Background

This research programme is fully funded by Citrus Research International, with emphasis on the sustainable management of important preharvest diseases, and pre- and postharvest insect pests of citrus in South Africa. The increased pressure on South African agriculture from the EU Green Deal, particularly on export crops such as citrus, necessitates the need to focus research on gaining a better understanding of the epidemiology of diseases and biology of insect pests in order to develop and advise on effective Integrated Pest Management strategies. The diseases currently under consideration in this programme are Phytophthora root rot, Botrytis grey mould and Alternaria brown spot. The complex of fruit flies (Bactrocera and Ceratitis species) and psyllids present or at risk of introduction to South Africa are the focus of our entomological research.

Objectives of the Research Programme

• Determine the efficacy of novel oomycete fungicides for the management of Phytophthora nicotianae from South African citrus orchards
• Assess the effects of root disease on systemic fungicide uptake; establish the mobility of three novel oomycete fungicides into plant tissues in healthy rootstocks and rootstocks compromised by Phytophthora nicotianae infection
• Test the in vitro and in vivo efficacy of biocontrol products, plant extracts and GRAS substances against Phytophthora citrophthora and P. nicotianae
• Validate the accuracy of the Alter-Rater risk model in predicting Alternaria infections in Nova orchards
• Determination of fungicide sensitivity of Botrytis cinerea isolates from South African lemon orchards
• Evaluate the pathogenicity of fungi and fungal complexes isolated from Eureka blossoms
• To evaluate the antifungal potential of indigenous South African plants against citrus fruit and foliar pathogens
• Identify the fungi responsible for blossom blight and rind distortion in Eureka lemons and Valencia oranges in Northwest South Africa
• Identify the bacterial and fungal microbial community present on and in the fruit rinds of soft citrus, lemons and oranges under nets and in open orchards
• Establish the role of temperature, relative humidity and fly physiology on the efficiency of fruit fly lures
• Establish the interacting roles of protein bait location, application density and fruit fly physiological state on response by fruit flies (Ceratitis capitata, Ceratitis cosyra and Bactrocera dorsalis)
• Define the locations and times of the year when targeted, pre-harvest fruit fly control measures can be applied to minimise infestation of citrus in a diverse agroecological landscape
• Investigate the influence of interactions between larval diet and gut microbiome on susceptibility of Ceratitis capitata and Bactrocera dorsalis larvae to post-harvest cold treatments for South African export citrus
• Develop novel monitoring tools for African citrus psyllids (Trioza erytreae) in South Africa

