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7/15/2013 - 7/21/2013

Soybean Vein Necrosis Virus Identified in Iowa

By Daren Mueller, Department of Plant Pathology and Microbiology

Soybean vein necrosis virus (SVNV) was first confirmed in Iowa last season. Last year we did not see SVNV until August. This past week we identified SVNV in several locations in Iowa. It is not known yet if earlier symptoms may increase chances of yield loss. We will continue to monitor and provide updates.

The virus belongs to the tospovirus group, which is vectored by thrips and possibly other insects. Symptoms often begin as chlorotic (light green to yellow) patches near the main veins, which may enlarge eventually becoming necrotic (brown) areas (Figure 1). The veins may appear clear, yellow or dark brown. The browning of the veins may be especially noticeable on the lower leaf surface (Figure 2), but this may not always occur.

Currently, there are no management recommendations for this disease. Other pathosystems that include thrips and tospoviruses, including tomato spotted wilt virus, focus on resistance and management of the vector. Because of the newness of this disease, there are no known sources of resistance. Insecticide application only should be considered in fields with a known risk of yield loss.

 

Figure 1. Foliar symptoms of SVNV on soybean


 

Figure 2. Browning of the veins on the lower leaf surface

 

Daren Mueller is an assistant professor in the Department of Plant Pathology and Microbiology. He can be reached at 515-460-8000 or e-mail dsmuelle@iastate.edu.

Hail and Fungicides Update 2013

By Adam Sisson, Integrated Pest Management, and Daren Mueller, Department of Plant Pathology and Microbiology

When crop hail damage occurred in the past, farmers could either replant or see if crops would grow out of the damage, depending on extent and timing of hail. However, there have been significant changes in production practices for corn and soybean, especially concerning fungicide use and the labeling of fungicides for the mitigation of plant stress such as hail injury.

One reason that fungicide applications are considered for a hail-damaged crop is that disease infection is more likely to occur after wounding. However, foliar diseases managed by fungicides do not require wounds for infection. It is also argued that crops could be more susceptible to fungal pathogens as a result of increased stress. Another reason fungicides are considered after hail damage is that physiological benefits gained from a fungicide application will help sustain or increase yield of damaged crops. It is important to note that claims by the chemical industry do not state fungicide applications recover yield potential lost due to hail damage. But some claims do suggest fungicide application to hail-damaged crops will protect the remaining green tissue and allow plants to maximize yield after sustaining damage.

In 2012, we used a custom made hail machine to shoot ice at soybean plants (Figure 1), and weed eaters in corn plots (Figure 2), to simulate hail damage. We examined various pesticides, pesticide application timings, and hail damage timings in corn and soybean at two locations in Iowa.

Figure 1. The business end of the hail machine “hailing” soybean plants. Photo courtesy Amy Asmus.

 

Figure 2. Corn ear injured from simulated hail using a weed eater. Photo courtesy Stith Wiggs.

 

In this first year of the study, averaging data from all 2012 treatments within each corn and soybean experiment indicated:

  • Non-injured plots yielded more than those receiving simulated hail;
  • Simulated hail applied at growth stage R4 (full pod) resulted in less yield loss than hail simulated at growth stage R1 (beginning bloom) for soybean;
  • Simulated hail applied at growth stage R2 (blister) resulted in more yield loss than hail simulated at growth stage VT (tasselling) for corn;
  • In general, an application of fungicide to corn seven days after a simulated hail event resulted in greater yields compared to applying the fungicide at two days after the simulated hail event;
  • Application of fungicide in corn usually resulted in increased yield over non-application across all treatment averages (both hailed and non-hailed); and
  • In soybean, fungicide application resulted in increased yield about 60 percent of the time across all treatments (both hailed and non-hailed). Breaking this down we see that:
    - Treatments with Headline® averaged 1.2 bu/acre more, Domark® averaged 0.4 bu/acre less, and Stratego® YLD averaged 2.8 bu/acre less than untreated controls.
    - Treatments with Headline® plus insecticide averaged 1.8 bu/acre more than plots with insecticide alone.

Current research suggests using fungicides is most beneficial in response to a disease threat, which did not exist during the dry conditions of 2012. These preliminary data, along with the need for testing these treatments in the context or a more “normal” season, suggest application of fungicides on hail-damaged crops merits additional research. We are repeating the experiment in 2013 with additional treatments and we will keep you posted.

 

Adam Sisson is an Integrated Pest Management extension specialist. He can be reached at ajsisson@iastate.edu or (515) 294-5899. Daren Mueller is an assistant professor in the Department of Plant Pathology and Microbiology. He can be reached at 515-460-8000 or e-mail dsmuelle@iastate.edu.

It May Be a Good Year for a Fungicide Application

By Alison Robertson, Department of Plant Pathology and Microbiology

Delayed planting coupled with the wet spring may mean this is a good year for a fungicide application on corn. Wet conditions have favored the development of some foliar diseases within the canopy, and many fields are delayed developmentally due to later planting. In the past two growing seasons, most of the corn in Iowa has been flowering by mid-July. This year, it will be another 10 days or more before widespread tasseling occurs in the state. When diseases start during early grain fill, this increases the risk of reduced yield.

Research has shown that greater yield responses due to a fungicide application consistently occur in the presence of disease. In Iowa, mean yield response was greatest when disease severity in a field at R5 was high. If disease severity on the ear leaf at R5 was <5 percent, mean yield response was 4.83 bu/A; however, when disease severity on the ear leaf at R5 was >5 percent, the mean yield response was 9.46 bu/A. Therefore, to increase the chance of a return on investment of a fungicide application, farmers should target fields that are at high risk for disease development during grain fill.

To identify fields that are at an increased risk for disease, and consequently an increased chance they will benefit from a fungicide application, consider the following three factors:

  1. Hybrid: Hybrids vary in their tolerance to various diseases. “Race horse” type hybrids are often susceptible to one or more diseases. Fields planted to susceptible hybrids may require a fungicide application to protect yield.
  2. Presence of disease: Gray leaf spot (Figure 1) and northern leaf blight often develop in the lower canopy and then move up the plant. Common and southern rust are more often found in the mid to upper canopy. Presence of leaf spots in the canopy means the weather has been favorable for disease development and inoculum is present in the field.
  3. Weather conditions during grainfill: For disease to happen, or continue to develop, certain environmental conditions need to occur (Table 1). 


Table 1. Conditions that favor foliar disease development in corn




Figure 1.  Early symptoms of gray leaf spot are small tan spots that are bordered by leaf veins and occur in the lower canopy. 

 

There are several fungicides registered for use on corn, and new products continue to be introduced. The Corn Disease Working Group, corn pathologists from across the United States, developed a Fungicide Efficacy Table that is available here.  The pre-harvest interval (PHI) of the products ranges from 7 to 45 days so always remember to check the label and consider days to harvest before applying a product.

Pre-tassel applications of fungicides may result in arrested ear syndrome. Research at Purdue University showed spray additives (NIS surfactants and crop oil) to pesticides applied between V10 and VT increased the risk of this disorder. Wet spring conditions across Iowa have resulted in uneven stands that range widely in crop growth stage. Try to wait to apply a fungicide until the field has completely tasseled. Some fungicides do not require the addition of a surfactant – check the label, however when fungicides are applied aerially, crop oil is used to ensure the fungicide product does not evaporate and is deposited within the canopy. 

 

Alison Robertson is an associate professor of plant pathology and microbiology and extension and research responsibilities. She can be reached at alisonr@iastate.edu or 515-294-6708.



This article was published originally on 7/22/2013 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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