Rust was the top reported item for most significant disease issue in a 2024 end-of-season survey of Florida blueberry growers. When rust is left unchecked or becomes significant, it can cause severe defoliation on blueberry plants. This is especially problematic in the evergreen system where plants don’t go dormant, and it is essential to keep the summer foliage healthy and intact through harvest. 

The species of rust pathogen found in Florida is Thekopsora minima, which is most active during late fall through early summer when temperatures are mild. Symptoms first appear as small angular yellow spots that turn red to black on the upper surfaces of leaves (Figure 1). Multiple rust lesions can occur on the same leaf, turning the leaves yellow or red over time (Figure 2) before causing defoliation. The most characteristic symptom of rust is clusters of yellow- or rust-colored spores produced on the underside of the leaf, opposite the lesions on the upper leaf surface (Figure 3), during extended periods of humid wet weather.

Figure 1. Rust symptoms on upper surface of leaf

Credits: D. Phillips, UF/IFAS

Figure 2. Multiple rust lesions on multiple leaves

Credits: D. Phillips, UF/IFAS

Figure 3. Rust spores on underside of leaf

Credits: D. Phillips, UF/IFAS

Management Strategies

The best strategies to effectively manage rust include horticultural and chemical methods to reduce the impact of the disease. Irrigation with overhead sprinklers favors disease development and spread, and should be avoided in favor of drip irrigation whenever possible to avoid extended leaf wetness. When overhead must be used, consider timing the application of irrigation to take place during the early morning hours, after dew formation, so that leaves will be dried by the morning sun. Pruning and plant spacing to increase airflow in the canopy and controlling weeds in row middles can encourage leaf drying and may also help reduce disease pressure.

Several fungicide products are available for blueberry growers to use to manage rust that have the potential to provide excellent control, but they have economic and environmental costs. Systemic fungicides move into infected leaves and can stop rust development early in the infection cycle. However, most products will only reduce or delay the amount of sporulation once symptoms are present. Fungicides do a better job protecting leaves against new infections by maintaining a protective residue on the leaves.  

In the central and southern regions of the state where the evergreen system is used, rust is usually less of a problem during summer, becoming more of an issue around the end of October. Disease pressure persists through harvest and summer postharvest pruning. The key to managing rust in this system is prevention. Once the disease becomes severe, options become fewer, less effective, and more costly. 

In the evergreen system, Chlorothalonil (sold as BravoTM and others) applications for rust management can be made starting late fall, before bloom. Chlorothalonil is a contact fungicide that cannot be used after bloom, and that some growers have concerns about causing leaf burn in the heat of summer. Chlorothalonil has efficacy for several diseases and applications made when disease pressure is generally low but starting to increase makes sense. 

Rust Detection

As the season progresses, growers should scout for rust by walking the rows, turning over leaves with spots, and looking for the orange spore clusters. As rust starts to increase on the interior lower canopy leaves, consider using ProlineTM (prothioconazole), which has stood out in some published research as an excellent choice among DMI products for rust. Other products with reported excellent effectiveness include Qulit XcelTM (azoxystrobin and propiconazole) and PropulseTM (fluopyram and prothioconazole). DMI’s with longer preharvest intervals (PHI) can also be considered if rust increases before bloom (e.g., IndarTM, TiltTM). They will have some efficacy, and this will leave QuashTM and ProlineTM (with a 7 day PHI) as options for any flare-ups closer to harvest. AboundTM and PristineTM both also have rust efficacy and make for good rotation partners with one of the DMI products. 

If applied at or after bloom, consider tank mixing a captan product with AboundTM or PristineTM, because of the widespread anthracnose ripe rot resistance to these products. One or more of these options employed in the late fall to pre-bloom period should do a good job of keeping rust severity low through harvest. 

The UF Blueberry Breeding Program and the Blueberry Pathology Program are currently collaborating on a project to address leaf rust at the genetic level. The project is supported by royalties generated through sales of our current varieties and will help us continue to improve rust response of future releases. The project has two primary objectives, studying the rust fungus diversity and identifying sources of rust resistance already available in the UF Breeding Program materials. So far, a Ph.D. Student, Christ-Mane Belizaire, has collected rust from across the Florida production area and has confirmed the identity of the rust fungus we fight in Florida. She has also phenotyped (rated for rust susceptibility) Stage 3 blueberry selections located in Waldo and Arcadia. Initial results demonstrate a wide and promising range of susceptibilities, from apparent immunity to highly susceptible. The Breeding Program team is currently identifying molecular markers that will point to the genes responsible for these different levels of rust resistance. Future work will characterize the nature of the resistance observed and hopefully add to the list of tools available at the genetic level to select for favorable leaf rust responses in future variety releases. Additional work will aim to test the durability of rust resistance that we find and to further optimize the integration of all rust management options available to Florida growers in pursuit of sustainable, efficient, and profitable production.

CREDITS

DR. PHIL HARMON, UF/IFAS

& DOUG PHILLIPS, UF/IFAS