True loose smut of barley is caused by the fungus Ustilago tritici. The disease cycle is exactly the same as that of loose smut of wheat. Infection occurs when the flowers develop. The seeds become infected internally and must be treated with a systemic fungicide to eliminate the pathogen.
Semiloose smut is caused by the fungus, Ustilago avenae. The disease typically causes small losses due to the use of resistant cultivars and fungicide seed treatment. It is most common when soil temperatures are cool and the soil is dry. Distinguishing semi-loose smut from true loose smut in the field is extremely difficult. Diseased heads typically appear earlier than healthy heads. Millions of fungal spores may be formed in diseased heads.
Ustilago avenae survives as teliospores on the seed surface. At seed germination, the fungus becomes active and infects young seedlings prior to their emergence from soil. As the plants develop, the fungus grows within the growing point of the young plant. At the boot stage, the fungus transforms the kernels and glumes into dark spore masses very similar to those produced by the true loose smut fungus. Spores are readily dispersed by wind or the action of harvesting equipment. Healthy seeds are contaminated upon contact with the spores.
Covered smut caused by the fungus, Ustilago hordei, can result in significant yield losses if left untreated. It is a relatively common disease throughout the barley-growing regions of the world. Problems with the disease occur over a wide range of temperatures and are most common under moist soil conditions.
Ustilago hordei survives as thick-walled resting spores on barley seed. Typically, spore germination coincides with seed germination, it is at this time that infection occurs. Once established, the fungus invades the actively growing tissue and keeps pace with growth of the plant. At flowering, the fungus grows through the floral tissue and forms masses of spores in place of healthy seed. Each kernel becomes a spore filled "smut ball" that is enclosed by a thin membrane until plant maturity. At harvest, spores are liberated and may contaminate healthy seed.
Barley stripe, caused by the seed-borne fungus, Drechslera graminea, is most common during periods of rain or high humidity. Few seeds are produced on infected plants, and losses generally coincide with disease levels in a particular field. Properly treated seed is the primary means of controlling this disease. Symptoms are first noticeable on the first two or three leaves produced by the plant and on many leaves produced later. A yellow stripe on the leaf sheath and basal portion of the leaf blade is common. Later, the stripe may extend the entire length of the leaf and the leaf may die. At heading, diseased plants are often stunted and appear a light tan color in contrast to the larger, green healthy plants. Spikes may be deformed or may fail to emerge. At heading, spores are produced on diseased leaves and spread by wind to nearby heads. Grain produced by diseased plants is typically brown and shriveled.
Drechslera graminea survives entirely as mycelium in the outer layers of the seed. When the seed germinates in moderately moist soil at temperatures below 55 degrees F, the fungus mycelium penetrates and infects the seedling. The fungus grows within plant tissue and occupies the actively growing plant parts.
Net blotch and spot blotch, common diseases of barley, are caused by the fungi, Drechslera teres and Bipolaris sorokineana, respectively. Net blotch is most common during rainy and humid weather, and early flag leaf infections can result in both reduced grain yield and weight. A net-like pattern on sheath leaves or flag leaves is the most recognizable symptom of the disease. Initially, small spots or streaks appear on the leaves, followed by an expansion into conspicuous longitudinal and transverse streaks, eventually combining into a network. Brown areas surrounded by greenish-yellow margins are commonly observed on infected leaves. Eventually, entire leaves may wither and die.
Drechslera teres survives in infected seed or on crop residues. Infection of seedlings is most common at 34 degrees F to 60 degrees F. Once infection is established, the fungus produces spores on leaf tissue during periods of high relative humidity. Spores are released and carried by wind to nearby barley plants where new infections can result. Infection of leaves can occur when leaves are wet for periods of 5 to 15 hours within a 46 degrees F to 91 degrees F temperature range.
Spot blotch is most commonly a problem during warm, humid weather. Yield losses of up to 36 percent have been attributed to this disease when the flag leaf has become severely diseased early in the season and has died prematurely. Seedling blight (B. sorokineana) in winter barley is favored by planting early in warm soil. Delayed planting of winter barley in cooler soil helps prevent blight.
Small brown spots surrounded by a yellow halo is the characteristic symptom of the disease. Both leaves and leaf sheaths on plants of all ages can become infected. Eventually, the spots coalesce and cover large areas of the leaves. Minute fungal fruiting structures containing spores can sometimes be observed in the larger spots. Severely diseased leaves may eventually die.
Bipolaris sorokineana survives in seed or infested crop residue. Leaf infection results from airborne spores produced on the seed or on residue. Epidemics of spot blotch can occur when there are prolonged wetness periods (greater than 16 hours) and the temperature is above 70 degrees F.
Note that net blotch is more common in cooler, humid weather, as opposed to spot blotch which prefers warm, humid weather. Most common broad-spectrum seed treatment fungicides are effective in eliminating seed-borne B. sorokineana and D. teres as well as preventing losses from seedling blight caused by them.