Diagnosing Leaf Diseases

1. Powdery mildew
Powdery mildew is favored by cool temperatures (<85° F), high nitrogen fertility, and high stand densities. It is characterized by white mycelium on the lower leaves and stems.  The fungal growth begins to turn brown at maturity or under conditions that become less favorable for the fungus.  Flag leaves can be attacked, and tiller numbers can be reduced, as well as yield and test weight. Triazole fungicides (Tilt, Stratego) are effective for control.  Applications at earlier growth stages (5 leaf stage) will maximize benefits. Late season fungicide applications (beyond the flag leaf stage) are less beneficial as significant damage will have already occurred. Since powdery mildew generally develops late in the growing season for barley, the potential for economic control of mildew in barley is not likely.

2. Stem rust
Many varieties of wheat are resistant to both stem and leaf rust, spores of which overwinter in southern states and then are carried north by winds. New races of rust appear periodically, making rust research and the release of new varieties necessary.  Leaf rust control with fungicide sprays is not usually economical, unless susceptible or moderately susceptible varieties are grown. Stem rust control with fungicides is rarely necessary.

3. Tan spot       and    4. Septoria leaf blotch
Common leaf diseases are tan spot, septoria leaf blotch, speckled leaf blotch, spot blotch, and net blotch. If enough leaf area is killed by leaf diseases—especially the flag leaf—seed set and grain fill are reduced. The results are lower test weights and reduced yield (losses from 10-40% are possible). Leaf disease-causing fungi and bacteria survive in infected cereal residue, in grassy weeds, and sometimes in seed. Most of these disease organisms require long periods of dew or high humidities for infection to occur. Chemical control of leaf diseases in spring wheat is practical under certain conditions: high yield potential, presence of disease, and persistent humid weather. Crop rotations and burying crop residue help reduce these diseases.

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5. Barley Yellow Dwarf Virus
The BYD virus is transmitted by several species of aphids, and affects barley and oats more than wheat. Symptoms include stunting and chlorosis (yellowing) of individual plants or groups of plants.  Symptoms are more severe when plants are infected in earlier stages of development.  No control measures of the BYD virus are recommended; control of aphids should be based on the recommended thresholds for that pest.

Xanthomonas streak
Leaves attacked by Xanthomonas (zan-tho-mone-us) exhibit small, linear, light brown water-soaked spots or streaks. These elongate under wet conditions, running with the veins and may exceed 2 inches or more on varieties susceptible to foliar diseases.  Heads can also be attached, with infected spikelets exhibiting  a dark staining of the veins. This symptom is referred to as black chaff.  Xanthomonas is seed-borne but may also survive in residue.  In wet conditions, circles of infected plants, 6-12 inches in diameter, may be observed in fields. No control practices are recommended except to avoid using varieties susceptible to foliar diseases, and seed lots known to be heavily contaminated.

Source: Jochum Wiersma, University of Minnesota Extension Small Grains Specialist, and The U of M Small Grains Field Guide, which includes more information about small grain diseases and other pests.  More information can also be found in NDSU plant disease publications online at www.ext.nodak.edu/extpubs/plntdise.htm.  Disease treatment decisions can be aided by following NDSU’s disease forecasting system, which can be found on the web www.ag.ndsu.nodak.edu/cropdisease or by calling toll-free, 1-888-248-7357.