5% fruit damage. No plant protection products are registered for use against the lemon borer moth. However, there are several pesticides that are registered for other pests on citrus that are effective, including Bt (DiPel) and mevinphos.

Crop and fruit quality management

(P. Cronjé)

General. It is important to keep managing and monitoring fruit pickers throughout the harvest season to reduce culling of export fruit, caused by picking injuries. Pruning of early- and mid-season cultivars should commence as quickly as possible after harvest, to allow ample time for flower induction.

Maturity indexing. It's done to predict the rate of change in fruit maturity, to harvest fruit at a maturity that would maintain 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 start 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 in order 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). It is important to maintain good records of the maturity indicators over several years, to identify and possibly manipulate possible 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 a must-read for any grower. Both are available from the CRI online shop at www.citrusres.com.

Pruning. Pruning of early and late cultivars should be done as soon as possible 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 to reduce 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 also 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 by pruning 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 especially important for the effective control of phytosanitary pests and diseases. Pruning tools must be regularly sanitised with a 10% Jik solution, to prevent spreading of viral diseases, and should be done at least after each row, and when moving from one orchard to another.

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 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 weekly parthenocarpic cultivars that have 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. Be careful not to girdle too deep into the trunk, or to remove a strip of bark. Moisture stress should be avoided at all costs during full bloom, fruit set and early fruit growth, as these periods are characterised by the cell division stage of fruit development, during which water supply is of critical importance.

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, ens.) word gebruik vir bemestingsaanbevelings.

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

1. Begin met stikstoftoedienings in Julie met die uitsondering van die Wes-Kaap en Hartswater wat in Augustus begin.
2. Afhangende van die klei-inhoud van die grond, word die stikstof (N) tussen een tot vier toedienings verdeel.

Waar druppers gebruik word

1. Begin met die stikstoftoedienings in Julie met die uitsondering van die Wes-Kaap en Hartswater wat in Augustus begin.
2. Verdeel die volumes wat per maand aanbeveel is in ten minste weeklikse toedienings.
3. Stikstoftoediening geskied volgens die fenologiese stadium van die boord. Gebruik gerus die breë riglyn hieronder.

Blaarbespuitings

1. Dien 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 en die konsentrasie van die voedingselement in die spuit-oplossing.

Om die loging van stikstof te beperk kan die volgende gedoen word

1. Dien stikstof in die middel van, of aan die einde van die besproeiingsiklus toe.
2. Voorkom oormaat van stikstof in die grondoplossing. Stikstofkonsentrasies wat hoër is as 150 – 200 mg/ het geen addisionele voordeel nie. Oormaat stikstof lei tot moontlike loging van die stikstof en moontlike probleme met vrugkwaliteit.
3. Stikstoftoediening moet verkieslik aan die einde van die groeiseisoen gestaak word, sodat die stikstof-inhoud in die grond gedurende die wintermaande kan afneem.
4. Die regte hoeveelheid water moet tydens besproeiing toegedien word. Dit is onvermydelik dat sekere hoeveelheid stikstof tydens besproeiing geloog word, maar oorbesproeiing versnel die proses en groot hoeveelheid stikstof kan dan verby die wortelsone geloog word. Reënval moet ook tydens besproeiingskedulering in ag geneem word.

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 with additional information such as tree age, tree vigour, and expected yield rootstock are used to compile a fertiliser programme.

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

1. Start with nitrogen fertilisation in July, except for the Western Cape and Hartswater, which should start in August.
2. Split the nitrogen application in one to four portions, depending on the clay content of the soil.

When drip irrigation is used

1. Start with nitrogen fertilisation in July except for the Western Cape and Hartswater, which should start in August.
2. Split the volumes recommended per month into at least weekly applications. Ensure that the water and fertilisers do not penetrate deeper than the upper root zone (30 – 40 cm).

Foliar spray of nitrogen

1. Spray the urea in July to improve flowering and fruit set, and after October to supplement nitrogen.
2. The 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 downward movement of nitrate

1. Inject nitrate in the middle of, or late in an irrigation event.
2. Avoid excessive concentrations of nitrate in the soil solution. Nitrate concentrations greater than 150 – 200 mg/ don't provide any additional benefit, and supplying more nitrogen fertiliser simply increases the risk of leaching by successive irrigation or rainfall events. Oversupplying nitrogen can also lead to fruit quality issues.
3. Complete the nitrogen supply programme by the end of the growing season to allow depletion of nitrate in the soil by winter.
4. Ensure that only the right amount of water is applied. 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. Allowing for expected rainfall when calculating the depth of water to apply, will also help reduce the likelihood of leaching.

