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7/18/2011 - 7/24/2011

Japanese Beetle Activity Increases in Iowa

By Erin Hodgson, Department of Entomology

For the last few weeks, I have been seeing Japanese beetle activity throughout Iowa. Ornamentals, trees and shrubs have large numbers of adults feeding and mating, and defoliation is becoming obvious in our small plot research. In addition, ISU field agronomists have been reporting defoliation in commercial soybean. Although Japanese beetles have been reported here since 1994, plant damage has been erratic. I strongly encourage growers and crop consultants to scout corn and soybean fields for this pest this year.

Life cycle

Japanese beetles have one generation per year. Adults emerge from grass in late June and begin to feed on low-lying plants, and eventually move up on trees and field crops 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 white 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 that emerge from the soil.

Damage and management

Adults prefer to feed between soybean leaf veins, but can ultimately consume most of the leaf (Figure 1). The treatment threshold for Japanese beetles in soybean is 30 percent defoliation before bloom and 20 percent defoliation after bloom. Most people tend to overestimate plant defoliation, but this reference can help with estimations. 

Japanese beetles on leaves
Figure 1. Japanese beetles are skeletonizers that cause leaves to look lacy. Photo by Mark Licht.

In corn, Japanese beetles can feed on leaves, but the most significant damage comes from clipping silks during pollination (Figure 2). Consider a foliar insecticide during tasseling and silking if: there are three or more beetles per ear, silks have been clipped to less than ½ inch, AND pollination is less than 50 percent complete. 
 
Japanese beetles on corn
Figure 2. Adults aggregate during tasseling and can clip corn silks. Photo by Mark Licht.

There are many insecticides labeled for Japanese beetle control; however, do not expect season-long control from a foliar application. Adults are highly mobile and move frequently in the summer. Japanese beetles release a strong aggregation pheromone, and are commonly seen feeding and mating in clusters. Beetles present during the application will be killed, but beetles migrating into sprayed fields may not be controlled. If soybean defoliation continues, additional applications may be necessary to protect the seed-filling stage. If corn pollination is complete, Japanese beetles may not be economically important anymore. Also consider a border treatment if Japanese beetles are aggregated in the edge rows.

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.

Begin Scouting for Sudden Death Syndrome in Soybean

Alison Robertson, Daren Mueller, Leonor Leandro, Greg Tylka and XB Yang, Department of Plant Pathology and Microbiology

In past years, sudden death syndrome (SDS) has appeared during the last week of July or the first week of August in Iowa. Therefore we anticipate symptoms of SDS will begin appearing in the state within the next couple of weeks.  Although we do not expect SDS to be as widespread or as severe as the 2010 growing season, there have been some Iowa counties that have received higher-than-normal precipitation. We expect the risk of SDS in these counties to be higher since disease development is favored by wet conditions. 

Begin scouting for SDS soon 

The first symptoms of the disease are usually found on more compacted and low areas of the field. First symptoms are seen on the leaves of infected plants as scattered, yellow spots between leaf veins (Figure 1). Large sections of leaf tissue between veins turn yellow as spots grow together. These yellow blotches soon turn brown, but the veins remain green (Figure 2). Eventually the leaves die and drop, but the petioles remain on the stem. Infected plants are also easily pulled from the soil because the roots are rotted. When split lengthwise with a knife, the internal tissue of the main or tap root will be gray to reddish brown, not healthy white (Figure 3).

There are no in-season management options for SDS, but scouting is still important for several reasons.

  •  First, this is a good time to evaluate soybean varieties for resistance to SDS. Growing resistant varieties, or avoiding very susceptible varieties, is the most effective way to reduce losses to SDS.
  • Also, identifying fields or parts of fields with SDS can help with future management practices. These management tactics include reducing soil compaction since the disease has been associated with compacted soil; planting fields with a history of SDS towards the end of a planting schedule when soils may be warmer and drier; and testing for the presence of soybean cyst nematodes.
  • Soybean cyst nematode is usually, but not always, associated with SDS and may increase its severity, especially in varieties that are SCN-susceptible. Therefore, management practices to reduce SCN populations, including SCN-resistant variety selection and preventing the spread of soil from field to field, may delay onset and spread of SDS.

