Volume 6, No. 2 , April 2003

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Index


Dear Reader,

Corn, cotton, and peanut planting will likely begin this month; soybeans will be close behind. Due to the wet soil conditions last fall, less wheat was planted. Some of that which was planted was written off due to poor growth. This means more full-season soybeans for Virginia. I generally suggest waiting until after the middle of May to plant due to cool soils early in the month. However, with more acreage and the need to get everything planted in a timely manner, there may be the urge to begin planting early. Some growers are even thinking about April plantings.

Early plantings can be successful, but seedling diseases will be more of a problem due to cooler soils. In addition, seed quality is not as good in the past due to the widespread phomopsis seed decay and purple seed stain problems from last year. Therefore fungicide seed treatments must be considered.

I'll devote most of this issue reviewing seedling diseases and fungicide seed treatments. Some seed will come pre-treated; but, in other cases a hopper/planter-box treatment may be desired. Several trade names will be discussed, but others not mentioned in this issue that have the same active ingredient may also be available. Therefore, emphasis should be placed on the active ingredient.

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Seed Quality in 2003 Still a Major Concern

Back in February, I attended a meeting of the Virginia Certified Seed Board. The meeting was called to review the status of peanut and soybean seed being submitted to the Virginia Seed Lab. Seed quality of soybeans in particular was of poorer quality than normal due to last fall's wet harvest conditions. The wet conditions led to higher incidence of Phomopsis seed decay and purple seed stain. Details on both diseases can be found in Virginia Soybean Update Vol. 5, No. 7.

Although seed lots with heavy infestations and low germination will not be certified and/or sold, seed quality and the resulting germination percentages will likely be lower than in the past. Furthermore, disease could still be present on the seed.

Germination percentages coming out of the seed labs in late January were a little unsettling. At that time, more that 50% of seed coming into North Carolina's seed lab had germination percentages of less than 80%. Maryland's lab was reporting that nearly 1/3 of soybean seed coming through their lab was less than 80% germination. Virginia seed was looking better (although not good), as only 17% of the samples were testing less than 80% germination. However, relatively few samples had been submitted to Virginia's lab at that time. By mid-March, things were looking worse. Only 43% of the soybean seed samples (869 samples) tested 80% germination or above. Another 18% was testing 70-79% germination. This leaves 39% with less than 70% germination.

The most obvious consequence of the above results is a short seed supply for maturity groups 4, 5, and 6. Normally, we would be able to obtain seed from the southern Mid-west or Delta states, but these areas had some of the same problems. This does not mean that there will not be enough seed for planting. However, obtaining extra seed for re-planting could be a problem. Therefore it is very important to get a good stand the first time through. Properly setting your planter or drill is probably the first and most important step. But almost equally important will be seedling diseases. Phomopsis seed decay and purple seed stain is a concern, but other seedling diseases common to Virginia need to be considered as well.

The best way to avoid seedling disease problems is to plant the seed in warm soils (>65O F) at the correct depth (0.75 to 1.25") to insure rapid emergence. Usually waiting until after May 15 will reduce the risk of poor emergence due to cool soils. However, cold fronts are common in late May, therefore the risk remains. Fungicide seed treatments are good insurance, especially when seed quality is questionable. But heed the following warning: seed treatments will not bring a dead seed back to life. Fungicides are no substitution for good seed quality. However, fungicide seed treatments can be beneficial, especially if planting into cool soils.

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Common Seedling Diseases

Poor emergence and/or death of seedlings are often caused by seed and seedling diseases. Various fungi and bacteria that cause these diseases reside in seed, soil, and crop residues. Due to the wet fall, many seed were infected with Phomopsis longicola (causing seed decay) and Cercospora kikuchii (purple seed stain). These infected seed have potential for lowering germination and emergence. Other fungi such as Fusarium, Pythium, Rhizoctonia, and Phytophthora reside in soil and crop residues. Below is a brief review of some common seedling diseases of Virginia.

Phomopsis seed decay: Seed infection by Phomopsis is generally minimal before the seed reach maturity. The disease can develop rapidly when mature soybeans are left in the field in warm, wet weather and harvest is delayed. As a result, this problem is more commonly seen in early-maturing varieties. Infected seed have a white, chalky appearance and are shriveled, elongated, and cracked. Still, some infected seed may not show symptoms. Storage reduces infection, but only after 1 year of storage will seed be free of disease. Seedborne Phomopsis can lower germination by killing young embryos during water imbibition and germination. Partially infected seed may also die before emergence (preemergence damping off). Plants that do emerge after planting may die from postemergence damping off or show poor vigor and stunting.

