WEEKLY UPDATES

Weekly Soybean Field Crop Report - June 28, 2007
Posted on June 29, 2007 at 08:43:05  
Soybeans
With dry conditions in many areas, yellow soybeans are common this year. Light green or yellow leaves are caused by various problems, including nutrient deficiencies, disease, pest, herbicide damage, or moisture stress. Yellow leaves with dark veins are classic symptoms of manganese (Mn) deficiency that results in yield loss. Symptoms of Mn deficiency are interveinal chlorosis (yellowing) on the newest trifoliates, although older trifoliates can also show symptoms. Leaf veins remain dark green. A foliar application of Mn, if applied early enough, will correct this deficiency. Potash (K) deficiency is evident in fields with poor root growth or low K soil levels. Potash deficiency appears as yellowing or browning of the leaf margins in older leaves. Plants will often outgrow K deficiency symptoms if adequate K is present in the soil and sufficient moisture is present.

Root rots and Soybean Cyst Nematode (SCN) will also cause yellow, stunted plants. SCN attacks the root system, preventing nutrient and water uptake. Symptoms appear in circular patterns within a field. Dig up those plants and look for root cysts. Fields with a history of SCN and late-planted fields are more likely to be affected as the crop has had less time to stay ahead of the SCN. In future years, rotate out of soybeans and when returning to soybeans use SCN resistant varieties.

Soybean aphids can be found, but populations remain low in most areas. A few “hot spots” have required insecticide treatment. Most of the highest populations have been found between London, Kitchener and Listowel. Earliest planted fields generally have the highest aphid populations. Evaluate each field individually. Populations may increase or decline quickly within a field, so once aphids have been found regular scouting will be necessary. If aphid levels reach 250 per plant and are increasing in number, a foliar insecticide treatment is recommended. Do not spray too early. Insecticides also eliminate aphid predators which can lead to a quick re-bound in aphid numbers. If this occurs, subsequent spray applications may be required.

-Submitted by Horst Bohner, OMAFRA Soybean Specialist
 

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