Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
Head Blight Update
The majority of Delaware small grain is just past or just entering flowering. Early flowering varieties are unlikely to be at risk for scab due to the environmental conditions just before and during flowering. Those plants just entering flowering could still be at a low level of risk depending on how much rain arrives over the next 3-5 days. Keep track of the wheat scab prediction center for updates on scab risk. http://www.wheatscab.psu.edu/.
Irrigation and Head Blight
The period 3 days before flowering and 5 days after flowering is the most critical for Fusarium head blight. Wet weather during this period does two things: 1) it increases local and regional inoculum potential (spores that infect the heads), and 2) it wets the heads and anthers, which allows the spores to germinate and more effectively colonize the head. Irrigation of wheat heads during the aforementioned periods increases your chance for elevated scab and vomitoxin. Infection can be particularly efficient during warm, wet conditions. The best thing you can hope for in wheat is dry weather during this period, which prevents development of spores and effective colonization of the head.
What if you applied a fungicide such as Caramba or Prosaro for head scab? Although it is true that these fungicides suppress (note I do not say control) head scab, they only provide roughly 45-50% reduction of scab or vomitoxin. When these fungicides are applied, they will cover the heads and any exposed anthers. Thus, any anthers that emerge after this point in time will not be protected from colonization by spores of the fungus. Wheat will continue to flower, exposing new anthers in the process. Irrigation of these unprotected anthers encourages the germination of the head blight pathogen. Therefore, irrigation during flowering can tip the balance in favor of the fungus, increasing the levels of scab or vomitoxin. Think of it as being similar to what happened in when the young men tossed sticks and stones at the tiger in the zoo. You may have some protection, but it’s best not to push your luck. As I stated last week, avoid irrigation during flowering if possible. If you feel as though you absolutely must irrigate then try to do so during the day so that the heads have a chance to dry during the evening. The evening tends to be the point in time when the majority of spores inoculate wheat heads.