Rust diseases are on the rise

In the past few years many “new” rust pathogens have found their way into the United States, including daylily rust. In 2005, guava rust (Puccinia psidii) exploded in San Diego County, Calif., on field-grown myrtle, and it happened again last year. As the name indicates, it was originally found on guava but it attacks many other members of the Myrtaceae family. During the same time frame, guava rust also appeared for the first time on some tropical crops in Hawaii. This disease has caused serious losses in the eucalyptus industry in many countries, including Brazil.

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Probably the worst new rust is gladiolus rust (Uromyces transversalis). This disease was quarantined, but in the spring-summer 2006 a very severe outbreak was found in Florida gladiolus production. The same disease was found in a very small commercial cut flower operation in San Diego County. It has not been found in operations of major cut gladiolus producers in other U.S. locations.

Every year, white rust on chrysanthemum (Puccinia horiana P. Henn.) has been encountered at ports of entry. Once in a while it has been found in production greenhouses, especially in the Northeast and along the West Coast. Last year was no exception, with cases reported in the Northeast.

Soybean rust (Phakopsora pachyrhizi), which is a quarantined disease, has finally entered the United States. It is the focus of quite a bit of attention from federal and local officials.

Chemical control of rusts

Researchers at Chase Research Gardens have been studying rust diseases and have found that these fungi do not always cooperate. We did finish three trials on hypericum rust (Uromyces triquetrus) once we discovered that one cultivar was particularly susceptible while many others were virtually immune. Over the past 10 years, we have worked on a variety of rusts including geranium, snapdragon, bellis (English daisy), solidago, rose and hypericum. Most of the results were consistent from crop to crop so some broad conclusions can be made.

In the first trial, products were applied as a foliar spray three times on a seven-day interval. The best control of rust in this trial (see Table 1) was achieved with the 6-ounce rate of Armada (an experimental product combining triadimefon and trifloxystrobin) or Heritage (azoxystrobin). Vitavax (oxycarboxin) provided very good control but caused severe stunting. This fungicide is labeled for seed treatment, but was trialed since Plantvax, a related compound, was so successful against rusts in the past. Some of the sterol inhibitors also caused significant stunting (triadimefon in Armada and Terraguard 50WP).

Effect of fungicides on prevention of hypericum rust and plant stunting