 |
Field and Forage Crops |
With some cotton cutting out and other cotton still susceptible to caterpillar and bug damage, this time of year can be difficult for making insect spray decisions. Certainly, with cotton so variable and with so many fields now past the point of insect damage, this is a good time of year to cull many cutout cotton fields from the insect scouting list.
Bollworms, European corn borers, and fall armyworms have a difficult time getting a start on cotton that has stopped producing blooms or squares. Bollworms and fall armyworms generally require a bridge of squares and/or blooms to become established before they move to larger bolls, and European corn borer female moths are not attracted to cotton plants which have cut out. We still have some very attractive, late maturing cotton fields scattered throughout the state. Also, even with our recent extremely hot, dry weather and wilted cotton plants in some areas, bollworms can still become established in these fields if they still have squares and blooms. On the basis of crop maturity, insect scouting efforts may be restricted to those cotton fields which are producing squares and blooms.
We appear to have a consistent, but generally low population of fall armyworms throughout much of the state. The easiest way to assess a possible fall armyworm infestation at this time of year is to look in white blooms. Often during the first week in September, these fall armyworms have a difficult time becoming established on large bolls, especially those feeding between the inner bract and outer boll surface of medium and large bolls.
The good news is that thresholds for bollworms and stink bugs can be raised significantly as the proportion of large insect-safe bolls increases. In many fields, if only small bolls are sampled for bollworms or stink bugs and the large safe bolls outnumber these smaller susceptible bolls by 3 to 1, for example, the treatment threshold may be raised 4-fold. That translates to about 12 percent bollworms in small bolls, or about 40 percent internal boll damage from stink bugs in the example. Stink bugs sometime abandon cotton fields for greener pastures (like soybean fields) at this time of year, so be sure to confirm at least some level of adult stink bug activity before treatment, even if quarter-sized boll damage thresholds are met.
Spider mite and cotton aphid activity have peaked in some areas, but levels for the most part have been on the low side. Often when cotton fields get to the point of defoliation (and certainly after defoliation), cotton aphids number diminish dramatically, presumably due to nutritional changes in the plant.
Overall, the potential for insect damage is diminishing, especially if our hot, dry weather continues. Most producers would opt for a good rain shower, or two.
Frogeye leaf spot caused by a fungus, Cercospora sojina, has been detected at several locations on soybeans in eastern North Carolina. Disease on these samples was fairly severe. More information on this disease can be found on the Internet at: http://www.ces.ncsu.edu/depts/pp/notes/Soybean/soy003/soy003.htm. Many, if not most, varieties are resistant to this disease. Symptoms are a leaf spot with a reddish brown border. Spots may appear throughout the canopy or may be layered in the canopy because older leaves are generally resistant. The fungus can also infect pods and may affect seed quality. Fungicides are not routinely recommended for use on soybeans in North Carolina, but fungicide sprays have provided significant yield increases when this disease is present. The fungicides Quadris and Topsin M are labeled as foliar spray for soybeans and will provide some control of this disease.
Sudden death syndrome (SDS) is the common name for a root-rot of soybeans caused by the fungus Fusarium solani f.sp. glycine. The disease was first found in North Carolina in 2001, although it may have been present earlier. Yield loss in 2001 was negligible. The likelihood that major yield loses from SDS will occur in North Carolina is unclear, but SDS is a potentially serious disease. The name sudden death refers to the early defoliation and death of the soybean plant. The common name stems from the fact that in wet years, foliar symptoms (a yellowing or browning between leaf veins) seem to spread rapidly through a field at or after the pod filling stage in susceptible varieties. Varieties very considerably in their susceptibility and seed catalogs typically rate varieties for resistance to this disease. Although considerable yield loss may occur, growers in Mississippi commented that they would like to see it again since overall they had excellent yields. Disease is usually more severe in high yield environments. Sudden death syndrome is often associated with the soybean cyst nematode, Heterodera glycines. Fungicides are not effective in managing this disease, because this is a root-rot pathogen. Disease is usually worst following corn or cotton, rather than following soybeans. The leaf symptoms resemble those of a number of soybean diseases (brown stem-rot, Phytophthora root rot, red crown rot, and Southern blight [white mold]). Disease is usually worst following corn or cotton, rather than following soybeans. Red crown rot of soybeans causes a similar disease and is more common following peanuts or soybeans in a rotation. For more information, see the following web sites:
http://www.ces.ncsu.edu/depts/pp/notes/Soybean/soy007/soy007.htm
http://www.ces.ncsu.edu/depts/pp/notes/Soybean/soy005/soy005.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 2, 2003 by Stephen J. Toth, Jr.