Stop chrysanthemum white ruse to protect crops

Chrysanthemum white rust (CWR) is widespread and endemic in Europe, South America and other countries, but so far, it’s not established in the United States. Several recent outbreaks and USDA interceptions at ports of entry into the U.S. from off-shore sources have caused more concern about the disease and its causes. Any introduction of this pathogen from offshore is a significant threat to the United States floriculture industry. CWR spreads rapidly and can cause major economic and crop losses.

CWR is a quarantine-action pest, regulated by federal and state regulatory agencies. Prevention and control of CWR depends on effective plant quarantine laws, healthy starter material, management of humidity and irrigation, and the proper selection and use of fungicides.

What is CWR?

To fight CWR, it’s helpful to know what it is and what it looks like. Here’s an overview.

Cause. CWR is caused by Puccinia horiana Henn., a filamentous fungus. It is an obligate parasite, so it only grows and reproduces on host plants.

Hosts. The host range is a number of chrysanthemum species and their close relatives.

Plant stages. Examine all stages of plant production, since the disease is capable of infecting young plants as well as the finished product. Growers who scout crops regularly will be most likely to find any potential infection early.

Source: Yoder Brothers

Symptoms and signs. The first symptoms of CWR are yellow spots on upper leaf surfaces, up to 4 millimeters in diameter. These may become sunken and necrotic. At first glance, to someone unfamiliar with this pathogen, the yellow spots may be mistaken for spray injury, insect damage or a virus. Any sign of yellow spotting should be closely examined for further evidence of this disease.

The subsequent development of prominent pustules on the lower surface of leaves is diagnostic for a rust-type pathogen. Pinkish or buff-colored pustules mature to waxy white. Pustules may also develop on stems, bracts and flowers.

Severely infected leaves dry up and hang along the stem.

Microscopic observation of two-celled teliospores in pustules is required to confirm a diagnosis for CWR.

Disease cycle. New infections are initiated by basidiospores (the airborne spores) released from pustules during periods of high relative humidity (96-100 percent) when temperatures are 40°F-73°F (optimum 63°F). Spores landing on a plant surface can germinate and penetrate in as little as two hours at optimum temperatures (63°F-75°F). A film of free water is required for infection.

For five to 14 days after infection, the fungus grows within the plant as a latent infection, after which chlorotic (yellow) spots, and ultimately pustules, appear. Teliospores (the spores that remain attached to the leaf) produced in pustules germinate in place to produce the next generation of basidiospores when temperature and humidity are favorable.

Dispersal. Long-distance dispersal of white rust depends on movement of infected plant material. Because cuttings may not display symptoms for as long as two weeks after infection, healthy cuttings are not a guarantee of safety. Basidiospores can be carried short distances by wind. These spores are so short-lived that even under ideal conditions they only survive long enough to be carried a few miles before dying due to desiccation. Infested debris carrying viable teliospores may also play a role in dispersal.

Survival. CWR survives for extended periods only in association with host plant tissue. Basidiospores are thin walled and rapidly desiccate. They are extremely short-lived, surviving only 5 minutes at 80 percent relative humidity and less than one hour at 90 percent relative humidity. Teliospores survive in dried plant debris for at most two weeks when buried in air-dried soil. If CWR is established on hardy outdoor plantings of chrysanthemums, it can overwinter with the host.

If you find white rust. Inform USDA, state or county officials if you find CWR. Regulatory officials will supervise eradication and treatment programs that may include:

* Destruction of infected plants and those immediately surrounding them.

* Fungicide treatment.

* Survey of surrounding premises.

* Trace back to attempt a determination of source and distribution.

* Trace forward to track shipments.

More extensive action may be taken if infection is generalized in an operation.

Official control programs of Animal and Plant Health Inspection Service are subject to revision. See the National Protocol for the United States: at www.aphis.usda.gov/plant_health/plant_pest_info/cwr/downloads/cwrplan.pdf.

Protectant fungicides offer help

In addition to exclusion, a fungicide program should be used for protection against CWR. Some fungicides are better for eradication while others are better protectants. Keep this in mind when you apply fungicides. Myclobutanil (Eagle, Systhane), which acts as a sterol biosynthesis inhibitor, can be used to eradicate early infections. Routine use of this fungicide on chrysanthemums is not recommended. It is not as effective as some other fungicides as a protectant for CWR and resistance is a high risk.

Here are some fungicides to use in a protectant program for CWR as sprays:

* Azoxystrobin (Heritage) systemic 1-4 ounces per 100 gallons of water every seven to 28 days.

* Chlorothalonil (Daconil Ultrex, Spectro 90 WDG with thiophanate-methyl) protectant 1-2 pounds per 100 gallons of water every seven days (minimum).

* Kresoxim-methyl (Cygnus) protectant 3.2-6.4 ounces per 100 gallons of water at a seven- to 10-day interval.

* Mancozeb (Fore, Dithane 75DF, Protect DF) protectant 1.5 pounds per 100 gallons of water at a seven- to 10-day interval.

* Triadimefon (Strike) systemic 1-2 ounces per 100 gallons of water at a seven- to 14-day interval.

* Triflumizole (Terraguard) protectant 2-8 ounces per 100 gallons of water at a seven- to 14-day interval.

Always read and follow label directions. The label is the law.

Here is an example of a prevention schedule. Begin this schedule in propagation on day 3, 4 or 5 after sticking cuttings. Apply at night when mist is not on.

Week 1 Heritage (azoxystrobin) spray.

Week 2 Daconil Ultrex (chlorothalonil) spray (this formulation is not phytotoxic).

Week 3 Dithane 75 DF (mancozeb) spray.

Week 4 Daconil Ultrex (chlorothalonil) spray.

Week 5 Medallion (fludioxonil), Chipco 26019 (iprodione) or 3336 or OHP 6672 (thiophanate methyl) spray or drench for other chrysanthemum diseases.

Repeat this schedule weekly until color break. Then use Heritage when color can be seen. Repeat Heritage weekly until the crop is shipped if the conditions are 63°F-75°F and wet. Do not apply chlorothalonil or mancozeb after color break since burn and residue, respectively, can occur. Heritage can cause growth regulator effects in chrysanthemum.

- Jyme Mariani and Jane Trolinger

Jane Trolinger is Yoder Brothers’ crop health manager at its Alva, Fla., facility, (239) 728-2535, Ext. 150; www.yoderbrothers.com.

Contact Yoder Brothers for a copy of the complete bulletin on chrysanthemum white rust, www.yoder.com and look for Grower Services/Cultural Tips.

Program summary for prevention

There are five simple steps to prevent chrysanthemum white rust:

* Plant mum cuttings from a reputable commercial source.

* Do not handle imported chrysanthemums in or near production facilities. Cut mums from any country known to have CWR are a threat to chrysanthemum crops.

* Maintain low humidity and dry foliage. Use fans, vents, plant spacing and heat to move air and reduce humidity. Use drip rather than overhead irrigation to minimize free water on foliage. Beware of impermeable barriers that retain humidity if using black cloth to control day length.

* Monitor crops regularly.

* Schedule regular applications of fungicides as a preventive measure. Fungicides known to be effective are Daconil Ultrex and fungicides containing mancozeb, including Dithane 75 DF and Fore and Protect DF. Read and follow label directions when using pesticides.

What goes into a federal quarantine

Federal quarantines require:

* Six-month post-entry quarantine of imported cuttings.

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* Prohibition of cutting import from infested countries.

* Inspection of imported cut chrysanthemums at port of entry.

* Certification programs for offshore cut flower producers wishing to export to the U.S.

* Eradication when CWR is found.