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Cool summer not ideal for Ontario soybean pod set

Aug. 21, 2014, Ontario – Soybeans are now at R4 (full pod) to R5 (beginning seed) growth stage, reports Horst Bohner in the latest field crop report from OMAF, but white mould has been found across the province and soybean aphid numbers are increasing. 

Although vegetative growth has been good, a relatively cool summer has not been ideal for pod set. Ideal daytime temperature for soybeans is about 28 C. Good growing conditions for the remainder of August and September will be necessary for high yields.

This has been the worst year in recent memory for poor root nodulation and nitrogen (N) fixation. Some fields did not turn dark green due to N deficiency. In some cases it was due to excess water, but in many cases, N deficiency was due to poor nodulation. This was a particular problem in first-time fields, but also observed in fields with limited soybean history. Even fields that were inoculated are showing poor nodulation. In cool years, nodules can be extremely slow to establish and may not occur at all in first time fields. Inoculant that was put on the seed may actually die in the soil before the plant allows the bacteria to invade the roots.

When conditions turn dry and cool immediately after seeding on a first time field as in 2014, the bacteria never have the opportunity to establish themselves on the roots before they die out in the soil. This problem has occurred previously in cool years so the general recommendation is to apply two inoculant products to increase likelihood of good nodulation on first time fields. Some first time fields that used only one product or a pre-innoculant have no nodules this year. The only remedy to a nodulation failure is to apply N fertilizer at first flower or early pod set.

White mould has been found across the province. Most infected fields have low disease pressure with pockets of dead plants, but severely impacted fields are more prevalent than usual this year. A low level of white mould pressure is common in Ontario fields and has limited impact on yield. Severe infestations reduce yields significantly. It is too late to spray a foliar fungicide to suppress white mould. Fungicides in soybean are largely preventative and must be applied during flowering and early pod set for a yield benefit. Take note of fields with white mould to make future management decisions. Fields with white mould should not be seeded to soybeans next year. Two-spotted spider mites have been a problem in limited fields this year. Mites feed on individual plant cells on the underside of leaves where each feeding site causes a stipple/dot. Severe stippling causes yellowing, curling and bronzing of the leaves. Damage is more severe in hot, dry weather. Fields should be sprayed if above threshold of one damaged leaf per plant, up to R6 growth stage (seeds within one pod on the top four nodes fills the pod cavity).

Soybean aphid numbers continue to build especially in Eastern Ontario. Some fields have been sprayed in that part of the province. Aphid numbers remain low or non-existent in southwestern Ontario. Fields need to be monitored until plants have reached R6 (full seed) stage, which will not occur until well into September for longer season varieties this year.