It is Time to Start Scouting for Wheat Diseases April 18, 2019
The wheat growing season in Nebraska has started. Although the wheat crop is still in the early stages of development and diseases are minimal, regular scouting for early disease detection is recommended. Scouting is especially important this season due to the excessive moisture we have had from the snow and some major storms, as well as a wetter than normal weather outlook in late April into early May.
Scout the entire field or a representative area of the field once every 7 to 10 days using one of several patterns, for example X, W, or Z to walk through the field. Make frequent stops to examine plants for disease symptoms in the upper and lower crop canopy. If you observe stunting, pull a few plants and examine the crown and roots for discoloration or rotting which is a symptom of root or crown rot caused by soilborne fungal pathogens. Other symptoms to look for are:
- fungal leaf spots indicative of Septoria tritici blotch or tan spot (Figure 1),
stripe rust and leaf rust pustules (Figures 2 and 3),
- powdery mildew (Figure 4), general yellowing that may be due to nutrient deficiencies,
- yellowing from the leaf tip down indicative of barley yellow dwarf virus (Figure 5),
- and a mosaic of yellow and green on leaves indicative of wheat streak mosaic or wheat soilborne mosaic (Figure 6).
Samples can be mailed to the Plant and Pest Diagnostic Clinic for accurate disease identification and management recommendations. Instructions for submitting samples to the clinic can be found at https://cropwatch.unl.edu/plantdisease/unl-diagnostic-clinic-lincoln.
Examination of recent wheat sample submissions to the clinic revealed virus symptoms. A double infection by wheat streak mosaic virus and wheat soilborne mosaic virus was confirmed in one sample. Although symptoms of these two virus diseases are similar (stunting and a mosaic of yellow and green on the leaves), the two diseases are quite different in their life cycles and management.
Wheat streak mosaic virus is transmitted by wheat curl mites that are transported by wind from infested volunteer wheat or late maturing host crops such as corn to newly emerged wheat in the fall. Mites can also be dispersed in the spring from infested wheat fields, but infections that occur in the fall are the most damaging. Symptoms of wheat streak mosaic in early spring are an indication that infections occurred in the fall. Management of wheat streak mosaic includes
- controlling volunteer wheat before planting in the fall,
- avoiding early planting,
- planting resistant or tolerant varieties, and
- avoiding planting in close proximity to late maturing mite and virus host crops such as corn and foxtail millet.
Wheat soilborne mosaic virus, on the other hand, is transmitted by a primitive soilborne organism in the Kingdom Protozoa known as Polymyxa graminis that is favored by excessive soil moisture. Zoospores (asexually produced spores capable of swimming in water) of P. graminis infect the roots of wheat seedlings in the fall during cool, wet periods. These infections result in severe stunting and yellowing in early spring. These symptoms tend to be most conspicuous in low lying areas of the field. Wheat soilborne mosaic can be managed by
- planting resistant varieties, and
- avoiding planting in low areas in the field where drainage is poor.
Wheat variety ratings for wheat streak mosaic and wheat soilborne mosaic are in the 2018 Fall Seed Guide.
Update from Southern States
Wheat soilborne mosaic has also been observed in Kansas. Stripe rust and leaf rust were found in south central Kansas this week and have been reported in southern states including Texas and Oklahoma. Because it is still early in the wheat growing season, it is most likely that these rust diseases will occur in Nebraska wheat fields. Fungal leaf spots, stripe rust, leaf rust, and powdery mildew can be controlled by
- planting resistant varieties,
- applying a fungicide timed to protect the flag leaf, and
- rotating crops (for fungal leaf spots and powdery mildew).