Soybean Update

Volume 8, No. 5, June 2005

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Index

Dear Reader
Soybean Rust Control Tips
Thrips in Soybeans: Should You Spray?
Manganese Deficiencies
Q&A: What fungicide should I buy now to prepare for soybean rust?
Soybean Aphid and Rust Scouting in Full Swing

Dear Reader,

It seems that everyone that I speak to wants to know the latest on soybean rust. So once again, we're devoting a significant portion of this month's newsletter to the topic. The most recent question concerns the effect of Tropical Storm Arlene. A tropical storm such as this would create ideal conditions for spread of the disease. But, the question remains, is there enough inoculum in Florida (the field in Georgia has been destroyed and there have been no other detections in that state) to begin an epidemic and a movement north. The question will be answered in a couple of weeks. If rust doesn't show up within the next 2-3 weeks in the storm's path, then the answer would seem to be no.

We've begun our soybean aphid and rust monitoring program again in 2005. So far, NO RUST OR APHIDS. We have found quite a bit of brown spot and some bacterial blight. We'll continue to check and let you know what we're finding.

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Soybean Rust Control Tips

Dr. Pat Phipps, Extension Plant Pathologist

Virginia is participating in a nationwide soybean rust monitoring, tracking, forecasting and scouting program in 2005. This program has already confirmed the presence of soybean rust on kudzu in Florida and volunteer soybeans in Georgia. No reports of soybean rust in commercial plantings of soybeans have been confirmed as of 10 June 2005 in the U.S. The locations of counties where the soybean rust has been confirmed are continuously being updated on a map of the U.S. at the USDA Soybean Rust Information Site (www.sbrusa.net). This web site also provides state-specific scouting recommendations and disease management information. Virginia-specific disease forecasts, tracking data, and fungicide recommendations will be issued on the Asian Soybean Rust home page for Virginia at (www.ppws.vt.edu/ipm/soybeanrust/index.htm). Virginia will offer a Soybean Rust Hotline for delivering status reports on soybean rust and disease alerts during the 2005 season. To access this hotline, dial 757-657-6450 ext. 103.

Several pieces of information should be considered for optimum timing of fungicide sprays for control of soybean rust. All factors need to be considered when making the decision to apply or not apply a fungicide spray. For growers in Virginia, this information could pay great dividends in saving fungicide costs and/or heavy yield losses to disease.

1. Disease Forecasts: Soybean rust spreads by windborne spores from its overwintering areas in the Deep South (South Florida, Caribbean, etc.) to the north where the fungus is unable to overwinter. The numbers of spores to become windborne over the past 30 days is thought to be very low because the fungus has not reached epidemic proportions in commercial soybean, kudzu, or other hosts in the Deep South. While soybean rust has been slow to develop so far, it is a well-known fact that the disease can appear suddenly, spread at alarming rates and move great distances when environmental conditions are ideal for wind dissemination of spores. Disease forecasts are useful in anticipating windborne spore movement from areas with ongoing epidemics of soybean rust and deposition of spores in areas where the disease has yet to be detected.

2. Disease Tracking: As the disease begins to move northward in soybeans, we must monitor reports of disease outbreaks as it moves closer to Virginia. Of great importance are disease severity and spore production on soybeans and other hosts. Certainly, we should be in full-alert status when the disease reaches North Carolina and/or Tennessee

3. Sentinel Plots: These plots have been planted at multiple locations in our soybean production area for early detection of when the fungus has reached Virginia. These plots were planted to early maturing varieties about 2 to 3 weeks prior commercial soybean. This means that sentinel plots will produce a canopy of foliage and become vulnerable to infection earlier than commercial soybeans. This occurs when plants begin flowering and proceed through the stages of pod and seed development. As a result, sentinel plots are expected to be the first to exhibit disease, which provides an early warning before commercial fields begin to show the disease. Sentinel plots must be scouted at least weekly for early detection of disease. Once disease appears in sentinel plots, they should be sprayed with a triazole fungicide or mixture of triazole/strobilurin fungicides in order to minimize opportunities for secondary spread of the disease to neighboring fields.