Highlights of the Research

• Novel oomycide efficacy: All four oomycete fungicides effectively inhibited Phytophthora nicotianae growth in vitro, with effective concentrations correlating to previous research in other countries. The results indicate that South African P. nicotianae isolates are sensitive to these oomycete fungicides. The preventative and curative in vivo applications of ethaboxam, mandipropamid and zoxamide yielded inconclusive results. Further orchard trials are recommended to fully investigate the effectiveness of these three agrochemicals against Phytophthora nicotianae.
• Novel oomycide uptake: Uptake and mobility of the novel oomycides were established in citrus rootstock cultivars using spectrometry. Chemometric models likened fungicide effects to phosphonate-induced resistance, pointing to a potential systemic action of the compounds.
• Alternative controls for Phytophthora spp.: Extracts from Artemisia afra were found to inhibit P. citrophthora and P. nicotianae in vitro at 78.2% and 62.1%, respectively. The in vitro inhibition of P. citrophthora by strains of Trichoderma asperellum and Bacillus amyloliquefaciens was 40.86% and 35.57%, respectively, and against P. nicotianae 52.31% and 55.56%, respectively.
• Alter-Rater validation: Three seasons of trials in a Nova orchard in MooiNooi were done to confirm infection periods predicted by the Alter-Rater model in PhytRisk. The findings suggest that the model will accurately predict Alternaria alternata infection periods after minor modifications to the algorithm. Another orchard spray trial was done over two growing seasons to compare the efficacy of the Alter-Rater fungicide spray programme with a standard calendar spray programme, an unsprayed control and a biological control programme. The results so far indicate that the model is accurately predicting spray timing intervals during a season with wet and hot conditions, as were experienced during the trial seasons.
• Botrytis cinerea fungicide sensitivity: Thirty B. cinerea isolates were screened in vitro for their sensitivity to four active ingredients from different FRAC groups. The results indicated that the isolates were sensitive to all four active ingredients tested. The results from this project will play a role in identifying the resistance patterns of B. cinerea and identify the mutations associated with the resistance of the isolates. This will improve the management of B. cinerea in lemon orchards.
• Fungal complexes in citrus blossoms: Results from in planta inoculation trials indicated that Botrytis cinerea and Colletotrichum gloeosporioides were the most virulent pathogens associated with blossom blight on lemons. These pathogens could threaten lemon production during flowering, resulting in disease development under favourable conditions. The outcomes of this project will assist the citrus industry in making effective and sustainable disease management decisions pertaining to blossom diseases.
• Plant extracts against fungal pathogens: Preliminary results show that several extracts are effective against the pathogens. Each extract displays different antifungal potency against the various pathogen species. No statistical conclusions can yet be made, but more data will further elucidate antifungal activity.
• Rind distortion etiology: Preliminary results show that multiple pathogens are present in blossoms and can infect all parts of citrus blossoms under various weather conditions. These pathogens may contribute to rind distortion, as rind distortion was observed on fruit in the orchard.
• Citrus rind microbiome: The species richness in the rind of all three citrus types under nets is less abundant than in open orchards.
• Fruit fly lure efficiency: Fly age influences the response of all species but is attractant specific, reflecting different requirements by B. dorsalis and C. capitata as they approach sexual maturity. Fly response generally increases with temperature, with lower thresholds ranging from 12.21 to 22.95°C depending on the species and attractant. Male B. dorsalis response to methyl eugenol drops by more than 90% when fed the attractant before testing.
• Fruit fly response to protein baits: Protein baits should be applied in the citrus tree canopy to optimise fly response. Higher levels of fly activity at warner temperatures increases of protein bait response. More flies visit protein bait when applied as many small spots in comparison to one single large point.
• Fruit fly control optimisation: In the Mooinooi area of North West province, South Africa, pest species like C. capitata, C. cosyra, and C. quilicii, were caught in high numbers during the dry winter season, which is inconsistent with the expectation that this period should have low fly numbers, with numbers decreasing leading into spring. Trap captures of these pests were concentrated in citrus, showing that fruit fly management in this landscape needs to target citrus orchards, even when fruit are not present, perhaps because of the sheltered microclimate and irrigation water available at these sites.
• Post-harvest control treatment for fruit flies: Removal of the egg chorion to prevent the transfer of bacteria from female flies to their offspring leads to slower development rates and lower survival in B. dorsalis and C. capitata.
• Citrus psyllid monitoring tools: Tested plant-derived odours did not improve T. erytreae catch during field cage or field trials, and weathering in the field did not affect the composition of odorants. However, temperature influenced odorant loss, and release rate from polyethylene bulbs decreased over time. Two artificial intelligence models showed promise as a workable means to speed up and improve the identification of psyllids on yellow sticky traps used for monitoring.

Industry and Societal Impact

The main emphasis of the preharvest disease program is the sustainable management of fruit, foliar, and soilborne pests and diseases in citrus. This involves the development of better monitoring methods, seeking and evaluating alternative, gentler management methods, enhancing chemical control strategies to deter resistance development, improving spray technology, and researching the epidemiology of various pathogens to optimize the timing of fungicide applications.

This research programme has a profound societal impact by ensuring a stable citrus supply, supporting food security and the livelihoods of farmers and agricultural workers. Reducing reliance on harsh chemicals and improving disease resistance management contributes to safer food production and healthier ecosystems, promoting human well-being. Additionally, the optimisation of insect pest management tactics and fungicide applications will minimize the environmental impact thereof and enhance resource efficiency, supporting sustainable agricultural practices that mitigate climate change effects. Ultimately, this research strengthens the resilience and sustainability of citrus farming in South Africa, benefiting both local communities and global food markets.

Capacity to solve the pest and disease problems facing the citrus industry has been grown through the training and graduation of young researchers.

• Tania Pogue (MSc Entomology, 2024)
• Dylan Pullock (MSc Entomology, 2024)
• Msizi Ramaoka (MSc Entomology, 2024)
• Eloff Theron (MSc Plant Pathology 2024)