Extension briefs for June and July 2021

Red scale (T.G. GROUT)

Those growers who have experienced red scale problems during the past season should consider applying narrow distillation range horticultural mineral oils as these comprise of 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 to be applied (generally 1.0 – 1.25% medium grade oil). When spraying oil, care must be taken to apply the oil as a full cover, film wet spray. Trees to be sprayed must not be under any stress either. 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 honeydew-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.

NA-OES-PRAKTYKE (P.J.R.CRONJÉ)

Vir die voorkoming van na-oes-fisiologiese afwykings is die korrekte produksiepraktyke, soos bemesting en besproeiing, asook snoei uiters belangrik. Daar is egter gedurende die oes en pakproses kritiese faktore waarop gelet moet word wat die voorkoms van skildefekte, asook algemene vrugkwaliteit kan beinvloed. Bepaal die optimum plukvenster vir elke kultivar per boord deur ‘n maand voor die beplande oesdatum met rypheid-indeksering (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 “drench”). Die paklyn het invloed op skilkondisie, asook die voorkoms van skildefekte, en dit is belangrik dat detail aandag aan alle bewegende dele, bv. rollers en borsels, asook wakstipe en aanwending geskenk word.

Ontgroening is ‘n belangrike aspek van die sitrus na-oes-hantering en moet optimaal bestuur word. Dis belangrik om te besef dat daar ‘n interaksie tussen kultivar, vrugrypheid en die effektiwiteit van ontgroening bestaan. 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 ‘n 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 tydperk 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.

‘n Stadiger ontgroeningstempo sal ‘n verlenging van die tyd in die ontgroeningskamer vereis en ‘n verkorte rakleeftyd tot gevolg hê. Vir meer inligting, raadpleeg Common defects associated with degreening of citrus deur Krajewski en Pittaway, wat by CRI in Nelspruit (kontak Bella 013 759 8000) bestel kan word.

Die koueketting se invloed op die vrugkwaliteit is van kardinale belang en sal nie net die voorkoms van skildefekte beïnvloed nie, maar ook algemene kwaliteitsaspekte soos vrugfermheid en rakleeftyd. Die belangrikste faset van die koueketting is dat dit nie onderbreek mag 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: Maturity indexing on mid-season to late cultivars should commence. Maturity indexing is done to predict the rate of change in fruit maturity, in order 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 a number of 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. Sugar content (Brix) is determined using a refractometer; the sugar:acid ratio calculated and fruit colour should be read from a colour chart.

All the parameters mentioned above 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 for any grower. Both are available from CRI. Contact Bella Thulare 013 759 8000, bella@cri.co.za.

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. Picking bags should at all times be free of leaves, shoots or sand and kept dry throughout. To avoid lesions on fruit, fingernails of pickers should be short and scissors and ladders handled correctly, i.e. no long stems and limited contact between ladders and fruit.

Picking of low-hanging fruit as well as collecting of dropped fruit, should be avoided. At each bin, two sorters should be stationed wearing gloves and the fruit quality of each picker monitored via the sorters, by an appointed team leader.

Pruning: Pruning for early and mid-season cultivars should be done as soon as possible after harvest. All of the following 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. Increase in 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 if the orchard has a history of alternate bearing. Follow-up pruning of regrowth in the summer is of critical importance. Pruning tools should always be sanitised with 10% Jik when moving from one orchard to another. Pruning also ensures better spray penetration during pest and disease control and much more cost-effective spraying.

Flower induction: Citrus trees require a rest period of about six weeks to ensure that adequate flower induction takes place. The two mechanisms involved in floral induction
in citrus involve (i) low temperature, or (ii) controlled-drought stress, both to ensure no root activity. In the absence of low-temperature rest, citrus trees should be exposed to controlled-drought stress for four to six weeks 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 the 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, nie-geaffekteerde 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 nie. Geen norm word gebruik nie, maar wel die relatiewe verskille tussen die twee monsters.