Grondgedraagde siektes

(M.C. Pretorius en J. van Niekerk)

Aalwurms. Grond- en wortelmonsters kan in die lente getrek word en na die Diagnostiese Sentrum in Nelspruit gestuur word vir ontleding, 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.

Afwykings van die geregistreerde dosisse om kostes te bespaar, lei tot oneffektiwiteit. Dit is belangrik om 'n program te volg, van twee of drie toedienings met twee maande intervalle (om toksiese vlakke hoog genoeg te hou in die grond vir 'n langer periode) – behalwe vir Velum. Volg etiket-inligting van die betrokke produk. Afwyking van hierdie aanwysings kan tot oneffektiewe beheer lei, en dus 'n mors van geld.

Phytophthora. 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 daar op die waarskuwings gelet word voordat die produk gebruik word. Dis om effektiwiteit te verseker en fitotoksisiteit te voorkom.

Om effektiewe werking van die fosfonate te verseker behoort opvolgtoedienings ten minste twee, maar verkieslik drie, toedienings twee maande 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 drie blaarbespuitings (met agt weke intervalle) aanbeveel. Die eerste bespuiting behoort direk na blomblaarval gedoen te word – nie tydens blom nie, aangesien dit die moontlikheid van blomblaarval kan verhoog. Dit word sterk aanbeveel om nuwe aanplantings en nie-draende bome op 'n fosfonaat-program 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

(P. Moyo)

Alternaria core rot. Alternaria core rot (also known as navel-end rot or black rot) is 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 called 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, such that both the secondary and primary style abscise during petal fall, resulting in 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 which remain quiescent until favourable conditions stimulate further growth of the fungus. Alternaria core rot is linked to fruits with large or malformed navel ends.

Score (50 ml/100 water) and Folicur (80 ml/100 water) are registered for control of the disease.

Botrytis on lemons. The role played by Botrytis and the damage that it can do to lemon fruit drop and the formation of ridging of the rind, is still not clear. Damage can be caused during blossom on lemon petals when prolonged wetting and cool weather occur simultaneously. Producers in the Eastern Cape enlisted on the Adcon system, can make use of their early warning forecasts for Botrytis. Benomyl is the most effective fungicide to control this fungus and should be sprayed at the balloon stage during blossom, which can also form part of the black spot control programme.

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 be taking its toll on management and may lead to insufficient diligence in the management of critical control points. Therefore, please note these suggested critical control points.

Critical control points for improved postharvest disease management

• Communicate with the pre-harvest managers and ensure up-to-date exchange of information:
  • Monitor the incidence of insect activity in the orchards and institute appropriate measures to reduce their populations, especially fruit fly and false codling moth numbers that could increase with hotter weather.
  • Monitor orchard practices and sanitation:
    • Advise the orchard managers and producers if sanitation is being improperly managed or neglected, as this can also play an overbridging role for FCM from one season to the next. It is important to follow the FMS correctly.
    • Removal of rotten and fallen fruit 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 definitely within 24 hours after picking.
• Ensure that all fungicide applications are replaced frequently as per protocol and kept clean.
• Keep the fruit dumping site where fruit will enter the packhouse, as clean as possible.
• Ensure proper removal of any rotten fruit before sanitising and treatment of the fruit.
• Have a sanitation action between fruit sorting and the first fungicide application.
• Ensure that fruit are dry before entering the fungicide treatment.
• Manage the concentration of imazalil
  in the fungicide treatment by doing
  regular titrations.
• Do not wax wet fruit, to avoid placing fruit quality at severe risk.
• Apply the correct amount of wax onto the fruit (i.e. 1.0 – 1.2 per ton 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 a day or week activity.
• Store fruit destined for the juicing factory as far away from the packhouse as possible and have them removed as soon as possible.

Below is a short checklist for critical measurements relating to chemical applications.

Checklist for chemical application

• Chlorine application
  • Solution pH: 6.5 – 7.5
  • Chlorine concentration: 75 – 100 ppm (free active chlorine)
  • ORP: 800 mV.
• Fungicide dip tank
  • Imazalil concentration: 500 ppm
  • 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
  • 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 ton 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 impeller or venturi type of wax blending tanks, the wax solution should be agitated continuously (24 hours a day, seven days a 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 ensure that the desired effect is gained.

Featured Image: Macro of Cottony Cushion Scale on a citrus plant