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Allen, T.

Wheat Disease Update: May 4, 2013

The majority of the winter wheat in MS is beyond the flowering stage of development.  However, there are instances where winter wheat has either just begun to flower or will likely reach flowering at some point in the next week depending on geographic location of the particular wheat field.  In some situations, foliar diseases continue to plague wheat fields.

Bacterial leaf streak.  Note the characteristic parallel lesions in the bottom left hand corner of the photo.

Bacterial leaf streak. Note the characteristic parallel lesions in the bottom left hand corner of the photo.

Bacterial leaf streak

Over the past week I’ve observed several fields with symptoms of bacterial leaf streak.  More often than not, the specific symptoms on leaves can be mistaken for Septoria leaf blotch.  At this time, most of the Septoria appears to be lower in the canopy, and in some plants that I observed near Greenwood on Tuesday afternoon I could find active fungal fruiting bodies of Septoria at the base of the plant on the main stem.  Bacterial leaf streak and Septoria leaf blotch will appear quite different.  In most of the cases where I have received calls about Septoria being present on flag leaf tissue the disease has been bacterial leaf streak rather than Septoria.  The symptoms of bacterial leaf streak will appear as linear lesions that run parallel with the veins and are prominent on the flag leaf (look at bottom left hand corner of the attached photo).  In severe cases lesions of bacterial leaf streak can coalesce (run together).  Early in the development of the disease the lesions will appear sunken into the leaf tissue and can be maroon to tan in color.  Conversely, Septoria will be present on leaves in the lower canopy and are oval to oblong in appearance and will generally have a prominent yellow halo around the lesion.  Mature lesions will have the characteristic pepper grains from the fruiting structures of the fungus present in the lesions.  Bacterial leaf streak situations would not benefit from a fungicide application.  Photos of the lesions associated with Septoria leaf blotch can be observed at: http://www.mississippi-crops.com/2013/04/20/wheat-disease-update-april-20-2013/

Barley yellow dwarf

I’ve observed several situations in eastern MS where BYDV can be clearly observed in some field settings.  I have also had some calls regarding BYDV.  Late infection will normally be observed as either a purpling or yellowing of the flag leaf tissue on either a single flag leaf or in clumped areas in the field.  Once the disease is observed there are no management strategies that are necessary.  In fact, rarely have I observed a situation where yield loss has occurred due to BYDV in MS.

Leaf rust

As opposed to 2012 I’ve seen much less leaf rust throughout MS wheat fields this season.  I don’t foresee a problem from leaf rust this year and have found the disease difficult to detect in most wheat fields.  For further information regarding the observational differences of leaf and stripe rust please refer to:

http://www.mississippi-crops.com/2013/03/30/wheat-disease-update-march-30-2013/

Stripe rust

Since some of the early reports of stripe rust I have not received many reports of additional stripe rust.  But, with that in mind, following the cooler wetter conditions we’ve encountered over the past couple of weeks we could still see some stripe rust development on susceptible varieties.  Be on the watch for stripe rust and make sure you know what variety is planted in each wheat field when it comes time to decide if a fungicide is necessary.  Fungicides will be more beneficial when applied to susceptible varieties if disease has been observed.

Fusarium head blight (scab)

Wheat in the flowering stage of development is most at risk to scab if (and I can’t repeat that word enough, IF) the correct environmental conditions allow for the fungus to produce conidia (spores) and allow infection.  The most important time for infection occurs during the 7 day period that surrounds flowering.  The best option to determine if wheat is at risk to scab is to use the predictive/risk model available at: http://www.wheatscab.psu.edu/ . The model is approximately 70% effective at predicting the risk of scab.  Presently, wheat in south MS may be at risk to scab development.  The green area on the MS map occurs due to a low number of weather stations in some of those areas.  However, based on a report I received today from south MS, most of the wheat in these areas is likely beyond risk to scab development.

Keep in mind, only a subset of the fungicides labeled for wheat are labeled to manage scab.  In addition, if you have made a tebuconazole application of 4 fl oz/A prior to flowering you cannot make another application with a fungicide containing tebuconazole (such as Prosaro).  See: http://www.mississippi-crops.com/2013/04/02/important-notice-tebuconazole-label-restrictions-in-wheat/ for additional information regarding tebuconazole and the products that contain tebuconazole.

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