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Differentiating Between Wheat Head Diseases and Disorders

July 5, 2016

Several diseases and disorders can develop in wheat heads and insect pests can also affect the development of the wheat head. It is important to scout and determine which diseases/disorders are affecting your crop. This may help in making management decisions in order to minimize the impacts of these diseases. The following diseases and insect injury are being observed in wheat this season.

Fusarium Head Blight (Scab)
FHB or scab is a fungal disease that develops in the wheat head. The scab pathogen infects wheat head through the flowers and infection is promoted by warm and humid weather. Typical symptoms include tan or brown spikelets or the entire wheat head may appear bleached. The peduncle of the affected spikes remains green or is slightly browned. Sometimes when the infected spikelets are wet, a salmon-pinkish color can be seen. The scab fungus produces a mycotoxin called Deoxynivalenol (or DON for short). This mycotoxin is poisonous to humans and livestock and the FDA recommended maximum rate is 2 ppm in food grade wheat. Scout wheat fields before the wheat heads dry out. Fields with high scab incidence should be harvested last and depending on the level of scab, could be blended to lower the DON level.

Root and Crown Rots
There are three common diseases of wheat roots and crowns namely take-all (Gaumanomyces graminis var. tritici), common root rot (Bipolaris sorokiniana) and Fusarium foot and crown rot (Fusarium spp.). Infection of wheat by the root and crown rot pathogens takes place early in the season but symptoms are seen after wheat heading, especially when plants are moisture-stressed. Wheat plants that have root and crown rots have entire spikes bleached. Sometimes this symptom may be confused with Fusarium head blight. The best way to determine if the plants have root and crown rots is to gently uproot the plants and examine the crown and sub-crown. For plants with take-all root and crown rot, the crown and first nodes have a glassy-black appearance. Plants with common root rot have the sub-crown discolored, while plants with Fusarium foot and crown rot have a reddish-brown crown area.

Wheat Stem Maggot Damage

During the larval stage, the wheat stem maggot (Meromyza americana) is a yellowish-white maggot. During the spring the female flies lay eggs in the leaves and stems of wheat, other cereal crops, and grasses. When the larvae hatch, they burrow into the stem and feed near the flag leaf node. Heavy feeding causes the peduncle to be excised from the stem leading to the drying of the entire head. Stem maggot damage can be determined by gently pulling out the bleached wheat head. The damaged wheat head should easily pull out of the stem. Although this pest can be found throughout much of the wheat production areas of North America, it is generally considered a minor pest. The current management recommendations are using delayed planting to avoid periods when adult flies are active in the spring, and also preventing the green bridge from occurring between volunteer plants and the new crop.

Source: Emmanuel Byamukama, South Dakota State University