Herbicide injury caused from photosystem II (PS II) inhibitors including s-triazines (such as atrazine and simazine), as-triazine (such as metribuzin), ureas (such as diuron and linuron). [Note: symptoms from metribuzin and urea herbicides are described in separate factsheets.] S-triazine and as-triazine carryover can be an issue in rotation to sensitive crops.
- Interveinal and marginal chlorosis on older leaves first (s-triazines).
- Leaf margin chlorosis on older leaves (as-triazines).
- Necrosis of older leaves first followed by seedling dieback.
- No root symptoms.
Plant Entry and Symptom Expression
Foliar or root absorbed and apoplastically translocated throughout the plant. PS inhibitors are often soil applied and quickly absorbed by young seedling roots and shoots. Chlorosis resulting from chlorophyll synthesis inhibition is visible on older leaves within about 7 days of application. Cell membranes are damaged resulting in tissue necrosis shortly thereafter. Damage is typically most common on broadleaf plants, less common on grasses.
PS II inhibitor injury may be confused with symptoms from:
- Iron or magnesium deficiencies but is usually observed on older leaves.
- Urea herbicides bind to the same metabolic sites as PS II inhibitors thus producing similar symptoms.
- Fast acting contact herbicides like gramoxone may show chlorosis throughout the plant within a few hours to one day but followed rapidly by necrosis.
- Some exterior paints.
Herbicide Mode of Action Category
WSSA – 5
HRAC – C1
Applied Weed Science: Including the Ecology and Management of Invasive Plants (3rd Edition), Merrill Ross & Carol Lembi, pages 167, 176-178, 260-270
Publication date: Dec. 14, 2015
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