Feature Articles

Disease and Pest Management in Roses
A 5-acre garden on the University of California at Davis campus holds 400 of the most commercially important rose varieties. Researchers are maintaining these plants as samples that are tested and shown to be disease free. Some rose varieties can carry rose mosaic virus without showing symptoms. In the past, some rose stock has even failed to show disease symptoms in the temperate California climate, but has later shown rose mosaic symptoms after being shipped to the eastern United States.

"We do a bio assay of these plants by grafting a bud onto an indicator variety and growing it for two years to make sure symptoms do not show up," says Mike Cunningham, rose program manager at  Foundation Plant Services. Cunningham made his remarks to rose experts from around the world as they toured highlights of California's floriculture industry following the recent Fourth International Symposium on Rose Research and Cultivation held in Santa Barbara, California, United States.

Before looking at rose research facilities at the University of California at Davis, tour participants also visited the new potted rose program at Greenheart Nursery in arroyo Grande, the Fioli Gardens in San Francisco, Rose Gene Technology in Watsonville and the ha of hydroponic roses a few miles away from Rose Gene Technology at California Pajarosa Floral. In addition to their 2-ha repository of disease-free roses, University of California Foundation Plant Services researchers do extensive research to learn how important diseases like rose mosaic virus are transmitted.\"We think that rose mosaic virus is transmitted among the roots of the plants," said Deborah Golino, director of Foundation Plant Services at the University of California at Davis. In order to test this theory, researchers are growing one healthy plant and a second virus-infected plant together in the same opt. The healthy plant will presumably contract rose mosaic virus through root contact with the diseased plant.

At another location on the Davis campus, tour participants heard about the latest developments in greenhouse cut rose integrated pest management.

The three major pest problems with cut roses are powdery mildew, two-spotted spider mites and western flower thrips. Recently University of California researchers have discovered a tribe of ladybug beetles that feeds on the fungus that causes powdery mildew on cut roses. Work has begun in an attempt to learn if releasing this type of ladybug can be used as an effective part of a mildew control program in commercial flower greenhouses. The predatory mite persimillis already provides effective biological control of spider mites. And this beneficial insect is widely available to greenhouse growers near the California coast because many coastal-area strawberry growers use it in their integrated pest management programs.

But there are no know effective biological controls for western flower thrips. In order to control thrips, while maintaining biological control of mites, researchers developed a unique system of spraying insecticides only on the areas of the roses where thrips are likely to congregate. "By spraying just around the bud we can reduce the water by 75% and lower the impact on the biological control in the lower part of the plant," says Michael Parella, University of California at Davis professor of entomology. A decade ago the California Pajarosa Floral cut rose nursery outside of Watsonville, California began working on biological control of spider mites, which are high on the list of cut rose pests. Their approach requires unusually vigilant scouting in order to discover the spider mites when they are still at low enough levels for the predatory mite persimillis to keep them under control. In order to maintain this biological control for spider mites, the insecticides that are used to control thrips are sprayed at the lowest possible levels and are directed high on the plant. Blue tape is also stretched prominently throughout the greenhouses in order to attract and trap western flower thrips.

The jury is still our, however, on the effectiveness of this particular control tool. "My feeling is that when you have millions of thrips coming in to the greenhouse, and you have a few thousand go to the blue tape, it mainly makes you feel good," said Steve Tjosvold, university of California Cooperative Extension farm advisor in Watsonville, California. California Pajarosa also has a head start in satisfying water-quality regulators because the water from the 17 acres of hydroponic roses at the facility is already recycled. Irrigation sets are calculated to leach 20% of the water out of the bottom, in order to prevent excess salts from building up in the medium. After water leaches from the bottom of the pots, it is captured and sent to a retention pond on the nursery grounds. From the retention pond, the water is sand filtered and then mixed with fresh well water to irrigate the roses.

In 1993 he converted to hydroponics  and began growing his roses in rockwool on rolling benches. Today there is no rockwool in the California Pajarosa greenhouses, as the firm has continued to evolve its methods and now grows in a mix of 50% coconut and 50% perlite. Coconut has proven to be better for cut rose production than rockwool, and costs about the same. Production is increase 20 to 30% by bending the weaker limbs downward. And the firm also increases its productivity by investing in relatively quick turnover of the plants, as the roses are replaced in just five to seven years.

Between 140 and 150 varieties of roses are grown at California Pajarosa. This extensive variety count includes around 50 varieties of spray roses, another 50 or more varieties of hybrid tea roses and the remainder in miniature roses. "We are not growing as many cut flowers as before, but we are growing more nursery plants, colorful bedding plants," says Tjosvold. Not long ago growers in Monterey and Santa Cruz counties grew $75 million in cut roses annually. That figure has dropped to just $20 million, as lower cost South American imports have taken the market, and production cut flowers of all types is down to $75 million. But as the cut flower trade has declined, local growers have taken advantage of the temperate coastal climate to build a $300 million nursery industry.

"The industry left here, so all the cut flower varieties are coming out of Europe," said George Marciel, sales director at Rose Gene Technology. "We go to the Netherlands every year and bring back the varieties we like." Every year Marciel brings back 12 to 15 new cut rose varieties that look like they would fit in the North American market. These varieties are then grown on natal briar rootstock in the Rose Gene greenhouses to learn how the fare under California coastal conditions. There has not been a dominant red rose variety in the United States market since Kardinal reigned supreme more than a decade ago. The red rose market today is fragmented among six and eight popular varieties, according to Marciel.

And tastes in cut roses are different in the United State, where cut flowers are usually occasion gifts; by contrast, cut flowers are frequently purchased to decorate homes in Europe. "In the United State, it is about how it looks, not how long it lasts, because it is still mostly gift-giving," Marciel says.

The Rose Gene Technology complex outside of Watsonville used to be a rose research facility. But circumstances have forced the company to diversify and today the complex is divided among a cut flower business, s starter plant nursery for cut rose growers and a potted plant breeding facility for garden roses. And Marciel believes the march of low-wage production could eventually impact the Netherlands. There are between 61 and 81 ha of cut rose greenhouses in the United States and Canada, which is down from a peak of 202 and 304 ha. The Netherlands, by way of comparison has 809 to 911 ha of cut rose greenhouses. "Africa is starting to encroach on what they are doing in the Netherlands," Marciel says. "You can see the long-term trend. I feel kind of like a blacksmith at the turn of the last century."

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