4. Scouting: Fields need to be scouted at least once a week from the start of flowering (R1) up to full seed (R6). The intensity of scouting should be increased as the disease moves closer and especially after disease forecasts for movement of windborne spores into Virginia. Scout fields in a zigzag pattern and use different entry and exit points on each visit. Priority should be given to areas likely to have extended periods of leaf wetness due to poor air drainage as in low lying areas, and to locations with heavy plant growth and a dense canopy. Attention should be given to checking the leaves that are midway or lower on plants. This is where the disease is most likely to develop first. Soybean rust begins by causing small yellow pinpoint spots that turn tan to brown or red brown with age. Lesions may be angular or circular and cluster near veins. Rust spores are produced in pustules that are powdery and tan or pale brown. Until soybean rust first appears within a county of Virginia, all suspect samples should be brought to a plant pathologist for confirmation of the disease. Soybean rust is very difficult to distinguish from other common diseases of soybean (brown spot, bacterial pustule, etc.) even with a hand lens in the field. Virginia Tech's Dept. of Plant Pathology has equipped Plant Disease Diagnostic Clinics at field stations with the resources for making gaining an accurate identification of the disease. A fungicide spray is recommended wherever soybean rust is present at barely detectable levels and up to levels of 10% of leaflets infected in the lower canopy. After disease incidence exceeds above 10% of leaflets infected in the mid-canopy, fungicide sprays may not be profitable or effective in disease control according to reports from Brazil.

5. Plant Growth Stage: Until research justifies otherwise, we are recommending sprays of fungicides be made for control of soybean rust only during the period from flowering (R1) until full seed (R6). Overall, it seems likely that fungicides may be the most profitable when applied in the period from flowering (R1) to beginning seed (R5). Thereafter, the level of profitability is expected to gradually decline. No fungicide sprays are expected to be profitable if applied prior to flowering (R1) or after full seed (R6).

The above items are foremost in determining the risk for disease outbreaks and when fungicides are likely to be profitable for control of soybean rust. To avoid costly mistakes, growers need to be aware of the situation in the five categories above. This means checking the Soybean Rust Home Page for Virginia and/or calling the Hotline at least twice weekly for updates on disease forecasts, tracking reports, and sentinel plot information. Growers must review scouting information from their fields and reports posted on the Soybean Rust Home Page for Virginia and/or Hotline to know the status of soybean rust and other diseases in soybeans. And finally, the growth stage of soybeans must be known to avoid spraying at times when fungicides are not likely to be profitable. Collectively, plant pathologists will use this information in formulating disease control strategies involving the use of fungicides. Other factors may be considered depending upon the field situation such as weather conditions, plant growth, canopy development, yield potential, the presence of other diseases, etc.

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Thrips in Soybean: Should You Spray?

Dr. Ames Herbert, Extension Entomologist

Several thrips species are feeding on soybean seedlings throughout much of Virginia causing some leaf striping and leaf curl. Insecticide sprays will definitely green up thrips infested fields but there is little evidence that thrips do economic damage to soybean. We tried for several years to document any impact on yield with field tests on a number of growers' farms in several counties. Although we found that several products are effective in killing thrips, we could never show a yield advantage. This has a lot to do with where thrips feed on a plant. In cotton and peanuts, thrips feed on the plant's growing points or buds. Damage to these plant tissues causes seedling deformity, slows maturity, alters the plant canopy later in the season, and destroys embryonic reproductive tissues all of which can result in yield reduction. This is not the case in soybean. In soybean, thrips mostly feed on leaves, and although leaves may be damaged, many more will form before plants begin to develop reproductive structures (flowers and pods). Plants have plenty of time to produce the canopy needed to reach yield potential. The only case where thrips can reduce soybean yield is if plants go through a sustained period of drought stress during or just after thrips damage has occurred. Again, we have not been able to show any yield advantage from treating thrips in soybean.

