Loose Smut

Family: Ustilaginaceae

About Loose Smut

Life Cycle

Ustilago nuda fungi cause loose smut for barley, whereas Ustilago tritici fungi cause loose smut for wheat. Depending on the severity of infection, this disease has the ability to cause significant damage to yield and is seedborne. This disease is also sometimes known as true loose smut, and is not the same fungus that causes false loose smut or covered smut. Plants are vulnerable to infection when they flower. At this point, wind can blow loose smut spores from other infected plants. Insects or rain water can also transmit the disease. Plants are most susceptible at the time of flowering because the spores can enter the wheat or barley and then infection will occur within 48 hours after flowering.

If loose smut fails to perpetuate the plant at this phase, infection is much less likely to occur because plants grow resistant to these fungi the week after flowering. The spores germinate once it has found an open flower. The mycelium will grow and extend into a developing seed’s embryo through the ovary. After it establishes itself here it will become dormant, which is why this disease is hard to spot. The kernels that have been infected will not look any different from seeds that are uninfected. This is where the fungus overwinters. When this seed is planted and germinates the following season, the fungus will be reactivated and continues to develop as the plant matures; seeds will not be capable of development if the head is infected. Instead, black teliospores replace the seeds. These powdery spores will stay as one mass until emergence occurs. At head emergence the mass of spores is released into the wind and the cycle continues. All infected areas are left barren once the spores have blown away.

Loose Smut Identification and Habitat

Identification

At head emergence, infection is easy to spot due to the release of powdery spores. Additionally, infected heads commonly mature at an earlier state than uninfected heads. The mass of spores is often olive-black and are encased in a gray membrane before rupturing. Due to the internal development of smut, it can be challenging to know whether or not a field is infected with this disease before head emergence. However, there are a few other symptoms that can be spotted. Plants infected with lose smut may develop erect and darker green than normal leaves. They occasionally have chlorotic streaks as well. When infection occurs, it is common for the fungus to reach all of the plants floral parts, meaning this disease can end up causing major economic damage if this occurs on large areas of wheat or barley fields.

Varieties

• • Ustilago nuda
• • Ustilago tritici

Habitat

This fungus has been known to infect wheat and barley plants worldwide. However, typically loose smut thrives in environments where the weather is most often cool and humid. The preferred temperature range is between 60 and 71 degrees Fahrenheit, especially during times with heavy dew or light rainfall.

Loose Smut Management and Control Methods

Cultural Control

To help in the management of loose smut, use resistant varieties whenever possible. Resistant varieties are often ones that have almost closed florets at the time of flowering. Additionally, use certified seeds and ensure fields meant for seed production are separated from fields for commercial use. Inspect fields regularly to spot first signs of loose smut during the early stages of heading. If fields are cleared of plant debris following harvest, spread can be minimized the following season. However, having satisfactory control is often dependent on using an integrated management system that includes the use of both cultural and chemical methods.

Chemical Control

Due to the fact that loose smut is a seedborne disease, seed treatment is often the most effective and least expensive way to manage this disease. A fungicide that has systemic action and acts as a seed disinfectant can kill the fungus hibernating in the seed embryo. A broad spectrum fungicide on foliage is also recommended to be applied along with the seed treatment. Seed treatment fungicides that have been effective with small grains include carboxin, benomyl, triadimefon, terbutrazole, carbathiin, ethyltrianol, or furmecyclox. A broad spectrum fungicide could be conazole. Always be sure to carefully read the label for cautions and proper application before use.

Alternative Loose Smut Names

• • True Loose Smut

Sources