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Practical Weed Science for the Field Scout: Corn and Soybeans
Photosynthesis inhibitors
Atrazine injury on soybeans appears first at the tips and margins of older leaves.
Drift from postemergence applications of atrazine on soybeans or other plants such as ivyleaf morningglory will appear as interveinal chlorosis and necrosis of older leaves first.
Metribuzin damage in corn. Note the necrotic leaf edges.
Soybeans injured from a preemergence application of metribuzin. Note the interveinal necrosis that has occurred on the unifoliate leaves.
Triazines chemical group
Herbicides
- Atrazine (Aatrex and others — corn, grain sorghum)
- Simazine (Princep — corn)
- Metribuzin, (Sencor and others — soybean, corn)
Mode of action and characteristics of activity
- Binding to D1 quinone protein of photosynthetic electron transport
- Controls many grass and broadleaf weeds, soil and foliar activity
- Translocates upward in xylem
- Activity and persistence affected by soil texture, pH and organic matter
- Injury and persistence greater on high pH soils
General symptoms
- Chlorosis and necrosis at leaf tips and margins on older leaves first (lower leaves) followed by interveinal chlorosis and lower leaf drop
- Photosynthesis inhibitors do not prevent seedlings from germinating or emerging
- Injury symptoms occur only after the cotyledons and first leaves emerge
- Initial injury symptoms include chlorosis of the leaf margins or tips
- In broadleaf plants, chlorosis between leaf veins may occur
- Older and larger leaves will be affected first because they take up more of the herbicide from the water solution and they are the primary photosynthetic tissue of the plant
- Injured leaf tissue will eventually turn necrotic
- Because of the chemical nature of the herbicide-soil relationship, injury symptoms are likely to increase as soil pH increases (above 7.2)
Injury from metribuzin on corn appears as chlorosis and necrosis along the leaf edges.
Phenylureas chemical group
Herbicides
- Linuron, (Lorox — corn, soybean)
- Tebuthiuron (Spike — pastures, noncropland)
- Fluometuron (Cotoran — cotton)
Mode of action and characteristics of activity
- Binding to D1 quinone protein of photosynthetic electron transport
- Control of some annual grasses and broadleaf weeds
- Primarily soil applied but have foliar activity
- Translocates upward in xylem
- Short to long soil persistence
General symptoms
- Very similar to triazines
Injury from bromoxynil on soybeans and corn. Notice the necrotic tissue, mostly along the edges of the leaf.
Injury from bentazon on soybeans appears as chlorosis, then necrosis of leaf tissue. Injury on corn appears as necrotic spots and streaks across the upper half of the leaf.
Benzothiadiazoles, benzonitriles and pyradazines chemical group
Herbicides
- Bentazon, (Basagran — corn, grain sorghum and soybean)
- Bromoxynil (Buctril — corn, grain sorghum, alfalfa, small grains)
Mode of action and characteristics of activity
- Binding to D1 quinone protein of photosynthetic electron transport
- Postemergence control of annual broadleaves
- Little translocation and soil activity
General symptoms
- Occasionally brown midveins
- Plant injury is confined to foliage that has come into contact with the herbicide
- Affected leaves will become yellow or bronze in color and eventually turn necrotic
- Injury symptoms can look similar to cell membrane disruptors
- Crop oil concentrates (COC), other additives, or warm weather may increase weed control and crop injury symptoms
IPM1007, revised June 2007
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