Plant pathologists and agronomists continue research to improve our understanding of the biology of the fungus that causes SDS and develop improved management options for the disease. The Iowa State University (ISU) soybean breeding program continues to develop and release germplasm with improved resistance to SDS that is available to all private soybean breeding companies. These Iowa State scientists collaborate with scientists at other universities.  Most of the SDS research at Iowa State is funded by soybean checkoff dollars from state, regional and national organizations, namely the Iowa Soybean Association, the North Central Soybean Research Program and the United Soybean Board.

ISU Research

Key research advances from the last five years of research on SDS at Iowa State include:

  • Development and release of soybean breeding lines with improved resistance to SDS that can be used by seed companies to develop resistant varieties adapted to Iowa
  • Discovery, identification and molecular characterization of a toxin produced by the SDS fungus that causes the disease and that the toxin needs to be exposed to light to cause the disease on the leaves
  • Discovery that the fungus needs to colonize the central part (or vascular system) of the roots so that the toxin can be moved up from the roots to the leaves in the cells that carry water up the plant
  • Discovery that soybean seedlings are most vulnerable to root infection in the first few days after planting, and that in cold soils the seedlings are vulnerable to infection for a longer period of time than when planting occurs in warmer soil
  • Discovery that the SDS fungus can survive in corn residue, including corn kernels dropped in field, and this may be a way the fungus overwinters from season to season

Several other ongoing projects include:

  • Sequencing of the entire genetic composition (genome) of the SDS pathogen, which will allow us to identify the genes involved in the ability of the fungus to cause disease on soybean
  • Identifying the mechanisms behind the interaction between the SCN and the SDS pathogen
  • Identifying soybean genes involved in resistance to SDS using molecular approaches
  • Continuing to screen soybean breeding populations adapted to Iowa for improved resistance to SDS
  • Evaluating the impact of crop rotation, planting date and seed treatment for SDS management in Iowa



Figure 1.  Early foliar symptoms of SDS.

 


Figure 2.  Advanced foliar symptoms of SDS

 


Figure 3.  Gray discoloration of the taproot (bottom) associated with SDS compared to a healthy soybean tap root (top).  

 

Alison Robertson is an associate professor in the Department of Plant Pathology and Microbiology with extension and research responsibilities; contact at alisonr@iastate.edu or phone 515-294-6708. Daren Mueller is an extension specialist with responsibilities in the Corn and Soybean Initiative and ISU's IPM program. Mueller can be reached at 515- 460-8000 or by email at dsmuelle@iastate.edu. Leonor Leandro is an assistant professor of plant pathology with research and teaching responsibilities. Leandro may be reached at (515) 294-8855 or by email at lleandro@iastate.edu. Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes. XB Yang is a professor of plant pathology with research and extension responsibilities in soybean diseases. Yang can be reached at (515) 294-8826 or by emailing xbyang@iastate.edu.

Aridity Index, a Guide to Rain and Temperature Impact on Corn

By Elwynn Taylor and Roger Elmore, Department of Agronomy

“It has been hot, but at least the soil moisture is good,” northwest Iowa farmer this week.

“Last year it was so dry I thought we would lose the crop, but temperatures were not so bad and it turned out great,” central Iowa farmer in 1995. 

Most everyone knows that “dry and hot” is worse than “cool and moist” when it comes to the impact of summer weather on Midwest corn yield. Naturally it is not all that simple, still the generalization is meaningful.

Years with better corn yields across Iowa tend to be a bit warmer than normal before the crop tassels and a bit on the cooler and wetter than usual side of usual afterward. The late Dr. Louis Thompson of Iowa State University Department of Agronomy based a corn yield forecast model on these well-known effects. The Thompson Model achieved international acclaim because it provided a useful assessment of the impact that temperature and rain have on Midwest corn production. Although the crop yield cannot be accurately forecast while it is still developing, it is of value to know the crop yield risk by week and by crop reporting district as the season progresses. A week-by-week index showing relative crop risk according to the interaction of temperature and precipitation has proven to give a realistic picture of localities with poor and areas with better than usual crop yield. The Aridity Index does this for you; the index is directly related to the probability of having a district yield that exceeds the historical trend line.



Figure 1- Aridity Index by crop reporting districts. Locations with an index higher than -4 are on track to likely have above trend yield (yellow through green and blue colors). Districts with an AI below -4 will have below trend line yields if weather conditions persist. The nature of the aridity index is such that major changes can be observed week to week. The individual history by week for each reporting district is available by clicking the original image on the website. View depicted is for the week ending July 18, 2011. 


