NPIPM:Tan spot
Authors: Byamukama, E., Wegulo, S., and Ali, S.
Causal Organism
Tan spot is caused by the fungus Pyrenophora trictici-repentis. There are eight known races of this fungus.
Symptoms and Signs
Tan spot produces tan to brown, lens-shaped lesions on the leaves. Lesions can vary from 1/8 of an inch to ½ of an inch in length and 1/16 of an inch to 1/18 of an inch wide. Lesions are surrounded by a distinctive yellow halo with a small dark spot in the center. Tan spot lesions can coalesce, leading to larger necrotic (dead) areas of the leaf. In advanced cases, tan spot can infect the wheat heads and the kernels will turn dark red. This change in color is known as red smudge. Red smudge usually develops under prolonged wet periods or high humidity during kernel development.
Disease Impacts
Tan spot lesions on young leaves often remain small and will enlarge as the leaf matures. Eventually the lesions will decrease the green area enough to affect the rate of photosynthesis, thus decreasing yield. Tan spot can cause significant decreases in yield especially when the disease severity is high at the time of grain fill.
Life Cycle and Epidemiology
The tan spot fungus overwinters as black pinhead-sized structures known as pseudothecia that develop on last season’s wheat infested straw. In the spring and early summer, the pseudotheica releases spores that are spread by wind and rain splash. Initial infections can also produce secondary spores. Tan spot has a wide range of temperatures (50-70 F) for infection to take place, but optimal temperature for infection and germination is over 60°F and in considerably wet growing seasons.
Management Approaches
Rotating wheat crops with oats and barley or with non-host crops such as canola, flax, corn, potatoes and alfalfa can reduce the incidence of tan spot. Tillage to reduce surface straw will reduce spread of airborne spores. Timely foliar fungicides are effective in protecting the flag leaf and the leaf below flag leaf. Fungicides are especially effective when there is prolonged rainfall and for wheat planted into wheat stubble.
Other Online Resources
Wegulo, S. 2011. Tan spot of cereals. http://www.apsnet.org/edcenter/intropp/lessons/fungi/ascomycetes/Pages/TanSpot.aspx
Fungicide efficacy table by NCERA 184 for fungicides effective against tan spot and other fungal diseases. http://bulletin.ipm.illinois.edu/wp-content/uploads/2015/04/NCERA-184-Wheat-fungicide-table-2015_V3.pdf.
