Stephen J. Toth, Jr., editor
Volume 20, Number 21, September 2, 2005

Field and Forage Crops

From: Jack S. Bacheler, Extension Entomologist

In North Carolina, cotton insects are winding down (or at least their potential for damaging cotton). I typically begin the damaged boll survey around September 1 and try to have it completed by September 15. Although it is not unusual to uncover moderate to high bollworm and/or stink bug damage to bolls that has taken place over the previous two months in some cotton fields, finding significant levels of live caterpillars or stink bugs on cotton from this time on is uncommon.

I had an interesting report of stink bugs this week from Scotland County. The agricultural Extension agent there noticed very high numbers of dead green stink bugs in about the first 50 feet of a cotton field across the road from recently harvested field corn. The grass strips along the road had also just been mowed. If this cotton field had not been a part of a recently sprayed stink bug test, the potential infestation probably would have gone unnoticed. This cotton field still had between 3 and 4 nodes above first position white flowers, and therefore could have sustained significant upper boll damage, at least the area bordering the corn. It might not hurt to check the borders of late cotton fields adjacent to corn fields that have recently been harvested. If a potential problem is noted, a single perimeter spray should do the trick.

With the exception of a few late cotton fields that still merit checking, this insect season is about over. The levels of beet and fall armyworms, soybean loopers, and white flies occurring lately do not appear to be a risk. If regrowth becomes a significant problem in some early opening cotton fields, cotton aphids and their honeydew can occasionally cause headaches for growers. But even very scattered September and October rainfall here is almost always enough to eliminate potential sticky cotton and sooty mold problems.

From: Stephen R. Koenning, Extension Plant Pathologist, and James E. Dunphy, Extension Soybean Agronomist

The soybean rust epidemic that many have been concerned about has not happened yet. Will we see soybean rust in North Carolina in 2005? Seems likely. Will soybean rust cause soybean yield losses? Probably not. These are our current thoughts on the potential for soybean rust in North Carolina.

Reasons why we will see soybean rust:

1. Recent trajectories at the Soybean Rust web site have shown a fair likelihood for transport of soybean rust spores to North Carolina from areas to the south of us, and rust-like spores were found in spore traps in Virginia a few weeks ago.

2. As the weather cools down and most, if not all, of the crop is in the reproductive stage, the chances for infection increase, provided we have extended periods of leaf wetness.

3. Much of our full season crop was planted late or had to be replanted.

4. Typically, 30 to 40 percent of the soybean crop is planted after June 15, and will be in the field later than full season beans.

5. Although the hurricane season has been kind to us thus far, most hurricanes strike North Carolina in September and the forecasters say this will be an exceptional year for hurricanes.

Reasons why soybean rust will have little or no impact on soybean yields:

1. Rust detection in South Carolina is at very low levels and many fields in Georgia have been sprayed with fungicides. For the most part, rust has been found in sentinel plots, and not in commercial soybean fields.

2. Recent weather has simply been too hot for infection to occur in much of the state.

3. The heavy rains in the Mid-South as a result of Hurricane Katrina may well lessen the chances of a soybean rust epidemic, since spores would be washed out with the rain. Soybean fields that have water standing for 48 hours or more will likely defoliate early. Early soybean defoliation and death from flooding will make these plants unsuitable hosts for soybean rust, and these areas will cease to serve as sources of rust spores.

4. Much of the Delta soybean crop is already at harvest maturity and thus ceases to be susceptible.

5. Kudzu and other hosts for soybean rust appear to be much less susceptible than suspected.

6. Even if a few spores of soybean rust have found a suitable environment for infection, it will probably require 3 to 5 weeks before rust is apparent in a field. This time frame puts us at late September or early October. Too late to affect much if any of the crop.

Sentinel plots continue to be monitored weekly and several samples from kudzu, taken in the southern part of the state were taken to the lab for examination. All were negative for Asiatic soybean rust and rust has not been found in North Carolina thus far. Downy mildew has been severe in much of the state, but the impact of fungicides on this disease are not known. Frogeye leaf spot and target spot is quite common in susceptible varieties this year and these diseases warrant fungicide application.