 
The Aridity Index is based on the observed temperature and precipitation relationships for the central United States. When rain is diminished to one standard deviation below normal, the impact on the crop is much the same as if the temperature had been elevated by one standard deviation. Daily observations of temperature and precipitation are available for every county.  Accordingly, the AI can be updated for each crop reporting district on a weekly basis (or conceivably by county on a daily basis). This index treats just the temperature/precipitation interaction. It does not consider the Growing Degree Days, the impacts of excess water, the fate of nitrogen in the soil of the farm, or any number of other factors impacting crop yield. Still many find it of real value when knowledge of district level crop response is important. The AI analysis is especially valuable when used in conjunction with USDA-NASS reports of state-by-state crop condition (if more than 50 percent of the crop acres are in good and/or excellent condition an above trend line yield is likely). 

The AI computer program and display was developed by Agricultural Meteorologist Darren Miller while he was a graduate student. 


 

Elwynn Taylor is extension climatologist and can be reached at setaylor@iastate.edu or by calling (515) 294-1923. Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. He can be contacted by email at relmore@iastate.edu or (515) 294-6655.

Is Foliar Fertilization Useful to Supplement Pre-plant Fertilization

By Antonio P. Mallarino, Department of Agronomy

It's that time of the season when many producers wonder if foliar fertilization could help improve soybean growth and grain yield, and some also wonder about corn. They wonder if pre-plant fertilization was adequate; if late planting dates, replanting and cold or excessively wet conditions limited early nutrient uptake; or if assumed adequate soil nutrient levels may limit yield when growing conditions are excellent.

Foliar fertilization should be viewed as a complement of pre-plant fertilization or a "rescue" treatment. Only a very small fraction of the needed amount of nutrients can be applied to the foliage because high nutrient concentrations in the leaf surface have toxic or salt-induced damaging effects, and repeated applications would be too costly.

Information from research

Hundreds of field trials conducted across the Midwest before the 1990s focused on foliar fertilization of soybean, with a few for corn, mainly during the reproductive growth stages (R1 or more advanced). Fertilizers included nitrogen (N), phosphorus (P), potassium (K), and sulfur (S), although several trials included micronutrients. Researchers thought that foliar applied nutrients at these stages would delay leaf senescence and minimize “seed starvation” when nutrient uptake from soil or N gas fixation by nodulated soybean was limited. A few Iowa trials in the middle 1970s suggested that spraying an N-P-K-S mixture at the R5 or R6 growth stages could increase yield by seven to eight bu/acre. However, several subsequent trials in Iowa and other Midwest states until the early 2000s showed inconsistent results, with an equal frequency of yield increases and decreases.

My ISU research group's most recent study with foliar fertilization of soybean at the R2 to R3 growth stages was in 2005 and 2006. The results were summarized in an ICM News article last summer. Treatments included foliar fertilization (3 gal/acre of 3-18-18 sprayed at either the V5 or R2/R3 growth stages or at both stages, and 3.3 gal/acre of 28 percent UAN at the R2/R3 stage), fungicide applied alone at the R2/R3 stage (Headline®), and a tank mix of the fungicide with 3-18-18 or UAN. On average, the fungicide increased yield by 2.9 bu/acre and delayed leaf senescence, although disease control was observed only for brown spot in three fields. Spraying 3-18-18 fertilizer on average did not affect yield, but spraying with UAN decreased yield. The UAN application caused moderate leaf burning and the 3-18-18 application caused no burning.

A great deal of research in Iowa since the the middle 1990s focused on spraying nutrients to soybean at early growth stages. My research group conducted about 100 trials in which treatments were spraying foliar fertilizers with or without a tank mix with glyphosate herbicide at the V5 to V7 growth stages. The commercial products tested (not all products were included in all trials) included 8-0-8, 3-18-18, 10-10-10 (N-P2O5-K2O) with the two N-P-K products with or without S and with or without the micronutrients boron (B), iron (Fe), and zinc (Zn). Product rates ranged from two to six gal/acre applied once or twice (spaced 8 to 10 days). Foliar fertilization increased yield in 15 to 30 percent of fields, and about 15 percent of fields on average. Detailed results were published in a 2008 ICM News article. The average response to the best treatment across all fields was 0.7 bu/acre. Yield with single or double applications did not differ consistently. Treatment differences were not consistent across fields, but yield increases tended to be higher for the three gal/acre rate of 3-18-18. Adding S or micronutrients did not produce higher yield, and the highest rate of 10-10-10 and 8-0-8 sometimes reduced yield (some leaf burn was observed). A recent study in northeast Iowa by ISU Extension field agronomist Brian Lang showed a small soybean yield response to foliar applied manganese (Mn) only in one of three years.

