C.O.R.N. Newsletter 2012-25

Dates Covered: 
August 7, 2012 - August 13, 2012
Editor: 
Rob Leeds

Weather pattern change is forecast for the next 2 weeks

The forecast worked out pretty much on track from last week with widespread 0.25 to 1.00 inch rains across much of the state of Ohio with isolated totals over 2 inches.
A pattern change is now forecast for the next 2 weeks. This means temperatures will average close to normal for August with rainfall near normal. Normals are highs in the low 80s and lows in the low 60s. Normal rainfall is now about 0.75 inches per week. The best chances for rainfall are the second half of this week and the second half of next week but some light showers may linger in the northeast part of the state this weekend.

Summer so far is on track to be a top 5 warmest summer on record across Ohio and much of the corn belt. For July it was also a top 5 warm month most places with Columbus having it warmest July ever. This was the case from Ohio to Wisconsin and most of the corn belt.

Stress Degree Days for corn continue to track more like 1988 and 1934 than 1936 with the SDDs total in Ohio at 304 through August 5. This is more than double the 140 when below trend line yields occur.

Longer range, nothing has changed to our outlooks. The National Weather Service Ohio River Forecast Center made the call that the drought had bottomed on July 23 and slow improvement could be expected into fall. This was likely one of the first calls on this change. This change is now confirmed with our recent pattern and our forecast going forward. Expect slow improvement into fall. Temperatures still will likely be above average but not to the extremes we have seen.

Further out as discussed several weeks ago, there is at least a 50 percent chance of an El Nino this winter and next spring. This would favor a drier winter. Ohio State University and NWS OHRFC research indicates El Nino growing years average below trend line yields especially for corn and wheat with hotter summers and variable rainfall but usually not as dry as this summer but lots of other issues like diseases. Heat stress is also common in El Nino summers. We will monitor this into fall and print more statistics about what could happen next year in the coming weeks.

Why the rains aren’t stopping mites

The question is being asked why the rains aren’t stopping the mites, which would actually be from a fungal pathogen that can decimate the mite populations under wetter and more humid conditions.   Although rainfall reduces risk of damaging spider mite populations, thunderstorms alone will not eliminate infestations, particularly when rain arrives after large mite populations are established and when rain is followed by dry, hot conditions.  Sources in other states’ newsletters suggest that the mite-killing fungus requires temperatures cooler than 85°F, with 90% relative humidity, to produce infective spores. Periods of at least 12-24 hours of relatively cool, moist, and humid conditions are necessary for the fungal pathogen disperse and infect a spider mite population in a field.  In “normal” years, these are conditions we often see in mid-August.  So although we are expecting these conditions in the near future, we still urge growers to monitor their fields and spray if the mites are alive and actively feeding.  But look closely at the mites, and before pulling the trigger to spray for them, make sure they are still active.  Do NOT make miticide applications based on injury symptoms alone; the mites already might be dead!

Another question is how long are the crops susceptible to mite feeding if environmental conditions do not take care of the problem.  As mentioned above, we usually would expect the mite populations to decline naturally in later August.  However, knowing how strange this summer has already been, we should not assume anything.  For soybean, we would usually consider the R6 stage, full seed, when treatment might not offer much protection or benefit.  Corn in the Midwest is a different story, mainly because this is a new problem for us.  However, in the article that we referred to last week out of Wisconsin, it is suggested that once corn has reached the hard-dough stage, no economic benefit will result from treatment. 

The last question or problem growers are having, is that the two main insecticides with mite activity, Lorsban (and generics) and dimethoate, are in short supply and difficult to find.  Thus, growers are already needing to use other materials.  We have already discussed bifenthrin products including some that are combinations of bifenthrin and a pyrethroid (Hero), and in combination with clorpyrifos (Tundra Supreme).  Presently, reports received suggest that all the bifenthrin products are working.  We have also received word that there is a new miticide labeled on corn, Zeal, made by Valent, which contains the miticide etoxazole.  Zeal now has a supplemental label for use on corn.  It is NOT labeled on soybean and cannot be used in that crop.

