eOrganic author:
Ann Hazelrigg, University of Vermont Extension
Smut Diseases
Loose smut of wheat (Ustilago tritici) and barley (U. nuda) fungi survive in the embryo of infected grain and are truly internal seedborne pathogens. Disease symptoms appear at head emergence when sooty olive-black spores replace healthy kernels. Spores are quickly dispersed by rain or wind after emergence leaving a bare rachis. Spores land on healthy flowering heads, germinate, and colonize the embryo of the developing seed or grain. This disease will build up across growing seasons if infected seed is planted and saved for planting future crops. This disease will not occur in a field unless infected seeds are planted. Yield reduction is proportional to the number of infected plants. Do not save or replant smutted seed. Purchase smut-free seed and choose disease-resistant cultivars where available.
Figure 1. (R) Brown spores of loose smut of wheat (U. tritici) and Figure 2. (L) barley (U.nuda). Photo credits: G.C. Bergstrom, Cornell University
Figure 3. Seedborne wheat loose smut. Photo credit: G. Sassi, University of Vermont
The pathogen causing false loose smut (U. nigra) of barley is externally seedborne and infects between germination and seedling emergence. Disease symptoms of loose smut and false loose smut are similar and become apparent after heading as dark brown heads with masses of spores. Affected heads tend to be taller and emerge slightly earlier than healthy heads. The rachis may become bare as the spores are dispersed. Plant disease-free seed and use resistant cultivars where available.
The pathogen causing covered smut (U. hordei) of barley is an external seed contaminant causing similar disease symptoms as loose smut at head emergence but with brown-black masses of spores. During harvest, spores can contaminate healthy seed, machinery, and soil. Seed and soilborne spores remain dormant over summer and germinate the following season under cool moist conditions, infecting the seedling before emergence. The fungus grows within the plant, eventually forming smutted heads. Plant disease-free seed and use resistant cultivars where available.
Identification of the different smuts may require laboratory diagnosis.
Foliar Fungal Diseases of Grains
Foliar fungal diseases may drastically reduce grain yields when infections are severe during the first few weeks after flowering. Several foliar diseases, described in detail below, are caused by pathogens that live internally as mycelium within the pericarp of the seed and overwinter in contaminated seed or on infected plant debris. These pathogens prefer moderate temperatures and rainy/humid conditions for growth. Repeated grain plantings in the same field and tillage practices that leave crop residue on the soil surface may have magnified the occurrence of foliar diseases in recent decades.
Tan spot (Pyrenophora tritici-repentis) symptoms begin as tan-colored flecks on upper and lower leaf surfaces that expand into oval-shaped lesions with yellow borders. As the disease advances, the lesions coalesce into blotches surrounded by chlorosis/yellowing. When wet, the center of the tan lesions darken as the fungus produces olive-brown spores. Bleaching of infected wheat spikes or browning of glumes may occur and kernels develop a red smudge on the seed coat caused by P. tritici-repentis infection.
Figure 4. Tan spot of wheat (Pyrenophora tritici-repentis). Photo credit: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo Bugwood.org
The spot blotch (Bipolaris sorokiniana) pathogen of barley and wheat infects all plant parts and the disease first appears on seedling roots, crown, and emerging sheaths as brown lesions less than 1 cm. Stressful growing conditions such as planting infected seed in dry, warm, nitrogen-deficient soils will promote disease development in seedlings. Preferring warm, wet weather, the fungus spreads by wind or rain to stems and lower leaves of mature plants producing numerous small brown spots that coalesce into blotches. Entire leaves may senesce early with severe disease pressure leading to complete defoliation, plant mortality, malformed kernels, and significant yield reductions. This disease, also called black point, causes blackening of the glumes and embryo tip of the infected seed harming seed quality, germination, and discoloring flour.
Figure 5. Severe spot blotch (Bipolaris sorokiniana) on spring malting barley. Photo credit: Gary Bergstrom, Cornell University
Net blotch of barley occurs in two forms; the Net form net blotch (NFNB) caused by the fungus Pyrenophora teres f. sp. teres and the Spot form net blotch (SFNB) caused by the P.t. f. sp. maculata pathogen. NFNB first appears as pin-point, brown spots that expand in a net-like pattern becoming surrounded by yellow or dead tissue as the disease extends along leaf veins. The pathogen travels into the stem from the leaf veins as the barley plant ages, surviving in the stubble for up to 3 years, and serves as the primary source of inoculum (airborne spores) for subsequent crop infections. SNFB produces dark brown oval lesions surrounded by a necrotic, yellow halo occurring initially on older leaves. The blotch symptoms develop when the spots elongate and merge together leading to yellowed leaves that senesce from the tips downwards with age. Although seeds are not a primary source of inoculum, SFNB disease reduces grain size and quality when greater than 10% of leaf surface is affected.
