Head disease of wheat

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Wheat acres rebounded significantly from the record low acres of 2009-2010. Now wheat in the southern portion of Missouri is or has headed. Wheat in the central portion of the stand is beginning to head. Wheat in the northern regions of the state is approaching heading. Thus far, there have been questions about Fusarium head blight or scab, loose smut and foliage diseases of wheat. However, there have been few actual reports of these diseases occurring in the field. Generally head diseases are most obvious right after flowering when heads are still green. As heads and plants begin to dry down it becomes more difficult to detect and diagnose head diseases of wheat.

From flowering through the early stages of grain fill is the time to scout for loose smut, Fusarium head blight or scab, Septoria and Stagnospora infections on heads, bacterial stripe and black chaff on heads and take-all. Loose smut is obvious as heads emerge from the boot and for several weeks after that. The kernels on infected heads are replaced with masses of powdery black spores. So the heads have a very distinct black, powdery appearance.

These spores are eventually dislodged by wind and rain, so later in the season the smutted stems are less evident and only the bare rachis will be left. Spores produced on smutted heads are wind carried to adjacent plants in the field and infect through the flowers. The fungus that causes loose smut survives within the embryo of wheat seeds. If infected seed is planted, the plants growing from those seeds will be infected and develop smutted heads the next season. If seed from a field that has a “small” amount of smut in one season is used for seed, the field planted with that seed may have a substantially higher level of smut.

Loose smut is best controlled by planting either disease-free seed or using a systemic fungicide seed treatment. Scab or Fusarium head blight of wheat was covered in last week’s issue of the Integrated Pest and Crop Management Newsletter. Symptoms of scab should become evident over the next few weeks. The characteristic symptom of scab on wheat is a premature bleaching of a portion of the head or the entire head. Superficial mold growth, usually pink or orange in color, may be evident at the base of the diseased spikelets. Bleached spikelets are usually sterile or contain shriveled and/or discolored seed.

Septoria leaf blotch has not been particularly widespread or severe thus far this season but with scattered pop-up rains and high humidity it could still develop. Foliage symptoms may be evident on the flag leaves. On the heads dark brown to black blotches may develop. Stagnospora nodorum may also cause leaf lesions but is usually more common on heads- again causing dark blotches on glumes of part or all of the head.

Bacterial stripe or black chaff is a bacterial disease that produces symptoms on both leaves and heads. Water-soaked lesions may develop on young leaves. These expand into reddish-brown to brownish-black streaks on the leaves. Glumes and awns show brown-black blotches or streaks. Fungicides are not effective against bacterial stripe or black chaff so the use of resistant or tolerant varieties and crop rotation are the main management options.

Take-all is one of the more common root and crown rot diseases of wheat in Missouri. The fungus which causes this disease may infect seedlings in the fall. Symptoms are usually most evident after heading as white heads on the wheat plants. Entire heads on infected plants may be bleached (white heads) and sterile. Infected plants are also stunted and slightly yellow, have few tillers and ripen prematurely. Plants with take-all typically have poorly developed root systems and roots are sparse, blackened and brittle. With sufficient soil moisture, a black-brown dry rot may extend into the crown and up the lower stem.

This shiny, black discoloration of the lower stem and crown may be seen if the lowest leaf sheath is scraped off with a knife or fingernail. A management program for take-all should including planting good quality seed of adapted, disease resistant varieties, planting in well-drained sites under good seed bed conditions, rotating with nonhost crops for one to three years, controlling weed-grass hosts and volunteer wheat, using seed treatment fungicides and maintaining good plant vigor with adequate fertility.

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