The state's soybean crop has emerged throughout most of the state and producers should be watching for early season insects. (Photo by Brett Hampton)

June 17, 2005

Corn nematodes tied to disease, crop losses

Figure 1. Eight-week-old corn root damaged by nematode. Figure 2. Nebraska field exhibiting corn nematode damage.

Disease caused by corn nematodes has been reported recently in several areas of Nebraska. Nematodes are microscopic worms in the soil that can feed on plant roots (Figure 1). Although you can’t see them, their effects can be dramatic. Corn nematodes are present in every corn field. Damage severity and yield loss depends on which nematodes are present and their population densities.

Unlike soybean, where we are primarily concerned with one type of nematode (soybean cyst nematode), there are several genera of nematodes that cause disease in corn. Among those nematodes historically reported on corn in Nebraska are needle, sting, lance, dagger, lesion, and stubby-root. Some of these nematodes do NOT require sandy soil – contrary to popular belief.

Symptoms caused by corn nematode feeding are nondescript and may closely resemble those of other problems, so they are easily misdiagnosed and frequently overlooked. Symptoms on upper plant parts, such as yellowing, stunting, misshapen ears, and yield loss may mimic such disorders as nutrient deficiencies or drought. Damaged roots, like those in the photo, may appear necrotic, pruned, or excessively branched, resembling symptoms associated with herbicide damage, soil compaction, insect feeding, or other diseases. The affected areas often occur in isolated spots in a field, similar to the one shown in Figure 2, or to a lesser extent, as variability in plant height.

Symptomatic areas may be easier to locate early in the season and some nematodes travel deep in the soil as the summer progresses, making early season a good time to sample for nematodes.

Nematode analyses can be conducted by many agricultural testing labs, including the UNL Plant and Pest Diagnostic Clinic. Soil samples should be collected to a depth of 8 inches and represent no more than 10 acres. It is also a good idea to submit symptomatic roots, in addition to soil, for nematode analyses because some nematodes spend most of their lives inside the roots. Unfortunately, corn nematodes are favored by many of the recent changes in cultural practices. The reduction in insecticide use in favor of transgenic corn and the shift in insecticide chemistries favors nematode survival.

Successful management of corn nematodes depends on accurate diagnosis. There is little to no information available on resistance in corn. Rotation to a nonhost crop is effective for some nematodes and control of grassy weeds is also important. Some chemicals provide control, but are not always economical. And, the use of good agronomic practices to reduce plant stress, particularly by maintaining optimal soil fertility and providing adequate soil moisture, reduces the extent of damage caused by corn nematodes.

There is no recent information on corn nematodes in Nebraska. This article was intended as a brief introduction and review of corn nematodes. You can look forward to more detailed information in future issues of Crop Watch.

Tamra Jackson
Extension Plant Pathologist

Figure 1. Magnification of uredinia in one lesion. Figure 2. Young lesions. Figure 3. Angular, older lesions. Figure 4. Widespread defoliation and plant damage.

With its wide host range, including many legume weeds like kudzu, and edible legume crop species, this fungal disease poses an unknown but potentially serious threat to future crops of soybean and possibly common bean (dry edible, snap) that will be grown during 2005 in the southeastern United States and other regions east of the Rockies. Environmental patterns (storm movement, wind currents) could easily move soybean rust spores from southern states to midwestern to western regions during the upcoming months, and if the environmental conditions in these regions are favorable to the establishment of the Asian soybean rust pathogen (see Disease Cycle and Epidemiology below), it could incite epidemics of the disease during June to September. Long-term survival of and chronic threats from this “tropical” pathogen to legume crops in more temperate climates, especially in frost-free areas, are purely speculative at this time. The other tropical rust (American soybean rust) is P. meibomiae, which has been present in the Americas (South, Central and the Caribbean) for a long time, but has not posed a threat to edible legumes.

The potential impact on dry edible and snap beans are of concern because we have many acres of these crops interspersed with or contiguous to soybean fields in the southeast, central and northern United States. Unfortunately, U.S. dry bean researchers and the industry at large have rather limited information on the genetic vulnerability of our diverse market classes, commercially grown varieties and advanced breeding lines to the genetically variable Asian soybean rust pathogen.

Preliminary evaluations at the USDA Ft. Dietrick Facility in summer 2004 and field observations in South Africa and Brazil in April 2005 suggest that dry bean varieties do vary in their reactions to the Asian soybean rust pathogen and are not affected as severely as soybeans. In addition, preliminary observations suggest that infection severity of susceptible dry beans diminished if they were located more than 10 feet from infected soybeans.

