Scouting Late Season Diseases  08/02/18 9:47:52 AM

COUNTY FAIRS---Please stop by our booth to rest your feet and have a cold drink and a snack at the Sarpy Co. Fair in Springfield, August 1st --August-5th and the Cass County Fair in rural Weeping Water August 8th-11th. SEE YOU THERE!!

 

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  • As of July 30th---Corn planted April 25th has received 2242 GDD and 17.38 inches of rainfall here in Plattsmouth

     

    Most of the corn in SE NE is between the DOUGH TO DENT stage of development.  We are still running about 230 GDD ahead of “normal” in regard to plant development.  The corn is on the “down-hill slide” in regard to the amount of soil moisture being used on a daily basis as we are using about 0.20” per day.  We will again use the following tables to stage the development of your respective corn hybrids.

     

     

Growing Degree Days for Corn Growth Stages for a 113 day Hybrid

Stage                                                   GDD (Growing Degree Days--base 50)        

Emergence                                                                 

120

 

2 leaf-V2                                                                    

200

 

V6-tassel initiation                                                     

475

 

V10                                                                            

740

 

V14                                                                            

1000

 

VT (tassel emergence)                                                

1150

 

Silking                                                                        

1400

 

R4 (Kernel dough stage)                                            

1925

 

R5 (Kernel dent stage)                                               

2450

 

R6 (physiological maturity-black layer)                    

2700

 

*courtesy of Hollinger (University of Nebraska)

 
     

Corn Water Use Per Day

   
     

Growth Stage                       

 

Inches Water Use/Day

1-4 leaf                                           

 

02-.05

5-8 leaf                                   

 

.05-.10

8-10 leaf                                

 

.10-.15

11-14 leaf                                           

 

.15-.20

14-18 leaf                            

 

.20-.25

19 leaf - blister                                    

 

.25-.30

Milk - soft dough                     

 

.20-.25

Hard dough - early dent        

 

.15-.20

     

Corn Water Requirements to Maturity

   

Stage of Growth                             

Approx. Days to Maturity   

Ave. Water Use/Day Maturity

     

Blister Kernel                          

45

10.5 inches

Dough                                             

34

7.5 inches

Beginning Dent                                              

24

5.0 inches

Full Dent                                              

13

2.5 inches

Maturity (Black Layer)                         

0

0

 

Late Stages of Corn Development--Use the following table to determine what stage your corn is in, how much longer it will take to mature, and the effects that stress during these stages can have on final grain yield.

 

 

Growth

Stage

Days To

Maturity

 

Description

Blister (R2)

40-50

Kernels are white and resemble blisters. Relocation of plant nutrients from vegetative structures to the ear has begun. Severe stress can easily cause tip kernels to abort.

Milk (R3)

35-42

Kernels are mostly yellow and contain a milky-white fluid. This is the “roasting ear” stage. Severe stress can still abort kernels as well as reduce kernel weight.

Dough (R4)

30-36

Continued starch accumulation is giving the kernel’s inner fluid a pasty consistency. Kernel abortion is not very likely, but severe stress can reduce test weight.

Dent (R5)

20-27

All or nearly all of the kernels are dented. The milk line slowly progresses to the kernel tip over the next 3 weeks or so. Kernel moisture at the beginning of the R5 stage is approximately 55%. When the kernel reaches ½ milk line, it will be roughly 40% moisture, 10 to 14 days from black layer, and have attained 95% of its maximum yield. Stress at this stage can still reduce kernel weight but not number.

Maturity (R6)

0

The black layer has formed and kernels have attained their maximum dry weight. Kernel moisture is roughly 30 to 35%. Stress at this point will not hurt final yield.

  

At our current pace, much of the corn in SE Nebraska should black layer (physiological maturity) sometime in the 20’s of August.   The corn kernel reaches physiological maturity and maximum dry weight with the appearance of the black layer near the tip of the kernel. This generally coincides with kernel moisture of 30 to 35%. You can see the black layer by removing the kernel tip with your thumbnail or by cutting the kernel in half lengthwise. The black layer is caused by collapsing cells that block the tubes translocating food into the kernel. Tip kernels will reach black layer first. Under stress, tip kernels will reach black layer relatively early, indicating the plant was not able to completely fill all the kernels that were pollinated.

Drydown after black layer formation varies greatly from hybrid to hybrid. Hybrid characteristics such as husk coverage can have a significant impact on field drying since kernels dry from the crown inward toward the cob, not from the cob outward. A general rule of thumb is that 30 growing degree units (GDU) are needed to lower the grain moisture each point from 30 to 25% and 45 GDU per moisture point are needed from 25 down to 20%. The time between black layer and harvest moisture largely depends on air temperature and air movement. During September and early October, mature corn should lose up to 1% moisture per day.

