The forecast calls for thunderstorms on Monday and Tuesday of this week, we could sure use the moisture, but we do not need the storms associated with it. Some areas have received very timely and beneficial precipitation over the last two weeks while others have not. As mentioned previously, corn and soybeans are requiring up to 1/3 of an inch of water per day. Moisture stress during corn pollination can considerably reduce potential yields, while the critical moisture stress period for soybeans will occur during pod fill. Now is the time to be scouting 1^st generation bean leaf beetles to determine potential 2^nd generation beetle populations. I have put out four pheromone traps for the western bean cutworm, which was a problem in some fields in Sac, Ida and Monona counties last year. We will monitor moth flight patterns (if they occur at all) across the region.

Corn development has reached VT (tassel) and R1 (pollination) in many of the earliest planted fields. The VT stage is initiated when the last branch of the tassel is completely visible and the silks have not yet emerged. VT begins approximately 2-3 days before silk emergence, during which time the corn plant will almost attain its full height and pollen shed begins. The time between VT and R1 can fluctuate considerably depending on the hybrid and environmental conditions. Under field conditions, pollen shed (also termed pollen drop) usually occurs in the late mornings and early evenings. R1 begins when any silks are visible outside the husks. Pollination occurs when these new moist silks catch the falling pollen grains. A captured pollen grain takes about 24 hours to grow down the silk to the ovule where fertilization occurs and the ovule becomes a kernel. Generally 2-3 days are required for all silks on a single ear to be exposed and pollinated. The shank and husks attain full size between the R1 and R2 stages.

Soybean Development has reached the R2 stage in many fields. R2 is the classification for full flower. At R2 root development has moved across the space of a 40-inch row and root development is tapping the lower soil depths. The soybean plant has accumulated approximately 25% of its final biomass weight and has attained 50% of its final node number. This is a period of rapid growth and dry matter accumulation, which will continue through R6 (Full Seed). The soybean crop is demanding from 0.2 to 0.3 inches of water per day from R2 through R5. The greatest impact of water stress will not occur until Pod Fill. Nutrient uptake at R2 has been minimal by the soybean plant; approximately 31%, 30% and 34% of the total Nitrogen, Phosphorus, and Potassium respectively have been accumulated in the plant at this time. Greatest nutrient demand occurs during pod elongation and fill (R3 – R7).

Herbicide Vapor drift is a likely explanation. Symptoms may take two to three weeks to show up. Vapor is likely coming from corn herbicides that contain dicamba. Rainfall after dicamba applications will cause vapor production to end. Dry conditions the last four weeks have allowed vapor to be produced. Soybean fields affected are usually north of suspect cornfields.

Environmental/surfactant effects: Surfactants/crop oils may produce leaf symptoms that mimic dicamba drift.

Herbicide drift symptoms on soybeans: ‘Strapped’ leaves, parallel veins – 2, 4-D, Hornet. ‘Cupped’ leaves – dicamba, (Clarity, Northstar, Distinct, others)

Yield effects on soybeans: Yield reduction is highly correlated to height reduction of soybeans. Soybeans with cupped leaves and little height reduction generally will have minimal yield reduction.

As mentioned in the previous newsletter, now is the time to be scouting first generation Bean Leaf Beetle for 2^nd generation control considerations. Peak 1^st generation emergence will occur around 1212 GDD’s following soybean emergence. ISU has developed a new control strategy (for 2^nd generation beetles) based on 1^st generation population numbers. Based upon a high density of first generation beetles, it is expected that the second generation will exceed the economic threshold.

The first thing to do is determine the week soybeans emerged, then refer to the accumulated GDD table and match dates. Begin sampling fields 1 week after peak emergence. If the sample is below threshold, scout again the next week. If the sample is still below threshold, scout 1 more week. If 1st generation beetles are above threshold, scout again mid August to confirm beetles are present, then spray.