In a stressful growing season, attack from plant patho-gens may begin to show up late in the year as patches of dying or wilted plants with drooping or diseased leaves. While soybean diseases may become apparent just prior to harvest, the infection may have occurred much earlier in the season. These above-ground symptoms are com-mon to several unique below-ground problems. A quick peek at roots and lower stems can help determine which of these pathogens might be at work in your soybean fields. Proper identification can help with future variety selection and management decisions.

Sudden Death Syndrome (Fusarium virguliforme) produces striking leaf symptoms (Figure 1), which alert us

to problems in the roots. Affected plants may die rapidly after first leaf symptoms ap-pear due to toxins pro-duced by the root rot-ting fungus. Split stems will generally show only minor discoloration in solid cortex areas, with normal white pith. Cool, moisture conditions early in the growing season often results in higher disease inci-

dence. Stress due to heat and drought may reduce occur-rence of SDS in some cases. Brown Stem Rot (Phialophora gregata) produces similar striking leaf symptoms mid-season as SDS, which may cause confusion of the two diseases. Tissues between veins become yellow and quickly turn brown, except for a narrow band of green tissue outlining the vein. However,

BSR is distinguished from SDS and other diseases by brownish discoloration due to an infection of the pith tissue in lower stems (Figure 2). This infec-tion impedes the movement of water and mineral nutrients needed for growth.

Northern Stem Canker (Diaporthe phaseolorum) is a re-emergent soybean disease that begins at points along lower stems, creating brownish-red lesions (cankers) ex-tending part way around and into lower stems (Figure 3). Affected plants often retain dead leaves even up to har-vest time. Roots and pith (the soft center are of stems) are generally not affected by stem canker. White Mold (Sclerotinia Rot) Sclerotinia white mold is favored by cooler night temperatures (50-60°F) and moist conditions in the plant canopy. In addition to cool temper-atures, the production of white mold apothe-cia requires moist soil and a closed canopy. If surface soil moisture is low or the soybean canopy is not closed during flowering, the fungus would not be able to produce apo-thecia. When scouting for this disease, pay attention to the fields that have a history of white mold and fields that have good soil moisture and a closed canopy. The first evi-dence of white mold is a chlorotic, girdling lesion covered with white, fluffy mycelium at one of the middle nodes. The evidence of the disease becomes conspicuous in August when dead tops start to show up in fields. It is most likely to show up in low spots of the field where plant popula-tions are high, in narrow rows, in tightly closed canopies, where plants become lodged and/or where less tolerant varieties are planted.

WALKING YOUR FIELDS® newsletter is brought to you by your local account manager for DuPont Pioneer. It is sent to customers throughout the growing season, courtesy of your Pioneer sales professional. The DuPont Oval Logo is a registered trademark of DuPont. PIONEER® brand products are provided subject to the terms and conditions of purchase which are part of the labeling and purchase documents. ®, TM, SM Trademarks and service marks of Pioneer. © 2013 PHII.

Late Season Soybean Diseases

WALKING YOUR FIELDS

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August 28, 2013 - Issue 5

Figure 3. Northern stem canker. Photo: L. Osborne, DuPont Pioneer

Figure 1. Leaf necrosis caused by SDS or BSR. Photo: DuPont Pioneer

Figure 2. Brown stem rot. Photo: L. Osborne, DuPont Pioneer

Figure 4. White Mold Photo: DuPont Pioneer

Yield information from monitors and GPS is an extremely important tool that you can use to make deci-sions in your business. However, it is only as accurate as the calibration and taking time to do this will help elimi-nate poor information later. Here are some tips to remem-ber when looking at yield monitor calibration:

Clear memory card and back up previous year’s data

Calibrate multiple times throughout the growing sea-son to ensure consistent data

Make sure your load is enough weight (3,000-6,000 pounds)

Make sure you have multiple loads at different speeds (3.5, 4.5, 5 etc.)

Calibration loads should be in a uniform area of the field, a good representation will improve accuracy

Differences in moisture and grain quality will require a new calibration to be accurate

In a year with variability, taking time to calibrate will pay big dividends!

Many corn fields in the region were planted mid to late May or early June, which means they will likely need until early October to reach maturity (black layer). As growers hope for a late autumn to get field work done before the snow flies, now is the time to weigh the cost of in-field grain drydown versus using artificial drying at harvest. There are three main factors that influence the rate at which corn dries, physiological maturity, weather and the corn hybrid. Corn that matures earlier typically dries fast-er due to more favorable drying conditions earlier in the harvest season. On the same note, later maturing corn has fewer warm days to aid drying and will dry slower. In a typical year, corn that matures on September 15 may

