Maize gets a number of diseases and they can all affect the final yield. One of the most damaging is northern leaf blight (NLB) caused by the fungus Exserohilium turcicum. It over winters in infected maize leaves, husks and trash from the previous crop.

The NLB fungal spores are produced when environmental conditions are favourable in the spring and early summer, then with the aid of rain splash and wind, will infect the new plant growing in the same or neighbouring paddock.

New developing leaves that are exposed to periods exceeding 12 hours of wet or damp conditions and temperatures between 18 to 27 degrees Celsius are susceptible to infection from NLB if the fungal spores are present. Then heavy dews and warm humid conditions will spread the disease rapidly and move the infection to the lower leaves and cob. The most common and most damaging time for infection in New Zealand is December to January.

Yield loss form NLB is driven by the loss of leaf area and so reduces photosynthesis area and yield. With weather conditions favouring infection from the early tassel stage, the crop yield can be impacted by as much as 30 percent, however with an infection later than this stage the impact is considerably less though there is an increased risk of lodging.

So what can I do to manage my risk I hear you ask?

1. Crop rotation: Growing maize crops back to back in the same paddock will increase the amount of inoculum passed from one crop to the next so try to avoid this.
2. Maize grain: Crops of maize grown after a maize grain crop are more susceptible than after silage maize because more stova is left after harvest and so infection risk is higher.
3. Cultivation: If you fully cultivate after harvesting, burying all of the crop residue, this will leave less plant material to infect the next crop. Direct drilling, minimal cultivation and strip tilling leaves trash on the surface, increasing the risk of infection for the following maize crop.
4. Hybrid selection: All major seed breeders publish the susceptibility of each hybrid to NLB. So when choosing your hybrid you can take this into account.
5. Planting date: Late planted crops are at higher risk from developing NLB as there is more inoculum in the environment from earlier planted and infected crops, and the later planted crops are more likely to be at an earlier growth stage when infected.
6. Fungicides: There are a few products with activity on NLB but they have to be sprayed either before or at early infection, protecting the plant from infection rather than trying to cure it. Remember, you can’t paint the leaves green so once they are brown and dead they stay brown and dead. The best time to prevent the worst impact of disease is just before or at early tassel stage and is usually done by an aircraft because of the crop height.
7. Monitor the crop through the high risk infection period and growth stage. Take account of the risk and make the decision to spray or not.

For help with monitoring your maize crop for northern leaf blight and to discuss an effective spray to control when identified, contact your local PGG Wrightson Technical Field Representative.