Agronomy Notes for the Week Ending July 18th, 2015

The week started out with a spotty rain shower moving thru parts of our area Monday afternoon followed up by another spotty rain shower Thursday morning in part of our Southern area, and ended with some rain thru most of the area Friday Night into Saturday morning. The rain event on Friday also had some strong winds that took down some tree limbs in spots and also caused some Greensnapping of corn in the Northfield area. In general the week was on the warm side but the moisture was welcomed for pollination that was taking place or soon would take place.

The rainfall also continued a trend which is we will receive measurable rainfall during the week of the Waseca County Fair, which was last week July 15-19. (In the last 40-50 years there is a 70-80% probability we will receive moisture during that time frame.)

Soybean plant development ranges from V4/R1 (Fourth Trifoliate and Beginning Bloom) on some replanted soybeans to R2 (Full Bloom) and I wouldn’t be surprised if some beans were almost R3 (Beginning Pod) at the end of the week on some early planted soybeans.

Planting Date Plant Stage Growth Stage
28-Apr Full Bloom to Beginning Pod R2-R3
03-May Full Bloom R2
10-May Full Bloom R2
01-Jul Fourth Trifoliate/Beginning Bloom V4-R1

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We started to find Soybean Aphid populations increasing slowly except in fields that lack a seed treatment that contained an insecticide package. These “Naked Beans” had 80-120 Aphids per plant where the treated beans Aphid counts were 0-20 Aphids per plant. This fields had the same variety, similar planting date and fertility levels, with the only difference being the lack of seed treatment. In my experience I tend to find Soybean Aphids sooner in fields that:

• The first or last planted soybeans
• By wooded areas that have buckthorn in them
• Soybeans with Seed Treatments with absent or less active insecticidal properties
• Fields with lower Potash (K) levels (140 PPM or less)

I would suspect that we will have some fields hitting a sprayable threshold during the week of July 25th but would hazard to guess we will see the vast majority of soybean fields sprayed during the last week of July and first week of August.

The Septoria Brown Spot we have found in fields is starting to get worse, due mainly to our more moderate temperatures and increased moisture levels, from rain, dews and higher humidity. The concern that comes about with Septoria is that it starts to defoliate the soybean plant from the bottom up. What this fungal disease does is infect a portion of the leaf and much like other diseases it moves and infects the rest of the leaf, which shuts down the photosynthetic capabilities of that leaf. This will move from the bottom of the plant upward from leaf to leaf and plant to plant. By reducing or shutting down the photosynthetic “factories” in the leaves you reduce that total area in the plant that can help fill beans in pods, which reduces yield not only by reducing the number of beans and pods but also reduces the actual size of the soybean seed.

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As you are out walking soybean fields’ plants that have Septoria Brown Spot will lose their leaves and will fall to the ground. The photo above shows affected leafs still attached to the petiole but if you look closely to the left of the affected leaves you can see leaves that have already fallen off of the plant.

As soybeans start to flower and enter full bloom (R2) the rapid uptake of potassium (K) and nitrogen (N) is starting. At R2 soybean plants have taken up 15% of that years N and K needs for the year, and by early to mid-August when soybeans plants have reached the beginning seed stage (R5) they will have taken up 70-75% of the N and K needs for the growing season. The implications of not having enough nutrients available to the plant now will become more visible and taking a tissue sample at this stage will help plan for that field’s fertility program in the future.

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