Wheat Disease Update: May 4, 2014

For the most part, wheat ranges from flowering complete to base of spike (Feekes 10.5.3) to some fields that have likely reached ripening stages (Feekes 11) in the southern part of the state.  For more information regarding wheat growth stages see: https://www.mississippi-crops.com/2014/03/20/identifying-wheat-growth-stages-using-the-feekes-scale/ or look at http://varietytesting.tamu.edu/wheat/docs/mime-5.pdf for more specific information on the appearance of growth stages past flowering.  Some things have certainly changed on the disease front.  I have looked at a lot of wheat throughout the state over the past week.  I have also received quite a few calls and picture texts of different wheat diseases as well as non-disease issues.

Wheat can be a tricky crop to scout for disease.  In addition to the major foliar diseases we observe on an annual basis (e.g., leaf rust, stripe rust, Septoria leaf blotch, Stagonospora glume blotch, Barley yellow dwarf virus), wheat can experience physiological leaf spotting that is not associated with a pathogen.  Moreover, herbicide injury can produce some spotting that will appear quite similar to fungal diseases.

For the previous wheat disease updates that could contain some additional information regarding specific foliar diseases, such as bacterial leaf streak, please see:

https://www.mississippi-crops.com/2014/04/22/wheat-disease-update-april-21-2014/

https://www.mississippi-crops.com/2014/04/11/wheat-foliar-disease-update-april-11-2014/

Septoria leaf blotch, note the small pepper grains inside the lesion.

Septoria leaf blotch

A few fields have been observed to contain small amounts of Septoria leaf blotch.  The easiest way to distinguish Septoria leaf blotch from the other diseases is to look for the small pepper grains embedded in the leaf tissue.  As opposed to paraquat injury (more teardrop shape) and tan spot (more oval; outlined below), the lesions are more irregular shaped.  In some rare cases the lesions can have parallel margins similar to bacterial leaf streak as well as stripe rust.  Lesions are typically light brown to reddish brown in coloration.  Within the lesions, if they’re of the correct maturity, dark brown to black pycnidia (the reproductive structures that look like pepper grains) will be observed in the plant tissue.  In most cases, the Septoria leaf blotch I have observed has been in the lower plant canopy.  In some rare cases I have observed extremely low levels of leaf blotch on flag leaves, mostly along field edges.  If you suspect Septoria leaf blotch, try to observe several infected leaves for the characteristic symptoms as there are some other diseases that can be easily confused.  For additional information as well as some good close up photos of the reproductive structures embedded in plant tissue see:

http://www.apsnet.org/edcenter/intropp/lessons/fungi/ascomycetes/Pages/Septoria.aspx

Glume blotch

The purplish discoloration observed on wheat heads is more often than not glume blotch.  Glume blotch occurs just about every year, and can be observed at low levels in almost all wheat fields.  The purplish discoloration can be more evident in wheat fields that exhibit irregular head emergence.  Extended periods of dew as well as temperatures between 68 and 81F are conducive for the development of glume blotch.  In general, if sufficient moisture is present at the time of heading the disease will be more likely to occur.

Tan spot of wheat, note tan lesion surrounded by distinct yellow halo.

Tan spot

Symptoms associated with tan spot appear similar to paraquat injury as well as Septoria leaf blotch.  However, the absence of the pepper grains inside the lesion is the main difference between Septoria leaf blotch.  Lesions tend to be tan in coloration, with a yellow halo around the lesion.  The lesions are generally diamond or oval-shaped and can be as long as ½ inch.  The most distinguishing characteristic is when the leaf is held up to the light.  A small, dark spot is normally present in the center of the lesion.  The pattern developed by the small dark spot in the center with the tan lesion surrounded by a yellow halo produces an “eyespot” appearance.  At present the only tan spot that I have observed this season has been in east MS.  Keep in mind that paraquat injury can look similar.  However, the lack of yellow halo as well as dark spot in the middle of a paraquat-induced lesion should help tell the two apart.

Barley yellow dwarf virus

Once heads emerge we tend to see a lot more BYDV in wheat fields.  This year in particular I have observed more BYDV than in recent memory.  Small circular areas of wheat plants in fields have been observed throughout MS this year.  Generally speaking, symptoms on flag leaves can range from light yellow to purple depending on the time of infection as well as the particular variety planted.  Remember, once BYDV is observed in a field, no management practice is necessary.  The insects that vector the virus have already come and gone and a fungicide application isn’t effective since the disease is caused by a virus.

Fusarium head scab

The risk assessment tool has not suggested that scab was a threat during 2014.  Generally speaking the occurrence of scab is associated with rain during flowering.  Based on the model, it appears the other environmental variables have not occurred for scab to be an issue this season.  Information regarding the environmental model can be located at: http://www.wheatscab.psu.edu/ .

Leaf rust

To this point, I’ve only received a single call regarding leaf rust and only observed a trace amount of leaf rust in some protected volunteer wheat plants.  One field in the Greenwood area was observed to have leaf rust by a Jimmy Sanders representative on Tuesday afternoon (4/29).  Rarely has leaf rust triggered a fungicide application.  Light leaf infection, on the order of less than 2% in most cases, is not enough to trigger a fungicide application.  In addition, the vast majority of the wheat in MS is beyond a growth stage where a fungicide would benefit yield.  Also, the environment at this time is not conducive for the widespread development of leaf rust.  Leaf rust prefers cooler temperatures when compared to stripe rust that prefers warmer temperatures.  Keep in mind that leaf rust pustules will be scattered on a leaf and appear more orange in coloration.

Stripe rust

Last week (4/25) I observed stipe rust on a few scattered leaves in a protected area under a rainout shelter in Stoneville.  Outside of those scattered plants I haven’t seen any stripe rust in a commercial field nor have I received a phone call for stripe rust.  At this late stage, immediately past flowering, if stripe rust were to occur in a field I doubt there would be enough time for a yield loss to occur as a result of the disease.