The effect of prohexadione-calcium application in ‘Fuji’ apple trees in South Africa

Thesis (MScAgric)--Stellenbosch University, 2023.

ENGLISH ABSTRACT: South African apple [Malus x domestica Borkh.] producers have observed that shoot regrowth occurs in vigorous ‘Fuji’ apple trees after applying the registered treatment of prohexadione-calcium (calcium 3-hydroxy-5-oxo-4-propionyl-cyclohex-3-enecarboxylate; Pro-Ca). The registered treatment includes two applications of Pro-Ca during the first four weeks after full bloom (WAFB). The first applied at 70 mg.L⁻¹ and a rate of 80% tree row volume (TRV) when shoots are 3 – 5 cm long in spring. The second application should occur at the same concentration, but four weeks later at 100% TRV. The effect of Pro-Ca treatments applied at different timings and rates on shoot growth and shoot regrowth in ‘Fuji’ apple trees in South Africa is unknown. The Pro-Ca treatments were applied to ‘Fuji’ Kiku® and ‘Fuji’ Akifu® apple trees in the Vyeboom and Witzenberg Valley regions, respectively. Trees were randomly selected and treated with Pro-Ca according to the TRV. Nine Pro-Ca treatments were used with Pro-Ca applied up to six times during the growing season, but no later than 6 WAFB. These treatments were compared to an untreated control (UTC). The application of 70 mg.L⁻¹ Pro-Ca (80% TRV) at full bloom, followed by 70 mg.L⁻¹ (100% TRV) four and six WAFB; or five 70 mg.L⁻¹ (40% TRV) applications applied from full bloom until four WAFB, suppressed shoot growth in ‘Fuji’ Akifu® trees without increasing shoot regrowth, but did not significantly suppress final shoot length in ‘Fuji’ Kiku® trees. The current registered treatment of Pro-Ca in South Africa may cause shoot regrowth to occur, which may negatively affect flowering, fruit yield and quality. The Pro-Ca treatments that included an initial 70 mg.L⁻¹ (80% TRV) at full bloom followed by 70 mg.L⁻¹ (100% TRV) four WAFB; or five 70 mg.L⁻¹ (40% TRV) applications within the first six WAFB, increased fruit diameter in ‘Fuji’ Kiku® trees without significantly affecting fruit yield. The application of Pro-Ca had no significant effect on return bloom, fruit set, fruit yield or quality in either ‘Fuji’ Kiku® or Akifu® apple trees. Few studies have shown that the application of Pro-Ca at full bloom on the vigorous ‘Fuji’ apple cultivar may suppress shoot regrowth and improve fruit quality. However, these studies were conducted over a single growing season. No study has observed the cumulative effect of Pro-Ca applied over two consecutive seasons in ‘Fuji’ apple trees in South Africa. The application of 70 mg.L⁻¹ Pro-Ca (80% TRV) at full bloom, followed by 70 mg.L⁻¹ (100% TRV) four and six WAFB on the same ‘Fuji’ Kiku® trees over two consecutive seasons suppressed shoot growth and improved fruit yield efficiency without significantly increasing shoot regrowth, or negatively affecting fruit set, fruit size or return bloom. The application 70 mg.L⁻¹ (80% TRV) at full bloom, followed by 70 mg.L⁻¹ (100% TRV) four and six WAFB, applied over two consecutive seasons to the same ‘Fuji’ Akifu® trees, suppressed shoot growth without significantly increasing shoot regrowth, or negatively affecting fruit set, fruit yield, fruit quality, or return bloom.