Estimating Corn Grain Yield

This annual process this time of year is more fodder for your friends and folks at the coffee shop.  HOWEVER, you can get some early estimate of corn yield as soon as two to three weeks after pollination. This estimate is only as good as the number of samples taken.  It also does not take into consideration kernel depth and test weight, which will be determined these first weeks of August. 

 

To make this estimate, calculate the number of kernels per acre, then divide that by the average number of kernels in a bushel of corn. Here’s how.

1. Walk into the field a set number of paces. Establishing a predetermined pattern eliminates the natural tendency to stop where the crop appears to be better than average.

2. Measure 1/1,000 acre and count the number of ears. The length of row to measure for 1/1,000 acre depends on your row spacing, as shown in the table below:

Row Width

Row Length Per 1/1000 acre

15 in

34 ft 10 in

20 in

26 ft 2 in

22 in

23 ft 10 in

30 in

17 ft 5 in

36 in

14 ft 6 in

Do not count ears that have only a few scattered kernels, because these won’t contribute much to yield.

3. Sample three ears from the measured row length. To avoid any sampling bias, take the third, sixth and tenth ears. Do not sample ears that were not included in the ear count.

4. Count the rows and average number of kernels/row on each ear. Do not count tip kernels less than half size. Multiply the number of rows by kernels/row to get kernels/ear.

5. Estimate yield by the following formula:

Bu/A = (number of ears x average number of kernels per ear)/80.

Some folks use 80 when the corn really has not experienced much developmental stress and the kernel depth and test weight appears to be good.  Others will use 90 in this formula as a bushel of corn in a stressed environment normally contains about 90,000 kernels. We drop the 1,000s because we’ve counted the ears in 1/1,000 of an acre. If kernel size is unusually large or small, adjust the divisible number to reflect this: use 90 for small kernels and 80 for large kernels. In good environments like this year, kernels should tend to be large—use 80. 

6. You will get a more reliable estimate of yield if you repeat steps one to five for each 10 acres in the field.

You have to sample representative ears to get a reasonable estimate of yield. Selective sampling of ears, or kernel sizes that vary markedly from normal, will cause these yield estimates to differ significantly from harvested yield.

 

End-of-Season Stalk Nitrate Test

The end?of?season corn stalk nitrate test is an excellent tool to evaluate the nitrogen status of corn. The test is based on the fact that corn plants deficient in nitrogen will remove nitrate from the lower portion of the stalk, and plants with excess nitrogen will accumulate nitrogen in the lower portion of the stalk. Sampling for this test is done by cutting the stalk at 6 and 14 inches above the soil to get an 8-inch segment. It’s recommended that 15 stalk segments be collected to represent that field.  The time to sample is one to three weeks after black layer. The samples can be sent to most soil testing labs for analysis. The concentration of nitrogen in the stalk sample determines if the field was in the low, optimal or excess category with respect to nitrogen fertility for the season.

 

Corn Stalk and Root problems—Remember Last Years Harvest Lodging??   Even though foliar disease pressure has been light to this point, that does not mean that we cannot experience ill-fated affects from Southern Rust, GLS, Goss’s Wilt, etc. in the months of August and September.  SCOUTING IS THE KEY to determining what/if and issue exists and scheduling harvest accordingly IF the stalks integrity has been significantly affected. 

 

Why do stalk quality/stalk rot problems develop?

 

For a corn plant to remain healthy and free of stalk rot the plant must produce enough carbohydrates by photosynthesis to keep root cells and pith cells in the stalk alive and to meet demands of grain fill. When corn is subjected to foliar fungus—Common/Southern Rust, Gray Leaf Spot, Goss’s Wilt etc., and/or extreme weather conditions such as drought or excessive wet weather, photosynthetic activity is sharply reduced and carbohydrate levels available for the developing ear are insufficient. The corn plant responds to this situation by remobilizing carbohydrates from the leaves, stalk, and roots to the developing ear. While this “cannibalization” process ensures a supply of carbohydrates for the developing ear under stress conditions, the removal of carbohydrates results in premature senescence and death of the pith cells in the stalk and root tissues which predisposes plants to root infection by stalk rot fungi.

Can differences in relative maturity between the hybrids and timing of environmental stress play a role?

Short-season hybrids are often associated with more lodging, but the timing of an environmental stress can also be a major factor.  Because short-season hybrids produce less photosynthate than later maturing hybrids there may be less carbohydrate available to fill grain and maintain stalk integrity. As a result, they may be more vulnerable to droughts, leaf blights, and other stresses than later-maturing hybrids. However, if full season hybrids are subjected to late-season droughts, hail damage, or leaf blight during grain fill, their stalk quality may be impacted more than earlier-maturing hybrids (because the latter may be closer to completing grain fill).

Some hybrids are more prone to cannibalizing their stalk for ear length and grain fill.  What stresses trigger this cannibalization, and what is the critical growth period when environmental stress has the most impact on resulting stalk quality?

