White rot continues to lay to waste fields of onions and garlic around the world, but plant pathologist Fred Crowe of Oregon State University at Madras says the search for a control is making progress.
“I'm a little upbeat from past years, but we have to realize that white rot is still a serious problem, and if you have it, it is hard to live with,” Crowe told the recent Garlic and Onion Seminar at Five Points, Calif.
Germination stimulants, the same compounds responsible for the characteristic odors and tastes of onions and garlic, may be a key to a solution to the disease, he said.
Sponsored by the American Dehydrated Onion and Garlic Association (ADOGA), the event drew more than 100 persons involved in the allium industry worth $410 million in California in 1997.
Crowe, who has collaborated with scientists in California, Washington and elsewhere, said the fungus, Sclerotium cepivorum, enters alliums through roots, starting as a small cluster of dying plants. A group of clusters can spread rapidly across an entire field, depending on temperatures and length of the season.
Its poppy-seed sized fruiting bodies, even when buried as deep as one foot in the soil, invade onion or garlic roots coming in contact with them. The disease then moves up the roots of those plants and crosses over to others.
Even if knocked down by methyl bromide fumigation, the disease leaves behind enough sclerotia to re-ignite an infection.
“You have to treat it repeatedly after every crop once you have a high population, but with germination stimulants we might be able to retreat periodically and at least start each crop with a lower population.”
The odor-producing compound, diallyl disulfide (DADS), a petroleum derivative, stimulates the fungus to germinate, and in the absence of alliums as a food source, it cannot reproduce.
The material is being developed commercially by United Agri Products in the United States and has been released in New Zealand. Crowe said an application, made with shank injection, costs about $300 per acre.
Crowe added that the concept was dormant for about 10 years because of regulatory questions about the compound. In 1996 EPA deregulated the garlic products from its oversight and research resumed.
“The petroleum products did so well in early trials that in recent years we have looked at natural products,” he said.
Crowe and his colleagues used high-grade garlic powders as a slow-release form of the stimulant in various plot trials in naturally infested fields in Walla Walla, Wash.; Madras, Ore.; Bakersfield; and Hollister. The materials were sprayed on the soil and tilled in.
The trials with the natural products performed on a par with those of the petroleum material for 97 to 98 percent reduction in the fungus.
The powder has to be distributed through the soil profile where white rot fungus may reside. “You have to treat down to the depth of the plow layer or the depth you may have moved sclerotia,” Crowe said.
Crowe has also worked on trials with fungicides, both in-furrow and seed treatment, for the disease. Garlic data is more advanced than that of onions, and he said he was excited about the amount of control from the products, even in higher infestations.
However, the extended season of the crops, 10 to 11 months, requires residual control that few materials provide. Other concerns are the costs of treatments and the possibility of registrations of fungicides being lost in FQPA review.
Michael Davis, plant pathologist at UC, Davis, reported that his trials on the bacterial onion disease Pseudomonas cepacia, or sour skin, showed a link between irrigation and incidence of the disease in trials at Five Points.
He found that the disease was more prevalent in plots irrigated by overhead sprinklers than those irrigated all season by furrow or those first irrigated by sprinklers until bulbing (or when the bulb is twice the diameter of the neck) and afterward by furrow.
In addition to natural infestation, he inoculated plots with dried tissue of infected onion and with sprays of the bacterium over the growing plants.
“Where we used furrow irrigation, only a trace of sour skin developed. Where we used sprinklers all season, almost one-third of the onions developed sour skin symptoms,” he said.
Although some controversy persists about whether thrips are truly harmful to onions and garlic, Rich Coviello, Fresno County farm advisor, said they are hard to control deep into the necks of plants.
“It's hard to get materials to them and we are losing the products that had a lot of fuming action to take them out. Some of the new material coming on don't have this fuming action.”
In his 1994 and 1995 trials at Five Points, he found that at about 500 cumulative thrips-days (CTDs) significant losses started to occur on Southport White Globe onions.
CTDs are calculated by averaging the number of thrips per plant on two adjacent sample dates, multiplying the average by the number of days between samples, and adding that to previous sample interval's CTDs.
“That's the level you have to watch in terms of timing treatment. A CTD would be the equivalent of one thrips per plant for one day, 50 thrips per plant for 10 days, or five thrips per plant for 100 days.”
Coviello said sucking thrips affect both yield and percent of undersized bulbs. He found that onion thrips were the predominant species, although western flower thrips were the majority early in the trials.
In offering some tips to avoid seedling diseases, Joe Nunez said conditions at planting are critical to promote rapid seedling germination and seedling growth.
“Plants can generally outgrow seedling disease problems when conditions favor rapid germination and growth. Garlic and onions, however, tend to be slow growing plants, and a considerable amount of their early growth is during the colder time of year.”
Among the precautions are proper drainage, breaking compaction layers, and turning under previous crop residue.
For the San Joaquin Valley, the two major sources of seedling disease are pythium species and Rhizoctonia solani, he said.
Pythium symptoms are water soaked seed or seedlings, or yellowed plants having root rot. Symptoms of rhizoctonia can be very similar, but the root may be more discolored than in a pythium infection.
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