Extension briefs for June and July 2024

Red scale (T.G. Grout)

Growers who experienced red scale problems during the past season should consider applying narrow distillation range horticultural mineral oils. These comprise the most IPM-compatible treatment option for red scale. The optimal period to apply this treatment is from budswell to budburst (mid-July to mid-August). The concentration to be used is dependent on the grade of the oil (generally 1.0–1.25% medium grade oil). When spraying oil, care must be taken to apply it as a full cover, film wet spray. Only spray stress-free trees. Prune the trees before spraying to help improve spray coverage of the tree framework, where many of the red scale insects survive. Other treatments for red scale would normally be applied in August, September or after petal fall, and will be discussed in later editions of Extension Briefs.

Ant control (T.G. Grout)

Ants are usually indirect pests on citrus and interfere with the behaviour of natural enemies, sometimes protecting and even transporting pest species. The winter months are a good time to control ants before honey-dew-producing pests such as aphids increase on the spring growth flush. Trees should also be skirt-pruned to prevent branches from touching the ground later in the season when bearing fruit. SAGA is a new bait that is effective against both the pugnacious ant and brown house ant and is available from River Bioscience.

Naoespraktyke (P.J.R. Cronjé)

Die korrekte produksiepraktyke – soos bemesting en besproeiing, asook snoei – is uiters belangrik vir die voorkoming van naoesfisiologiese afwykings. Daar is egter gedurende die oes en pakproses kritiese faktore waarop gelet moet word, wat die voorkoms van skildefekte, asook algemene vrugkwaliteit kan beïnvloed. Bepaal die optimum plukvenster vir elke kultivar per boord deur maand voor die beplande oesdatum met rypheidsindeksering (interne kwaliteit en skilkleur) te begin. Daar moet gepoog word om die temperatuurlading op die vrug te beperk, vanaf die plukproses tot in die pakhuis, deur die vrugte se veldhitte so gou as moontlik te verwyder (bv. deur vrugte gouer te stort/doop). Die paklyn het invloed op skiltoestand, asook die voorkoms van skildefekte, en dit is belangrik dat noukeurige aandag aan alle bewegende dele – bv. rollers en borsels, asook wakstipe en aanwending – geskenk word.
Ontgroening is belangrike aspek van die sitrusnaoeshantering en verg optimale bestuur. Daar bestaan ‘n interaksie tussen kultivar, vrugrypheid en die effektiwiteit van ontgroening. As vrugte te vroeg (onvoldoende kleur-ontwikkeling aan boom) in die ontgroeningskamer geplaas word, sal die verlangde kleur nie ontwikkel nie en vrugte neig om geler of bleker voorkoms te kry. Gedurende ontgroening is daar egter aspekte wat streng beheer moet word, nl. etileenkonsentrasie (1-3 ppm), temperatuur en relatiewe humiditeit (95%+).

Optimum ontgroeningstemperatuur

Satsuma-mandaryne 18 tot 21 °C
Clementine- en Nova-mandaryne 19 tot 22 °C
Nawellemoene 21 tot 23 °C
Ander lemoene 23 tot 24 °C
Pomelo’s en suurlemoene 24 tot 25 °C

Ontgroen altyd meer sensitiewe vrugte teen die laagste temperatuur. Die etileenbehandeling is die effektiefste as vrugte van dieselfde kleur gelykertyd behandel word. Die duur van die behandeling moet ook so kort as moontlik wees. Hou die CO2-vlakke onder 0.3% (3000 dpm) deur die ventilasie sodanig te stel; hoë CO2-vlakke verlangsaam die ontgroeningsproses deur as kompeterende inhibeerder teen etileen op te tree. Stadiger ontgroeningstempo sal verlenging van die tyd in die ontgroeningskamer vereis en ’n verkorte rakleeftyd tot gevolg hê. Vir meer inligting, raadpleeg die Common defects associated with degreening of citrus deur Krajewski en Pittaway, wat hier bestel kan word: https://www.citrusres.com/shop/.
Die koueketting se invloed op vrug-kwaliteit is van kardinale belang en sal die voorkoms van skildefekte, asook algemene kwaliteitsaspekte soos vrugfermheid en rakleeftyd beïnvloed. Belangrik: die koueketting mag nie onderbreek word nie, m.a.w. as vrugte tot op die verlangde temperatuur verkoel is, mag die temperatuur nie weer styg nie.