Symptoms on common bean

Chlorotic leaf spots develop into angular, tan to reddish brown or purple leaf lesions, 0.02-0.16 inches in diameter, within a week after infection (Figure 1). Infection is more apparent in older and aging leaves in the lower to mid levels of plant canopies. Up to 20 tan to brown uredinia (rust pustules), each less than 0.01 inch in diameter, develop in each lesion. Uredinia open by a pore to produce many pale brown to light tan or nearly white urediniospores. Sporulation occurs predominantly on the abaxial (lower) leaf surface (Figure 2). The angular lesions (Figure 3) resemble those of common bacterial blight and angular leaf spot both of which lack the microscopic conical uredinia on the lower surface of the leaf (Figure 2). Severe infection may cause premature defoliation of older leaves (Figure 4).

Disease cycle

Urediniospores, the primary means of disease spread, are distributed in the air by wind and rain, and can remain viable for one to two months, depending on environmental conditions. A dew period (free moisture) of 4-12 hours is required for the urediniospores to germinate and for infection to occur. Urediniospores do not germinate below 46ºF or above 86ºF. Maximum germination and infection occur at about 68ºF. Upon germination, penetration of leaves is direct or through stomates to produce the angular lesion that usually contains multiple uredinia. Production of urediniospores starts about 10 days after infection and continues for several weeks. The optimal temperature for postinfection disease development is about 75ºF. During the rainy season in the tropics (South Africa and Brazil), the prevalence of P. pachyrhizi on common bean often increases, whereas that of common bean rust, caused by Uromyces appendiculatus, may decline. Germination of teliospores has not been reported, and their role in the life cycle is unknown. Early disease development on common bean, as with soybean, occurs on older leaves inside the plant canopy.

Management

Little or no attention has been given to developing management tools for Asian soybean rust in common beans due to the minor importance of the disease in common bean. General integrated pest management recommendations for dry bean diseases include the following, and may also reduce Asian soybean rust impacts:

1. Rotate out of dry beans for at least two years.
2. Eliminate bean debris and sources of volunteer beans in the fall and again in the spring.
3. Plant high quality, certified, treated seed of disease resistant varieties, if available and suitable for your market needs.
4. Follow recommended production practices to avoid stress from extremes of moisture, temperature, and soil compaction;
5. Manage water and fertilizer inputs to provide adequate, but not excessive amounts to avoid highly vigorous canopy development.
6. Carefully scout fields to detect foliar infection as early as possible.
7. Get confirmation of disease diagnosis from appropriate experts;
8. Monitor reports on weather patterns, disease forecasts, and confirmed sightings in your region.
9. When infection is confirmed in or near your field, implement a timely program of fungicides and bactericides with protectant and systemic modes of action.
10. Rotate fungicide chemistry, apply labeled rates, and stay within recommended spray intervals.
11. Adjust the combine at harvest to maximize seed quality and reduce loss of seed which can germinate next spring.
12. Thoroughly incorporate each season’s crop debris + pathogens to reduce carryover and potential disease pressure the following season. Rely on cultivation and herbicide in next year’s rotation crop to reduce volunteer bean emergence and possible infection by pathogens which can then be spread to next year’s host crop.

These fungicides are likely to be effective against soybean rust (and common bean rust) of dry bean: maneb, Bravo/Echo (chlorothalonil), Endura (boscalid), Quadris/Amistar (azoxystrobin), Headline (pyraclostrobin).

Section 18 Emergency Label requests have been made by various bean-producing states for: Tilt/Propimax/Bumper (propiconazole), Folicur (tebuconazole), Laredo (myclobutanil), and Quilt (propiconazole + azoxystrobin). Check with local officials on the label status, restrictions and pre-harvest intervals for your state.

It is assumed that many currently grown commercial varieties may be susceptible to some degree, but resistance probably will be found in common bean, when it is screened. Research to identify common bean varieties with resistance to the Asian soybean rust pathogen and studies to characterize this resistance have been initiated.

Additional information on the status of soybean rust in the United States is available at these web sites:

“Apple growers are expressing concern over the possibility of soybean rust developing in the Midwest, not because apples are a host for the disease, but because one of the available fungicides for rust management is toxic to certain varieties of apples,” said Doug Jardine, K-State Research and Extension state leader in plant pathology.

The fungicide, azoxystrobin, labeled as Quadris for Asian soybean rust control, is phytotoxic to Macintosh and Macintosh-derived apple varieties, Jardine said. Azoxystrobin is also sold under the trade names Abound and Heritage. If apple trees are subjected to the fungicide, leaves and twigs could die and fruit may drop.

“Conditions favorable for drift of the fungicide have caused problems in other parts of the country where, for instance, azoxystrobin was used in grape vineyards adjacent to apple orchards.”

Jardine said, however, that Quadris has been used on wheat in Kansas for several years with no reported problems.

