What is in this article?:
- California garlic, onion growers tackle white rot challenge
- DADS cost very high
- Biotech approaches
- White rot control continues to be the biggest challenge facing the research-driven marketing order for California garlic and onion producers.
- California produces 91 percent of U.S. garlic production. Fresno, Kings and Kern are the top garlic producing counties with 17,731 acres planted last year. Onion acreage is split between fresh market and processing with the total at nearly 49,000 acres statewide.
There are some biotech approaches to allium white rot reported New Zealand plant geneticist Colin Eady. Eady, who has been working on white rot resistant allium varieties, said that in theory they can be developed, but regulations on where and how biotech varieties are grown have hindered field trials.
Imperial County entomology farm advisor Eric Natwick said onion thrips must be managed because the pest vectors iris yellow spotted virus (IYSV). The disease reduces yield, bulb size and causes storage problems in garlic and onions.
Desert onion and garlic growers first experienced the virus problem seven years ago.
Infested fields have a silvery appearance, Natwick said.
Natwick said control options for the disease-vectoring thrips include biological control with natural predators. However, the predators may not be high enough in numbers to control onion thrips until later in the season — when the damage has already been done. Avoiding planting allium crops downwind of small grains — which attract thrips — can also help. Host plant resistance is years away from development, he added.
Cultural practices that can help control thrips include use of sprinklers which wash the thrips from the plants. Water droplets also inhibit thrips movement. Burying cull piles and removal of volunteer onions also can slow the spread.
Insecticide sprays can be effective, but chemicals must be rotated to avoid resistance. Natwick advised checking labels to make sure products are approved by the California Department of Pesticide Regulation. Sampling five plants in four areas of a field is recommended. If more than 30 thrips per plant are found, fields should be treated, he said. That number can be lower for younger plants or higher for older plants. Timing of applications is critical to knocking down thrips populations. Sprays are most effective when weather is cooler. Use of surfactants is also recommended to get the material down into the leaves.
Tom Turini, a UCCE farm advisor in Fresno County, said IYSV has been found there, but at low levels. In a 2011 field trial with processing onions, Turini said insecticide applications began when there were five to 10 thrips per plant detected. The materials used did reduce the thrips population in the fields, but he said there were no yield improvements.
In the Klamath Basin where the growing season is shorter, UCCE Siskiyou County Director Steve Orloff said growers need to protect their onion crops from thrips for eight weeks. His trials involved both chemigation and sprays with systemic and contact materials.
Garlic growers are still experiencing periodic outbreaks of Rust, fungal disease that devastated many fields in the 1990s, said UCCE farm advisor Steve Koike from Monterey County. Since those first Rust infestations, Koike said effective fungicides were registered and the disease has been managed. What is new are changes in the biology of Rust. Outbreaks of Rust in the 1990s did not affect leeks, but in the last few years Rust has been found in that crop. Koike said a 2011 trial found that Quilt Xcel at 21 ounces or Quadris at 12 ounces worked in Rust suppression. Rust is still a production factor in allium crops and no tolerant cultivars have been developed. Rust management guidelines have been initiated, Koike added.