Crop and fruit quality management (P.J.R. Cronjé)

Maturity indexing: On mid- to late-season cultivars should commence. It’s done to predict the rate of change in fruit maturity, to harvest fruit at an optimal maturity. The aim is to define changes or rate of change in acids and sugars, and to build up a database over several years. 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 and sugar content (Brix°) is determined using a refractometer. The sugar:acid ratio calculated, and fruit colour should be read from a colour chart. These parameters should be plotted on a graph over time. Once plotted, trends will become apparent, harvest dates can be estimated, and problem areas in internal and external quality parameters can be identified and manipulated.
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 to order from CRI at www.citrusres.com.
Pickers’ training and monitoring: Picking bags should always be carried on the side of the waist to avoid crushing of fruit between the body and the ladders, bins, or trees. Bags should always be free of leaves, shoots or sand and kept dry throughout. Short fingernails will prevent lesions on fruit. Clippers and ladders must be handled correctly, i.e., no long stems, and limited contact between ladders and fruit. Avoid picking of low-hanging fruit, and collecting dropped fruit. At each bin, two sorters should be stationed wearing gloves, and the fruit quality of each picker monitored by the team leader via the sorters.
Pruning: For early and mid-season cultivars, pruning should be done as soon as possible after harvest. These should be removed during pruning: old, broken, and dead shoots/twigs; weak and entangled shoots crossing each other or hanging downwards; any rootstock regrowth, water shoots and excessive regrowth from main frame shoots on the inside of the tree. Light levels of above 30% of full sunlight are necessary for optimal photosynthesis – at least one “window” cut should be made to allow adequate light distribution and improve bearing wood. Increased photosynthesis and light distribution will lead to increased fruit size and internal fruit quality (Brix°), better fruit colour development, increase in rind integrity, as well as a more uniform fruit distribution across the tree. Pruning can also be used as a thinning technique: Prune more heavily after a light crop if a heavy crop is expected, and when the orchard has a history of alternate bearing. Follow-up pruning of regrowth in summer is of critical importance. Pruning tools should always be sanitised with 10% Jik when moving from one orchard to another. Also, pruning ensures more cost-effective spraying, and better spray penetration during pest and disease control.
Flower induction: Citrus trees require a rest period of about six weeks to ensure adequate flower induction. The two mechanisms involved in floral induction in citrus involve (1) low temperature, or (2) controlled-drought stress, both of which prevent root activity. In the absence of low-temperature rest, citrus trees should be exposed to controlled-drought stress for a four- to six-week period during June and July, if possible.

Bemesting/Fertilisation
(P.J. Raath and J.G.K. Coetzee)

Blaarontleding/Leaf analysis: The very last opportunity to get leaf analyses done is in June and July. Leaf analysis is still the best diagnostic method to measure the nutritional status of trees.
Blaar-analise kan egter nie as die enigste norm vir die daarstelling van ’n bemestingsprogram gebruik word nie, maar moet as ’n aanvulling tot grond-analises gesien word. Dit is ook nuttig in gevallestudies waar blare vanaf “siek” bome gemonster word, asook vanaf naburige, gesonde bome. Die gesonde/beter bome se blare dien dan as direkte kontrole, sodat die tyd van monsterneming en al die ander veranderlikes, minder krities of nie ter sake is. Geen norm word gebruik nie, maar wel die relatiewe verskille tussen die twee monsters.
Grondontledings/Soil analyses: Soil analyses should be done every second year on sand soils, and every two to three years on clay soils, preferably during the same time as the leaf analyses. Take soil samples from the same spot every year, for accurate historic dataset development for the soils. Sampling areas can be marked with GPS coordinates. Separately sample sections where trees are performing poorly.
Gereelde grondontledings in bestaande boorde verseker die handhawing van optimale groeitoestande. Die belangrikste voordeel van grondontledings in bestaande boorde is dat veranderings in grondvrugbaarheid bespeur kan word en korrektief opgetree kan word. Dus vóór voedingsprobleme wat boomprestasie benadeel, kan ontwikkel.
Other analyses: Analyse manure or compost to determine the nutrient value and occurrence of unacceptable elements (e.g. heavy metals) in the organic material. The amount of organic material applied is determined by the concentration of nutrients in the compost or manure. The nutrient content of organic material is much lower than inorganic fertilisers, and often in an inappropriate ratio compared to the trees’ nutritional requirement. Beware of applying excessive amounts of nutrients that are out of sync with the trees’ requirements.
Bemesting/Fertilisation: Potential yield is used as an indicator for the amount of fertiliser needed. Use historic yield data to determine a realistic target yield and basis from which the fertilisation requirement is compiled. The fertilisation programme should then be adjusted, using results of both soil and leaf analyses as well as evaluations during the season.
The fertilisation programme for each season starts in July or August of the previous year, depending on the production area. Due to the properties of the specific chemical ion, the properties of the soil and the physiological requirements of the trees, certain basic rules should be adhered to when applying fertilisers to citrus. See Table 1.
Blaarbespuitings word gebruik om tekorte reg te stel, of om die bome se fisiologie te manipuleer. Vir manipulasie is dit hoofsaaklik lae biuret-ureum of kaliumnitraat wat gebruik word. Indien nodig, moet beide bespuitings ses tot agt weke voor die 50% blomstadium toegedien word. Dit is meestal Julie of vroeg-Augustus. See Table 2.