require only about 10-15 days to reach 20 percent mois-ture, while corn that matures on September 25 may need 30 days to reach the same moisture level (D.R. Hicks, 2004). Drydown is linked to heat units (GDUs). Under ideal weather conditions, corn may lose up to one point of moisture per day. As the days get cooler, GDUs de-crease and drying slows. A rule of thumb is that 30 GDUs are required to lower the grain moisture each point from 30 percent to 25 percent. Forty-five GDUs per moisture point are required from 25 percent to 20 percent. Also consider that we typically expect no more than about 10 GDUs per day in late September, and only about 3-5 GDUs per day in late October. That means that field dry-ing of corn may take two to three times longer for late maturing fields. Drydown is also hybrid specific. The amount, thickness and tightness of husks affect drydown; the more insulated the ear, the longer it takes to dry. Up-right ears are more prone to capture moisture in the husks. Lastly, corn with moderate test weight dries faster than hybrids with heavier test weights. The ideal harvest moisture for corn is between 22-25 per-cent. Waiting for corn to dry to 18 percent moisture in the field certainly saves on the energy bill; but it also increas-es the likelihood of excess harvest losses due to stalk lodging, ear drop and detrimental weather all of which can affect your bottom line. In addition, there may not be enough heat units this fall for in-field drydown. Ohio State University research indicated no additional in-field grain drying occurred after early to mid-November (Minyo, Geyer & Thomison, 2009). “Phantom yield loss” may also encourage growers to har-vest at slightly higher moistures this fall. Growers occa-sionally report harvesting part of a field early and finishing the field when it is dryer, only to discover the later har-vested portion of the field is yielding several bushels less per acre than the first harvested corn. Purdue University confirmed these claims indicating that grain corn dried in the field has the potential to yield one percent less per point of moisture. For example, corn that was 200 bushels/acre at 28 percent moisture would only yield around 180 bushels/acre at 18 percent moisture if left in the field too long. This research also confirmed that the ideal moisture level for corn grain harvest is 25 per-cent. Harvesting wetter than that can damage kernels and of course significantly increase drying costs (Nielsen et al., 1996).

Calibrating Yield Monitors

Corn Drydown & Harvest Timing

Estimated Cost to Dry Corn to 15% Moisture

Harvest Moisture

LP gal/bu

LP $/gal

LP $/bu

Drying Cost $/bu*

Drying cost $/point*

35 0.472 1.35 0.637 0.645 0.032

30 0.337 1.35 0.455 0.461 0.031

25 0.219 1.35 0.296 0.299 0.030

20 0.109 1.35 0.147 0.149 0.030

Based on: NCH-51 Hybrid Maturity-Energy Relationships in Corn Drying, Iowa State University; *assumes electrical cost of $0.115/kwh

Delaying harvest may increase risk of lodging, ear drop or kernel loss and result in reduced yields. Photo: DuPont Pioneer

>>

Artificial drying costs will vary this season depending on LP gas prices, which currently range from $1.25 - $1.40 per gallon across the Midwest. Nonetheless, harvesting at a higher moisture level this year may increase grower profitability, specifically when growers anticipate medium to high harvest losses. The table on page two was devel-oped by Iowa State University, and customized by DuPont Pioneer agronomists to illustrate different drying scenarios and costs based on an average LP price of $1.35/gallon. If you have questions about moisture levels, drydown and harvest, contact your local sales representative for assis-tance. (Sources: Minyo, Geyer & Thomison. 2009. How will delaying corn har-vest affect yield, grain quality and moisture? Ohio State University Ex-tension. Nielsen et al. 1996. Kernel dry weight loss during post-maturity drydown intervals in corn. Purdue University.)

The results of the fall 2011 tillage are evident in the 2012 crop. Particularly in corn on corn cropping systems. Fall 2013 is shaping up to be a similar environment and may present some of the same opportunities and challenges. It may be helpful to review and adapt for this environ-ment. Challenges of 2011 Primary Tillage

Hard, dry soil led to equipment stress, frequent breakdowns, and dramatically increased wear on soil contacting blades and points.

Primary tillage standards on many makes and models are set up on wide spacing (30”). To increase residue coverage many are equipped with wings on the points. These wings on wide spacing can cause large boulders of soil and eventually seedbeds that exhibit areas of loose dry soil intermingled with firm moist conditions.

Too much speed. To fracture the soil and form con-sistently sized chunks that are field level, we may need to reduce ground speed to 4.5 to 5 mph. Each soil and type of tillage equipment is different, but slowing down may improve the results.

Opportunities for Tillage in a Dry Soil

Primary tillage is most effective at lifting soil compact-ed layers and producing an even shattering effect across the width of the tillage tool.

Soil smearing with ground contacting points is mini-mized

Leveling devices on primary tillage tools may work more effectively.

Appropriate levels of residue incorporation can be accomplished which increases soil to residue contact enhancing residue breakdown without layering resi-due and causing seedling residue interactions (corn on corn concern)

Residue sizing and corn root ball management can be more effective with the use of corn head choppers or stalk choppers combined with vertical tillage tools prior to primary tillage.

Deep tillage will not ‘dry out your soils’.

Make every attempt to incorporate and reduce large residue for next spring planting.

Will Fall Tillage be Similar to 2011?

Large depressions from primary tillage. In this case there is a 12” difference between the high and the low behind the digger. In a spring like 2012 where the conditions were dry this may lead to areas of dry loose soil slumping into the depressions in contrast to the firm moist areas. This can lead to planter performance issues on planting depth and seed drop due to engagement of gauge wheels. This then leads to differing emergence rates due to seed-to-soil con-tact and water uptake into the kernel. In a wet spring and summer, these factors are less of a problem. Photo: Kimberly Ag Consulting. Maxwell, IA.

Variable ear placement and ear sizes based on emergence timing and interaction of seedlings with residue in the de-veloping root system. By digging up runt plants and com-paring to larger better developed neighbors you can start to understand causes of delayed emergence or delayed growth. Photo: Kimberly Ag Consulting. Maxwell, IA.

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