Northwest Iowa Crop Update Newsletter by Todd Vagts ISU Extension Crops Specialist Counties Served:  Carroll, Calhoun, Crawford, Ida, Monona, Pocahontas and Sac.
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Volume 2, Number 23

Northwest IA Crop Update, August 26, 2002
(Word Document)

In this issue 
Western Research Farm Field Day
Assess corn and soybean fields now for agronomic and pest problems
Short husks and incomplete pollination
Corn Smut in area corn fields
Cercospora Leaf Blight in soybeans
SDS in soybeans

Introduction

The Iowa State University Western Research and Demonstration Farm will host its annual fall livestock and forage field day on Tuesday afternoon, August 27.  Mild growing conditions continue which will help with grain fill and hopefully add test weight to area corn and soybean crops.  Asses area corn and soybean fields now for insect, weed or pathogenic pest problems.  Corn smut may be common is corn field this year due to plant stress and favorable fall conditions.  Cercospora leaf blight and SDS has shown up in area soybean fields.  Scout for these diseases and others so appropriate agronomic and crop selection decisions can be made in following years.

Western Research and Demonstration Farm Livestock and Forage Field Day

Please plan to attend the Iowa State University Western Research and Demonstration Farm

Livestock and Forage Field Day on Tuesday, August 27, 2002 - Castana, Iowa, Time 1:00 – 5:00 p.m.

The afternoon sessions will highlight research at the farm addressing current and future livestock and forage issues.  Location:  The Livestock and forage Field Day will be held at the Western Research and Demonstration Farm, Located four miles east of Castana on county highway E34

Growing Degree Day Accumulation and Crop Development
Scout and evaluate corn and soybean fields now while you have time.  Many disease, insect and weed pest problems can be found in area corn and soybean fields now.  Obtaining a positive ID on the disease, weed or insect will be very important when making crop rotation, hybrid and variety selection decisions for next year.  If you, your crop consultant, seed or Coop agronomist cannot absolutely identify what the problem is, contact your local extension office and send plant or insect samples to ISU for help.  The ISU plant disease clinic http://www.extension.iastate.edu/Pages/plantpath/pdcintro.html, weed, insect and soil fertility specialists are there to help you identify problems and develop remedial actions. 

 Short husks and poor pollination.  A few reports have come in of husks that do not completely cover the ear or complete kernel rows that did not pollinate.  Both of these problems are most likely associated with moisture deficit stress.  Short husks are most likely due to moisture stress that led to a lack of turgor pressure, resulting in smaller, shorter husks.  This is the same result that you see as shorter plants and smaller leaves in moisture stressed plants.  The cob has higher priority for water, therefore it was able to grow and extend to normal length, whereas the husks did not.  Entire kernel rows that did not pollinate also most likely resulted from moisture stress.  Either the silks did not have enough turgor presser to fully extend out of the husk or pollen drop was low (due to heat sterilization or improperly timed silk emergence and pollen drop) and the chance for landing on a silk was reduced, particularly if the silk was shaded by some obstruction. 

Pest Management
Smut may be common in corn fields this year.  Common corn smut, caused by the fungus Ustilago zeae or Ustilago maydis, is a well-known disease that usually does not cause economic damage. Smut can infect any part of the plant, especially actively growing meristematic tissue. Infections of the stalk, ear, or tassels result in smut galls, which are distorted growths that have a shiny, silvery-white surface. These galls eventually rupture to release millions of powdery black teliospores.  The spores released by the smut galls usually do not infect plants during the same season, but fall into the soil where they can survive many years. If the spores fall onto susceptible plants parts, such as silks, infection may occur in the same season. Silk infection results in ear infection.

By harvest, the smut galls have mostly ruptured and dried, leaving nothing but masses of black spores that become airborne when the plant is disturbed. Harvested kernels are not infected but may have many spores clinging to them externally. These spores are not a threat of further infection, nor are they harmful to livestock or humans. 

Higher incidence of smut this year may be related to poor pollination in specific fields. The extended dry spell during the mid-season was detrimental to pollination for many hybrids. When pollination is poor, the silks remain susceptible to smut for a longer duration, and eventually they become infected.

There are no fungicidal treatments for smut. Some hybrids and inbreds are more resistant, so avoid those that seem consistently susceptible. Hybrid decisions, however, should not be based on experience in a single year. The disease is favored by excess nitrogen or manure, herbicide injury or damage by cultivation, hail, or blowing soil. Avoiding these conditions, where possible, reduces the disease incidence. Crop rotation has little effect because the spores survive for several years.

Excerpted from “Common smut more common than usual” Integrated Crop Management, 11/19/2001.  Gary Munkvold, ISU Extension Plant Pathologist.  http://www.ipm.iastate.edu/ipm/icm/2001/11-19-2001/commonsmut.html

 Cercospora leaf blight in soybeans has shown up in a Crawford county soybean field.  Symptoms of Cercospora leaf spot are mottled purple-to-orange discoloration of the uppermost leaves. The infected leaves become purple and have a leathery appearance in August and become orange or bronze when the plant is approaching maturity. Severe infection may cause early defoliation. The fungus also can infect seeds and cause a purple discoloration of the seed coat that can cause poor seed vigor and reduced germination if planted next year.  If you find such bad spots, consider tillage to bury infected residues that may harbor this pathogen.  For more information, visit this web site:  http://www.ipm.iastate.edu/ipm/icm/2000/9-18-2000/predefol.html

Pictures at this site:  http://www.extension.iastate.edu/carroll/crops/field_problems/case_6_02.htm

Sudden Death Syndrome in soybeans has been found in several locations in Calhoun County.  The SDS pathogen infected plants last spring under cool moist conditions.  Early planted fields are at highest risk for infection.  X.B. Yang (ISU Extension Pathologist) describes symptoms to look for as scattered yellow spots between leaf veins. These spots eventually coalesce to form brown streaks between the veins (interveinal necrosis). Only the midvein and major lateral veins remain green. Leaflets drop eventually.  Diseased plants have deteriorated taproots and lateral roots. The root cortex is light gray to brown, and the discoloration may extend up into the stem 2 inches above ground. Sometimes bluish fungal colonies can be seen on the root if soil moisture is high.

SDS can spread rapidly throughout a field, detection and positive I.D. is essential to protect soybean yield and profitability.  Once a field has been identified with infection of SDS, select SDS tolerant varieties in subsequent years.  No resistance to this disease is available in any soybean varieties.  Read the most recent ICM article for more information on SDS:  http://www.ipm.iastate.edu/ipm/icm/2002/8-19-2002/sdsprevalence.html

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