“Use of Quadris on soybean rust would simply increase the likelihood for trouble should there be orchards or backyard apple trees adjacent to soybean fields,” he said.

Apple varieties grown in Kansas that are susceptible to azoxystrobin include Akane, Courtland, Gala, Macintosh, Mondial Gala, Royal Gala, Starkspur Mac, and Summer Treat. The Quadris label contains a warning with regard to this problem and applicators who spray in the vicinity of apple trees should take the time to read it, Jardine said.

The purpose of this article is to summarize preliminary data from UNL studies conducted at Concord and North Platte in 2004, with the objective to test and compare glyphosate tank-mix with other herbicides to control above mentioned weed species.

We used a labeled rate of glyphosate (Roundup WeatherMax at 22 oz/ac) tank-mixed with “half rate” of seven common broadleaf postemergence herbicides: Classic 25DF (0.3 oz/ac), Cobra/Phoenix 2EC (5 oz/ac), Raptor 1SC (3 oz/ac), Pursuit (Extreme)(3 pt/ac), Reflex/Flaxstar 2EC (8 oz/ac), Scepter 70DG (1.44 oz/ac), and Ultra Blazer 2 SC (12 oz/ac). Each tank-mix contained appropriate amounts of additives such as AMS (2.5 lbs/ac), NIS (0.125% v/v) and/or COC (1% v/v) as indicated on the product label. Each tank mix was applied at three growth stages of the weed, targeting:

1. 2- to 5-inch weeds (early POST);
2. 6- to 12-inch weeds (mid POST); and
3. 12- to 20-inch weeds (late POST).

The level of weed control at 21 days after herbicide treatment varied from 10% to 100%, depending on the weed species and tank-mix used. Weed size also was an important factor that determined the overall level of weed control (Table 1).

Yonts recommends producers with limited water supplies, such as those in the Central Nebraska Public Power and Irrigation District, Republican River Basin or the North Platte Valley, eliminate runoff from the field and deep percolation below the soil's root zone.

"To some extent, when you go into a restricted water supply for several years, it's a new way of irrigating and you have to change your thought process from what's been the normal thing to do," Yonts said.

Typically in order to uniformly furrow irrigate a field, some runoff and deep percolation occurs as a result of trying to adequately irrigate the end of the field.

"Producers who normally need 15 inches of irrigation water, but only are getting 10 inches this season, will want to apply their water as effectively as possible," Yonts said. "If water is limited, this growing season and perhaps in the future producers may want to totally eliminate runoff and deep percolation from their fields."

This will result in fields not being uniformly irrigated, but it does mean that water available for irrigation will be used for crop production. To eliminate runoff and deep percolation, Yonts recommends producers run shorter set times on their furrow irrigation system.

"Not running water across the soil for as long of a time period reduces deep percolation which puts water below the crops' root zone where it can't be used," he said.

By avoiding deep percolation, water will be kept in the crops' root zone where it will be used by the growing plants. In addition, Yonts recommends producers not let water run off at the end of the field.

"If water supplies are limited and water stress on a portion of your planted acres is a sure thing, let the stress occur at the bottom end of the field," he said. "If there is no deep percolation or no runoff, you have nearly a 100% application efficiency."

Traditionally, furrow irrigation has a 50% to 60% application efficiency. A center pivot irrigation system typically has a 90% application efficiency.

"Not everyone can switch to a center pivot irrigation system, so managing furrow irrigation better will increase its efficiency," he said. "It comes down to the producer. Producers can try to irrigate their fields like they normally do and lose some of their water to deep percolation or runoff, or they can use all of their available water to make sure a portion of their field does receive adequate water and let the end of the field be stressed."

Rank weed species competitiveness in soybean

Field studies were conducted at two locations in eastern Nebraska in 2002 and 2003 to determine and compare the values for competitive indices among weed species as influenced by soybean row spacing and the weed emergence time relative to the crop’s growth stage. This study is also a Master’s degree project for Shawn Hock.

Soybeans were planted in 7.5- and 30-inch rows. Seven broadleaf and four grassy species were planted at three soybean growth stages: crop planting (VP), crop emergence (VE), and 2nd trifoliate stage (V2). The species included common lambsquarters, redroot pigweed, common waterhemp, common sunflower, common cocklebur, Pennsylvania smartweed, giant ragweed, yellow foxtail, giant foxtail, fall panicum, and barnyardgrass. Soybean yield data, weed biomass, and weed seed production were collected at the season end.

