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With the hot temperatures that we had in the last couple weeks many growers are wondering where are we at compared to normal? Below is the GDU information for several different areas of Wisconsin. It is interesting to note that we have several areas that are ahead of the normal deviation. We can start to make management decisions based upon these GDUs with regards to insects and overall GDU units to tassel and other significant effects in our cropping system.

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July 24, 2013 - Issue 4

GDU Update

Growing Degree Units in Wisconsin (May 1 - July 16, 2013)

GDU Location

2012 2013 Vs. Normal

Appleton 1306 1053 -30

Chilton 1298 1057 28

Waupun 1333 1104 -14

Milwaukee 1348 1048 0

Madison 1463 1212 43

Platteville 1434 1228 10

Whitewater 1390 1134 6

Monroe 1510 1287 74

Extreme, early-season moisture, strong wind events and corn rootworm feeding all contribute to increased poten-tial of corn root lodging. If root lodging has occurred, take time to assess the cause of lodging—don’t just assume it is rootworm feeding. Causes of Root Lodging in Corn

Wet conditions at planting, causing sidewall compaction

and restricting root growth

Wet conditions early in the growing season

Strong wind and rain events during critical root develop-

ment and prior to brace root formation

Reduced root growth due to nitrogen deficiency

Hybrid differences in root formation

Corn rootworm (CRW) feeding

Yield loss and lodging due to CRW feeding is affected by the timing of root feeding, weather, soil type and hybrid type. Ex-treme root lodg-ing often results from root feeding by CRW larvae in June, followed by summer storms. Once rootworm larvae have damaged or destroyed roots near the soil surface, they work their way down the root system. The later the root lodging occurs in the growing season, the less able corn is to straighten up without pro-nounced goose-necking. Closely monitor root lodged fields and consider harvesting at higher moistures, if stalk rot becomes an issue.

Understanding Water Usage in Corn Water is acquired, used and lost through evapotranspira-tion (ET). In this process, water is removed directly from the soil surface to the atmosphere by evaporation and through the plant by transpiration. Plant transpiration is the evaporation of water from leaf and other plant surfac-es. For corn, evaporation often accounts for 20 to 30 per-cent and transpiration 70 to 80 percent of total ET over time.

Soybean aphids are right on schedule and were re-cently reported in southern Wisconsin. Even if you planted soybean seeds treated with a nicotinoid insecticide to delay aphid population establishment, you will still want to scout your fields for this pest from V5 through R5. The soybean aphid over-winters in buckthorn and migrates to legumes (soybeans, alfalfa and clo-vers) in July. It’s less than 1 mm in length but when left untreated, economic infestations can reduce yields by more than 10 bushels/acre. Take Action Count the number of aphids on 30 to 50 plants throughout the entire field and calculate the aver-age number of aphids per plant. If aphid populations reach 250 per plant and are increasing during the R1 – R5 growth stages, consider a foliar insecticide to control the population. Your local Pioneer sales rep can help with product recommendations. There are also several bene-ficial insects that feed on soybean aphids and can natu-rally control the aphid population. They include Asian lady beetles, damsel bugs, green lacewing larvae, and insidi-ous flower bugs.

Root Lodging in Corn

Role of Water in Corn Development

Soybean Aphids

Photo: Marlin E. Rice

Corn rootworm damage reduces a

plant’s structural support and makes

it more susceptible to lodging. CRW

injury to nonBt corn roots on left

compared to Bt hybrid on the right.

Taken from an Iowa State University

research plot.

Table 1. Estimated Corn Evapotranspiration and Yield

Loss per Stress Day During Various Growth Stages

Growth Stage

Evapo-transpiration* (inches/day)

% yield loss per day of stress

(min-ave-max)

Seedling to 4 leaf 0.06 ---

4 leaf to 8 leaf 0.10 ---

8 leaf to 12 leaf 0.18 ---

12 leaf to 16 leaf 0.21 2.1 - 3.0 - 3.7

16 leaf to tasseling 0.33 2.5 - 3.2 - 4.0

Pollination (R1) 0.33 3.0 - 6.8 - 8.0

Blister (R2) 0.33 3.0 - 4.2 - 6.0

Milk (R3) 0.26 3.0 - 4.2 - 5.8

Dough (R4) 0.26 3.0 - 4.0 - 5.0

Dent (R5) 0.26 2.5 - 3.0 - 4.0

Maturity (R6) 0.23 0.0

Adapted from Rhoads and Bennett (1990) and Shaw (1988). From: What Happens Within The Corn Plant When Drought Occurs? Joe Lauer, Uni-versity of Wisconsin-Extension, 2003.