Purple seed stain: Seed discoloration ranges from pink or pale purple to dark purple and occurs as specks to large blotches that may cover the entire seed surface. Heavily infected seed can produce diseased seedlings and reduce stand. Cotyledons often shrivel, turn dark purple, and fall prematurely. Infection may spread from the cotyledon to the stem, producing necrotic areas that girdle the stem and kill young plants. Less severely affected stems survive, but are stunted. In humid and warm weather, the fungus produces a velvety, grayish white mold on infected cotyledons and stems of seedlings.

Fusarium/Pythium Complex: Fusarium and Pythium are soil borne fungi that cause pre-emergence and post-emergence damping off diseases. Both organisms appear to be responsible "Essex Syndrome", a disease complex that causes reduced stands in scattered fields in counties across the Middle Peninsula and Northern Neck of Virginia. Crop rotation and sanitation practices such as disking and/or plowing may reduce the severity of problem. Seed treatment fungicides are only partially effective.

Pythium damping off: This disease is often referred to as the Pythium complex because at least six species of Pythium exist. The fungi can attack soybean at various growth stages, but it is of primary importance to the emerging seedling, causing pre- and postemergence damping off. If infected before emergence, seedlings have a short, discolored root. After emergence, the plant can also be infected. In a few days after symptoms appear, seedlings collapse and have a rotted appearance. Symptoms are very similar to Fusarium. Although this disease is widespread in many Midwestern states, it has not been a common problem in Virginia.

Rhizoctonia damping off: Rhizoctonia is probably the most widespread seedling disease of soybean in Virginia. Preemergence damping off symptoms are very typical of this seed rot, but usually go unnoticed. More common is infection of seedlings before the first trifoliate leaf develops. Reddish brown, sunken lesions appear on the hypocotyls at the soil line. Infected seedlings may or may not die. Dead plants will have a shrunken appearance. Surviving plants can be stunted and yellow and have poor root systems. Diseased plants will first appear where water and nutrients are limited. Patches within the field are sometimes elongated, following the path of tillage.

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Seed Treatment Fungicides for Soybean

Seed treatment fungicides are not usually needed in Virginia when high-quality seed are planted into warm (>65O F) soils at the correct depth. Warm, moist soils promote rapid germination and emergence, therefore the disease does not become well established during the vulnerable preemergence stage. Warm conditions are present after wheat harvest, so there is little need for seed treatments for soybeans being double cropped. On the other hand, planting soybean into cool soils in April or May does entail some risk. In addition, no-till fields generally have cooler soils. Under these circumstances, seed treatments should be considered.

Keep in mind that fungicides will not improve on inherently poor seed quality (not due to infestation with fungi). They will however improve germination where seed quality is low due to fungal infection and where seeds must be planted into cool soils that delay emergence and increase likelihood of fungal infection. If low seeding rates are used (limited seed supply, expensive seed, etc.), then I would also suggest a seed treatment. In a year like 2003 when seed supplies are low and levels of infection with Phomopsis and purple seed stain are high, fungicide seed treatments may be valuable.

Seed treatments are available as dusts, flowable suspensions, liquids, or wettable powders. Some products are available for commercial use in slurry- or mist-type applicators. Others are labeled for on-farm use and are usually referred to as hopper-box or planter-box treatments. Regardless of type of application, good coverage is essential for consistent control of disease. Commercial seed treatment equipment provides the best coverage. Many varieties will come pre-treated this year. However, if seed are pretreated, they must be planted; they cannot enter into the food, feed, or oil market. For good coverage with on-farm application, add _ of the seed to the hopper add _ of the fungicide and mix thoroughly, then the other _ of the seed, and finally mix in the other _ of the fungicide. Always read and follow label directions, and wear personal protection equipment when making this or any chemical application.

Fungicide seed treatments can be divided into contacts or systemics. Contact fungicides protect only the seed surface, while systemics are absorbed by the emerging seedling and inhibit or kill the fungus inside the soybean plant. Systemics usually have a longer residual than contacts. Examples of contact fungicides are: captan, fludioxonil, PCNB, and thiram. Systemic examples include: azoxystrobin, carboxin, mefenoxam, metalaxyl, and thiabendazole.

These products can also be divided into fungicides that are effective against the water mold fungi, Pythium and Phytopthora or those effective against fungi other than water molds (Rhizoctonia, Fusarium, Cercospora, and other soil- or seed-borne fungi. This classification is important since the only fungicides effective against Pythium and Phytopthora (damping off only; not seedling root and stem rot) are those containing metalaxyl and mefenoxam. These fungicides do not, however, control the other soil- and seed-borne fungi.