The most competitive weed found in this study was common sunflower, producing twice as much dry matter than any other species. Common cocklebur was the next most competitive weed followed by giant ragweed and then velvetleaf. Common waterhemp was more competitive than redroot pigweed but less competitive than velvetleaf. Common lambsquarters was the next competitive and slightly more competitive than the grassy species. Giant foxtail was the most competitive grass, followed by barnyard-grass, fall panicum and yellow foxtail. In general, competitive indices were affected by row spacing and emergence date. Weeds growing in 30-inch rows were more competitive than those in 7.5-inch rows. Weeds also were more competitive when emerging with the crop than when emerging a week or two later.

The major practical implications of this study are:

1. It’s important to properly identify weed species and their composition before making weed management decisions since weed species differ in their competitiveness,
2. Planting soybean in narrower rows will reduce the competitiveness of most weed species, providing a competitive advantage to the crop.
3. Scout fields to determine weed emergence times relative to the crop stage, because data shows that weeds emerging a week or two after the crop are much less competitive than those emerging with the crop.

This study was partially funded by the North Central Regional Weed Science grant.

Stevan Knezevic
Extension Weeds Specialist
NEREC Haskell Ag Lab

The UNL Extension clinics begin each day with 7:30 a.m. registration at the Agricultural Research and Development Center near Mead and start at 8 a.m.

Participants can attend one or both of the clinics as subject matter will be different each day. July 14 topics will include: corn plant distribution -- population, twin rows and equidistance; corn rootworm technology; hands-on crop scene investigation (CSI); diagnostic lab update; Liberty Link vs. Roundup Ready weed control system; relay cropping -- wheat and soybeans; and soybean rust.

July 15 topics will include: occasional tillage for improvement of no-till systems and crop nutritional disorders; managing irrigation for maximum profits; root dynamics; corn nitrogen credits; hands-on crop scene investigation; nutrient management tools; and water optimizer software demonstration. Presenters include UNL extension educators and specialists.

Early registration is recommended to reserve a seat and resource materials. Cost for attending one clinic is $130 for those registering by July 7 and $180 after. Cost for attending both clinics is $225 before July 7 and $275 after.

Planning can provide extended options

Plan how long you hope to keep this stand. Repeated harvests when alfalfa is young will stress plants, making them susceptible to insects, diseases, and winter injury or death. For long-term stands, at least one of the year’s remaining harvests need to wait until plants are blooming well, at least 40 days after the previous cutting. You can wait now with this cutting or wait later this year.

Plan pest control. If you have good soil moisture, this is a prime time for weeds to get started in your alfalfa. Also, insects like potato leafhoppers are being found in fields. Be ready to spray pesticides as soon as problems become apparent.

Early first cuttings lead to potentially early second cuts and extra flexibility to time hay harvests the rest of the year. Use this opportunity to produce the kind of hay you need and want instead of allowing weather and the calendar determine your harvest schedule.

Bruce Anderson
Extension Forage Specialist

USDA’s Nebraska Agricultural Statistics Service: For the week ending June 12 variable temperatures and continued rainfall aided crop and pasture development but complicated hay harvest. With the ongoing rains, weed control has fallen behind in many fields. Precipitation since April 1 continued at average to above normal levels across all districts. The north central counties have recorded twice their normal precipitation since April 1 and the northeast has received 50% more than average.

Wheat condition remained stable and rated 7% very poor, 16% poor, 33% fair, 36% good, and 8% excellent. Fields were reported to be 93% headed, behind last year at 98% but near the average of 92%. Twenty percent of the fields were reported as turning color, well behind last year at 52% and the average at 37%.

Corn condition rated 2% poor, 23% fair, 59% good, and 16% excellent. Conditions continue higher than last year and normal.

Soybean planting moved to 98%, near last year at 97% and in line with the average. Soybean emergence at 92% was ahead of last year and average at 88%.

Sorghum planting was at 90% complete, behind 94% last year and 91% for average. Seventy percent of the crop had emerged.

Oat condition rated 3% poor, 25% fair, 53% good, and 19% excellent. Fifty percent of the crop has headed, compared to 67% last year and 53% for the average.

Alfalfa conditions improved slightly and rated 2% very poor, 9% poor, 38% fair, 40% good, and 11% excellent. First cutting was 62% complete, behind last year at 78% and average at 77%. Wet conditions have impacted quality.

Proso millet planting remained slow and was 17% complete, behind last year at 65%.

Dry bean planting progressed to 68% complete, behind last year at 79% and the average at 76%. Fourteen percent of the crop had emerged.

About Crop Watch | Agricultural News | Events | Archives | Markets Ag Links | Weather | Photos | Search Lisa Jasa, Crop Watch Editor | Publications | IANR

Copyright 2005 by the University of Nebraska

Published by University of Nebraska Cooperative Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture

The University of Nebraska-Lincoln does not discriminate on the basis of gender, age, disability, race, color, religion, marital status, veteran's status, national or ethnic origin, or sexual orientation.