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7/27/2009 - 8/2/2009

Check SCN Resistant Soybean Roots for SCN Females

By Greg Tylka, Department of Plant Pathology

There is some concern in Iowa these days about how well SCN-resistant soybean varieties are controlling the nematode. Reports of soybean cyst nematode reproduction on resistant soybean varieties with the common source of SCN resistance, PI 88788, have increased in recent years.

How would you know if your SCN-resistant soybean varieties are not effectively controlling the SCN populations in your fields?  You can’t necessarily tell if plants are infected with SCN by looking at the crop. Up to 40 percent yield loss can occur on susceptible varieties with no visible symptoms above ground.

The SCN females are easily observed on soybean roots with the unaided eye. A crude yet somewhat effective way to check the effectiveness to resistant soybean varieties in SCN-infested fields is to look for SCN females on the roots of resistant plants. The SCN females are small, round, white objects on the roots and are about the size of a period at the end of a sentence. (See image below.)

This is the time to check
Now would be a great time to check the roots of SCN-resistant soybean varieties. Checking a few times through August would be better than just a one-time assessment. 

If numerous SCN females are observed on roots of resistant soybean varieties, having an HG ("HG" for Heterodera glycines, the scientific name for SCN) type test performed on the SCN population in the field may be warranted. The HG type test is a greenhouse test that assesses SCN reproduction on the different sources of resistance used in breeding SCN-resistant soybean varieties. Detailed information about HG type tests can be found in the Integrated Crop Management Newsletter article titled What's your type?: An HG type test for SCN populations, Nov. 13, 2006.

Additional information about the biology, sampling and management of SCN can be found on the Web at www.soybeancyst.info.


 

Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes. Tylka can be contacted at gltylka@iastate.edu or by calling (515) 294-3021.    


 

scn females on root

Adult SCN females on root of an SCN-resistant soybean variety in central Iowa.

Goss’s Wilt and Northern Corn Leaf Blight Showing Up in Iowa

By Alison Robertson, Department of Plant Pathology

Goss’s wilt
This past week I have received several more reports of Goss’s wilt.  Reports of the disease come from south of Highway 3, North of I-80 and east of I-35.  For the most part, it seems that the disease is occurring in random fields; however this past week I visited a 200 acre field in Boone County in which many of the plants were infected. Many of the plants had extensive leaf blight occurring on the top two to three leaves of the canopy (Figure 1).  Most of the lesions occurred around holes made by hail. 

Goss’s wilt lesions are large, have wavy margins and are brown, yellow, gray in color. They may be elliptical or V-shaped and usually extend down a leaf vein (Figure 2). The tissue neighboring the lesion is water soaked. It is common for the bacteria that cause this disease to ooze out onto the leaf surface, so the lesions often have a shiny appearance.  Most characteristic though, are the dark green “freckles” that occur within the lesion (Figure 3). In the Plant Disease and Insect Clinic, we check for “streaming” to diagnose a bacterial disease.  I did this in a glass of water in my office (Figure 4). 

Goss’s wilt disease can progress rapidly under the right conditions (warm 80 degree F and wet) resulting in extensive leaf blight and death of the canopy.

Since this is a bacterial disease, a fungicide application will not control the disease. The pathogen is able to survive in infested surface crop residue for 10 months.  The recommended management practices include rotation to a non-host crop, any type of tillage that buries the infested residue and helps with decomposition, and tolerant hybrids.

An excellent review of this disease is available from University of Nebraska, Lincoln.


goss's wilt leaf blight symptoms

Figure1. Leaf blight symptoms of Goss’s wilt

 

goss's wilt lesions

Figure2.  Characteristic lesions of Goss’s wilt

 

goss's wilt freckling

Figure3.  Characteristic freckling seen on Goss’s wilt lesion

 

goss's wilt oozing

Figure 4.  Oozing caused by Goss's wilt

 

Northern Corn Leaf Blight
Northern corn leaf blight (NCLB)is also making an appearance. This disease could be mistaken for Goss’s wilt because it also causes large elliptical lesions on corn leaves. Since Northern corn leaf blight is caused by a fungal pathogen, Exserohilum (Helminthosporium) turcicum, a fungicide could be used to manage this disease.