*Evapotranspiration is the total of the water used by the corn plant and water lost to evaporation.

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Bacterial leaf blight – Cool wet weather is the optimum conditions for disease develop-ment. Both brown spot and bacteri-al leaf blight can be found on the same plant. Leaves tend to appear ragged as dead areas on leaves will fall out as the leaf is retained. Phytophthora Root Rot – Scout one to two weeks after excessive rains as phytophthora prefers warm, saturated, compacted soil. Phytophthora can be present from VE through R6 growth stage, and it displays seed rot, seedling blight

and root/stem rot phases. Phytophthora can rot soy-bean seeds and cause damping-off which can be diagnosed by the dark brown discoloration of the stem, usually beginning at the soil line. Symptoms of the root/stem rot phases include a smaller than nor-mal root mass and a dis-colored tap root. By the time you notice above-ground symptoms like an uneven, stunted stand, the plant has already been infected below ground for several weeks. Plants may also be light-green due to minimal nodulations on the roots. During later growth stages, dark-brown to red-brown lesions progress up the stem from the roots. Wilting and plant death may soon follow, especial-ly if the plant is stressed. White Mold (Sclerotinia Rot) – Sclerotinia white mold is favored by cooler night temperatures (50-60°F) and moist conditions in the plant canopy. In addi-tion to cool temperatures, the production of white mold apothecia (mushroom-like structures) requires moist soil and a closed canopy. If surface soil moisture is low or the

soybean canopy is not closed during flowering, the fun-gus would not be able to produce apothecia. When scout-ing for this disease, pay attention to the fields that had

white mold previously and that have good soil moisture and a closed canopy. The first evidence of white mold is a chlorotic, girdling lesion covered with white, fluffy myce-lium at one of the middle nodes. The evidence of the dis-ease becomes conspicuous in August when dead tops start to show up in fields. White mold is a disease of high management. It is most likely to show up in low spots of the field, where plant populations are high, in narrow rows, in tightly closed canopies, where plants become lodged and/or where less tolerant varieties are planted. Sudden Death Syndrome (SDS) is a disease of growing

importance in Wis-consin. Since its intro-duction, it has contin-ued to spread in the state affecting more and more acres. Large yield losses can be attributed to SDS when infections are severe. Symp-toms typically show up in July and August even though the in-fection occurred much earlier in the year. Though it is a root disease, it is usually identified by leaf symptoms which begin with scattered yellow, diffused spots between the veins.

The spots will continue to expand into brown lesions sur-rounded by chlorotic areas. Leaves can dry up or curl and will detach from the petioles as the disease progresses. The pith of the stem will stay white, which will help distin-guish SDS from brown stem rot, which can have similar leaf symptoms. SDS does cause a root rot which may have a surface blue fungal growth in moist conditions. SDS often occurs in fields with an infestation of soybean cyst nematode. Varietal resistance along with soybean cyst resistance is the main method of management. Fusarium Root Rot is a disease becoming more preva-lent across the state. Plants suddenly die and when dug

up tap roots are often rotted off. Side hill areas that are less well drained are often parts of fields where Fusarium is most noticea-ble. There is no known varietal resistance to Fusarium. Seed treatments and proper drainage are key manage-ment reminders.

Soybean Diseases

Plants wilting among healthy plants is often a symptom of phy-tophthora.

Photo: DuPont Pioneer

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Soybean plant showing symptoms of white mold.

Leaf blight symptoms

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Sudden death syndrome symptoms show up on leaves in July and August.

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Fusarium root rot. Photo: DuPont Pioneer

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