Late season diseases are early this year

Based on visits to some fields and samples sent in to the lab here in Wooster, there are some late season diseases that are appearing.  They are:  Phytophthora root and stem rot, Diaporthe stem canker, Sudden Death Syndrome, Soybean Cyst Nematode (SCN), Charcoal rot, and Frogeye leaf spot.  At this stage the infections have taken place, at this time fungicides will not prevent any further disease development.  All of these diseases are managed with host resistance.  Scouting fields to see where these problems are will greatly help identify what disease resistance package the variety should have in the field the next time.  Links to pictures of these diseases and fact sheets can all be found at
http://oardc.osu.edu/ohiofieldcropdisease/t01_pageview2/Home.htm

Phytophthora stem rot – the Phytophthora sojae populations in Ohio are very diverse and we have found that the Rps genes are not effective in all fields.  If stem rot is developing that indicates that the field resistance (tolerance, partial resistance) part of the package was not up to snuff.  Read the fine print on the scales – Universities and Companies all use different scales – and choose a variety with the highest level of this resistance for the next soybean crop.

Diaporthe stem canker – this disease can be distinguished from Phytophthora stem rot by how much of the stem is girdled.  For Phytophthora the stem lesion will begin in the roots and move up the stem.  For Diaporthe canker the lesion typically is at the 3rd or 4rth node of the plant and there will be small black dots (fungal structures) in the lesion.  This is a residue borne disease so rotation will be important in these fields as well as choosing a resistant variety.

Sudden death syndrome – symptoms were severe in a couple of fields, with the yellowing and brown necrosis between the veins.  The plants had not begun to defoliate.  This was also a SDS susceptible and SCN susceptible variety.  For those parts of the state where this is a perennial problem, adding SDS to the resistance package will be important.

Soybean cyst nematode – it is getting easy to pick out the areas of the fields with high populations.  They are about half the height of the neighboring beans.  These are also the areas of the fields that will turn yellow first.  It is worth taking out a shovel and digging these roots up to see the females, there will be some shiny white ones on the roots.

Charcoal rot – this is very apparent in the areas of the state that have been hit with drought.  This also causes premature yellowing and dying of soybean plants.  To distinguish this from these other diseases, split the tap root open and there will be black flecks embedded in the tissue.  These are the microsclerotia which will serve as survival structures.  Most of the research on this disease has been done in Kansas as well as Tennessee.  One management strategy for this, in addition to resistance, is planting soybean at reduced populations.  We have several studies this year in Ohio which are comparing high and low plant populations to see if this will be a viable management strategy for our conditions in Ohio.

Frogeye leaf spot – found one leaf spot in one field, this leaf spot has purple margin with gray center and on the underside of the lesion the conidia (spores) look like whiskers.  This fungus can only infect new growth and once the leaf has expanded it is resistant.  The key for this disease is to check at the end of the season, this can overwinter in Ohio.  We have had some experience here in the state if it builds up in a field one year and the same or another susceptible variety is planted back into the same field we can run into some serious disease loss issues. 

Testing nitrate levels in drought-stressed corn

There has been a lot of interested in testing drought-stressed corn for nitrate levels. Rightly so! Nitrate poisoning is a real concern for livestock production. Under normal growing conditions, nitrate is quickly converted to nitrite, then to ammonia, and finally into plant proteins and other compounds. When plant growth is slowed or stopped, nitrate can accumulate in the plant. Drought, frost, cool, cloudy weather can cause nitrate to accumulate. Rainfall following an extended dry period may cause an immediate increase in nitrates for 2 to 5 days until the plant can concert the nitrate to protein.

OSU Extension in Defiance County coordinated the collection and testing of 64 corn plant samples from various farms in Defiance and Paulding County.  The plants were collected on July 31 and August 1 from fields with moderate to severe drought stress, normal to reduced nitrogen applications and low and high harvest height.  Samples were delivered to the lab on each day of sampling.  The range of percent Nitrate on a dry basis was less than 0.01 to 0.27.   Samples less than 0.44 percent nitrate on a dry basis is considered safe to feed under all conditions.  Percent moisture ranged from 63% to 76%.  Keep in mind, rainfall received locally on August 4-7 will potentially effect the nitrate levels in the field corn.

A & L Great Lakes Laboratory in Ft. Wayne, IN summarized nitrate samples received into the lab in a report dated Aug 6th.

http://www.algreatlakes.com/PDF/Corn_Forage_Nitrate_Summary_8_06_2012.pdf

 

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About the C.O.R.N. Newsletter

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.