Controlling rust

The past 20 years have seen an onslaught of “new” rust pathogens on ornamentals. Among them: Solidago, aster, Bellis (also infects common groundsel throughout the West), chrysanthemum (brown rust), Hypericum, sunflower, iris, veronica, geranium, rose, snapdragon, mint, Dianthus and Heuchera to name a few.

Rust is also being seen on ornamental grasses (switchgrass - Puccinia emaculata) and tropical grasses. For example, Prospodium rust on lantana and Tecoma and Plumeria rust (Coleosporium plumeriae). Canna, for the most part, appears to be limited mainly to the Southeast. Most recently I saw some fuchsia rust on unrooted cuttings. This was also reported in England in the most recent Hardy Ornamentals Notes newsletter from ADAS (www.adas.co.uk). With that said, it may be easier to list plants that don’t get rust than those that do.
 

Recent outbreaks

In 2001, daylily rust (caused by Puccinia hemerocallidis) was found in Florida, and by the end of 2003 it was reported in nearly every state, including California. Since that time it has received a lot of attention from researchers looking for the best way to control the disease. The difference between effective fungicides for prevention, eradication, and ability to kill rust spores was an important focus of this work.

In 2005, guava rust (Puccinia psidii) developed in San Diego County on field-grown myrtle (Myrtus communis used for cut foliage). Since then, it’s become a costly addition to apply preventive fungicides every year. As the name indicates, it was originally found on guava, but does attack other members of the Myrtaceae family. During the same time frame, guava rust appeared for the first time on some tropical crops in Hawaii. The disease has caused serious losses in the eucalyptus industry in many countries, including Brazil, and has been reported on many hosts in Florida. Hawaii has stopped importation of Myrtaceae plants and products that are known to host the rust into the state from all sources domestic and foreign in an effort to keep this disease from becoming an even more serious concern in their native forests (Ohia in 2005).

Gladiolus rust (Uromyces transversalis) is also newly reported in the U.S. This rust was originally described in 1898 and has been listed under federal quarantine. Interceptions of the disease were reported in 2004-2006 (Mexico, Brazil, and Colombia). In 2006, the disease had a severe outbreak in Florida that cut gladiolus production and a minor outbreak (mainly gardeners) in southern California. It has since become more or less established throughout coastal California and is found in many gardens as well as some large commercial cut flower operations. It continues to be under quarantine, although little funding is currently available to manage this costly rust.
 

Trial work on fungicides

Much of the most helpful information developed in the past 10 years was conducted by Dr. Steve Jeffers (Clemson University) and Dr. James Buck (University of Georgia) and their students. They looked at a variety of rust diseases on ornamentals and researched the ability of a variety of fungicides to kill rust spores. They found that the best products were chlorthalonil, mancozeb, azoxystrobin and copper pentahydrate.

In direct contrast to the ability to kill spores, these fungicides worked to different degrees when applied to plants (daylily, geranium and sunflower) either before or after inoculation.

Azoxystrobin could prevent disease when applied up to 15 days before infection on sunflower while it provided curative control around five to seven days after infection occurred.

Chlorothalonil did not provide curative control but was very effective preventively on geranium (15 days), but not on daylily or sunflower.

Systhane gave relatively consistent prevention and eradication for about five days before or five days after infection occurred on the three crops.

Propiconazole was similar to systhane for eradication but not effective preventively. Jeffers’ and Buck’s full research papers can be found in the scientific journal Plant Disease (89:255-261 and 88:657-661).

Determining which fungicides work best for controlling gladiolus rust was addressed by Jeffers, Buck, and Dr. Cristi Palmer (IR-4 Project) by working with a local plant pathologist in Mexico. Trials ran for three years, starting in 2010. Fungicides were applied at two-week intervals and disease severity was recorded weekly for a minimum of seven weeks after the initial fungicide application. Fungicides were understandably less effective under high disease pressure (2010) than when disease pressure was lower (2011 and 2012). Fungicide treatments generally reduced disease severity significantly. Triazoles (FRAC 3) were more effective than quinone outside inhibitors (FRAC 11). Combining fungicides in different mode-of-action groups was more effective than applying them individually. Rotations of fungicides, either with individual products or with combinations of two products, provided excellent rust management. This work was published as well in Plant Disease 97:1491-1496.

Dr. Aaron Palmateer (University of Florida) also has performed some helpful trials on rusts in South Florida. His trial on canna rust demonstrated that weekly applications (starting the day before inoculation) that FRAC group 11 fungicides (pyraclostrobin/ boscalid and azoxystrobin) both significantly decreased disease compared to controls.

Treatments with FRAC 3 fungicides (triticonazole and systhane) did not significantly decrease disease levels. It is interesting that on one rust, FRAC 11 may be better while on another FRAC 3 may be better.

The work from gladiolus rust does demonstrate that rotation and combinations are, however, the best overall approach to rust control.

Conclusions

We are still relying on protectant products available in the 1970s, such as chlorothalonil and mancozeb, for prevention of rust. As the work by Mueller et al. shows, these remain some of the best products for killing spores and preventing disease.

The newer, systemic fungicides like FRAC 3 - sterol inhibitors (i. e. propiconazole, myclobutanil, triadimefon and triticonazole) as well as FRAC 11 - strobilurins (i.e. azoxystrobin, pyraclostrobin, trifloxystrobin and fluoxastrobin) provide some eradicant benefits. Use a 14- to 21-day interval with protectants like mancozeb when conditions are unfavorable for rust and you are in a preventive mode. If an outbreak occurs, use of sterol inhibitors and strobilurins with a wetting agent will be your most effective means of eradication.

Finally, be sure to rotate between classes of fungicides and tank mix a protectant and systemic eradicant to make sure resistance does not develop.

A.R. Chase is the part-owner of Chase Agricultural Consulting LLC and Professor Emeritus of Plant Pathology at the University of Florida. www.chaseagriculturalconsulting.com