Late Season Fungicide Applications in Wheat

Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu; @Delmarplantdoc

I have had some questions regarding fungicides and application timings in wheat approaching flowering. In particular, the question of efficacy on canopy and head diseases has come up in conversations. The first thing to note is that the fungicides that are recommended for Fusarium Head Blight (FHB) suppression (Caramba, Prosaro, Proline) are effective in controlling foliar diseases. Commonly encountered foliar diseases include Stagonospora leaf blotch, tan spot, powdery mildew, and the rusts. However, it is important to understand that although these products are labeled for FHB, they are only effective if applied at the correct timing. Applying these products before flowering will not result in FHB suppression. This is because the FHB pathogen infects flowering grain heads (Figure 1).

Consequently, application of materials before flowering are not going to be as efficacious for FHB as they will be if you make these applications from the start of flowering (when about 50% of your main tillers are flowering) to 5-6 days after the start of flowering (Figure 2).

Figure 1. The Fusarium Head Blight Disease Cycle. The FHB pathogen grows on crop residue (corn, small grains predominantly). When it rains for 7-10 days prior to flower, the fungus can produce spores. Spores can be rain dispersed or moved long distances on air currents. When grain is flowering, spores that land on the head or anthers colonize these tissues and move into the grain head. Once in the head they can choke off water and nutrient movement, resulting in bleaching. The FHB pathogen can produce vomitoxin (DON) under the right conditions. Applications prior to flowering are not efficacious for FHB suppression.

Figure 2. Wheat heads just starting to flower. Flowering starts at the center and moves to the top and bottom of the heads. Anthers will be yellow for a short period after flowering. This yellow color indicates that they carry pollen. Fungicides should be timed such that roughly 50% of your main tillers are starting to or have recently flowered. Applications at this timing ensures that the glumes and anthers are protected from FHB. These same fungicides are very effective in managing foliar diseases of the canopy. Our research trials since 2013 have shown that in the majority of cases, the flowering timing is just as efficacious if not better for managing our most common foliar diseases (Table 1). This is because in most cases, these diseases do not reach the flag leaf or leaf below the flag leaf until near the flowering stage. This doesn’t mean that earlier applications cannot be profitable and efficacious, in fact in some cases, such as early infection by stripe rust, applications at flag leaf may be warranted, but it does mean that you can expect good to excellent protection of the major tissues contributing to wheat yield in many growing situations in Delaware and Maryland.

Table 1. An example of a wheat fungicide trial we conducted at the Wye, Maryland, in 2016

FB = followed by. FGS = Feekes growth stage where 5 occurs with 2^nd shot of N, 8/9 is flag leaf emergence/early boot, and 10.5.1 is flowering. Different letters within a column indicate statistical differences between treatments. In this trial, leaf blotch complex was managed the best when products were applied at flowering. These treatments also resulted in the greatest test weights and yields under these conditions.