By Jennifer Elwell. Sources: Chad Lee, Travis Legleiter, Raul Villanueva, and Carl Bradley

May is the prime time to scout and take action for wheat for diseases, insects and weeds.  

While approximately 10,000 acres of wheat that were underwater for several days in April are lost, most farmers across Kentucky have reported only a small portion of their wheat fields were impacted.  Further good news is that the condition of Kentucky’s surviving wheat crop has improved, according to the May 5 USDA-NASS Crop Progress and Condition report.  

By May, wheat is typically flowering and developing seeds. This grain-filling period is critical for producing high yields because kernel size and weight are determined during this stage. Yields will be reduced by any stress (high temperatures, low soil moisture, nutrient deficiencies and pests) occurring during grain fill.   

Even though we have little control over weather scenarios, decisions can be made to address pest stressors.     

Pre-harvest Weed Control  

After wheat has headed, watch for emerging warm-season weeds. A preharvest treatment after the hard-dough stage (30% or less grain moisture) may be needed to control weeds – such as Pennsylvania smartweed, ragweeds and johnsongrass – and improve harvesting efficiency of wheat, especially where wheat stands are poor and weed infestations are heavy. However, research has shown preharvest treatments are not effective in preventing the production of viable seed of winter annuals weeds such as Italian ryegrass. 

Glyphosate and specific formulations of 2,4-D are examples of herbicides registered for preharvest weed control in wheat. Glyphosate and 2,4-D formulations registered for preharvest weed control typically require a 7-to-14-day pre-harvest interval. Drift to nearby sensitive crops is a concern when using these treatments. Preharvest treatments can injure wheat or reduce seed germination or seedling vigor, and are not recommended for wheat grown for seed production.  

Late Season Insect Pests 

Scouting for insect pests, such as aphids, armyworms, and the cereal leaf beetle, should continue through May. Early detection, correct identification, and assessment of pest problems allow appropriate management decisions to be made. Regular field monitoring is the best means of getting the information needed to follow the recommended treatment guidelines.  

For spring aphids, use the rating scale to assess direct aphid damage and consider control measures if an average rating of 2 (moderate) or higher is recorded.  

Visit each field at least once a week to check for armyworms. First, check field margins and lodged grain. If armyworms are present, begin surveying in the standing grain. Armyworms longer than 1¼ inch may have completed most of their feeding. If the grain is nearly mature and no head clipping has occurred, controls are not advised. 

Cereal leaf beetles can be seen on the leaves from early April until mid-May. Their distinct yellow eggs are laid from mid-April until late May; the larvae are active and feeding from late April through mid-June. Controls may be warranted when there is an average of more than one larva and/or adult per stem. 

Late-season disease  

The risk of Fusarium head blight (FHB) has increased due to rains and generally cloudy weather that occurred in early May. According to the FHB risk map (www.wheatscab.psu.edu), large differences in risk can be observed for susceptible varieties compared to moderately resistant varieties. Wheat is generally now beyond the time where a fungicide application can be made (several days past early anthesis at this point in time), except for maybe a few exceptions with later-maturing wheat varieties. It will be important to start monitoring symptoms of FHB in wheat fields over the next few weeks to determine if adjustments to combine fan speeds should be made, which can help blow out the lightweight "tombstone" kernels, which may have the highest levels of deoxynivalenol (DON; "vomitoxin"). Although the time for a fungicide application to this year’s wheat crop has likely passed, information about how well different fungicides perform for management of FHB and other diseases can be found in the Wheat Disease Efficacy Guide, which is updated annually, and available at the Crop Protection Network website (https://cropprotectionnetwork.org/).

Other diseases that have been observed already this year by Dr. Bradley’s Research/Extension program include Stagonospora and Septoria leaf blotch, leaf rust, and based on symptomology, possibly a viral disease that resembles wheat streak mosaic, wheat soilborne mosaic, or wheat spindle streak mosaic. Unfortunately, specialized assays must be done to determine if and which virus is causing these symptoms. These assays are not done at the UK Plant Disease Diagnostic Lab (UK PDDL), but samples may be sent to the UK PDDL for examination and determination if the samples should be sent on to a different external lab that does the virus assays. Other diseases that may be present currently in some fields or that may be observed in the coming weeks include powdery mildew, barley yellow dwarf, glume blotch, black chaff, bacterial leaf streak, and stripe rust.

In general, management of important wheat diseases includes an integrated approach. An integrated approach includes planting the most disease-resistant varieties available and applying an effective fungicide at the appropriate wheat growth stage (if warranted from disease risk and/or scouting observations).     

Additional recommendations on managing late-season wheat and preparing equipment for harvest can be found at https://graincrops.ca.uky.edu