Stripe Rust & Aphids Confirmed in Nebraska Wheat

Photos of 3 wheat fields: with stripe rust, leaf rust, and healthy
(From left) Figure 1. A hot spot of stripe rust in a research plot at the ARDC near Mead on April 14. Figure 2. Leaf rust in a research plot at the ARDC near Mead on April 14. Figure 3. A healthy wheat field in Thayer County on April 13. (Figures 1-8 by Stephen Wegulo)

Stripe Rust & Aphids Confirmed in Nebraska Wheat April 15, 2016

Water-stressed wheat field
Figure 4. A field with symptoms of stress from lack of moisture in Nuckolls County on April 13.
Powdery mildew in wheat
Figure 5. Powdery mildew in a research plot at Havelock Farm on April 14
Powdery mildew in wheat
Figure 6. Yellowing of the lower leaves caused by powdery mildew in a research plot at the ARDC on April 14
Septoria tritici blotch in wheat
Figure 7. Yellowing of the lower leaves caused by Septoria tritici blotch in a research plot at the ARDC on April 14
Yellowed wheat
Figure 8. Large areas of yellow wheat in a grower’s field in Jefferson County on April 13

In surveys of wheat fields on April 8 and April 12-14, stripe rust (Figure 1) and leaf rust (Figure 2) were found at trace to low levels in south central and southeast Nebraska. Samples from several wheat fields in Banner County in the Nebraska Panhandle had both stripe rust and leaf rust. This is the same county in which both diseases were active last fall, indicating the possibility that the two rusts overwintered.

The surveys on April 8 and April 12-13 were in the southernmost tier of counties from Gage County in the southeast to Red Willow County in the west central part of the state.  Low levels of disease, mainly fungal leaf spot diseases, were seen on the lower leaves in some fields.  Other fields had very little or no disease and looked healthy (Figure 3). Symptoms of stress from lack of moisture were apparent in several fields (Figure 4).

On April 14, surveys were conducted in research fields at Havelock Farm in Lincoln (Lancaster County) and at the Agricultural Research and Development Center (ARDC) near Mead (Saunders County).  The predominant disease at these two locations was powdery mildew (Figure 5) that was active mainly in the lower canopy.  A few hot spots of stripe rust (Figure 1) were found in one field at the ARDC.  This same field also had trace levels of leaf rust (Figure 2).

A symptom that was common in many fields is yellowing of the lower leaves (Figure 6).  Several factors are involved in yellowing of the lower leaves early in the growing season.  They include environmental conditions such as low temperatures and lack of adequate sunlight (lower leaves are shaded by the upper, actively growing leaves), natural senescence coming out of the winter, and inadequate nitrogen.  If there are no symptoms or signs of disease on the yellow leaves, environmental conditions are the most likely cause.

Yellowing of lower leaves can also be caused by diseases including powdery mildew (Figure 6) and fungal leaf spot diseases (Septoria tritici blotch or tan spot, Figure 7).  If diseases are the cause of yellowing, symptoms or signs of disease will be visible on the leaves.  These include dark brown or black spots, necrotic (dead) areas on the leaves or, in the case of powdery mildew, a white powdery fungal growth on the stems and leaves.

Large areas of yellow wheat as seen in one of the fields surveyed on April 13 (Figure 8) are indicative of lack of nitrogen.  This commonly results from the physico-chemical properties of soil in these areas, including pH, that tie up nitrogen, preventing its availability to the wheat crop.

Management

Given the presence of stripe rust and leaf rust in the state, it is recommended that wheat fields be scouted regularly for early disease detection.  Dry conditions during the last two weeks have slowed or stopped stripe rust development.  However, rain is forecast in the next several days for most of the state and especially the western half.  Stripe rust, leaf rust, and fungal leaf spot diseases will become active and continue to spread following rain events. 

If you see stripe rust in your field and favorable weather conditions (moisture, wind, and cool to moderate temperatures) are forecast, consider applying a fungicide to protect the wheat crop.  The recommended timing is at 50% to 100% flag leaf emergence.  However, if the risk of stripe rust development and spread is high, an earlier application at the jointing growth stage may be warranted.  Consider the yield potential, resistance level of the wheat variety planted, and the price of wheat when making the decision to spray. 

Keep in mind that stripe rust can form new races that can overcome the resistance in varieties rated as resistant, and resistance can be overwhelmed if disease pressure is high.  Therefore, even if you have planted a resistant variety, consider a fungicide application as a second line of defense.  Fungicides that are effective in controlling rust diseases are also effective in controlling fungal leaf spot diseases.  A list of wheat fungicides and their efficacy ratings is provided in Table 1 compiled by the North Central Regional Committee on Management of Small Grain Diseases (NCERA-184).

Aphids in Wheat

Aphids are being found in Nebraska wheat fields.  Regular scouting of wheat for aphids is important because numbers can increase rapidly.  There are several aphids in wheat which may be found in Nebraska.  Identification is key because the damage potential varies among species.  See NebGuide G1284, Cereal Aphids, for more information.

Wheat-aphids
(From left) Figures 9-12. Birdcherry oat aphid, corn leaf aphid, English grain aphid, and greenbug.

If populations are below threshold level, note the level of beneficial predatory and parasitic insects present.  Lady beetles, minute pirate bugs, syrphid fly larvae,and parasitic wasps can play an important role in suppressing aphid numbers. See Extension Circulars Beneficial Insects I and II (EC 1578 and 1579) for more information on beneficial insects found in Nebraska field crops. 

A variety of insecticides may be used to control economic populations. See the 2016 UNL Guide for Weed, Disease and Insect Management in Nebraska (Extension Circular 130) for a list of insecticides labelled on wheat, aphid control rates, and restrictions.

Aphids can transmit barley yellow dwarf virus to wheat.  Symptoms of barley yellow dwarf include yellowing or purpling of flag leaves from the tip to the bottom and from the edges to the midrib which can significantly reduce yield. Barley yellow dwarf cannot be controlled once it occurs. Controlling aphids can reduce infections.  However, it is recommended that aphids be controlled based on actual aphid damage (see Table 2) rather than the potential for transmitting barley yellow dwarf. This is because aphids that fly into the field after an insecticide spray can still transmit the virus.

Table 2. Number of small grain aphids per stem to justify chemical control at various wheat growth stages
Type of aphidSeedlingBoot to headingFloweringMilky ripeMilk- medium dough
Greenbug 5-15 25 >25 >25 >25
Corn leaf aphid 20 30 >25 >25 >25
Bird cherry-oat aphid 20 30 5 10 >10
Source: The High Plains Integrated Management Guide, section on bird cherry-oat aphid.