Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
We have been assessing small grains fields throughout the state this week. The majority of fields are relatively disease free, a result of dry conditions during the preceding weeks. However, there are a couple of wheat fields showing symptoms of glume blotch. Glume blotch is caused by a fungus, Stagonospora nodorum. Symptoms of Glume blotch include grey/brown/purple lesions that typically occur on the upper third of the chaff (Fig. 1). As lesions develop on the glume they typically increase in size and produce rows of small, black pin-head structures. These structures are not easy to see unless using a good hand lens. Stems may also be infected.
Grain in infected heads tends to shrivel, resulting in poor test weight. If you notice symptomatic heads, you also should have noticed foliar symptoms earlier in the season. Foliar symptoms include brown lesions that begin as very small black/dark brown flecks or spots. On occasion these spots may be surrounded by a yellow halo. Over time lesions expand into oval shaped lesions with dark brown centers.
Figure 1. Heads infected to various degrees with Stagonospora nodorum, causal agent of glume blotch. Note greyish lesions on the glumes that extend to the base of the kernel.
Figure 2. Severe infection of wheat heads by Stagonospora nodorum.
Where did glume blotch come from? Fungal spores originate from wheat residue or volunteer wheat. Research has shown that Stagonospora nodorum can persist in residue for up to three years. The fungus can also be spread through infested seed. Stagonospora nodorum produces spores during wet periods and moderate temperatures and are spread short distances via rain splash. Typically spores spread from lower portions of the plant, resulting in a slow disease “creep” up the plants over time. This differs from other diseases such as powdery mildew and rusts, which can reproduce and spread rapidly. Conditions that favor disease include lush, dense canopies and persistent, wet conditions.
Management
As with most diseases, an integrated approach is your best bet for managing this disease. Residue management and rotation to non-host crops such as vegetables and soybean, can significantly reduce your chances of glume blotch in fields. In fields with a history of glume blotch, growers should be sure to select varieties with good glume blotch resistance. In addition, in fields with a history of disease reducing seeding rates and avoiding excessive nitrogen can help to reduce excessive lush growth and promote canopy drying. Avoid excessive irrigation and avoid irrigation at night if possible, as this serves to spread the disease and promotes infection. Remember that it’s always raining when the pivot is on; more water equals greater potential for disease development. Within the season, scouting fields to catch the disease before it can pose a threat is essential, particularly during wet seasons or if you are irrigating your fields.
Fungicide treatments that protect the flag or head and flag can be important in minimizing damage by Stagonospora nodorum in affected fields within the season. A recent article published in the journal Plant Disease gives us some insight into fungicides and their role in protecting wheat against this disease. Researchers assessed different fungicides and programs (pseudostem erection (FGS 5), flag leaf (FGS 8), full head (FGS 10), and FGS 5+FGS 8) for their ability to reduce the risk of Stagonospora blotch and the probability for fungicide profitability. The study was replicated 8 times over a 3 year period. What did they find? Fungicides applied at FGS 8 reduced the risk of glume blotch reaching the flag leaf by 55 and 75%, compared to 62 and 69% and 67 and 70% for the FGS 10 and FGS 5+8 programs, respectively, and only 32 and 37% for the programs where fungicides were only applied at FGS 5. All programs except those where fungicides were only applied at FGS 5 had more than an 80% chance of resulting in a positive yield response. However, the chance of obtaining a the yield increase required to offset application cost ranged from 44 to 60% for FGS 8, FGS 10 and FGS 5+8 programs, compared to 22 and 25% for FGS 5 programs. No differences between the fungicides tested (Prosaro and Headline) were detected. Therefore, fungicide applications targeted at or after FGS 8 are more likely to be profitable under disease favorable conditions. The article can be found at this address: http://apsjournals.apsnet.org/doi/abs/10.1094/PDIS-02-15-0218-RE.
Additional information on fungicides and glume blotch can be found through University of Delaware Factsheets and also the Field Crop Disease Management Blog.