Barley is the fourth largest food crop in the world and disease has a major impact on its contribution to human food supplies. The major diseases affecting barley are fungal, particularly mildew, head blight, smut, Rhyncosporium, and Ramularia. Barley Yellow Dwarf Virus and Barley Stipe Rust are the most prevalent virus infections. In addition, a number of insect pests attack barley, particularly aphids, but also worms and beetles.

Many barley diseases are common to wheat, and this makes combating them critical to the security of food resources. Barley is more adaptable than wheat and other cereals, being able to grow at higher altitudes and in drier and more saline environments. However, this wider ecological range means that a broader range of diseases may develop.

Barley cultivation has always been a constant battle between managing growth and fighting disease. The development of high barley yields and of specific malting varieties has continued for decades and has been a perpetual leapfrogging of increased resistance to disease being overcome by the evolution of new virulence factors increasing disease prevalence.

The incidence of most plant diseases varies according to climate, geography, and agricultural practice and differs considerably according to the resistance of varieties to specific diseases. Barley is no exception, and high levels of disease are typically associated with moist conditions and often with cool temperatures which typically favor fungal growth. Viruses are often spread by aphids, whose populations can rise rapidly when conditions are favorable, resulting in a subsequent surge in crop disease. See aphids.

Barley disease also varies with the age of the plant with some diseases affecting young shoots, others prevalent on leaf and shoot, and others only evident on maturing seed grains.

The different tissue locations of barley diseases are listed below:

Barley disease symptoms on leaves include a yellowing often with the development of darker necrotic spots or patches, although mildew grows externally as white powdery masses of fungus. Infection of stems causes blackening and weakening leading to collapse, while infection of roots results in local rotting, poor plant support, and stunted growth. On seed heads infection can discolor and shrivel grains as the fungi digest the internal structure. Ergot is distinctive as the barley ears show the fungus protruding between the corns.

Some diseases are systemic but require environmental triggers to cause plant damage, while others reside in the seed and develop in the next generation of plants. Ramularia, for example, develops progressively as the plant grows, but symptoms may not appear until the level of light is sufficient to trigger synthesis of phytotoxins.

While most diseases reduce plant growth and the yield of corns, some also provide longer lasting impact and affect grain quality. Fusarium infection is particularly relevant for this as many species produce poisonous mycotoxins during growth. These toxins are part of the fungus attack on the plant but are also harmful to animal health, producing neurological effects at high concentrations and carcinogenic effects at low levels. Mycotoxins may survive malting and pass into beer, and brewers require malt to be checked for purity before use.

Other effects of barley infected with fungi are the production of hydrophobins. These act to reduce the surface tension to assist in spore release by the fungus but can induce early flocculation of yeast during fermentation. Similar factors may be involved in inducing gushing in beer by enhancing gas release on dispense.

Management of barley diseases depends on pesticides, particularly fungicides, but also on good agricultural practices. Many diseases may survive over winter on crop residues left in fields and their removal is therefore important. Pesticides are often used in preventing and limiting barley disease but require timely application and are often overcome by resistant mutations developing in the disease organism.

Breeding barley varieties for resistance to specific diseases may only provide short-term relief and more integrated approaches are now being adapted, including a mixture of partial resistance, crop rotation, and hygienic management. In addition, early warning of disease outbreaks is required to allow local action and to anticipate variety choice and practices for future years. In many cases the gross appearance of barley disease may be too late to treat. The use of molecular testing to identify the disease in the barley tissues before disease appears is desirable alongside modeling to predict the course of disease and direct treatment. Future approaches to barley disease management in the light of global climate change and increased sensitivity to the environment will require more understanding of specific diseases and inventive action.