The Bean Report – August 16, 2023

Soybeans range from R5.5 to R6 (full seed) with lower leaves dropping.

• Rainfall last week was very welcome for soybeans as they were filling seeds. Moisture for pod filling stages (R5-6) is critical to soybean yield. Plants experiencing moisture stress will mature earlier, reducing seed weight and the number of seeds per pod.
• Spider mites have been noted in greater numbers in some fields. The threshold for control is more than 20% of leaves affected. Since they are often congregated to field edges or patches, spot spraying is often effective. The insecticide option for control is dimethoate (Cygon, Lagon), which has a 30-day pre-harvest interval.
• Once soybeans have reached R6 (full seed), they are considered safe from soybeans aphids.
• Disease pressure has been low across the province. Phytophthora root rot and northern stem canker are showing up more often in fields with tighter soybean rotations or in areas of poor drainage. If large amounts of Phytophthora symptoms are noted in a field, consider sending soil for pathotype testing. This soil test will tell you the best major gene resistance to use in that field in future years to aid in variety selection. Scouting and Management of Phytophthora Root Rot →
• Patches of potassium (K) deficiency are visible in soybeans growing on sandier soils during pod filling stages. Symptoms are chlorosis of the leaf margins are soybeans are moving K from the leaves to the seeds. In previous research in Manitoba, soybean yields have not responded to K fertilizer treatments at low-K sites. Soybeans remove large amounts of K in the seed (1.1 – 1.4 lbs K2O/bu) so it is important to balance K fertility throughout the rotation to support yields of other crops.