Stresses which increase the likelihood of stalk rot problems in corn include drought; loss of leaf tissue due to foliar diseases, insects or hail; injury to the root system by insects or chemicals; high levels of nitrogen in relation to potassium in the soil; compacted or saturated soils restricting normal root growth; and high plant populations. Several of these stress conditions occurred this season. Saturated soils early in the season led to problems with root development involving restricted and limited growth. Protracted periods with high temperatures and limited rainfall stressed have plants as we enter grain fill. And finally, in some areas environmental conditions were particularly favorable for foliar diseases. In some locales, second-generation European corn borer also caused stalk injury and may also contribute to lodging problems. Environmental stresses (drought, leaf blights, hail) during grain fill probably have the greatest impact on stalk quality.

Six Common Stalk Rots

 

Corn stalk rot development often is related to the growth stage of the plant and is very dependent on environmental conditions. Following is a brief description of the more common stalk rots, geographic location and timing of attack.

Anthracnose Stalk Rot

Description:  Large, dark brown to shiny black areas of streaks. It’s the only major stalk rot that is airborne, not soil borne. Top dieback precedes stalk lodging. Upper two or three leaves may turn yellow or reddish and drop off.

Location:  Corn following corn and surface residue remains, such as in minimum till and no-till.

Timing:  Late season under wet, humid and warm conditions.

 

Gibberella Stalk Rot

Description:  Leaves on infected plants turn dull grayish-green; lower internodes soften and turn tan to dark brown. Identified by pinkish-red color in pith tissues.

Location:  More common in central to northern Corn Belt.

Timing:  Late season under cool, wet conditions. Severe plant stresses increase likelihood of  Gibberella.

 

Diplodia Stalk Rot

Description:  Leaves wilt, become dry and appear grayish-green, resembling frost damage. Small, pinpoint black specks appear around the nodes, which are brown and spongy. Affected plants die suddenly.

Location:  More common in continuous corn and reduced tillage.

Timing:  Dry early-season conditions and warm, wet weather two to three weeks after silking. High N, low K, high populations, and leaf loss through disease, hail or insect damage make plants more susceptible.

 

Charcoal Stalk Rot

Description:  Black specks on vascular strands and a charred appearance on interior of stalks denote charcoal stalk rot.

Timing:  Hot, dry weather conditions increase the chance of charcoal stalk rot.

 

Pythium Stalk Rot

Description:  Usually confined to a single internode just above the soil line. Diseased stalks will appear brown, water-soaked and soft.

Timing:  Can develop during extended hot, wet or very humid weather. Usually becomes evident shortly before or after tasseling. It’s the only stalk rot to attack plants prior to pollination.

 

Fusarium Stalk Rot

Description:  Similar to Gibberella and generally less common. Rotting affects roots, plant base and lower internodes. Whitish-pink to salmon discoloration in pith.

Timing:  Dry early-season conditions and warm, wet weather 2 to 3 weeks after silking favor the disease. High N, low K (and generally, fertility imbalances), excess plant population, and leaf loss through disease, hail or insect damage make plants more susceptible.

 

Late-Season Soybean Diseases

Soybean diseases such as sudden death syndrome (SDS), brown stem rot, charcoal rot, Rhizoctonia stem rot and Phytophthora root rot can cause early senescence. Documentation of what is the causal agent will help in management for these affected acres for the next soybean crop.  From a distance, the symptoms of yellow or chlorotic soybeans appear similar in the field. The first symptoms to appear are yellow or chlorotic areas in-between the leaf veins that turn tan to brown. Eventually, plants will defoliate early leaving stems, petioles and pods. Determining which pathogen is responsible can be a challenge.

 

Here are some key characteristics to look for to determine which disease may be causing the problem:

  1. Sudden death syndrome is caused by Fusarium solani f. sp. glycinea. The tap root is very discolored and there are few, if any, secondary roots. A blue stain may be present on the root. The pith is white. As leaves drop, petioles will remain attached to the plant.
  2. Brown stem rot foliar symptoms are similar to sudden death syndrome, but the tap root is not discolored. When stems are split, the pith is intact but chocolate brown in color. Leaves will wither and stay on plants.
  3. Charcoal rot is associated with high soil temperatures. Light gray or silverish lesions first develop on the tap root. Microsclerotia will form under the epidermis of the stem tissues, giving it a grayish black color resembling a "sprinkling of finely powdered charcoal".
  4. Rhizoctonia stem canker will have a brick-red sunken lesion at the base of the stem.
  5. Phytophthora stem rot symptoms include a dark brown discoloration of the stem progressing up from ground level. Internally, the stem pith will be discolored.

 

John W. McNamara

Agronomist

Wiles Bros. Inc.

606 Wiles Road

Plattsmouth NE. 68048

(402) 298-8550--Office

(402) 499-3870--Cell

(402) 298-7174--Fax

 

 

 

 
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