Evaluation of adjuvants in fungicide spray application for the control of alternaria brown spot in South African citrus orchards

Van Zyl, Johannes Gideon (2019-03)

Thesis (PhDAgric)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Citrus fruit and foliar diseases are mainly controlled through pre-harvest application of fungicides. Fungicides are only as effective as the application process and for effective disease control deposition of a uniformly distributed quantity of active ingredient(s) is required on the intended target(s). Adjuvants have the potential to improve fungicide deposition on a target surface. The influence of adjuvants on the deposition of fungicides, especially at the high spray volumes used in South African citrus production is unknown and was therefore investigated. A previously developed deposition assessment protocol, using a yellow fluorescent pigment as tracer for copper oxychloride (CuOCl) deposition, was improved through photomacrography and digital image analyses which proved accurate in determining the quantity and quality of deposition on citrus leaves. Spray deposition benchmarks indicative of the biologically efficacy of CuOCl against Alternaria alternata [causal agent of Alternaria brown spot (ABS) of mandarins] was developed. The deposition assessment protocol and deposition benchmarks was used to evaluate two organosilicone adjuvants (Break-Thru S240 and Break-Thru Union) at reduced spray volumes in dense and less dense citrus canopies in two separate orchard spray trials. Deposition quantity generally increased with increasing spray volume, but normalised values showed better spray efficiency at lower volumes. In pruned and less dense canopies, a beneficial effect of adjuvants was observed in terms of deposition quantity, efficiency and uniformity, especially at reduced volume applications. Some improvement in deposition quality was generally observed with the use of adjuvants. These benefits were not as evident in very dense canopies, illustrating the importance of canopy management when spraying at reduced volumes. Commercially available adjuvants [Break-Thru, Nu-Film-17, Citrole100, Villa51, Wetcit, Entrée and Exit] were evaluated in three orchard spray trials on different citrus types, cultivars and spray volumes. In trial one, adjuvants improved deposition quantity and canopy penetration. In trial 2 and 3, deposition quantity was generally higher at higher spray volumes, but spray efficiency was significantly better at lower spray volumes. Adjuvants generally improved deposition uniformity and deposition quality, but these benefits were significantly influenced by spray volume and the specific adjuvant treatment. Poor performance by adjuvants was ascribed to high spray volumes and/or too high adjuvant concentration used, which led to increased levels of run-off and poor deposition parameters. The effects of adjuvants on deposition quantity, quality and biological efficacy of CuOCl against ABS on mandarin leaves were determined in laboratory trials. Adjuvant treatments varied significantly in deposition quantity and quality and disease control achieved. Higher deposition quantity, beter quality and higher Cu residues was realized at pre- vs. post-run-off volumes. Adjuvants did not improve deposition parameters compared with the control treatment at both spray volumes. Leaf infection analysis indicated that CuOCl with adjuvant sprays (post-run-off volume) realized similar and in some cases slightly better control (although not significant) than copper oxychloride alone, but that deposition and Cu residue loading in some of these adjuvant treatments were markedly lower. This anomaly could be ascribed to direct or indirect effects of the adjuvant and was investigated further. In vivo and in vitro studies were done to identify possible direct adjuvant effects on pathogen development and potential synergistic effects between the adjuvants and CuOCl. Adjuvants alone did not influence conidial adhesion, appressorium formation, germ tube length and percent viable conidia. Adjuvant sprays together with CuOCl reduced conidial adhesion, germ tube length and percent viable conidia numerically; however, not significantly compared with CuOCl alone. Adjuvants also caused conidium/germ tube stress similar to CuOCl, but did not inhibit germination or growth. In the in vitro microtiter assay, adjuvants together with CuOCl improved germination or growth inhibition compared with the CuOCl treatment alone, although not at significant levels. The findings in Chapter 6 did not fully explain the anomalous findings in Chapter 5, and future studies should focus on developing methodology to support histopathology studies on sensitive leaf surfaces, as well as development of a more sensitive method of measuring deposition quality, especially on a microscopic scale.