Northern corn leaf blight lesions are elliptical or cigar-shaped, gray-green to tan in color, with a distinct margin between the infected and healthy tissue (Figure 5).  Under high humidity they look “dirty” due to large numbers of spores produce on the surface of the lesion (Figure 6).

Infection is favored by cool (65-80 degree F), wet conditions. Free water needs to be present on the leaf surface for 6-18 hours for infection to occur. Lesions develop within 7-12 days.

Yield losses of over 30 percent have been reported if the disease is present on the upper leaves of the plant at the silking. Losses are minimal if disease development is delayed until dent stage. Like other foliar pathogens, Northern corn leaf blight predisposes corn to stalk rot.

Hybrids with resistance to NCLB are available so check with your seed dealer. Rotation to soybean or alfalfa can be beneficial as well since the pathogen survives in infested crop debris.  What about now?  A fungicide application may be an option; however, there are no tried and tested thresholds available. Thus far I have heard of only one field in which several lesions were present on the ear leaf. Before making the call on a fungicide application, be sure to scout to determine disease pressure in the field. Then consider hybrid susceptibility to disease, current and predicted weather conditions, previous cropping history and economics (cost of fungicide plus application, price of grain, drying costs, standability).

northern leaf blight cigar shaped lesions

Figure 5. Cigar-shaped lesions of northern corn leaf blight  (Credit G. Coates)

 

northern leaf blight sporation

Figure 6. Sporulation on the surface of a northern corn leaf blight lesions

 

 

Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. Robertson may be reached at (515) 294-6708 or by email at alisonr@iastate.edu.

Bean Leaf Beetle Activity Noticed in Soybean

Erin Hodgson, Department of Entomology

Last week, I heard a few reports of bean leaf beetle adult feeding in soybean. Because cold temperatures last winter were extremely hard on overwintering adults, there was some question if bean leaf beetles would be a problem this year. Since Iowa is slightly behind in degree days, some areas of the state are experiencing delayed first generation feeding damage as fields are coming into bloom.

Life Cycle.  Bean leaf beetles have two generations per year in Iowa. The overwintering generation attack emerging soybean in the spring, and lay eggs that produce the first generation that usually emerges in late June and July. These first generation populations usually peak in the late vegetative or the early reproductive soybean stages. It is the second generation adults that peak and feed during the pod-fill stage  in late summer can be very significant. The red and yellow phase of bean leaf beetles is not an indication of age, although this rumor is widely circulated.

bean leaf beetle

Scouting for bean leaf beetles can help prevent economic loss, even with patchy populations this year.

 

Sampling.  Soybean fields in the reproductive stages can be sampled for bean leaf beetle by using either a drop cloth or a sweep net. Here are the procedures for each method.

Drop cloth
• Walk 100 feet in from the field edge and scout each field and each variety separately.
• Place a 3-foot wide strip of cloth on ground between the rows.
• Bend the plants on one row over the cloth and shake them vigorously.
• Count the number of beetles on the cloth and determine the average number of beetles per 3-foot of row.
• Repeat the procedure four times for each 20 acres of the field.
• Consult Table 1 for the number of beetles per 3-foot of row necessary to justify insecticide treatment.
• If the number of beetles is below the economic threshold, sample your fields again the following week and a third week if necessary.

Sweep net
• Walk 100 feet in from the field edge and scout each field and each variety separately.
• Take 20 sweeps and determine the number of beetles per sweep.
• Repeat the procedure four times for each 20 acres of the field.
• Table 1 shows the average number of beetles per sweep that justifies insecticide treatment for the second generation adults.
• If the number of beetles is below the economic threshold, sample your fields again on following week and a third week if necessary.
 

 

Table 1. Bean leaf beetle economic thresholds in reproductive-stage soybean.*table

*Economic thresholds are based on a row spacing of 30 inches and a plant population of eight plants per foot of row. For narrow-row soybeans (8-inch rows) and a plant population of three plants per foot of row, multiply the above economic thresholds by 0.7.

 

Management. Bean leaf beetle feeding on soybean pods can lead to significant reductions in seed quality and yield throughout Iowa. It is important to recognize bean leaf beetle injury. There are clearly defined economic thresholds for various plant stages. However, managing bean leaf beetles in soybean during the pod set and pod fill can be frustrating to growers and crop advisers because adults may be feeding on pods for a couple of weeks before the population reaches the economic threshold. In this situation, some loss in seed quality and quantity occurs before an insecticide application can be economically justified. There are several products registered in Iowa for bean leaf beetle (Table 2). Follow label directions and pay attention to spray guidelines.

