Wheat Disease Update: April 16, 2016

As we approach wheat heading throughout the state, additional foliar diseases have been observed. On Monday (April 4, 2016), wheat leaf rust and wheat stripe rust were observed in east MS, specifically in Lowndes County.  In addition, since the initial observation of wheat rusts in east MS, stripe rust was observed in some wheat plots in Starkville, MS.

Barley yellow dwarf virus (BYDV)

Over the past week yellow to purple leaves have become evident in some wheat fields in northwest MS. The confusing thing about Barley yellow dwarf has to do with the name.  Quite frequently, the symptoms associated with the disease that include yellow to purple leaves are clearly observed.  However, the dwarfism that has been reported to occur will not occur with later infections.  In the years I have worked in Mississippi I have only seen dwarfed plants on one occasion.  Be mindful that BYDV can result in minimal yield losses as a result of infection that occurs at any time during the growing season.  Once observed, insecticide applications are not necessary.  In addition, a fungicide application will not prevent yield loss as the disease is caused by an insect vectored virus.

Fusarium head blight, or scab, can produce pink colorations as a result of the fungus on the wheat head. Disease incidence and severity are closely related to rainfall that occurs at flowering (Feekes 10.5.1).

Fusarium head blight (scab)

Last year, scab was one of the most widely observed diseases throughout the wheat production area. The incidence and severity of scab are related to the environment encountered at the time of flowering.  In general, rainy weather occurring at the time of flowering can increase the amount of scab.  Last year in particular, early-maturing varieties at some variety trial locations appeared to have more scab than later maturing varieties.  However, the incidence of scab was more closely related to rainfall at flowering.  Fungicide applications made shortly before flowering (Feekes 10.5.1) can reduce the severity of scab.  But, no fungicide application is 100% effective since the fungicide product needs to be applied to a plant part (the head) that stands straight up in the air.  In addition, in any biological system slight nuances in the environment can overpower the effectiveness of the fungicide.

To more closely monitor the potential risks associated with the environment and potential for scab incidence refer to the predictive model at : http://www.wheatscab.psu.edu/.  Be mindful that Mississippi does not have near as many weather stations as some other states in our region.  Therefore, the coverage of environmental data to build the risk model may not be a specific as in other states.  To determine the potential risk in your given area within the state first make sure you are looking at the “winter wheat” risk model.  Also, if coloration of the map does not occur within your browser, change browser as the model only works for me in Mozilla Firefox and not within Internet Explorer.

Leaf rust

Over the past several weeks, leaf rust has been observed in numerous wheat fields throughout the state. In general, leaf rust is not an economically important disease, unless the planted variety tends to be susceptible to the fungus.  For the most part, the commercially available varieties planted in MS have some level of resistance to the disease.  In addition, leaf rust tends to be a disease observed in the lower canopy.  Leaf rust pustules are typically scattered on the leaf surface.  The sporulation that occurs as a result of leaf rust is generally more orange in color.  Check the disease package of varieties and scout fields for the presence of rust diseases.  Base any potential fungicide application on the growth stage, presence of a particular disease, expected yield, and overall management practices as employed during the season.  Keep in mind, that a fungicide will not make up for yield in situations where a full complement of fertility was not applied.  Fungicides reduce the potential yield losses associated with a foliar disease, in general they do not make or increase yield where disease has not occurred.

Stripe rust

Since the end of March, stripe rust has been observed in numerous wheat fields in both east and west MS. Stripe rust can be a yield-reducing disease on susceptible varieties.  When the disease is observed on the flag leaf fungicide applications may be beneficial.  Flag leaf observations generally produce the more characteristic “striped lesions”.  The expansion of the lesion can be observed without the presence of pustules and sporulation.  Stripe rust runs parallel to the leaf veins.  Sporulation from the pustules is generally more yellow in color than the sporulation associated with leaf rust.  To date, stripe rust has been observed in Bolivar, Lowndes, Monroe, and Oktibbeha counties in various stages of wheat (from late tillering to headed wheat).  If a fungicide application is deemed necessary, make fungicide application decisions based on the presence of the disease, overall yield potential of the crop, specific growth stage when the disease was observed, and the economics involved in your particular field situation.

Septoria leaf blotch of wheat. Note the brown/tan lesion surrounded by a yellow halo. The small black pepper grains in the leaf tissue are fungal reproductive structures.

Septoria leaf spot

Septoria leaf blotch has been observed in several commercial wheat fields. Typically observed in the lower canopy, Septoria leaf blotch can be diagnosed by the yellow halo around tan to brown spots with black pepper grains embedded in the plant tissue.  Septoria leaf blotch only reduces yield if the disease moves up into the canopy to either the leaf below the flag leaf or onto the flag leaf itself.  Using a 20x hand lens can aid in the observation of the fungal reproductive structures in the leaf tissue.  In general, Septoria can be more often observed on nutrient stressed leaves in the lowest part of the plant canopy.  However, do not confuse water injury or nutrient stress with Septoria leaf blotch.