Editor's Note: Thrips have been suspected in transferring various viruses that attack soybean. However, evidence for such transfer is very, very limited and usually circumstantial. We (Dr. Herbert, Sue Tolin, and I sampled many fields a few years ago and rarely detected any viral diseases. It is our opinion that thrips are not a significant vector of viral diseases and those viral diseases are not a big issue in Virginia. So, controlling thrips to control viruses is cannot be justified in most cases. Many varieties are resistant to some viruses; therefore, soybean breeding and variety selection is the best means to control viral diseases.

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Manganese Deficiencies

Manganese (Mn) deficiencies are common in Virginia soybeans. These deficiencies are not necessarily due to low Mn levels in the soil. Instead, like many micronutrients (nutrients that are needed by the plant in small amounts), Mn availability to the soybean crop is directly related to soil pH.

When pH levels reach 6.5 or above, Mn deficiencies will likely appear, especially on sandy soils. On some soils, Mn deficiencies will occur on soils with pH levels as low as 6.2. In addition, the saturated soil conditions can cause soil pH levels to remain high. However, upon drying, pH in these soils will fall to normal levels rather quickly. Generally Mn deficiencies are more common on our wetter soils.

Use the following guidelines for Mn applications:

Scout your fields. Mn deficiencies may or not materialize. The only sure way to determine a deficiency is to observe the deficiency symptoms. The characteristic symptom is yellowing between the veins on new leaves. Mn is an immobile nutrient. Therefore, it will not move out of older leaves to the new leaves. Symptoms will appear when the plant can no longer extract sufficient amounts of the nutrient from the soil. The typical symptom is shown above.
If Mn deficiencies are suspected due to high pH and no symptoms have yet appeared, you may want to consider taking a tissue sample. This can reveal deficiencies before symptoms appear. I would suggest this especially if lime was recently applied.
To overcome a deficiency, apply 3/4 lb chelated Mn (elemental basis) or 1 lb inorganic Mn (elemental basis) per acre to foliage upon appearance of symptoms and prior to flowering. More than one application may be required to correct a severe deficiency.
Note that many Mn products recommend applying lower rates of Mn. However, the label usually states that these are maintenance rates. Once a deficiency occurs, these lower rates will not correct the deficiency and the rates stated above will be needed.
An alternate method of application can be used. Before a deficiency is evident an on soils that commonly show a deficiency, especially on soils recently limed, a lower rate (~ 1/2 of that shown above) can be combined with another scheduled application. Such an application may include a postemergence herbicide. This may be a sufficient rate to prevent a deficiency from occurring. But, continue to scout the field and take future corrective measures if deficiencies reappear. Symptoms like that shown on the previous page will still require the full rate. I will remind you that this is a preventative treatment. A deficiency may not occur. Furthermore, these are only maintenance rates and another application will likely be needed if the field is truly deficient.
Be reminded that some Mn formulations in combination with glyphosate herbicide (Roundup, Touchdown, many generics, etc.) will result in reduced weed control of certain weeds. Other herbicides have not shown to interact. If including Mn with glyphosate, use the EDTA chelated formulation as it has shown not to interact.

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Q&A: What fungicide should I buy now to prepare for soybean rust?

This is a difficult question because it requires one to guess the following: 1) What diseases are present in the field, 2) When do the diseases reach a threshold for treatment, and 3) Does soybean rust appear before or after common soybean diseases? Each of these questions would have significant impact on the answer. Remember that the Section 18 approval only allows use of approved fungicides (mostly triazoles) AFTER soybean rust is confirmed in the state on soybeans. So to use a Section 18, you must wait until rust is confirmed in the state. And, you would want to use a Section 18 product since the triazoles are more effective in controlling rust after the disease has infected the crop. On the other hand, frogeye leaf spot, brown spot, anthracnose, and Phomopsis at levels seen in 2004 would best be treated with a strobilurin such as Quadris or Headline. If and when soybean rust appears the triazole fungicides are probably the best choice, or a combination product containing a triazole and a strobilurin. The combination products may cost more, but could be most effective where common disease are above threshold and rust is also a problem. Strobilurins are most effective on the common foliar diseases of soybean and are also effective as a preventative (before disease arrival) rust spray. Again, after rust is detected in your or your neighbor's fields, a triazole must be used.