 

bean leaf beetle table 2

Portions of this article originally appeared in previous ICM News articles (2007 and 2000). 

 

Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at ewh@iastate.edu or phone (515) 294-2847.

Degree Days - Cool July With Good Crop Conditions

By Rich Pope, Department of Plant Pathology

Iowa corn is tasselling and silking this week. We think of the silk date as a marker for the final 60 to 65 days in which the corn reaches physiological maturity. That means we might expect dry down to extend into October across the state. Soybeans are setting pods, and with timely August rainfall, pods should fill nicely. Last week we lost approxiamtely 50 degree days to average across Iowa, making July 2009 as one of the ten coolest in history.  

Degree day accumulation map for July 26

Scouting for pests now has the potential to  influence yield profits. Soybean aphids are present in many areas, particularly central and northern Iowa, but most populations are below the economic threshold. Corn diseases like eyespot (especially in north central and central Iowa) and gray leaf spot (especially in southwest and south central Iowa) should be monitored.  For soybean, we are seeing a few fields with sudden death syndrome symptoms being expressed. 

 

Rich Pope is a program specialist with responsibilities with Integrated Pest Management. Pope can be contacted by email at ropope@iastate.edu or by calling (515) 294-5899.

Week of July 27 Crop and Weather Report

Iowa State University Extension climatologist Elwynn Taylor, integrated pest management specialist Rich Pope, and soybean agronomist Palle Pedersen are interviewed by Extension Communications specialist Doug Cooper to give us the weekly crop and weather report.

Taylor says July has been one of the coolest on record, so far. Most of the corn and soybean crops should be safe from a normal frost, although some areas of the state are as much as two weeks behind.

Pope recommends to continue scouting of corn and soybean fields and reports that most of the state's crops are in good to excellent condition.

Pedersen tells how the cool weather can slow down development of the soybean plant, and if prolonged can reduce yield potential.

Use Treatment Thresholds for Western Bean Cutworm

By Erin Hodgson, Department of Entomology and Rich Pope, Department of Plant Pathology

Western bean cutworm adults have begun emerging in Iowa this year, particularly in the southwest and southeast. Corn fields in the late whorl stage are most attractive to egg-laying females, with damage becoming evident in August and early September. University of Nebraska has developed a degree day model for 50 percent adult emergence. This year, 50 percent emergence of western bean cutworm moths in southern and central Iowa was approximately July 21. So now is the time to start scouting for egg masses before larvae enter developing corn ears.

Description. The eggs are about the size of a pinhead, but usually laid in masses on the flag leaf. The eggs are white when first laid. They turn tan and finally purple just before the larvae hatch.

western bean cutworm eggs

Western bean cutworm eggs turn purple when mature, indicating larvae will soon hatch.

 

Young larvae are approximately 0.25 inch in length and are tan with a faint diamond-shaped pattern on their backs. As the larvae mature, they become a pinkish tan or pale brown and reach a body length of 1.5 inches. Adult western bean cutworms have forewings with a mixture of buff, tan and grey with a cream-colored stripe extending nearly down the front edge; two light spots and a "boomerang" touch the stripe. Other late-season caterpillars and moths in corn can be easily confused with Western bean cutworm, but a 2007 ICM News article can help with identification.


 

western bean cutworm adult

These moths have one generation per year and can be overlooked for other late-season corn insects.

 

Damage. Before tasseling, young western bean cutworm larvae feed on pollen, but eventually larger larvae will feed on shed pollen, leaf tissue, silks and corn kernels. Larvae in the ear will feed on the tip, middle and shank. One larva per plant usually does not cause severe damage to the ear. But several larvae feeding on one ear could substantially reduce yield because western bean cutworms are not cannibalistic, compared with corn earworms. Sometimes heavy feeding can promote fungal pathogens in the ear. Western bean cutworm larvae do not tunnel into stalks; however, their damage is often confused with corn earworm and European corn borer.

western bean cutworm damage

Corn can experience severe damage when multiple larvae are feeding within an ear.