AFRIKAANSE OPSOMMING: Sitrus vrug- en blaarsiektes word hoofsaaklik deur voor-oes spuit toediening van swamdoders bestuur. Swamdoders is slegs so effektief soos die spuit toedieningsproses. Vir effektiewe siektebestuur word ‘n homogene verspreiding van die regte kwantiteit aktiewe bestandeel of bestandele verlang op die nodige teiken(s). Byvoegmiddels besit die potensiaal om swamdoder deposisie op verlangde teiken oppervlaktes te verbeter. Die invloed wat byvoegmiddels op deposisie parameters het, veral teen die hoë spuitvolumes wat in SuidAfrikaanse sitrus produksie gebruik word, is onbekend en was gevolglik ondersoek. ’n Voorheen ontwikkelde deposisie assesseringsprotokol is verbeter deur die gebruik van ’n geel fluoresserende pigment wat dien as ’n “tracer” vir koperoksichloried (CuOCl) deposisie, fotomakrografie en digitale beeld aneliese. Die verbeterde protokol kon deposisie kwantiteit sowel as kwaliteit akkuraat op sitrus blare bepaal. Deposisie drempelwaardes aanduidend van die biologiese effektiwiteit van CuOCl teen Alternaria alternata [die veroorsakende organisme van Alternaria bruin vlek (ABS) van mandaryne] is ontwikkel. Die verbeterde deposisie assesseringsprotokol en drempelwaardes is gevolglik gebruik om twee organosilikoon byvoegmiddels (Break-Thru S240 en Break-Thru Union) by verlaagde spuit volumes in digte en minder digte sitrus lowers in twee aparte boorde te evalueer. In die algemeen het deposisie kwantiteit toegeneem met toename in toedieningsvolume. Genormaliseerde deposisie kwantiteit waardes het beter spuit effektiwiteit uitgewys by laer toedieningsvolumes. Daar was ’n verbetering in deposisie kwaliteit waargeneem met die gebruik van byvoegmiddels. Hierdie voordele was nie so duidelik in digte lowers nie, wat die belangrikheid van lowerbestuur, wanneer verlaagde spuit volumes gebruik word, uitgewys het. Kommersiëel beskikbare byvoegmiddels [Break-Thru, Nu-Film-17, Citrole100, Villa51, Wetcit, Entrée and Exit] is op verskillende sitrus tipes, kultivars en spuit volumes in vier boord spuitproewe geëvalueer. Beter deposisie kwantiteit sowel as lower penetrasie was verkry deur die gebruik van benatters in proef 1. Deposisie kwantiteit was in die algemeen hoër by hoër spuitvolumes, terwyl deposisie effektiwiteit beduidend beter was by laer spuit volumes in proef 2 en 3. Byvoegmiddels het in die algemeen deposisie uniformiteit sowel as kwaliteit verbeter, maar hierdie voordele was noemenswaardig beïnvloed deur spuitvolume en die spesifieke byvoegmiddel behandeling. Swak resultate met die gebruik van byvoegmiddels is toegeskryf aan hoë spuit volumes en/of te hoë byvoegmiddel konsentrasie wat gebruik is. Hierdie invloede het gelei tot verhoogde afloop en daarvolgens swakker deposisie vlakke. Die invloed van byvoegmiddels op deposisie kwantiteit, kwaliteit en die biologiese effektiwiteit van CuOCl teen ABS op mandaryn blare is in ’n laboratorium studie geëvalueer. Byvoegmiddel behandelings het betekenisvol verskil kragtens deposisie kwantiteit, kwaliteit en ABS beheer verkry. Hoër deposisie kwantiteit, beter kwaliteit en hoër Cu residuvlakke is by voor- vs. na-afloop spuitvolumes verkry. Byvoegmiddels het nie deposisie vlakke in vergelyking met die kontrole behandeling by beide spuitvolumes verbeter nie. Blaar infeksie analiese het uitgewys dat CuOCl tesame met byvoegmiddel spuite (teen na-afloop spuit volumes) dieselfde en in sekere gevalle beter (alhoewel nie betekenisvol nie) siektebeheer as die CuOCl alleen spuit gelewer het. Tog is waargeneem dat deposisie en Cu residu in sommige gevalle, afhangende van die behandeling, laer was. Hierdie anomalie kan toegeskryf word aan moontlike direkte en indirekte effekte wat byvoegmiddels kan hê en is daarvolgens verder ondersoek. In vivo en in vitro studies is gedoen om moontlike direkte byvoegmiddel effekte op patogeen ontwikkeling sowel as potensiële sinergistiese effekte tussen byvoegmiddels en CuOCl te identifiseer. Byvoegmiddels op hul eie het nie konidiale vashegting, appressorium ontwikkeling, kiembuislengte of die persentasie lewendige konidia noemenswaardig beïnvloed nie. Byvoegmiddel spuite tesame met CuOCl het konidium aanhegting, kiembuis lengte en die persentasie lewendige konidia verlaag, tog nie betekenisvol in vergelyking met die CuOCl alleen spuit nie. Byvoegmiddels het ook konidium/kiembuis stress veroorsaak wat vergelykbaar was met die CuOCl spuit, alhoewel dit nie ontkieming of groei beïnvloed het nie. In die in vitro “microtiter” toets het byvoegmiddels tesame met CuOCl ontkieming of groei vertraging gewys in vergelyking met die CuOCl alleen behandeling, maar tog nie betekenisvol nie. Die bevindings in hoofstuk 6 kon nie ten volle die teenstrydige bevindinge van hoofstuk 5 verduidelik nie. Daarom moet toekomstige studies fokus op die ontwikkeling van metodes om histopatologie studies moontlik te maak op sensitiewe blaar oppervlaktes, sowel as die ontwikkeling van meer sensitiewe metodes om deposisie kwaliteit te meet, veral op ’n mikroskopiese skaal.

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