Wheat Disease Update - UNL CropWatch, May 17, 2013
|Figure 1. Tan spot on the lower leaves of wheat.||Figure 2. A young Septoria tritici blotch lesion on wheat in Saunders County on May 15.|
May 17, 2013
|Figure 3. An older Septoria tritici blotch lesion with pynidia (asexual fruiting structures) on a wheat leaf in Lancaster County on May 15.||Figure 4. Septoria lesions on wheat in south central Nebraska on May 9.|
Limited precipitation and warm temperatures over the last several days have lowered the risk for disease development in wheat. Stripe rust was found in research plots at the Agricultural Research and Development Center near Mead last week. Inspection of these plots on May 15 showed that stripe rust development and spread was slowed considerably due to warm temperatures, including record breaking temperatures over 100° F on May 14.
As of May 15, there was very little leaf rust reported in Oklahoma and only a single pustule of leaf rust was reported in Kansas during the second week of May. A survey of wheat fields in Lancaster and Saunders counties on May 15 did not find leaf rust or new stripe rust infections. There have been no reports of rust diseases elsewhere in the state.
Current weather forecasts (10 days) and the movement of rust from southern states indicate that the risk for development of damaging levels of rust diseases in Nebraska wheat fields is low. However, this risk can vary with local environmental conditions. Irrigated fields and areas that receive precipitation are at a higher risk than dryland fields and areas that remain dry. This week stripe rust was detected at low levels in several counties in south central Kansas, with a few fields having high enough levels to justify a fungicide application.
For more information see Differentiating the 3 Rust Diseases of Wheat in this week's CropWatch.
Leaf Spot Diseases
Figure 5. Typical tan spot lesions on a wheat leaf.
Figure 6. Wheat drilled into wheat stubble.
Figure 7. Powdery mildew on the lower stems and leaves of wheat.
Currently, the most commonly observed diseases in Nebraska wheat fields are the fungal leaf spots, mainly Septoria tritici blotch and tan spot. Symptoms (brown to dark spots accompanied by yellowing) initially start on lower leaves (Figure 1) and progress upward during favorable environmental conditions (warm, wet weather). Lesions of Septoria tritici blotch are broadly elliptical with a tan center and often have a distinct yellow margin (Figure 2). In older lesions (Figure 3), asexual fruiting structures known as pycnidia can readily be seen as tiny, black specks in the necrotic area of the lesion. However, pycnidia are not always present in Septoria lesions (Figure 4).
Tan spot lesions are characterized by a dark necrotic center surrounded by a yellow halo (Figure 5) and pycnidia do not form in these lesions. Late in the growing season when necrosis is extensive, it can be difficult to distinguish these two diseases from each other and from other fungal leaf spots. Both diseases overwinter on crop residue, therefore the risk for their development is higher when wheat is drilled into wheat stubble (Figure 6).
Powdery mildew is characterized by superficial patches of cottony mycelium on the surface of leaves and stems (Figure 7). It is favored by warm, humid conditions and therefore is more common in wheat fields with thick stands and in irrigated wheat. As with the fungal leaf spot diseases, powdery mildew starts in the lower canopy and moves upward with time during favorable environmental conditions. Powdery mildew has been observed in wheat fields in south central Nebraska during the last week.
In late April to early May, wheat soilborne mosaic, a virus disease, was widespread in southeastern Nebraska. This disease is favored by cool, wet weather. With the current higher temperatures, symptoms of wheat soilborne mosaic (yellowing and stunting) are expected to fade. When warm to hot conditions prevail, typically in late May to June, virus symptoms in wheat are indicative of wheat streak mosaic virus and/or Triticum mosaic virus.
Continue scouting wheat fields for disease as the growing season progresses, especially since wheat development has been delayed due to the prolonged cold temperatures in early spring. When scouting, check for disease symptoms in the upper as well as the lower canopy of the wheat crop. Wheat rusts, fungal leaf spots, and powdery mildew can be managed by applying a fungicide timed to protect the flag leaf. Fungicide label instructions and restrictions should be observed before and when applying any fungicide. Particular attention should be paid to pre-harvest intervals when fungicides are applied late in the growing season.
Nothing can be done once virus diseases have infected wheat. For the next growing season, minimize damage from wheat streak mosaic and Triticum mosaic viruses by controlling volunteer wheat and grassy weeds before planting and planting resistant cultivars. Minimize damage from wheat soilborne mosaic by planting resistant cultivars and not planting in poorly drained, low lying areas of the field.
Extension Plant Pathologist, Lincoln
Extension Plant Pathologist, Panhandle REC
Extension Educator, Clay County