Latest Research and Best Practices to Fight the Disease

“This is the story of Johnny Rotten, It’s better to burn out than it is to rust.” 

Several years ago, I wrote an article for The Blueberry News on leaf rust and referenced the song “My My, Hey Hey” on the Neil Young album titled Rust Never Sleeps. Evergreen production was a fairly new practice then, and a shift in the rust management paradigm had already begun. 

The disease “helps” defoliate plants in the deciduous production system at the end of November into December until the eventual hydrogen cyanamide (Dormex) application. The goal then was to keep healthy leaves through floral bud differentiation in early fall. The question was whether fungicides in late fall were worth the investment, was it “better to burn out than it is to rust?” But for evergreen producers that wasn’t really a question then and certainly not now. 

Healthy leaves through harvest are needed to realize a full crop and maintain plant vigor. Rust is just one of the issues that can impact foliage. Good fungicide options exist, but they are expensive, and plant resistance to rust is a long-term goal being pursued at UF that we hope will help reduce our reliance on fungicides to produce a crop. A brief review of our research progress and management options follows. 

Research Progress

Over the past two years, we have collected and measured spores from more than 100 rust samples from across the state. We sequenced and compared part of the rust fungus genome from 37 of those samples to determine if we have one consistent species across our production areas. With the help of the UF Blueberry Breeding Program, we have also sequenced a draft whole genome of the rust pathogen—no easy feat! Christ-Mane’s results so far have been very promising. The spore sizes, shapes and colors varied somewhat across the state, but all fell within those described for the pathogen Thekopsora minima. Our partial sequence data for the 37 rust samples also indicates that one consistent species of rust causes disease on blueberries in the state, so we are fighting one enemy rather than two or three. Our draft whole-genome has excellent coverage and quality and will serve as a valuable resource moving forward as we characterize the interaction between our resistant blueberry varieties and the rust fungus.   

On the resistance front, we have characterized the rust response of more than 500 blueberry genotypes in the breeding program’s stage III plantings. We have seen tremendous variability in rust susceptibility, with some selections like 1975-4 displaying not only more severe rust symptoms at each rating date, but also developing rust up to a month earlier (early November) than our least susceptible genotypes (mid-December). Initial genetic mapping efforts have identified some interesting gene targets for us to further explore that show promise for potential marker-assisted selection tools.  

Christ-Mane has also put in some serious hours learning what environmental parameters influence disease development, how to collect and manipulate the pathogen, and she now has an artificial inoculation protocol that can reproduce field observed levels of severity in small-scale inoculations (Figure 1). Additional work will seek to scale this inoculation protocol up to screen many genotypes at once, early in the selection process. These inoculations can help serve as validations for the potential marker-assisted selection tools employed in the future.  

 

Figure 1. Artificial rust inoculation.

Credits: C. Belizaire, UF/IFAS

 

Reviewing How to Protect Your Blueberries Against Rust

Managing leaf rust well requires combining good horticultural practices with timely fungicide use. The strategy is different for deciduous and evergreen production systems.

Best Practices

Here are some horticultural practices that may reduce rust:

1. Use drip irrigation whenever possible because overhead irrigation keeps leaves wet for a long time, which helps rust infect. If overhead irrigation is necessary, avoid running it in the early evening. Wet leaves overnight extend ideal conditions for the disease. A better option is to irrigate early in the morning when dew is already present and when leaves will dry quickly.

2. Trim plants to open the canopy. Better air movement helps leaves dry faster and makes it less favorable for the fungus to grow.

3. Remove and destroy infected leaves and stems during pruning. Keeping the field clean of diseased and dead plant material may help reduce the amount of fungus lingering on infected leaves.

4. Apply fungicides when disease is likely and before it becomes severe. Many of the fungicide options are systemic and have a limited ability to move inside the leaves. These can help stop early infections, but only before symptoms are visible. They can also prevent new infections from occurring, but will only slow the spread rather than remove or kill the disease. The best protection comes from keeping a consistent layer of fungicide on the leaves during periods that favor disease development (typically November through harvest for the evergreen system, although it can occur earlier). 

Rust slows during the summer but may become a concern again starting as early as late September depending on the weather. The disease stays active through winter and harvest in evergreen production (Figure 2), typically requiring more fungicide sprays than deciduous production. Fungicides used for fruit rot may also help suppress rust that carries through winter months.

Figure 2. Rust spores on blueberry leaves.

Credits: C. Belizaire, UF/IFAS

Fungicides That Have Been Effective

Here are some fungicide options that have been reported to be effective for the last 3 years:

Chlorothalonil products such as Bravo® can be used in late fall before bloom. They work well but cannot be used after bloom. 

When rust begins to increase in the lower canopy, good systemic options include:

• Proline® (prothioconazole)

• Propulse® (fluopyram and prothioconazole) 

• Quilt Xcel® (azoxystrobin and propiconazole). Initial data on Cevya® (mefentrifluconzole) also looks promising

Before bloom, products such as Indar® or Tilt® can be rotated or tank-mixed with Bravo® to help reduce rust while saving the lower PHI options of Quash® and Proline® (7 day PHI) and Cevya® (0 day PHI) for use closer to harvest.

Quadris® Flowable and Pristine® also work well and are good choices for rotation. At or after bloom, they should be tank mixed with captan because resistance to anthracnose ripe rot is common.

Using one or more of these products from late fall through the period before bloom usually keeps rust levels low through harvest. Additional information is available in our extension publication on blueberry leaf rust (https://edis.ifas.ufl.edu/publication/PP385).

Take-Home Message

Blueberry leaf rust can be a serious problem, particularly in evergreen blueberries and especially during mild wet winters. By following good horticultural practices and using fungicides when needed, growers can protect their plants, but keep in mind—Rust Never Sleeps.

CREDITS

DR. PHIL HARMON, UF/IFAS

& CHRIST-MANE BELIZAIRE, UF/IFAS