Grondontledings/Soil analysis

Soil analyses should be done annually and preferably during the same time as leaf analyses. A good practice is to take soil samples from the same spot every year to ensure that an accurate historic dataset is developed for the soils. Sampling areas can be marked with GPS coordinates. Sections where trees are performing poorly should be sampled separately.

Gereelde grondontledings word benodig in bestaande boorde om te verseker dat optimale groeitoestande gehandhaaf word. Die belangrikste voordeel van grondontledings in bestaande boorde is dat veranderings in grondvrugbaarheid bespeur kan word en korrektief opgetree kan word voor voedingsprobleme wat boomprestasie benadeel, kan ontwikkel.

Other analyses

If manure or compost is used it should be analysed 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 what the trees’ nutritional requirement is. Therefore, be careful not to apply excessive amounts of nutrients that are out of sync with the trees’ requirement.

Bemesting/Fertilisation

Potential yield is used as an indicator for the amount of fertiliser to be applied. 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 tweaked using the results from both soil and leaf analyses.

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.

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 in Augustus.

POSTHARVEST PATHOLOGY (W. DU PLOOY and L. MAMBA)

While most packhouses now enter their peak period, important and basic practices such as sanitation and fungicide application must not be neglected.

Packhouse sanitation

The main aims of this practice are:

• To remove any infected fruit from the packhouse environment
• To sanitise surfaces with appropriate treatments

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

Surfaces that should be considered in sanitation

• Packline surface – treat daily with quaternary ammonium compound (QAC), but rinse with clean water afterwards; alternatively use chlorine or another registered, food safe sanitising product.
• Fruit surface – one of the first steps when entering the packhouse should be to apply fruit disinfectant treatment. Chlorine (Calcium Hypochloride) is the most popular active for this step. Remember to strictly manage the Cl concentration, pH and ORP. Suitable alternatives such as liquid or powder PAA formulations can also be used, with the precaution that the PAA levels must be managed carefully. In the case of PAA, there is no need to adjust the pH.
• Floors and walls – wash weekly with a registered QAC, or a registered soap. These 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.
• Orchard bins and picking bags must be washed and sanitised before being used again

Sources of inoculum (spores)

• The orchard – spores naturally occur in the orchards so orchard sanitation is key to reduce the inoculum load.
• Waste fruit – have a protocol in place to remove these fruit a.s.a.p. and to keep the waste in sealed containers.
• Juice fruit – this fruit is often treated with fungicides, and sits in the packhouse area for some time until it’s shipped to the juicing plant. Any decay that develops on such fruit therefore presents a resistance risk.
• Retention samples – this fruit was also treated with fungicides and holds a similar resistance risk as above.
• Post-degreening fruit – degreening conditions are hugely favourable for disease development and unfortunately some sporulating fruit are often tipped into the packhouse system.

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

• Treat all harvested fruit a.s.a.p., but within 12 – 18 hours, as most fungicides cannot control infections older than a day.
• Don’t let fruit sit from Friday afternoon to Monday morning without any fungicide protection. In this case it would be prudent to drench the fruit on Friday.
• Degreen fruit for the absolute minimum period of time.
• Do not exceed the volume limit of your packline: exceeding the limit will result in accelerated agitation and movement of the fruit, which, in turn, will result in bruising and injuries, and will reduce the efficacy of sanitation and fungicide treatment.
• Fruit will move too quickly 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 and negative impact on quality.
• Limit the time from harvest to pack and 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, with decayed fruit noted on the carton and removed or isolated.
  • These infected/discarded fruit should be placed in a plastic bag immediately, and the bag sealed tightly. This is to reduce the risk of releasing inoculum (spores) into the packhouse atmosphere.
  • Fruit in retention samples has been treated with fungicides, and holds a serious resistance risk if spores are spread into the packhouse.
  • Green mould sporulation inhibition is a very important criterion to note, as the loss of sporulation inhibition is indicative of either fungicide resistance or poor fungicide application. To prevent spores spreading from these 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 in the packhouse, and should be totally enclosed with controlled access. This room should be cleaned and treated with a suitable soap and sanitiser, at least once a month.