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What is smut disease in plants?

Although they are not seen as commonly as their close relatives the rusts, smut fungi can affect a range of garden plants, including some widely-grown vegetables and ornamentals. Some smuts induce obvious growth deformities, while others result in leaf spots. Any of these substances may pollute the soil for lengthy periods of time.

Contents

Quick facts

Common name Smut
Scientific name Several, mainly Entyloma, Urocystis, or Ustilago species
Plants affected Many, including ornamental species like winter aconite and crops like sweetcorn.
Main symptoms Spots, blisters, deformed development, and the formation of black spores differ depending on the smut species.
Caused by Fungus
Timing Spring/summer

What are smuts?

Smuts are fungal diseases that can affect leaves, stems, flowers and sometimes storage organs such as bulbs and corms. Some generate massive amounts of black spores that, when shed off the plant, resemble soot (or smut, hence the common name).

Smuts are highly specialized diseases with limited host ranges. For example, the smut that affects sweetcorn is caused by a different species of fungus from the one attacking carnations.

Several grasses are afflicted by smut diseases, albeit they are seldom an issue on garden plants and cultivars. However, on members of the grass family grown as valuable cereal crops, such as wheat and barley, smuts (or bunts, as some of them are called) can be a significant problem.

Symptoms

You may see the following symptoms:

On Allium species (including onions, leeks, chives & shallots), Anemone, Gladiolus, globe flower, onion, Ranunculus, Viola, winter aconite

  • Smuts on these hosts often harm the leaves and/or stems.
  • On the damaged plant portions, dark streaks or blisters form.
  • The blisters finally burst, releasing a powdery substance containing millions of minute dark brown or black spores.
  • Growth may be stunted, and afflicted Allium species’ leaves are often bloated and twisted.
  • Corms of Gladiolus can also be affected

On carnation and Scilla:

  • Anther smuts assault these hosts, affecting the male reproductive components of the flower.
  • The anthers are loaded with black smut spores rather than pollen grains.
  • When the blooms open, the spores are discharged, detracting from their attractiveness.

On sweetcorn:

  • Affected plants are often stunted and distorted
  • Several kernels on the cob (number and location vary) become very enlarged and grayish in color.
  • These kernels explode, releasing a large quantity of black spores.
  • Swellings and blisters may also develop on the leaves, stems and inflorescences. Some of them will also burst, releasing spores.

On Calendula, Cosmos, Dahlia, Eschscholzia, forget-me-not and Gaillardia:

  • Smuts on these hosts form leaf spots but do not produce the black spore masses that other smuts do.
  • Older leaves are usually affected first
  • Leaf spots are typically round, but are often bordered by leaf veins to produce a more angular look (e.g., on Dahlia).
  • The spots are often pale green or yellow when new, but gradually darken to light or dark brown. They might have a yellow rim. They stay tiny and off-white in color on forget-me-not.
  • Calendula, Cosmos, Dahlia, Eschscholzia, and Gaillardia leaves that have been severely impacted may turn brown and shrivel.

Control

Non-chemical control

  • Most afflicted plants should be eradicated, and the soil should be cleared of that specific plant kind for many years.
  • Dahlia tubers are not colonised by the smut fungus, and can be stored and re-used if all of the foliage is removed. The next year, they should be planted in a new location.
  • Lightly-infected gladiolus corms can be saved by hot water treatment, although this is probably worthwhile only for particularly valuable stocks. Soak the corms for 45 minutes at 47°C (117°F).
  • Just the first leaf generated by the onion smut infects the plant. Plants raised in a disease-free seedbed can therefore be transplanted into contaminated soil without risk of infection
  • If an infection arises in a greenhouse (for example, on carnations), sterilize the area before restocking with the same host.
  • Several hosts have resistant cultivars available (e.g. sweetcorn). Yet, smuts frequently exist as a variety of genetically separate ‘races’. Since cultivars may not be resistant to all races, they cannot be depended on to provide comprehensive control.

Chemical control

Fungicides are not available to combat smut illnesses.

Biology

The smuts that produce masses of black spores on the hosts indicated are Urocystis and Ustilago species. Entyloma species produce leaf patches without apparent spore masses (often referred to as white smuts). Entyloma smuts produce resilient resting spores that are embedded within the affected leaf tissue and released into the soil as the fallen leaves rot down.

Spores from all smut species may infect soil for extended periods of time (15-20 years in the case of onion smut). They finally germinate, resulting in the production of a second spore type that infects the plant. In some cases (e.g. Entyloma smuts) spores can also germinate in situ on the plant, and the disease may then spread rapidly during the growing season.

Some smuts are able to colonise the entire plant internally in what is known as a systemic infection (even though symptoms and spore production may be confined to isolated parts of the plant). Others stay concentrated in the sections of the plant that create symptoms.

Entyloma smut of Eschscholzia californica (California poppy) was documented for the first time in the UK by the RHS in 2012.

 

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