Integrated disease management of citrus black spot and ‘Emperor’ brown spot (CT20009)

The project team conducted a survey with growers to investigate management strategies for Citrus Black Spot (CBS) and Exocortis Viroid (EBS). The project team also established six glasshouse and field trials, engaged with industry stakeholders, and prepared an article for Australian Citrus News.

The grower survey focused on mandarin production in the Gayndah-Mundubbera citrus growing region. Results showed that growers consider an effective spray program as the most important tool for disease management. Growers had already implement Integrated Disease Management (IDM) practices such as tree hygiene and disease and weather monitoring. Growers expressed concerns about potential fungicide loss and expressed a need for more efficient utilization of registered chemistries and the development of new chemistries. The findings of the grower survey will be shared through an article in Australian Citrus News.

The glasshouse trials aimed to develop a reliable inoculation assay for CBS, established a protocol for breaking CBS latency, and evaluate new, softer chemistries for CBS and EBS control. The progress of these trials will be reported in the next milestone in November 2023.

The field trials consisted of several evaluations. Firstly, they assessed the early and pre-rainfall application of copper fungicide sprays for controlling CBS in Imperial mandarins. Secondly, they examined copper phytotoxicity. Lastly, they tested newly registered SDHI products in combination with tree hygiene practices. The trial assessing the effectiveness of early and pre-rainfall copper fungicide sprays has been completed, including harvest and initial assessments, in April 2023. The remaining field trials will be harvested and assessed in June 2023, with all results presented in November 2023.

The project team have completed a literature review that outlines the major factors that contribute to the occurrence of fruit injury following copper fungicide application. The effect of pH on the solubility of copper fungicide spray mixtures was shown to be an important factor.

The Project Team engaged with one of the citrus industries most experienced and respected consultants regarding the best strategy to investigate copper phytotoxicity solutions.  It was agreed to examine pH effects as a first step and to do so at Bundaberg Research Station on a progeny block from the breeding program.

A row of irradiated 06C015 was chosen for this experiment because they were a uniform genotype, relatively small but still with some fruit, not for sale or consumption, and potential tree damage would not impact the breeding program. 

Laboratory experiments were conducted to produce pH response curves for both potassium hydroxide and citric acid, so that the correct amount could be used during field spraying (confirmed using in-field pH measurement).

Plastic tarps were placed temporarily between plots to ensure there was no spray drift between treatments, which worked extremely well.

Clear differences were seen between pH treatments, both in terms of the physical appearance of the spray mix and deposit on the trees.  Differences in phytotoxicity were also observed and are currently being confirmed.