 

Scouting. Examine twenty consecutive corn plants in at least five locations in the field. Check the upper three or four leaves of each plant for egg masses or young larvae. Continue scouting for 7-10 days after adult peak flight. Timing of the application is critical. If the tassel has not emerged when the eggs hatch, larvae will move into the whorl and feed on the developing pollen grains in the tassel. As the tassel emerges, the larvae will move down the plant to the green silks and then into the silk channel to feed on the developing ear. Remember to scout refuge corn and all Bt corn without the Cry1F protein.

Management. Consider using a Bt corn hybrid, like Herculex™, which contains a Bt protein (Cry1F), to protect against significant western bean cutworm damage. However, there are treatment thresholds for non-Bt refuge corn. If eight percent of the field corn plants (four percent for sweet corn) have an egg mass or young larvae are found in the tassel, consider applying an insecticide. If an insecticide is needed, apply it when 90 to 95 percent tassel has emerged. If the tassels have already emerged, the application should be timed for when 70 to 90 percent of the eggs have hatched. Once the larvae reach the ear tip, control is nearly impossible.

If an insecticide application is needed, corn fields should be checked for the presence of spider mite colonies. If mites are found, select a product that does not stimulate mite growth. Mite flares are especially a concern in moisture-stressed fields. There are several products registered in Iowa for corn leaf aphid (Table 1). Follow label directions and pay attention to spray guidelines.

 

western bean cutworm table

 

Portions of this article originally appeared in the July 10, 2006 ICM article, Western bean cut worm, 2006. (link to : http://www.ipm.iastate.edu/ipm/icm/2006/7-10/wbc.html).

 

Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at ewh@iastate.edu or phone (515) 294-2847. Rich Pope is a program specialist with responsibilities with Integrated Pest Management. Pope can be contacted by email at ropope@iastate.edu or by calling (515) 294-5899.

Japanese Beetles Expanding Range in Iowa

By Erin Hodgson, Department of Entomology

Since my June 30 ICM News article about Japanese beetle activity in Iowa, there have been many reports of adults feeding in corn and soybean. Adult Japanese beetles are metallic with white tufts along each side of the body. Several extension field agronomists have also reported seeing beetles in new areas this summer. Japanese beetles have a wide host range, feeding on more than 300 different plant species. With so much corn and soybean in Iowa, it's probably no surprise they are expanding range here.

Life cycle. Japanese beetles have one generation per year in Iowa. Adults emerge from grass in late June and immediately begin to feed on low-lying plants such as roses and shrubs. Adults eventually move up on trees and field crop foliage to feed and mate. Mated females move back to grass in August and September to lay small egg masses in soil cavities. The eggs hatch into small grubs that feed on roots underground until late September when the temperature cools. The almost fully-grown grubs burrow down in the soil and remain inactive all winter. In the early spring, grubs become active again and feed until turning into resting pupae. The pupae hatch into adults and emerge from the soil.

Damage and Management.  Japanese beetles release a strong aggregation pheromone, and are commonly seen feeding and mating in clusters. Adults are also highly mobile and move frequently in the summer. In soybean, adults prefer to feed on the upper leaf surface. The treatment threshold for Japanese beetles in soybean is 30 percent defoliation before bloom and 20 percent defoliation after bloom through seed set. See Table 1 for labeled products for Japanese beetle in soybean.


japanese beetle on soybean 

Japanese beetles remove tissue from soybean leaves, resulting in a bronzed, lacy appearance.

 

In corn, Japanese beetles can feed on leaves, but the most significant damage comes from clipping silks during pollination. Consider a foliar insecticide during tasseling and silking if there are three or more Japanese beetles per ear and pollination is not complete. Abundant soil moisture and rapid silk growth should be taken into consideration in making treatment decisions. See Table 2 for labeled products for Japanese beetle in corn.

japanese beetle on corn

Adults aggregate during tasseling and can clip corn silks.

 

Table 1. Insecticides labeled for Japanese beetle adults in soybeanjapanese beetle thresholds in soybeans

 

Table 2. Insecticides labeled for Japanese beetle adults in cornjapanese beetle thresholds in corn


 

Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at ewh@iastate.edu or phone (515) 294-2847.



This article was published originally on 8/3/2009 The information contained within the article may or may not be up to date depending on when you are accessing the information.


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