Table1.

Table2.

Postharvest pathology

(W. du Plooy and L. Mamba)

Even during peak periods, important basic practices like sanitation and fungicide application remain vital in packhouses.

Packhouse sanitation:

The main aims of this practice are to:

1. remove any infected fruit from the
   packhouse environment
2. sanitise surfaces with appropriate treatments.

These actions will reduce the inoculum (spore) levels, minimising the risk for future infections and fungicide resistance development.

Surfaces to consider in sanitation:

• Packline surface: treat daily with a registered, food-safe sanitising product. It is important that the product does not leave a residue and is quick and easy to use.
• Fruit surface: a fruit disinfectant treatment should be one of the first steps when entering the packhouse. Chlorine (Calcium Hypochloride) is the most popular active for this step. Remember to strictly manage the chlorine concentration, pH and ORP. Alternatives such as liquid or powder PAA formulations can also be used. In the case of PAA there is no need to adjust the pH. However, the PAA concentration levels must be managed carefully, and fruit must be dried timeously (both to prevent rind injury).
• Floors and walls: wash weekly with a registered soap and disinfectant product. In these areas, a product that leaves a residue (such as a QAC product) can be used. Importantly, a dirty surface can’t be sanitised. Rather use a registered soap before sanitation. The areas to clean include the packhouse, degreening rooms, and cold rooms.
• Degreening rooms and cold rooms: need to be washed and then – where possible – fumigated with a suitable product that can reach into crevices and folds. Never fumigate when there is fruit in the room, to avoid the risk of residue transfer.
  -Orchard bins and picking bags: must be washed and sanitised before re-use.

Sources of inoculum (spores):

• The orchard: spores naturally occur in the orchards, so orchard sanitation is key, to reduce the inoculum load.
• Post-degreening fruit: degreening conditions are hugely favourable for disease development and unfortunately, some sporulating fruit are often tipped into the packhouse system.
• Waste fruit: have a process in place to remove this fruit without delay, and to keep the waste in sealed containers.
• Juice fruit: this fruit is often treated with fungicides and sits in the packhouse area until it’s transported to the juicing plant. Therefore, any decay on such fruit holds a resistance risk.
• Small fruit: this fruit, if not properly graded out, can fall between brushes or gaps between units and lie unseen on drip trays in the packhouse.
• Retention samples: shelf-life fruit has also been treated with fungicides and holds a similar resistance risk as juice fruit.

Recommendations that can reduce the risk of infection and/or resistance development:

• Treat all harvested fruit promptly (within 12–18 hours), as most fungicides cannot control infections older than a day.
• Don’t let fruit stand from Friday afternoon to Monday morning without any fungicide protection. In this case it would be prudent to drench the fruit on Friday.
• Keep fruit degreening brief.
• Do not exceed the volume limit of your pack-line. This is to avoid accelerated agitation and movement of the fruit, which will result in bruising and injuries, and reduce the efficacy of sanitation and fungicide treatment. Spread the fruit out evenly, so that none skip the treatments. Risks related to rushing fruit through the various control points:
  • The chlorine treatment: poor disinfection, spores surviving.
  • The fungicide dip tank: loading suboptimal residue, poor infection control.
  • The wax applicator: poor wax deposition on the fruit; suboptimal residue loading, poor moisture retention, negative impact on quality.
• Limit the time from harvest to packing and the eventual introduction to the cold chain.

Some advice in managing retention fruit:

• Have a dedicated person managing this task.
• Fruit should be evaluated every day, and decayed fruit noted on the carton and removed or isolated.
  • Immediately place the infected/discarded fruit in a sealed plastic bag to reduce the risk of releasing inoculum (spores) into the packhouse atmosphere.
  • Green mould sporulation inhibition is a very important criterion to note, as the loss of sporulation inhibition indicates either fungicide resistance or poor fungicide application. To prevent spores spreading from this infected fruit, fruit exhibiting sporulation inhibition should be kept in sealed transparent plastic bags and incubated for another week to rate sporulation inhibition (white mycelium with few to no green spores).
  • When the sporulation inhibition rating is concluded, the bagged fruit must be destroyed (with the bag) well away from citrus orchards and packhouses, to prevent spores from entering any production area.
• Do not keep fruit for longer than the shipping period.

Ideally, the retention room should be some distance away from the clean area, and completely enclosed with controlled access. This room should be cleaned and treated with a suitable soap and sanitiser at least once a month.