- Pat Phipps, Extension Plant Pathologist

Fungicides approved for control of Asian Soybean Rust in Virginia

Preventative or "Curative" * Product Class Active Ingredient Rate/Acre Registration

Preventative Bravo/Echo Nitrile Chlorothalonil 16-36 fl oz Full label

Preventative Quadris Strobilurin Azoxystrobin 6.2-15.4 fl oz Full label

Preventative Headline Strobilurin Pyraclostrobin 6-12 fl oz Full label

Curative Tilt/PropiMax/Bumper Triazole Propiconazole 4-8 fl oz Section 18

Curative Domark Triazole Tetraconazole 10-13 fl oz Section 18

Curative Laredo Triazole Myclobutanil 4-8 fl oz Section 18

Curative Folicur Triazole Tebuconazole 3-4 fl oz Section 18

Curative Orius Triazole tebuconazole 3-4 fl oz Section 18

Curative Headline SBR Strobilurin + triazole Pyraclostrobin + tebuconazole 7.8 fl oz Section 18

Curative Quilt Strobilurin + triazole Azoxystrobin + propiconazole 14-20 fl oz Section 18

Curative Stratego Strobilurin + triazole Trifloxystrobin + propiconazole 5.5-10 fl oz Section 18

*Preventative fungicides must be applied before arrival of rust into the local area. Curative products must be applied before incidence exceeds 10% (very early detection).

Preventative or "Curative" *	Product	Class	Active Ingredient	Rate/Acre	Registration
Preventative	Bravo/Echo	Nitrile	Chlorothalonil	16-36 fl oz	Full label
Preventative	Quadris	Strobilurin	Azoxystrobin	6.2-15.4 fl oz	Full label
Preventative	Headline	Strobilurin	Pyraclostrobin	6-12 fl oz	Full label
Curative	Tilt/PropiMax/Bumper	Triazole	Propiconazole	4-8 fl oz	Section 18
Curative	Domark	Triazole	Tetraconazole	10-13 fl oz	Section 18
Curative	Laredo	Triazole	Myclobutanil	4-8 fl oz	Section 18
Curative	Folicur	Triazole	Tebuconazole	3-4 fl oz	Section 18
Curative	Orius	Triazole	tebuconazole	3-4 fl oz	Section 18
Curative	Headline SBR	Strobilurin + triazole	Pyraclostrobin + tebuconazole	7.8 fl oz	Section 18
Curative	Quilt	Strobilurin + triazole	Azoxystrobin + propiconazole	14-20 fl oz	Section 18
Curative	Stratego	Strobilurin + triazole	Trifloxystrobin + propiconazole	5.5-10 fl oz	Section 18

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Soybean Aphid and Rust Scouting in Full Swing

We now have three scouts combing nearly 100 soybean fields in the Commonwealth for soybean aphid and rust. Two are working out of the Tidewater AREC and another out of the Eastern Shore AREC. In this second year of this program, we're doubling our efforts to include more soybean growing regions and we're scouting on a weekly basis. Below is a map of most of the fields that we're scouting. Yes a couple of the sites seem to be in the Bay; evidently, the GPS unit was not exactly accurate that day. We'll get another reading this week. But as you can see, we should be able to detect soybean rust as it moves in and provide up-to-date recommendations. Check on the web site (www.ppws.vt.edu/ipm/soybeanrust/index.htm) or the hot line (757-657-6450 ext. 103) often.

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Sincerely,

David L. Holshouser

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