From the standpoint of Phomopsis, there is less information regarding the effectiveness of specific compounds. However, products containing thiram (various products) or PCNB and thiabendazole (Rival) provide the best control. This is not to say that other active ingredients are not effective; they are. There is probably not enough difference between the active ingredients to justify choosing a more expensive product over a less expensive one (i.e., captan or thiram).

In general, the most important seedling diseases in Virginia will be Phomopsis and Rhizoctonia. For these diseases, products that are effective on these fungi will be most useful. It is not clear whether or not products that also control Pythium are needed (except in the cases of "Essex syndrome").

A quick word about nitrogen-fixing bacteria (Bradyrhizobia) is warranted. Some fungicides will injure and kill these bacteria. Captan and PCNB, in particular, will reduce bacteria populations. If inoculating with nitrogen-fixing bacteria, better choices are thiram, carboxin, or azoxystrobin. Metalaxyl or mefenoxam are also compatible with the bacteria. Regardless of fungicide, always apply inoculants last and right before seeding.

Finally, I need to remind you that anything that reduces the speed of emergence (rain, low temperatures, soil compaction, crusting, planting dept, herbicide injury) increases the risk of seedling disease. Growers need to carefully monitor seeding depth (0.75 to 1.25") and be mindful of changes in different soil types. Maintaining residue via no or reduced tillage will reduce crusting. Fungicide seed treatments are not a cure for poor agronomic practices.

The following table lists common fungicide seed treatments. The list is probably not complete from the trade name standpoint, but is basically complete from the active ingredient standpoint. Keep in mind that our main concern in Virginia is Rhizoctonia. Phomopsis and purple seed stain are particular concerns for this year. Unfortunately, there is little information on the effectiveness of these fungicides on Cercospora, which causes purple seed stain. However, it is generally believed that the non-water mold fungicides have some activity.

      Application Method Disease Listed on Labela
Trade Name Manufacturer Active Ingredient Commercial Treater On-farmb Pythium Phytophthora Rhizoctonia Phomopsis Fusarium
Allegiance Dry Trace Chemicals Metalaxyl - + + + - - -
Allegiance-FL Gustafson Metalaxyl + - + + - - -
Apron Flowable Wilbur-Ellis Metalaxyl + - + + - - -
ApronMaxx RTA Syngenta Mefenoxam Fludioxonil - + + + + + +
Bean Guard Trace Chemicals Captan Carboxinc - + - - + + +
Bean Guard Allegiance Trace Chemicals Captan Carboxin Metalaxylc - + + + + + +
Enhance Vitavax-Captan 20-20 Trace Chemicals Captan Carboxin - + - - + + +
Hi Moly / Captan Trace Chemicals Captanc - + - - + + +
Maxim XL Syngenta Fludioxonil Mefenoxam + - + + + + +
Prevail Trace Chemicals LLC Carboxin PCNB Metalaxyl - + + + + + +
Protector-D Trace Chemicals Thiramc - + - - + + +
Protector-L Trace Chemicals Thiramc - + - - + + +
Protector-L Allegiance Trace Chemicals Thiram Metalaxylc - + + + + + +
Rival Flowable Gustafson Captan PCNB thiabendazole + - - - + + +
RTU-Vitavax-Thiram Gustafson Carboxin Thiram - + - - + + +
Stiletto Trace Chemicals Carboxin Thriam Metalaxyl - + + + + + +
SoyGard Gustafson Azoxystrobin Metalaxyl + - + + + + +
Vitavax CT Helena Carboxin Thiram - + - - + + +
Vitavax M Helena Carboxin Thiramc - + - - + + +
Vitavax T-L Trace Chemicals Carboxin Thiram - + - - + + +
Vitavax- PCNB Gustafson Carboxin PCNB + - - - + + +
Vitavax- 200 Gustafson Carboxin Thiram + - - - + + +
Vitavax-34 Gustafson Carboxin + - - - + - -
aa (+) = disease listed on label; (-) = disease not listed on label.
bb Labels lists as on-farm, but not necessarily "hopper-box" or "planter-box" application. In some cases, it is only labeled for an on-farm mechanical treater, in order to maximize coverage. Check the label for details regarding application.
c Contains molybdenum, a micronutrient sometimes deficient in acidic soils.

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Sincerely,

David L. Holshouser


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