Misidentification costs twice: you pay for a pass that doesn't work, and the real pest keeps going. Ten minutes with a hand lens beats a $12/acre guess.
Weeds: the ones that decide programs
- Waterhemp vs Palmer amaranth: both glossy, hairless stems. Palmer has long petioles (fold a leaf back — petiole longer than the blade) and often a watermark chevron; waterhemp leaves are narrower and lance-like. Both are resistance factories — identify precisely because programs differ.
- Marestail (horseweed): rosette then bolting stem, no basal branching — controlled far more easily small and in fall/burndown than in June.
- Giant ragweed: big lobed leaves, fast; escapes shade you out.
- Grasses: check the ligule/auricle to separate foxtails, barnyardgrass, and volunteer corn — grass herbicides differ sharply.
Diseases: what fungicides actually target
- Tar spot (corn): raised black specks that don't rub off (that's how you separate it from insect frass or saprophytes). Now a major Midwest yield robber.
- Gray leaf spot: rectangular lesions bounded by leaf veins, moving up from the lower canopy.
- Southern/common rust: orange-to-brown pustules that do rub off; southern rust is aggressive and mostly upper-leaf.
- Frogeye leaf spot (soybean): gray centers with reddish-brown borders — and the poster child for FRAC 11 resistance.
- White mold (soybean): white fluffy growth on stems in dense, cool, wet canopies — timing at R1–R3 or not at all.
Insects worth a pass
- Soybean aphid — small, pear-shaped, tailpipes (cornicles); count on the upper stem.
- Corn rootworm beetle — silk clipping matters at pollination; also informs next year's traits.
- Japanese beetle — dramatic defoliation that often stays below threshold; measure defoliation %, don't eyeball panic.
- Spider mites — stippling and webbing in hot dry years; check leaf undersides.
Free ID resources worth bookmarking
Your state extension's crop pathology and weed-ID guides, the Crop Protection Network field guides, and university plant diagnostic clinics (send a sample when it matters — a $20 diagnosis can save a $10,000 misapplication).
Descriptions are field shorthand — confirm with your extension diagnostic guide or a clinic. Then check timing: fungicide timing tool.
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