Primary symptoms

Herbicide injury can cause malformation (e.g., leaf twisting) of young plants and yellowing, burning, and dying of the leaves.
Fertilizer burning can cause seeds to not germinate, or the seedling may emerge and then die. Burned plants may look wilted or stunted.

Confirm the problem by checking the tables below.

Summary

Herbicide. Herbicide injury can cause malformation of young plants and yellowing, burning, and dying of the leaves. The malformation (such as twisted leaves which do not unroll properly, or distorted roots) results from applying herbicides from certain groups such as 2,4-D at the wrong growth stage or at too high a rate. This problem can occur with phenoxy herbicides (e.g. 2,4-D), dinitroanaline herbicides (e.g. pendimethalin), benzoic acid herbicides (e.g. dicamba), and amide herbicides (e.g. alachlor or metolachlor). Leaf chlorosis and death can result from overapplication of triazine herbicides (e.g. atrazine). Leaf burning results from getting a directed spray of a contact herbicide such as paraquat on the maize or from applying too high a concentration of another chemical. You can distinguish herbicide injury from foliar disease by looking for burning which occurs in patterns from the sprayer nozzle and which only affects leaves of a certain age which were exposed when the chemical was applied.

Fertilizers. If fertilizers (especially Nand K) are placed in contact with the seed or too close to young plants they can cause burning, especially if soil moisture is deficient. If the problem occurs at planting, the seeds may not germinate, or the seedling may emerge and then die. Burned plants may look wilted or stunted. When fertilizer is sidedressed, it should be placed no closer than 10 cm to the stem.

Foliar applied insecticides or fertilizers can occasionally cause burning of the maize leaves. This can be minimized by applying early in the morning or late in the evening, to avoid intense, direct sunlight on the leaves.

Is chemical injury a problem?

Evidence

Estimate the proportion of plants affected by the problem. If more than 10% of the plants are damaged badly enough to reduce yields (that is, if more than 30% of the leaf area on those plants is damaged), a problem exists. It is very important to look at several fields belonging to different farmers to decide if there is a problem with the technology in use or if a specific farmer simply made a mistake during that particular season.

If the injury has reduced the plant stand, measure the remaining population density. If the reduction in stand causes the density to drop below 70% of the optimum for your variety, measured under non-limiting conditions, the injury is important. (See plant density).

Possible solutions: Chemical injury

• Change to a safer product or use lower rates. Investigate "safeners" available for use with specific herbicides.
• Change the timing or method of application of the product.
  • Use a better design of shield. If the weeds were taller that the crop at application, causing problems when the farmer tried to apply a directed spray, this indicates a land preparation problem (see land preparation).

Possible solutions: Fertilizer injury

• Use a planting stick with two points to make a separate hole for fertilizer.
• Sidedress fertilizer when soil moisture is adequate and place the product at least 10 cm from the stem.
• Design experiments to examine the effect of timing on fertilizer application. In some environments the application of fertilizer at planting can be avoided.

Table 7. Common herbicides in maize, recommended range of concentrations*, and recommended time of application.

* Note that the concentrations given here are as kg of active ingredient per hectare. You will have to calculate the amount of commercial product as: kg of commercial product = kg active ingredient / (fraction of active ingredient in the commercial product).