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Wheat disease monitoring

By Harold Watters, Ohio State University Extension agronomist

We are monitoring wheat disease in an effort led by OSU Extension wheat specialist, Pierce Paul.

We follow growth stages of wheat to know when to apply herbicides safely but also to know when, or if, we should apply fungicides. Growers who rely on the height of the crop as an indicator of crop development may miss Feekes Growth Stage 6, a critical growth stage for herbicide application, and Feekes GS 8, a critical stage for managing foliar diseases with fungicides. Do not rely on the height of the plants or calendar dates alone (especially this year) to make your management decisions. Walk fields, pull tillers from multiple places, remove the lower leaves, and examine these tillers for the presence of nodes and the emergence of the flag leaf. At Feekes GS 8 the tip of the flag leaf, the fourth leaf above the first node, is visible. Plants of different heights and sizes may all be at the same growth stage. Feekes 8 marks the beginning of the period during which we recommend that fields be scouted to determine which disease is present and at what level.

Results from university studies have shown that the greatest benefits from foliar fungicide applications were obtained when applications were made between Feekes 8 and 10. This is largely because most of our major foliar diseases usually develop and reach the flag leaf after Feekes 8-9. For Head scab, we need to protect the plant at flowering so closer to Feekes 10.5 — that will be later in May.

 

Septoria and Powdery Mildew

Septoria develops best under cool, wet conditions with frequent rainfall, whereas powdery mildew likes cool, humid conditions.

  • Scout for powdery mildew and Septoria on the lower leaves. Unlike head scab, fungicide applications for these and other foliar disease do not have to be made at one specific growth stage. Instead, applications are based on disease thresholds, weather conditions, and variety susceptibility.
  • For instance, if it stays cool and wet and a few lesions are observed on the leaves below the flag leaf, a fungicide should be applied to protect the flag leaf if the variety is susceptible.
  • On the other hand, if it stops raining and warms up, you may want to save your fungicide application for head scab and late-season diseases like Stagonospora and rust, as warm weather usually prevents both powdery mildew and Septoria from spreading up the plant.
  • If you still plan to apply a fungicide to control early-season diseases, choose one like Propiconazole or Tebuconazole that are cheap, but effective. Rarely are two fungicide applications necessary or economically beneficial in Ohio, but, if an inexpensive fungicide is applied early in the season, then it may be feasible to make a second application at flowering to manage scab and late-season diseases.

 

Stagonospora and rusts

Stagonospora is very similar to Septoria in that it develops best under wet, rainy conditions, but unlike Septoria, it likes warm instead of cool weather condition. So, although Stagonospora can affect the crop at any growth stage, it tends to be most severe late in the growing season. In fact, conditions that are favorable for head scab are also favorable for Stagonospora leaf and glume blotch. It therefore means that a single application at flowering is often effective against both head scab and Stagonospora.

This is also true for the rust diseases. Since the rust fungi cannot overwinter in Ohio, spores have to be blown up from the south, and this usually occurs during the latter half of the season. In most years, the first symptoms of rust are observed between the boot and flowering growth stages, making a fungicide application at flowering also effective against these diseases. However, it is not uncommon for rust to develop early in the season, particularly in the southern half of the state.

 

Head scab on wheat

It is still too early to apply a fungicide to manage head scab. Use the scab forecast system (http://www.wheatscab.psu.edu/) to monitor concerns. If you plan to spray for head scab, Prosaro or Caramba should be your fungicides of choice. The new fungicide, Miravis Ace, which seems to be just as effective as Prosaro and Caramba, based on a limited number of trials, may not yet be available. Stay away from the strobilurins when it comes to head scab management — these fungicides tend to increase rather than reduce vomitoxin contamination.

I know that the idea of “protecting the crop” with a “preventative treatment” seems to suggest that the fungicide has to be applied before the crop reaches the critical growth state — flowering in the case of wheat. But results from more than 20 years of scab research show that you are better off applying a few days “late” rather than a few days “early.” Remember, with head scab you are also trying to reduce grain contamination with vomitoxin, and fungicides are certainly more effective against this toxin when applied at or 4 to 6 days after flowering for wheat.

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