By Ozgur Batuman, Sanju Kunwar and Ana Redondo
There are new products that potentially can be added to a grower’s toolbox in coming seasons for managing citrus bacterial canker.
Citrus canker is an infection by a species of bacteria (Xanthomonas citri subsp. citri) that causes fruit blemishes and fruit drop. It makes fruit unsightly and lowers its marketability. Every year, Florida citrus growers must put forth considerable effort to prevent citrus canker in their groves. However, the disease persists because the climate for growing citrus trees in Florida is also ideal for the bacterial canker.
The main source of the bacteria comes from infected trees in the grove. The disease can spread to non-infected areas via wind-driven rain, irrigation, mechanical means such as by farm equipment and contaminated tools, and the movement of infected plant materials within and between groves.
Currently, the main strategy to manage citrus canker is through copper sprays, which are costly and require repeated applications as fruit tissues expand. Although copper sprays effectively control canker, some growers avoid using them. This is because long-term use of the bactericide poses a big risk of developing a copper-resistant strain of this bacterium and over-accumulation of copper in the soil, which is detrimental to tree health and the environment. To reduce copper dependency, growers are interested in alternative products to control citrus canker.
In 2021, the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) tested new products for their potential to be used as an alternative or supplement control for citrus canker. The efficacy tests were performed on young seedling trees grown either in the greenhouse or in a commercial grove in Florida.
The first trial was conducted in a greenhouse and evaluated the potential of a novel plant defense inducer (PDI1) and Actigard (acibenzolar-S-methyl), a growers’ standard, applied as a foliar spray on two-year-old Valencia trees to suppress canker. The results of the trial showed that PDI1 applied at 500 parts per million (ppm) significantly reduced the severity of canker symptoms on leaves and did not cause any phytotoxicity (Figure 1A and 1C).
Plant defense inducers such as PDI1 are a new class of compounds that activate a plant’s natural immunity against diseases. The advantages of using plant defense inducers over copper sprays are that they are less toxic to the environment, do not accumulate in the fruit, are economical and can be used as a preventive measure. A foliar spray of Magna-Bon CS 2005 (copper sulfate pentahydrate; 150 ppm) was used as a control, which also performed well and had less disease than in the non-treated control (Figure 1A).
The second trial was conducted in a commercial grove and evaluated the potential of another novel plant defense inducer (PDI2) and a newly labeled product MC1 (copper sulfate pentahydrate; 24 ounces per tree). Both were applied as a foliar spray to suppress canker on Hamlin trees that were four to six years old. Kocide 3000 (copper hydroxide) was used as a control check.
In the grove, the two new products, PDI2 and MC1, were similarly effective as Kocide 3000 and reduced the number of symptomatic fruits compared to the non-treated control (Figure 1B). All three products marginally reduced the number of symptomatic leaves and number of dropped fruit due to canker.
Importantly, there was no visible phytotoxicity with any of the compounds tested in the greenhouse or in the grove.
It is important to note that 2021 was not as conducive for citrus canker as in past years, due to relatively dry spring months. Thus, disease pressure was considered to be low during the trials. UF/IFAS researchers expect these compounds to have better efficacy under high disease conditions.
The two novel plant defense inducers (PDI1 and PDI2) and MC1 compound could be good copper alternatives that are non-toxic and potentially more cost-effective for citrus growers to manage canker in Florida. UF/IFAS researchers cautiously believe that at least PDI2 and MC1 will be immediately available to growers as they are already labeled for citrus in Florida — provided they continue to hold their efficacy in repeated trials in the grove.
The next steps are to repeat the use of these products in commercial citrus groves and investigate what factors might affect their efficacy against citrus canker. Considerations include timing and application frequency and effects on fruit yield.
This information is important for protecting the environment and human health from the harmful effects of repetitive use of copper bactericides and preventing copper-resistant canker bacterium expansion within and between citrus groves in Florida.
Acknowledgments: The authors acknowledge grower and industry collaborators for supporting their research and Extension activities by providing access to their land and chemicals to conduct canker trials.
Ozgur Batuman is an assistant professor, Sanju Kunwar is a post-doctoral researcher, and Ana Redondo is a research assistant — all at the UF/IFAS Southwest Florida Research and Education Center in Immokalee.
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