AFRIKAANSE OPSOMMING: Die effek van prohexadione-calcium toedienings in ‘Fuji’ appel bome in Suid-Afrika. Suid- Afrikaanse appel [Malus x domestica Borkh.] produsente neem waar dat hergroei van lote in groeikragtige ‘Fuji’ appel bome voorkom nadat die geregistreerde prohexadione-calcium (calcium 3- hydroxy-5-oxo-4-propionyl-cyclohex-3-enecarboxylate; Pro-Ca) behandeling toegedien is. Die geregistreerde behandeling sluit in twee Pro-Ca toedienings in die eerste vier weke na volblom (WNV). Die eerste teen 70 mg.L⁻¹ en ‘n volume van 80% boomry-volume (BRV) wanneer lote 3 – 5 cm lank is in lente. Die tweede toediening moet plaasvind teen dieselfde konsentrasie, maar vier weke later en teen 100% BRV. Die effek van verskillende Pro-Ca behandelings, op verskillende tye en teen verskillende BRVs, op lootgroei en loot hergroei in groeikragtige ‘Fuji’ appelbome in Suid-Afrika is onbekend. Die Pro-Ca behandelinge is toegedien op ‘Fuji’ Kiku® en ‘Fuji’ Akifu® appelbome in die Vyeboom en Witzenberg Vallei-streke, onderskeidelik. Die bome was lukraak gekies en behandel met Pro-Ca volgens die BRV. Nege Pro-Ca behandelings was toegedien in die eerste ses WNV. Hierdie behandelings is vergelyk met ‘n onbehandelde kontrole (OK). Die toediening van 70 mg.L⁻¹ Pro-Ca (80% BRV) tydens volblom en 70 mg.L⁻¹ (100% BRV) vier en ses WNV; of vyf 70 mg.L⁻¹ (40% BRV) Pro-Ca toedienings vanaf volblom tot vier WNV, het lootgroei geinhibeer in ‘Fuji’ Akifu® sonder om loot hergroei te verhoog, maar nie in ‘Fuji’ Kiku® nie. Die huidige geregistreerde Pro-Ca behandeling in Suid-Afrika mag dalk loot hergroei aanmoedig, wat moontlik blom, vrugopbrengs en -kwaliteit nadelig beïnvloed. Die toediening van 70 mg.L⁻¹ (80% BRV) Pro-Ca tydens volblom en 70 mg.L⁻¹ (100% BRV) vier WNV; of vyf 70 mg.L⁻¹ (40% BRV) Pro-Ca toedienings binne die eerste ses WNV, het vrugdeursnee verhoog in ‘Fuji’ Kiku® sonder om vrugopbrengs beduidend te affekteer. Die toediening van Pro-Ca het geen beduidende effek op opvolgblom, vrugset, vrugopbrengs en -kwaliteit in beide ‘Fuji’ Kiku® en Akifu® appel bome gehad nie. Sommige studies het bevind dat die toediening van Pro-Ca op ‘Fuji’ appelbome die lootgroei inhibeer en sodoende vrugkwaliteit verbeter, maar hierdie studies is oor ‘n enkele seisoen uitgevoer. Geen studie het die effek van kumulatiewe Pro-Ca toedienings oor twee opeenvolgende seisoen in ‘Fuji’ appelbome in Suid-Afrika nagevors nie. Die toediening van 70 mg.L⁻¹ Pro-Ca (80% BRV) tydens volblom en 70 mg.L⁻¹ (100% BRV) vier en ses WNV op dieselfde ‘Fuji’ Kiku® appelbome oor twee opeenvolgende seisoene het lootgroei inhibeer en vrugopbrengs effektiwiteit vermeerder, sonder om loot hergroei beduidend te verhoog, of vrugset, vrugkwaliteit of opvolgblom negatief te affekteer. Die toediening van 70 mg.L⁻¹ Pro-Ca (80% BRV) tydens volblom en 70 mg.L⁻¹ (100% BRV) vier en ses WNV op dieselfde ‘Fuji’ Akifu® appel bome oor twee opeenvolgende seisoene, het lootgroei inhibeer sonder om loot hergroei te verhoog of vrugset, vrugkwaliteit of opvolgblom negatief te beïnvloed.

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