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Ohio State University Extension


C.O.R.N. Newsletter 2004-24

Dates Covered: 
July 27, 2004 - August 2, 2004
Steve Prochaska

Soybean Production Issues Highlighted at Research Station Field Day

Authors: Greg LaBarge

The Northwest Agricultural Research Station Field Day will be this Thursday, July 29th from 6-9 pm. Researchers from Ohio State University will be on hand to share field research plots on major issues facing Ohio’s Agronomic Industry.

Soybeans will be a major topic with three field presentations. Dr. Anne Dorrance has some very visual results to share on strategies to control Ohio’s major soybean disease, Phytophthora. Soybean aphids were an extreme problem in 2003, Dr. Ron Hammond will discuss how this pest is expected to development in 2004. Dr. Jim Beuerlein will share ideas for larger soybean yields. Corn standability has come into play the past couple harvest seasons, Dr Peter Thomison will share thoughts on How harvest delays will effect corn standability?

The Northwest Agricultural Research Station is located one mile east of State Route 235 between Hammansburg and Oil Center Roads at 4240 Range Line Road, Custar.

Two-spotted Spider Mites and Soybean Aphids

Authors: Ron Hammond, Bruce Eisley

Some areas of the state have been experiencing dry conditions, and a few reports have been received of twospotted spider mites starting to cause yellowing and bronzing along field edges. Although not currently a widespread problem in the state, mites could become a larger concern if dry conditions continue to exist. If mites begin to build up in fields, growers have two choices in materials to spray which will do an adequate job in control, Lorsban and dimethoate. The only other insecticide labeled for twospotted spider mites is Warrior, but it is labeled for suppression only, and thus, is not usually recommended. Growers seeing such yellowing along field edges should check for mites with a hand lens and consider a field-edge treatment. If the problem exists throughout the field, an entire field spray should be considered.
A reason why we are mentioning this problem with the mites at this time relates to the ongoing watch for the soybean aphid. As of this week, Ohio and the other Midwestern states are continuing to observe extremely low soybean aphid populations; thus, we do not believe there is a reason to make insecticide applications for aphid control. However if growers observe yellowing in their field, especially along the edges, it is of extreme importance that they make the correct identification of the cause of that discoloration because both pests in certain instances can cause yellowing. Twospotted spider mites and soybean aphids are arthropods (mites are not insects) that a grower needs a hand lens to make the correct identification. If a grower sprays a twospotted spider mite outbreak thinking it is a soybean aphid problem with an insecticide other than Lorsban or dimethoate, the mite population will not be controlled, possibly leading to even greater mite densities. And making entire field applications rather than just field edge sprays will be a waste of money and perhaps could lead to further outbreaks of other insects because of destruction of beneficial insects. This is a good example of the need to make correct identifications and to take the appropriate action for the specific pest.

Phytophthora sojae Is Impacting Stands

Authors: Anne Dorrance

The downpours of last week are clearly evident in many fields this weekend. A drive through the state – you could really see the tile lines in the soybean fields. Plants were deep green and taller on top of the tile lines while those in the wetter areas of the field were yellow and stunted. In walking through several fields—the characteristic stem lesion of P. sojae was evident – but in my field plots – it is only occurring in varieties with partial resistance scores of 5 or greater. (Some companies use the reverse score—so check the fine print on your seed catalogue). Some of the beans were truly flooded out. The area of the field smells – soggy – like dead worms. The Rhizobium nodules are dead. When you dig he plants up – the nodules, those round bumpy things on the roots can be crushed between your fingers if they are dead. If they are healthy- they will be firm and a nice pink color on the inside. If the plants have a Rps gene that is effective or if they have high levels of partial resistance – it will take some time for them to grow new roots. Last year, we had several cases where folks sent stunted, yellow seedlings in to see if they had root rot. During the 2 days in shipment- the plants grew new roots! Some of these soybeans could use some oxygen in the root zone and they will be in much better shape. Those plants that were attacked by Phytophthora will continue to die throughout the summer – thinning the stands in the wetter areas of the field.

If your stand has thinned to the point where you are considering replanting, be sure to use a seed treatment. The highest labeled rates of Allegiance and ApronXL are needed to protect seeds from Phytophthora sojae.

Soil pH and Nutrient Availability

Authors: Robert Mullen

When discussing nutrient management, the analogy of crop production and vehicle fluid maintenance have often been used. Nitrogen and phosphorus are the gas and oil of crop production, respectively. The amount of gas (N) necessary is directly related to the length of the trip each season (yield potential). If we know how far we are going, we know how much to put in the tank (our tank is never empty, but unfortunately we do not know how much gas will become available nor do we know for sure how long the trip will be – due to weather). The amount of oil necessary is not related to the length of the trip. We just have to make sure the crankcase is full in order to complete the journey. If nitrogen and phosphorus are the gas and oil, then soil pH is the air pressure of the tires. If the tires are flat, you are not going to get far. Understanding that soil pH controls the availability of most nutrients is important for proper nutrient management. Maintaining the proper pH for crop production is a must for efficient use of other crop inputs.

Nitrogen – While soil pH does not directly control N availability per se, it does affect soil microbial activity. Acid soil conditions can limit microbial activity and slow mineralization of N (from organic matter) as well as nitrification. High soil pH can result in significant loss of N by volatilization, especially when urea-based fertilizers are surface applied.

Phosphorus – P availability is strongly influenced by soil pH. Availability of P is maximized when soil pH is between 5.5 and 7.5. Acid soil conditions (pH < 5.5) cause dissolution of aluminum and iron minerals which precipitates with solution P effectively “tying” it up. Basic soil conditions (pH > 7.5) cause excessive calcium to be present in soil solution which can precipitate with P decreasing P availability.

Magnesium – Mg availability is affected by soil pH. Highly weathered, low pH soils can be deficient in Mg. Mg is leached out of the soil due to excess hydrogen, aluminum, and iron which compete for cation exchange sites.


Most micronutrient metals are directly affected by soil pH. As the pH decreases, the availability of iron, manganese, zinc, boron, and copper all increase. Molybdenum availability, on the other hand, decreases as soil pH decreases. This fact is especially important for legume production. The nodules of leguminous crops contain the enzyme nitrogenase which is rich in molybdenum. Thus if soil pH is low and available molybdenum is low, legumes will appear N stressed and production will suffer.

Understanding soil pH should also help in diagnosing crop production issues on specific soils. From the example above, if alfalfa is being grown in a soil with a pH less than 6.5 and the stand is poor and the leaves are yellows, the first thought should be molybdenum might be deficient. The best method to remedy the situation would be to lime the soil and increase soil pH, which is why it is important to soil test prior to planting. Surface application of lime to increase soil pH would be slow and would not impact molybdenum availability for quite a while. Similarly, if the soil pH is high (> 7.5) and the new leaves are exhibiting interveinal chlorosis, manganese and iron deficiency should be the first consideration.

Due to the influence of soil pH on nutrient availability (not too mention aluminum toxicity), maintaining proper soil pH is vital to a good fertility program.

Archive Issue Contributors: 

State Specialists: Anne Dorrance, (Plant Pathology), Robert Mullen (Soil Fertility), Peter Thomison (Corn Production), Mark Loux (Weed Science), Extension Agents and Associates: Greg Labarge (Fulton), (Harold Watters (Miami), Alan Sundermeier (Wood), Roger Bender (Shelby) and Steve Prochaska (Crawford).

Crop Observation and Recommendation Network

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.