Loose smut
Encyclopedia
Loose smut of barley is caused by Ustilago nuda. It is a disease that can destroy a large proportion of a barley crop. Loose smut replaces grain heads with smut, or masses of spores which infect the open flowers of healthy plants and grow into the seed, without showing any symptoms. Seeds appear healthy and only when they reach maturity the following season is it clear that they were infected. Systemic fungicides are the major control method for loose smut.
Ustilago nuda infects barley (Hordeum vulgare L.) but there are many strains of Ustilago which infect many different cereal crops in a similar manner. Ustilago tritici, for example, is loose smut of wheat. Unlike some Basidiomycete fungal pathogens, Ustilago spp. infect only one host plant, and do not need to move to an alternate host to complete their disease cycle.
After landing in an open floret, the teliospores give rise to basidiospores. Without dispersing to any alternate host plant, the basidiospores germinate right where they are. The hyphae of two compatible basidiospores then fuse to establish a dikarytic state.
After germination inside the ovary, the fungal mycelia invade the developing embryo in the seed. The fungus stays alive in the seed until the next growing season, when it is planted along with the seed. As the developing plant grows, the fungus grows with it. Once it's time for the flowers to form, teliospores are produced in place of the flowers and develop where the grain would be.
Plants which are infected with Ustilago spp. actually grow taller and flower earlier than their healthy counterparts. This gives the infected plants an advantage in that the flowers of uninfected plants are more physically and morphologically susceptible to infection. The teliospores in the smutted grain heads disperse to the open flowers of the healthy plants, and the cycle continues.
Environment becomes a factor once teliospores have been produced and are ready for dispersal. In order to get to the open flowers of healthy plants, the teliospores must be moved by wind or rain or possibly insects. If dry, calm conditions persist for the entire time the flowers are open, the infection rate will be low. Wind and moderate rain, as well as cool temperatures (16-22 degrees Celsius) are ideal for the dispersal of spores. Conditions in Eastern Europe, Western Siberia, and the Northern Caucasus regions are some of the most conducive to loose smut.
Recent studies have also investigated the use of foliar systemic fungicides, particularly carboxins, for treatment once infected seeds have already been planted. Although it has been relatively successful, this method is more expensive and more labor intensive than simply treating the seed before planting. Buying treated seed from a certified seed company will significantly reduce the chance of seeing loose smut on a crop.
Another option is heat treating the seed to kill the fungus before planting, but this is a delicate process because too much heat will kill the plant embryo and not enough will allow the fungus to survive. For this reason, systemic fungicides are the preferred treatment method.
In places like the United States where there are ample resources and technologies such as fungicide seed treatment, loose smut is not an especially important disease.
Hosts and Symptoms
The major symptom of loose smut is the "smutted" grain heads, which contain masses of black or brown spores where the grain would normally be. The spores completely replace the grain head so that there is no grain to be harvested on infected plants. It maybe be possible to identify infected plants in the field before they reach the flowering stage by looking for plants which are taller and more mature than the rest of the field. By causing its host plant to grow slightly taller and mature slightly sooner than the surrounding uninfected plants, the fungus is given a competitive advantage by ensuring that healthy florets will be open and easier to land on.Ustilago nuda infects barley (Hordeum vulgare L.) but there are many strains of Ustilago which infect many different cereal crops in a similar manner. Ustilago tritici, for example, is loose smut of wheat. Unlike some Basidiomycete fungal pathogens, Ustilago spp. infect only one host plant, and do not need to move to an alternate host to complete their disease cycle.
Disease Cycle
The disease cycle of loose smut begins when teliospores are blown to open flowers and infect the ovary either through the stigma or directly through the ovary wall. There are multiple mating types for Ustilago spp. so infection will only occur if two compatible mating types are present in the same flower.After landing in an open floret, the teliospores give rise to basidiospores. Without dispersing to any alternate host plant, the basidiospores germinate right where they are. The hyphae of two compatible basidiospores then fuse to establish a dikarytic state.
After germination inside the ovary, the fungal mycelia invade the developing embryo in the seed. The fungus stays alive in the seed until the next growing season, when it is planted along with the seed. As the developing plant grows, the fungus grows with it. Once it's time for the flowers to form, teliospores are produced in place of the flowers and develop where the grain would be.
Plants which are infected with Ustilago spp. actually grow taller and flower earlier than their healthy counterparts. This gives the infected plants an advantage in that the flowers of uninfected plants are more physically and morphologically susceptible to infection. The teliospores in the smutted grain heads disperse to the open flowers of the healthy plants, and the cycle continues.
Environment
Loose smut is unique in that it needs to infect the seed in the previous growing season in order to be a pathogen of the plant in the current growing season. This means that regardless of environmental conditions, if an infected seed is planted as long as the growing conditions are good for the plant, they will be good for the fungus. However if there are environmental stressors on the plant such as drought or extreme heat, the plant will be more susceptible to injury because of the added stress from the fungal infection. If the plant doesn't survive to its reproductive stage, the fungus will not get to reproduce either.Environment becomes a factor once teliospores have been produced and are ready for dispersal. In order to get to the open flowers of healthy plants, the teliospores must be moved by wind or rain or possibly insects. If dry, calm conditions persist for the entire time the flowers are open, the infection rate will be low. Wind and moderate rain, as well as cool temperatures (16-22 degrees Celsius) are ideal for the dispersal of spores. Conditions in Eastern Europe, Western Siberia, and the Northern Caucasus regions are some of the most conducive to loose smut.
Management
The most widely used method of control for loose smut is growing only clean seed. Since the fungus survives in seeds, growing only uninfected seeds will guarantee a smut-free crop unless spores enter from a nearby source. Seeds are typically treated with a systemic fungicide to kill any fungus that may be inside. Since the fungus grows up with the plant, it's very important that the fungicide used to treat the seed be systemic and not just external.Recent studies have also investigated the use of foliar systemic fungicides, particularly carboxins, for treatment once infected seeds have already been planted. Although it has been relatively successful, this method is more expensive and more labor intensive than simply treating the seed before planting. Buying treated seed from a certified seed company will significantly reduce the chance of seeing loose smut on a crop.
Another option is heat treating the seed to kill the fungus before planting, but this is a delicate process because too much heat will kill the plant embryo and not enough will allow the fungus to survive. For this reason, systemic fungicides are the preferred treatment method.
Importance
If uncontrolled, loose smut can wipe out entire crops, since it replaces the grain. In areas where people depend on their grain crops for survival and don't have the money or technology resources to control it, the disease can be devastating. Not only does it wipe out the crop, growers cannot even try again next year since any seeds they were able to harvest will be infected and will not produce seed the following season.In places like the United States where there are ample resources and technologies such as fungicide seed treatment, loose smut is not an especially important disease.
Varieties
Varieties of Ustilago nuda are:- Ustilago nuda var. foliicola TrotterTrotterTrotter may refer to:* Del Boy, Rodney and Grandad - Fictional Trotter Family from BBC sictom Only Fools and Horses* Clan Trotter, a lowland Scottish family* Trotter, a horse trained for harness racing* Italian Trotter, a breed of horse...
- Ustilago nuda var. hordei C.N. Jensen,
- Ustilago nuda var. nuda (C.N. Jensen) Rostr.
- Ustilago nuda var. tritici G.W. Fisch. & C.G. Shaw