Several reports about detections of Asiatic soybean rust-like spores have appeared in the news media and rumor mills. Due to the low number of spores detected, no one can positively identify these as Asiatic soybean rust. There are hundreds of rust fungi in North America and spores look very similar. Also, it is not possible to evaluate the viability of these spores. We have received reports on spore traps located in North Carolina, and these have been negative thus far. You can get up-to-date information on the distribution of soybean rust on the web (http://www.usda.gov/soybeanrust/), and see what specialists in other states are recommending in terms of whether to spray or not.

Target spot, a fungal disease caused by Corynespora cassicola, has been identified on several samples sent to the Plant Disease and Insect Clinic at North Carolina State University in 2005. In 2004, this disease resulted in early defoliation of certain soybean varieties and resulted in yield losses of as much as 50 percent. While we don't understand why this disease has been a problem in 2004-2005 when we had not seen it since about 1981, it has appeared on soybeans throughout the Southeast this year. At this time we suspect that only a few varieties are susceptible to this disease. The use of a foliar fungicide may well be warranted on susceptible varieties. Please consult the online version of the North Carolina Agricultural Chemicals Manual for recommendations for fungicides.

Frogeye leaf spot is caused by the fungus Cercospora sojina. Most soybean varieties currently grown are resistant to this disease, and the use of resistant varieties is the preferred method of control. Although frogeye leaf spot is seed borne, it tends to be worse in fields of continuous soybeans. Only newly formed leaves are susceptible to this disease, and fully expanded leaves are resistant until they start to senesce. Immature leaves become infected with periods of rain or high humidity, but infection will be limited by dry weather. So, as the soybean plants put on new layers of leaves, frogeye may be present or absent depending on weather conditions during leaf expansion. This can lead to a situation where frogeye is layered in the canopy at different levels. Frogeye has caused yield losses of 30 percent in some fields, so the general recommendation for susceptible varieties is the application of a strobilurin-type fungicide (http://ipm.ncsu.edu/agchem/chptr6/604.pdf), especially if continued wet and/or humid weather is expected. We do not have a threshold for number of spots or percent leaf area affected to justify fungicide application. If wet and/or humid weather persists as plants start to senesce, older leaves become susceptible again, and the plant may defoliate early. Early defoliation can result in smaller seeds which will translate into yield loss. Also, pod infection can cause a reduction in seed quality or contribute to seed rot. For more information on frogeye leaf spot, see Soybean Disease Information Note No. 3.

In the past ten years or so, foliar fungicides have not been recommended routinely for use in soybeans in North Carolina. The exceptions to this rule have been in situations where frogeye leaf spot was found on a susceptible variety, or in instances where a grower received a premium for edible beans or seed beans. Fungicide trials typically showed a one to two bushel yield increase with the use of materials like Benlate. With soybean prices generally in the $5.00 per bushel range, this expense could hardly be justified. More recent work with Quadris at 6 ounces per acre showed yield increases of 1.5 bushels per acre in North Carolina, although some locations in Virginia and elsewhere showed yield increases on the order of three to five bushels per acre or more with strobilurin-type fungicides (Headline or Quadris). With soybean prices approaching $7.00 per bushel, the chances of recouping chemical costs from fungicide application at the R3 stage certainly improve, and may allow producers to sleep better at night. Also, consider the rate per acre of the fungicide you choose to use. In general, higher rates will provide longer residual control, so the possibility exists that one application at the higher rate may eliminate or reduce the need for a second application. Remember though, that soybean should not be sprayed beyond the R5 stage and much of our full season soybean crop, especially early maturing varieties, are nearly "out of the woods" as far as soybean rust is concerned. Approximately 5,000 copies of the book Using Foliar Fungicides to Manage Soybean Rust have been delivered to county Extension offices and it is also available online at: http://www.oardc.ohio-state.edu/SoyRust/index.htm.

Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age or disability. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

Last modified on September 5, 2005 by Stephen J. Toth, Jr.