What are the Three Fungal Leaf Diseases of Wheat?
The Extension Connection Column by Jeff Stachler
What are the Three Fungal Leaf Diseases of Wheat?
Good Day! I hope all is well!
I had to turn in my article early, so there is no weather data. The temperature forecast for the coming week will mostly be below normal, but some days will be above normal.
Crops have been getting planted, with being gone over the weekend, I’m not sure how far along we are.
The three fungal leaf diseases of wheat include tan spot, Septoria / Stagonospora nodorum blotch, and Septoria tritici blotch. Tan spot is the most common of the fungal leaf diseases.
Tan spot is caused by the fungus Pyrenophoa tritici-repentis that produces three different toxins in wheat. It has eight different races with Race 1 being most prevalent in North Dakota.
Early season leaf symptoms produce small, oval to diamond-shaped spots (1/8 to 1/2 inch long and 1/16 to 1/18 inch wide) that are found sporadically on wheat leaves. The spots will enlarge and turn tan, often with a yellow border, and have a small dark brown spot resembling an “eye-spot” near the center. The center “eye-spot” is best seen when holding the leaf up to the sunlight. If kernel infection occurs, the kernels will develop a reddish discoloration on the seed coat, commonly called “red smudge”.
The tan spot pathogen overwinters on last season’s wheat residue. In the early summer, spores are dispersed by wind and infect wheat in a wide range of temperatures. During prolonged wet periods (24 hours or more), spores are produced and blown on the other wheat leaves forming new infections.
The pathogen causing Septoria / Stagonospora nodorum blotch is Parastagonospora nodorum.
The pathogen causes water-soaked and small yellow (chlorotic) lesions on the lower leaves of the wheat plant. Lesions become yellow and eventually red brown. Mature lesions are generally lens-shaped without the distinct yellow border typical of tan spot lesions. As the disease progresses, lesions develop an ashen gray-brown center containing brown-black pepper grains which are difficult to see in a brown lesion. Wet weather after flowering can lead to lesions forming on the glumes (leafy structures surrounding the grain). Infected glumes may be covered with dark brown to dark purple lesions with ash gray areas.
The Septoria / Stagonospora nodorum blotch pathogen over winters on wheat residue similar to tan spot. The fungal spores generally require 12 to 18 hours of leaf wetness for infection. The disease develops most rapidly between 68 and 81 degrees Fahrenheit.
Septoria tritici blotch is caused by the pathogen Zymoseptoria tritici.
Initial wheat leaf symptoms for Septoria tritici blotch develop on the lower leaves as yellow (chlorotic) flecks and expand into irregular brown lesions. The lesions are restricted to the leaf veins, giving the appearance of parallel sides. In the lesion are pycnidia (pepper-grains) that are sphere to ball-shaped and gray-brown in color. It can be hard to differentiate Septoria tritici blotch from Septoria / Stagonospora nodorum blotch at times.
The Septoria tritici blotch pathogen can survive for several years in the form of vegetative strands (mycelium), pycnidia and/or perithecia in wheat residue. Spores are released and spread by wind and rain splash. The pathogen requires more than 24 hours of wetness and is most destructive between 50 and 68 degrees Fahrenheit.
Integrated disease management for these diseases includes host plant resistance, seed quality, crop rotations, residue management, and fungicides. Host plant resistance is the easiest way to manage these diseases, but not all varieties have the resistance. Choose quality seed that is pathogen free as the diseases can cause reduced germination and poor seedling vigor. Seed treatment fungicides are available to control these diseases in the seedling phase. Rotate away from small grains to reduce the pathogens. Burying crop residue will reduce these diseases, but will increase soil erosion. According to the 2021 North Dakota Field Crop Plant Disease Management Guide, all foliar fungicides will effectively manage these leaf spot diseases, except hydrogen peroxide, peroxyacetic acid, phosphoric acid, and tebuconazole. When using fungicides, be sure to rotate modes of action to reduce resistance with these diseases.
Scout fields early and often to look for disease symptoms.
