Summer crop scouting
Brian Rees
Saturday, July 21, 2007
Even though the temperature is in the low 80’s as I am writing this month’s article, earlier this week we have been reminded that it is summertime in Kansas.
The hot temperatures and drying winds had a pretty dramatic effect on both plants and animals in the area.
That can be a really good thing with respect to one plant pest in the area that isn’t normally a problem here. Last week Doug Jardine, a visiting plant pathologist from K-State, and I found a few soybean aphids in the area that were verified the following day by Jeff Whitworth, entomologist with K-State Research and Extension in Manhattan. This was the first report of soybean aphids in Kansas this year, although Jeff found some in Riley County earlier this week.
This extremely small pest is usually not a problem in Kansas as they don’t like our normal weather conditions – hot and dry. Optimal conditions for reproduction of soybean aphids are in the 75-80 degree temperature range with moderate humidity. Once the temps are over 95, with consistency, the aphids go away.
Soybean aphids damage soybeans by sucking plant sap, resulting in yellow and stunted plants, puckered leaves, and reduced pod set. Yield losses will depend on aphid numbers, crop stage, and the overall condition of the crop. Numbers tend to build more rapidly on later plantings.
Yield risk is the highest when high aphid numbers are present at beginning flower, and can be most damaging when the plant is also under stress from drought. Treatment thresholds have been established through ?” pod stage (R-2). Again, this isn’t normally an issue here because numbers do not reach threshold levels due to our hot, dry summers. This may be one time we want to see 95-plus degree temperatures for a while.
While scouting for soybean aphids in the mornings, it might be a good time to make some good notes regarding any other controllable crop-related issues that have impacted your production this spring. One observation of mine this spring has been classic signs of potassium deficiency on both corn and soybeans. The symptoms of yellowed leaf margins, especially on the lower leaves, were more pronounced in no-till fields but were found in conventional-tilled fields as well. Many were in fields that do not test low in potassium.
I don’t have a “for sure” reason why this was the case this year, but I feel it has something to do with the temperatures, rainfall patterns, and planting conditions. Many of the fields I saw with deficiency symptoms were planted when it was just a little wet, resulting in some sidewall compaction and restriction of root growth, especially early in the season. Then, we had plentiful (or excessive) rainfall that allowed the plants to grow without working very hard for water or nutrients. Once the rains stopped, the plants were ready to grow rapidly but did not have the root system in place to take in the nutrients to support that growth. In a couple of instances where it was tried, an application of potassium fertilizer eliminated the symptoms. So what do you do to deal with or eliminate this situation in the future? First and foremost, I would encourage a soil test to find out for certain if there is an adequate level of potassium in the soil. If this is the case — and historically potassium levels have been plentiful in the area — I would look at using potassium in your starter fertilizer to get the plants past deficiency issues. If your soil test comes back low in potassium, using fertilizer would be a huge benefit to you. Potassium is removed in large quantities in whole crop harvest – hay and silage for instance — and also in large amounts in soybeans. Potassium is now being recognized as being deficient in areas of the state that previously have not seen deficiencies and K-State has potassium fertilization recommendations available for all areas of the state.
Additional information on soybean aphids and crop nutrition is available from the Lyon County office of K-State Research and Extension, 618 Commercial Street or call me at 620-341-3220.