Organic fungicides show promise for wire stem control in broccoli

Wire stem is an occurring disease problem for broccoli and other leafy vegetable crops grown in the Southeast. In addition to conventional fungicides, researchers at Clemson University's Coastal Research and Education Center in Charleston, S.C., are testing some promising naturally occurring biological fungicides to control the disease.

Wire stem gets its name from symptoms that occur on the stem at the soil level. A dark, water-soaked lesion initially appears on the stem. Later stems become wiry and slender at the point of the lesion. Diseased plants grow poorly, are stunted, and may eventually die, especially if there is inadequate moisture shortly after transplanting.

If infected plants remain alive, the stem becomes tough and woody; plants that survive usually mature late and fail to produce a marketable head.

The disease is caused by the soil fungi Rhizoctonia solani. Similar root-damaging diseases occur in tobacco, peanuts and most other agronomic crops. The causal fungi commonly occur throughout the Southeast.

Though cover crops are generally beneficial, building up organic matter in the soil and to vegetable production, green manure cover crops may indirectly contribute to problems with Rhizoctonia-related diseases.

These fungi are very adept at colonizing green organic matter.

To avoid an increased risk of wire stem, growers should leave plenty of time between cutting the cover crop and planting vegetables, allowing the green cover crop to begin to decay.

Clemson University Plant Pathologist Tony Keinath says Rhizoctonia is common, but in test plots his research team added innoculum to be certain to get high levels of the disease-causing fungi in his tests.

In addition to looking at commonly used fungicides for the treatment of wire-stem in broccoli, they looked at two unnamed, naturally occurring biological fungicides. Compared to untreated test plots in the field, the biologicals are doing a good job of reducing wire-stem damage, according to Keinath.

In broccoli, Keinath says the first symptoms of wire stem are a noticeable blue coloration of the plant, followed by severe wilting, stunting, and usually death of the plant. Once symptoms are evident in a field there is little that can be done to arrest spread of the disease.

“The good news, Keinath says, is that there are at least two fungicides that do a good job of preventing the disease. Amistar, which is a granular strobilurin and PCNB both do a good job on the disease,” he notes.

One of the biologicals being compared in the test is a non-pathogenic Rhizoctonia fungus, which is an un-named experimental product at this time. The second biological goes by the tradename BioYield, which is a liquid concentrated beneficial bacteria made from Bacillus bacterium.

In addition to chemical treatment to manage wire stem, several cultural practices are commonly used to reduce the risk of the disease occurring.

These include:

  • Choose a fertile, well-drained site for the plant bed. Poor drainage favors seedling disease.

  • Avoid excessive amounts of nitrogen fertilizer. Succulent plants are more susceptible to infection.

  • If seeding plants, seed when the soil temperature reaches 69°F (21°C) and seed as shallowly as possible so that germination and emergence are rapid.

  • Discard transplant seedlings that show symptoms of wire stem.

  • Avoid banking or throwing soil up around plants during field cultivation.

  • Harvest early.

In the South Carolina tests, which are being conducted at Clemson University's Coastal Research and Education Center in Charleston, the biological fungicides showed 14 percent to 15 percent infected plants, compared to 1 percent for the conventional fungicides at two weeks post transplant. Untreated plots showed 40 percent of plants infected with wire stem.

Though control of wire stem may not be as good with the biologicals as with conventional fungicides the one-time cost may be lower. However, a potentially bigger benefit from the biologicals is in promoting more vigorous plant growth. Looking at the side-by-side comparisons, it appears the two biological treatments look bigger and greener than plants treated with conventional fungicides.

This first year of testing of the biologicals will be followed by spring and fall testing in 2008. This is part of a four-state test that includes two sites in Tennessee, and one each in Kentucky and Arkansas.

Keinath says wire stem is a bigger problem on heavy soils. In general, he says growers on the sandier, coastal soils do not have a problem with the disease, he adds.

South Carolina does not produce as many fall crop vegetables, in particular broccoli, as it could produce, Keinath contends. The high cost of new production in the state and throughout the U.S. has kept U.S. vegetable production flat over the past few years. Broccoli, for example, reached a high of 8 million hundredweights produced in the late 1990s, but has remained steady at near 7.5 million hundredweights over the past three years.

In 2007, over $32 billion in fruit and vegetable products have been imported into the U.S., indicating a significant demand for these products. The cost of U.S. fruit and vegetable products has risen 3 percent to 4 percent per year over the past few years, roughly twice the growth rate of other food products.

Hopefully, research like the broccoli work being conducted by Keinath in South Carolina will provide U.S. growers with more cost-efficient means of growing vegetable crops to allow domestic production to more efficiently compete cost-wise with imported products.

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