By: Pierce Paul, Ohio State University Extension plant pathologist
It is wet and rainy outside and the forecast calls for more rain throughout this the second week of May (May 14–19). Therefore, growers’ concerns about diseases and the need for fungicides are understandable. However, although most of our common diseases of small grain crops are favored by wet, humid conditions, it does not automatically mean that you have to apply a fungicide this week. The timing has to be correct to get the best results with the fungicide you apply, to protect the crop when it is most susceptible to the disease in question, and to attack the fungus when it is most vulnerable. Unfortunately, there is no single timing that works best for every single disease, as the growth stage at which the crop is most susceptible and the conditions under which the greatest damage occurs vary with the disease. Here are a few guidelines:
Head Scab on Wheat and Barley: It is still too early to apply a fungicide to manage head scab in the northern half of the state. Although the crop is now flowering in southern counties, the scab forecasting system (http://www.wheatscab.psu.edu/) indicates that the risk of head scab is low (the map is green), suggesting that conditions have not been favorable for the scab fungus to infect. However, if you still 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, is probably not yet available. STAY AWAY from the strobilurins when it comes to head scab management. These fungicides tend to increase rather than reduce vomitoxin contamination.
In the north, this year’s crop is about a week or so behind – we are currently between jointing and boot. 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 and heading in the case of barley. But results from more than 20 year of scab research (mainly on wheat) 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 and at or 4 to 6 days after heading for barley.
Continue to monitor the crop and the weather. Barley will begin heading-out later this week and into next week, while wheat is still about a week away from heading in the south and about two to three weeks away from flowering in most areas of the state. There is still ample time to apply a fungicide for head scab and vomitoxin control, if conditions become favorable during the next few weeks. Prosaro or Caramba should be your fungicides of choice for head scab management. The new fungicide, Miravis Ace, which seems to be just as effective as Prosaro and Caramba, based on a limited number of trials, is probably not yet available. STAY AWAY from the strobilurins when it comes to head scab management. These fungicides tend to increase rather than reduce vomitoxin contamination.
Septoria and Powdery Mildew: Septoria develops best under cool, wet conditions with frequent rainfall, whereas powdery mildew likes cool, humid conditions. However, so far this season, there have been no reports of Septoria or powdery mildew in our wheat fields. This suggests that conditions have not been favorable for either disease to become established. But this week’s rain could certainly change that, favoring both diseases and making a fungicide application warranted if your variety is susceptible.
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 leave 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.
However, 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. This was certainly the case in 2014 and 2015 when stripe rust was reported in more than 20 counties. This particular rust disease likes cool, wet conditions similar to those favorable for Septoria and powdery mildew.
Now is the time to scout fields for rust, and if you have to make an early application, choose a cheap and effective fungicide like tebuconazole. This would allow you to save your more expensive fungicide in case you need it to manage head scab and vomitoxin.
Foliar Diseases of Barley: Based on what I have seen over the last few years, scald, net blotch, and spot blotch will likely be the leaf diseases of greatest concern in Ohio this season. However, barley also has its own Septoria, Stagonospora, powdery mildew, and rusts, and strategies for managing these diseases are very similar to those described above for wheat. In addition to Septoria and powdery mildew, be on the lookout for scald as it also develops best under cool, wet conditions. As the season progress, net and spot blotch will likely increase in severity as they are favored by warm, wet conditions. In the case net blotch, excessive nitrogen fertilizer also favors disease development.
Results from studies conducted in North Dakota show that fungicides are most effective against foliar diseases of barley when applied between boot and heading. Therefore, you should be able to effectively control most leaf diseases as well as head scab with a single application at heading or shortly after. However, you should still scout to see if an earlier application is needed and use a cheap and effective fungicide if you have to make such an application.
See the attached fungicide efficacy chart HERE for more details. Always read product labels before making an application.