Some winter wheat growers and others involved in wheat production in Nebraska wondered why they saw fusarium head blight this year in wheat fields that were sprayed for the disease at flowering (the optimum timing) with a triazole fungicide (the most effective fungicide class for this disease).
There are several reasons.
Suppression versus control
Fungicides only suppress FHB. They do not completely control it as they do fungal foliar diseases such as leaf rust, stripe rust and fungal leaf spots. This means that even if you apply a fungicide to manage FHB, the disease still will develop, but with much less damage, less incidence and severity — and less DON or vomitoxin — than if a fungicide was not applied.
Development of FHB despite fungicide application is due in part to the nature of the disease, the anatomical structure of the wheat head and the development of the wheat plant.
Nature of the disease
The fungus invades the head and grows in the tissues inside the head. This makes it impossible for the fungicide to come into complete contact with the fungus. So, the disease is only partially controlled, which is what the term suppression means. Research trials have shown up to nearly 70% suppression when a triazole fungicide is applied at flowering.
Anatomical structure of the head
The wheat head is thick, round and upright, unlike the leaf that is laminar (in the form of a thin plate) and horizontal or oblique (angled). The upright nature of the head limits thorough coverage because the surface area on which the fungicide-water mixture lands is small, and some of the mixture that lands on the head rolls off.
Development of the wheat plant
The wheat plant has tillers whose development is staggered, leading to uneven flowering within the field. Uneven flowering also can result from unevenness in the soil structure, fertility or moisture within the field, causing plants in some parts of the field to mature sooner than those in other parts.
The optimum timing of a fungicide application for management of FHB is during the flowering period. Most infections occur during this time because the flowers help the fungus to invade the wheat head.
Because tillers differ in height and age, and wheat in some parts of the field can mature sooner than in other parts, it is impossible to apply a fungicide when heads on all tillers have emerged or are flowering.
Heads that emerge after fungicide application are not protected by the fungicide, and therefore in a favorable environment for the disease, FHB will develop on them regardless of fungicide application in the field.
Wegulo is a Nebraska Extension plant pathologist.