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Confronting a new fungal nightmare.

Confronting a New Fungal Nightmare

On an August day in Virginia in 1971, peanut stems near moist soil were discovered to be covered with what looked like cotton.

The plants soon began to wilt and brown. A few days later, the cotton, which was really the vegetative stage of a fungus, gave way to small black sclerotia--bodies that looked like mouse droppings.

What U.S. peanut farmers were seeing for the first time in 1971 was sclerotinia blight, destined to become their worst nightmare. It took two decades, but the blight caused by the soilborne fungus Sclerotinia minor has become a major problem--not only in Virginia but in the Southern Plains a thousand miles away where one-fifth of the nation's 1.6-million-acre peanut crop is grown. Today the fungus also attacks lettuce growing in New Jersey, New York, and California.

In recent years, about a third of Oklahoma's 105,000 peanut acres and more than 20,000 acres in Texas have been infested by Sclerotinia. Some fields have suffered yield reductions of as much as 25 percent, says Hassan A. Melouk, an Agricultural Research Service plant pathologist at Stillwater, Oklahoma.

There are at least a half dozen possible ways in which Sclerotinia blight spreads; the planting of infected seed is only one. The pathogen, which survives in soil as long as 7 years, also lurks in dead plant material, live weed hosts, farm equipment, ruminants, and even living birds.

Sclerotinia blight is surely frustrating to Oklahoma farmers who have been permitted, up till now, to use only one fungicide. Each of two or three applications in a growing season costs nearly $40 per acre.

Despite governmental programs aimed at supply-demand stability, consumers may feel the financial pinch of increased peanut prices in years when the disease contributes to a nationwide drop in yields.

But there is some relief in sight from a new seed treatment that will help in reducing the disease's spread. The treatment consists of a combination of three fungicides that have been registered for other purposes. They are captan, pentacloronitro-benzene, and thiophanate-methyl.

In laboratory studies, the combination can cut the incidence of seed infection by 99.6 percent. Other seed treatments reduced infection by less than 80 percent.

While this chemical combination may help slow the disease spread, the importance of other measures, such as weed control, remains.

What's needed, he believes, is an integrated approach--one that combines disease-resistant varieties, chemicals, and possibly, biological controls.

It should be noted that there may be state and local regulations restricting the use of certain fungicides. Also, because registrations of fungicides are under constant review by the U.S. Environmental Protection Agency, growers should consult their county agricultural agent or state extension specialist to be sure the intended use is still registered.

Spanish peanuts, the small-seeded, round peanuts that are popular cocktail-party fare, differ in more ways than just seed size from runner peanuts.

It's been found that plants bearing Spanish peanuts are generally less susceptible to the Sclerotinia fungus than are the runner types. An especially resistant Spanish variety, Tamspan 90, has been developed and released by Melouk and researchers at Texas A&M University at College Station.

In 3 years of tests in nine Texas counties, Tamspan 90 produced peanuts worth 10.5 percent more than yields from an old standby variety, Starr. Where the Sclerotinia fungus was most prevalent, the new variety produced peanuts with a 48 percent higher value than that of Starr.

In 1990, companies began to increase seeds of Tamspan 90 for peanut producers to plant for the first time this year. A seed producer in west-central Oklahoma's Caddo County, whose seed yields normally range around 3,000 pounds per acre, says Tamspan 90 yielded more than 5,000 pounds per acre in 1990--his best crop in 20 years.

The first resistant varieties of runner-type peanuts may debut soon after the resistant Spanish peanuts; researchers of ARS and Oklahoma State University, Stillwater, have joined in the search. "We've tested several cultivated lines that have looked promising the last couple of years," Melouk says.

Breaking the Cycle of Infection

Some infected peanut seeds appear to be less likely to transmit the fungus to the next generation, an observation that might pay off in terms of new breeding lines. Melouk and two graduate students are researching the mechanism by which the fungus moves from an infected seed to a germinating seedling.

Seed infection, which one cannot detect by simply looking at the seed, becomes evident after the fungus on a seed fragment has been placed, or plated, on a growth-promoting agar medium for 7 to 10 days. To cut the time involved, Melouk is working on an enzyme linked immunosorbent assay (ELISA) that may allow diagnosis of infection in 1 day.

ELISA could speed his search for effective chemical and biological fungicides and for disease-resistant peanut breeding lines. In studies last year, Melouk and Carolyn Bowen, a graduate student, plated and looked at 26,000 seeds.

While no biological control system is yet commercially available for controlling S. minor in peanuts, Melouk and scientists at Oklahoma State, in cooperation with a major agribusiness company, are experimenting with a parasitic fungus, Sporidesmium sclerotivorum. In other studies on lettuce, ARS plant pathologist Peter B. Adams and his colleagues at Beltsville, Maryland, found that as little as 0.2 pounds of S. sclerotivorum per acre can compete with other microbes in the soil to reduce sclerotinia lettuce drop by more than 50 percent.

For now, Melouk sees resistant peanut varieties as a first line of defense against the disease, while he emphasizes the need to minimize presence of fungi in the soil. "With an average of one or two sclerotia per 100 grams of soil, 30 to 50 percent of peanut plants growing in that soil could become infected," he says.

No Easy Solutions

Rotating crops on infected soil will not, by itself, make the sclerotinia blight problem go away. Melouk and his colleagues have found that the fungus survives on weed hosts.

Recently they found viable sclerotia in the stems of both live and dead Eclipta prostrata, a plant in the sunflower family that often infests irrigated peanut fields. The weed may remain as a fungal reservoir on land that has been out of peanut production for several years.

Too much irrigation can also increase fungal infection. S. minor thrives in wet conditions when late summer heat begins to wane.

Here's another no-no for farmers. Don't move cattle from fields where they have grazed infected peanut crop residue to fields that aren't infected. That advice is based on studies conducted by Melouk and scientists of Oklahoma State's departments of plant pathology and animal science. The researchers fed a heifer sclerotia-infested oats and peanut hay for 10 days. Samples of feces and contents of the rumen collected 9 days after the feeding period contained an abundance of sclerotia, and 8 percent were capable of causing blight in peanut plants.

PHOTO : Peanut stems showing typical symptoms of Sclerotinia infection including fluffy white mycelia, black sclerotial bodies, and tan lesions. (K-4291-6) Background photo: Spanish, Virginia, and runner peanuts. (K-4297-4)

PHOTO : Plant pathologist Hassan Melouk examines pod sets of healthy, freshly dug runner (left) and Spanish (right) peanut plants. (K-4293-11)

PHOTO : Seed peanuts. (K-4295-19)

PHOTO : Research assistant Carolyn Bowen evaluates the effectiveness of seed protectants against Sclerotinia minor. (K-4298-9)
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Title Annotation:sclerotinia minor on peanut crops
Author:Hardin, Ben
Publication:Agricultural Research
Date:Nov 1, 1991
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