Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
Barley is headed out in the majority of fields. If you planted untreated seed, you may notice black heads (Figure 1). The heads, when touched, will produce a black “dust.” This is covered smut, a fungal disease, and the black material is actually a mass of fungal spores. The fungus infests the seed and infects the germinating seedling, growing within tissues undetected. The fungus then invades the developing head and replaces kernels with masses of spores. Covered smut can be controlled by planting certified, disease free seeds and/or treating with a fungicide seed treatment. Chemicals such as Raxil and others work well.
Figure 1. Covered smut in barley.
We are seeing a few interesting things in wheat. In some fields, symptoms that may be attributed to soil borne viruses have been observed in several fields (Figure 2). Soilborne mosaic and spindle streak virus both are transmitted by protists that exist and persist in soils. When temperatures are cool and soil is damp the organisms can infect roots and transmit the virus. The virus then reproduces within plant tissues and can cause stunting, chlorosis, and physical deformities, as well as yield loss. Symptoms can look like nutrient deficiency in many cases, and symptoms vary with variety, making confirmation based on visual symptoms difficult. In addition, both viruses degrade rapidly, resulting in false negatives in viral tests. Yield loss is dependent on symptom development in upper leaves, particularly in the flag leaf, but also the leaves immediately below the flag leaf. Symptoms typically dissipate when temperatures warm up. Cool temperatures forecasted in the next week may favor symptom development. Unfortunately there are no control options at the current time for symptomatic fields. More information on the viruses can be found here: http://www.apsnet.org/edcenter/intropp/lessons/viruses/Pages/SoilborneWheatMosaic.aspx
Figure 2. Wheat foliage with putative soilborne virus symptoms.
As I mentioned previously, stripe rust was confirmed in areas of the Eastern part of Maryland between Easton and Denton last week, and we confirmed it at very low levels at research plots in Middletown and Harbeson, Delaware early this week. The disease is also present at moderate levels in a few fields in the south central portion of Maryland, where wheat is just starting to head out (Figure 3). Remember that stripe rust is a cool weather pathogen, and can reproduce rapidly under the appropriate conditions on susceptible varieties (7 days from infection to new spores). If cool weather and rains persist, there is a chance that you could see elevated levels of stripe rust in some areas.
Figure 3. Top- the blue circle denotes an area of a wheat field infected with moderate levels of stripe rust. Bottom, a close up of affected plants within this area.
The next thing you will be thinking about is flowering and if you will need a fungicide to potentially suppress Fusarium head blight (FHB). Fungicides including Caramba, Proline, and Prosaro are recommended for FHB management, and also will provide control of our other major diseases, provided that they have not infected the flag leaf or glumes prior to application. The flowering stage is very challenging to determine. You have 5-6 days from the start of flowering (when you see roughly 50% of your main tillers flowering from the center of the head) to make an application of the aforementioned fungicides without dropoff in efficacy. If you are not clear on how to determine flowering, see Pierce’s video here: https://www.youtube.com/watch?v=ybZVW_YbhxY&feature=youtu.be. Remember that flowering also includes secondary tillers, which flower a couple days after the main tiller, which is one reason efficacy of these fungicides for suppressing DON and FHB is only around 45-50%. Keep an eye on the fusarium head scab prediction center (www.wheatscab.psu.edu) for updates. Make sure to check it around noon for the report including the previous evening’s weather data.
There is a link to the Fungicide Efficacy for Control of Wheat Diseases Table developed by North Central Regional Committee on Management of Small Grain Diseases below (click on the table for a larger PDF version.)
