Turcicum leaf blight
Long, elliptical, greyish-tan lesions — often described as cigar-shaped — running along the leaf, starting on the lower leaves and moving up. In a wet, moderately warm season it can take most of the canopy.
Common rust
Small, powdery, cinnamon-brown pustules scattered on both leaf surfaces, later turning dark. Favoured by cool, humid weather.
The question that decides whether to spray
Not "is the disease present" but "how much green leaf will be left during grain filling?"
Lesions on the lower leaves at knee-high stage matter little; the plant has time and those leaves contribute least. The same disease reaching the ear leaf and above around tasselling is a direct yield loss, because those upper leaves feed the cob.
So: scout from tasselling, look at the ear leaf and the two above it, and treat when the disease is climbing into that zone.
Management
- Resistant hybrids, the strongest and cheapest lever. Ask what is currently recommended for your district.
- Rotation and residue management, since both survive on maize debris. Continuous maize on the same field builds pressure steadily.
- Balanced nutrition, particularly potassium. A potassium-sufficient crop resists both noticeably better.
- Adequate but not excessive plant population — a crowded, humid canopy is what they need.
- A protectant application at or just before tasselling in a high-pressure season, rotating the FRAC group.
Other things on maize leaves
Fall armyworm damage is ragged holes and windowing with frass in the whorl, not lesions. Downy mildew gives long pale stripes and a downy growth on the underside in humid mornings. Nitrogen deficiency gives a V-shaped yellowing from the tip of the older leaves.
None of those is helped by a fungicide.
After harvest
Chop and incorporate the stubble rather than leaving whole stalks standing. Both pathogens overwinter there, and so does stem borer.
Read the label and the leaflet on the pack before use. The approved crops, dose and waiting period printed there are what count.