Unfortunately, as explained in an ICM News article last summer, plant tissue testing is not a good index of P and K fertilizer needs for corn or soybean. This is also the case for other nutrients, mainly due to a lack of recent calibration research in Iowa or neighboring states showing how nutrient levels in plant tissue relate to crop response to fertilization. Iowa State University research continues addressing this issue for N, P, K, and S, however, and a new project is beginning to focus on micronutrients.

So what can be recommended?

  • Consider foliar fertilization carefully, and don't expect an economic yield increase in most fields. The probability of a yield response increases when pre-plant fertilization rates were lower than recommended; there are nutrient deficiency symptoms; or soil or climatic factors (other than drought) limit nutrient uptake in late spring or early summer.
  • Plant tissue testing together with soil testing for areas within a field that show deficiency symptoms or poor growth compared with unaffected areas may be useful to make decisions about foliar fertilization. You must be aware of possibly misleading results, however, because several growth factors can increase or reduce the nutrient concentration in tissue.
  • Although there are no clear rules, avoid high rates of N, K, or S because may cause leaf burn and a yield decrease. Also avoid spraying early or during the day in days with prognosticated high temperature (temperatures below about 80 to 85 degrees should be safe mainly for low-salt products), and do it in the evening instead.

 

Antonio Mallarino is a professor of agronomy, with research and extension responsibilities in soil fertility and nutrient management. Mallarino can be reached at apmallar@iastate.edu or by calling (515) 294-6200.

Crop Minute - Scouting Corn Fields

In the July 18, 2011  crop minute, Alison Robertson, ISU Extension plant pathologist, tells producers to begin scouting corn as tasseling and silking starts. She says to target scouting to fields with leaf diseases, low-laying fields and to fields planted with hybrids having a greater susceptibility to leaf diseases. She also gives advice on when to make fungicide applications.

Foliar Fungicides on Corn

Alison Robertson and Daren Mueller, Department of Plant Pathology and Microbiology

It never ceases to amaze me just how quickly corn grows. It does not seem that long ago that the corn was knee-high and now crops are 12 feet tall. Across the state, crops are flowering and pollination is underway. With tassels, comes the question, “Should I spray a fungicide?” 

University and industry data continue to report mean yield responses due to a fungicide application that vary from 4 bu to as much as 15 bu per acre. With grain prices as they are, it may appear to be worth risk to spray, but fungicide decisions should be based on more than just the price of grain.  

Four years of fungicide trials in Iowa and across the Corn Belt at numerous universities has clearly shown that yield responses are greater when disease is present in the field (Table 1). The data in the table shows the disease severity at R5 (dent), and the mean yield response that occurred when disease severity on the ear leaf was low (less than5 percent) and more than 5 percent of the ear leaf was diseased. 

But how much disease should be present at tasseling? Good question and one we are working on. Fungicide trials to establish thresholds for applying a fungicide are in progress in Iowa, Illinois, Wisconsin and Ohio. This work is being funded by a grant from the USDA RAMP (Risk Avoidance and Mitigation Program) program. For now though, the threshold remains to consider a fungicide application if fungal disease is present on the third leaf below the ear leaf or higher on 50 percent of the plants at tasseling and the hybrid is susceptible to the disease. If the hybrid is moderately susceptible, consider other risk factors for disease development such as current and predicted weather conditions during grain fill, disease history of the field and previous crop. If the hybrid has good genetic resistance or tolerance to the disease, no application is needed. Following these guidelines should lead to a greater response to fungicide application due to disease pressure.

 

 

How will current and forecasted weather conditions affect disease development?

The hot, humid conditions forecasted for the next week to 10 days will slow development of eyespot, common rust and northern leaf blight. High humidity favors gray leaf spot (GLS) sporulation and infection. Temperatures in the mid 80s favor GLS development and lesion expansion, but temperatures in the mid 90s slow disease development.


 

Daren Mueller is an extension specialist with responsibilities in the Corn and Soybean Initiative and ISU's IPM program. Mueller can be reached at 515- 460-8000 or by email at dsmuelle@iastate.edu. Alison Robertson is an associate professor in the Department of Plant Pathology and Microbiology with extension and research responsibilities; contact at alisonr@iastate.edu or phone 515-294-6708.



This article was published originally on 7/25/2011 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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