Stagonospora leaf blotch (SLB) (Parastagonospora nodorum) symptoms begin on the lower leaves with spores splashing to upper leaves, and appear as small rounded to oval-shaped, dark-brown lesions with yellow halos. These lesions fuse together, forming gray blotches with black pycnidia (fruiting bodies). The pathogen then colonizes wheat heads from the tips of glumes causing Stagonospora glume blotch (SGB). Symptoms appear as water-soaked spots that grow to purple-brown lesions. The diseased heads form shriveled grain kernels of inferior quality. Duration of leaf wetness (greater than 8 hours) and rain-splash are significant contributors to disease development.
Figure 6. (L) Stagonospora glume blotch. Figure 7. (R) Stagonospora leaf blotch on wheat. Photo credit: Gary Bergstrom, Cornell University
The Fusarium Head blight (FHB) (Fusarium graminearum/Gibberella zeae) fungus overwinters on infected residues or within or on the surface of seeds. Seed and soil associated inoculum can lead to scab seedling blight but is not a significant source of inoculum for FHB epidemics. The primary inoculum sources for FHB outbreaks originate from either within-field cereal crop debris or from windborne spores transported from debris in area fields. After flowering, diseased spikelets show premature bleaching which may progress throughout the entire head. The fungus colonizes the developing grain causing it to shrink and wrinkle inside the head giving the infected kernels a rough, shriveled appearance, ranging in color from pink to light brown. In warm, humid/wet weather, spores are dispersed via wind and/or rain. Under certain conditions, the fungus can produce a mycotoxin (i.e., deoxynivalenol also known as vomitoxin or DON) that poses a health threat to humans and animals.
Figure 8. Bleached spring wheat heads from Fusarium head blight. Photo credit: Gary Bergstrom, Cornell University.
The Snow mold (Microdochium nivale, M. teres) fungus produces pink-gray cottony growth (mycelium) on dead or living wheat or barley. Plants may recover but retain leaf damage. The pathogens causing snow mold may survive in soil, crop residue and on seed as external contaminants. Microdochium spp. prefer cool-cold, wet weather and may become apparent once snow melts.
Figure 9. Snow mold in wheat. Photo credit: Michael Pace, Utah State University Extension
Managing Diseases in Grains
The best way to avoid foliar diseases in grains is to start with high quality pathogen-free seed, take advantage of resistant cultivars when available, avoid saving seed from diseased plants/fields and rotations of at least 3 years out of the susceptible crop. Scouting fields and getting a positive identification of diseases is important. Every state land grant university has a Plant Diagnostic Clinic that can assist with identification of diseases in grains. Although conventional growers may take advantage of fungicide seed treatments to control some diseases, organic growers do not have this option. Hot water or steam treatment of organic seed is currently being tested to determine the impact on disease and germination.
IMPORTANT: Before using any pest control product in your organic farming system:
- read the label to be sure that the product is labeled for the crop and pest you intend to control, and make sure it is legal to use in the state, county, or other location where it will be applied,
- read and understand the safety precautions and application restrictions, and
- make sure that the brand name product is listed in your Organic System Plan and approved by your USDA-approved certifier. If you are trying to deal with an unanticipated pest problem, get approval from your certifier before using a product that is not listed in your plan—doing otherwise may put your certification at risk.
Note that, although OMRI and WSDA lists are good places to identify potentially useful products, all products that you use must be approved by your certifier. For more information on how to determine whether a pest control product can be used on your farm, see the related article, Can I Use This Input On My Organic Farm?
Funding for this factsheet was provided by the USDA NIFA OREI grant Value-added Grains for Local and Regional Food Systems II, grant number 2020-51300-32379.
Ann Hazelrigg is an Extension Associate Professor and director of the Plant Diagnostic Clinic at University of Vermont Extension.
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