Sclerotium On Alliums – How To Manage Allium White Rot Symptoms


By: Tonya Barnett, (Author of FRESHCUTKY)

Crops like garlic and onions are a favorite for many home gardeners. These kitchen staples are an excellent choice for overwintering in the vegetable patch and for growth in containers or raised beds. As with any crop, it’s important to pay close attention to the needs and growth requirements of the plants to ensure the best results possible.

This also means routine observation of potential pest and disease issues which may damage plants or diminish yields. One specific issue, allium white rot, should be monitored carefully, as it can result in complete loss of allium plants.

What is Sclerotium on Alliums?

Sclerotium on alliums, or allium white rot, is a fungal issue. What causes white rot specifically? Allium white rot is caused by a fungus called Sclerotium cepivorum. Even in small quantities, these fungal spores can quickly spread to infect large plantings of garlic and onions.

When conditions are ideal, with temperatures around 60 degrees F. (16 C.), the fungus is able to germinate and reproduce in the soil.

Allium white rot symptoms include yellowing of leaves and stunted plants. Upon closer inspection, growers of onions and garlic (and related allium plants) will find that the bulbs have also been affected. Bulbs of infected plants may appear dark in color and covered with a white, matted “fuzz” or black specks.

Treating Sclerotium White Rot

When allium white rot symptoms are first noticed in the garden, it is imperative that you promptly remove and destroy any infected plant matter. This will help to prevent the spread of the infection in the current season’s crop, though it may not prevent it completely.

Allium white rot can remain in the garden soil for up to 20 years after the initial infection. This makes it especially detrimental to home gardeners and those growing in limited spaces.

As with many soil-borne diseases, the best strategy is prevention. If allium plants have never been grown in the garden before, use plantings are disease free from the start. When buying, make certain only to purchase seed or transplants from a reputable source.

Once allium white rot has been established in your garden, controlling it may be difficult. Long-term crop rotation will be essential, as infected areas of the garden should no longer be used to grow onions or garlic. It will also be important to avoid the spread of the spores through the use of contaminated garden tools or even foot traffic on cultivated areas.

Though the use of fungicides has provided some control, these options are seldom realistic for home gardeners. Select studies suggest that the use of solarization in the growing space has also helped to reduce the viability of the fungus present in the garden soil.

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Wood-decay fungus

A wood-decay or xylophagous fungus is any species of fungus that digests moist wood, causing it to rot. Some species of wood-decay fungi attack dead wood, such as brown rot, and some, such as Armillaria (honey fungus), are parasitic and colonize living trees. Excessive moisture above the fibre saturation point in wood is required for fungal colonization and proliferation. [1] Fungi that not only grow on wood but permeate its fibrous structure and actually cause decay, are called lignicolous fungi. In nature, this process causes the breakdown of complex molecules and leads to the return of nutrients to the soil. [2] Various lignicolous fungi consume wood in various ways for example, some attack the carbohydrates in wood and some others decay lignin. The rate of decay of wooden materials in various climates can be estimated by empirical models. [3]

Wood-decay fungi can be classified according to the type of decay that they cause. The best-known types are brown rot, soft rot, and white rot. [4] [5] Each produce different enzymes, can degrade different plant materials, and can colonise different environmental niches. [6] The residual products of decomposition from fungal action have variable pH, solubility and redox potentials. Over time this residue will become incorporated in the soil and sediment, so can have a noticeable effect on the environment of that area. [6]


Treating Sclerotium White Rot: What Causes White Rot On Alliums - garden


The closeup on the right shows the fluffy nature of the fungus. With a more advanced case the roots would have rotted away completely.

This is a serious fungal disease caused by Sclerotium cepivorum, which affects all members of the Allium or Onion family. The fungus grows on the roots of the plant, causing them to rot. The leaves show yellowing and wilting due to the lack of roots. The bulbs remain in a stunted form with most roots and the basal plate rotted away. A white fluffy mould covers any of the outer skin which is underground.
It persists as small, black, dormant structures called sclerotia, which can remain in the soil for up to 15 years and are very easily transferred to other sites, even on the soles of shoes. Infested areas should not be used again for growing onions or garlic for about 15 years.


Sclerotia on the surface of a garlic bulb - about 0.5mm in diameter

Infected plants should be removed and destroyed by burning or sealed in bags for dumping - never place on the compost heap. A portion of the surrounding soil could be removed with the infected bulb to hopefully prevent further infection. Any remaining bulbs should be well spaced to reduce the chance of them being affected. It can be brought in on onion sets and garlic cloves so some growers raise their crops from seed instead. Even shop-bought onions may carry the sclerotia so peelings should not be composted.
There is no chemical control for gardeners. Fortunately the fungus is very specific in what it grows on so other types of crops can be grown in the area, but if space allows it would reduce the possibility of carryover to put the area down to permanent planting instead, eg. fruit bushes, rhubarb or asparagus. Any tools used for cultivation should be disinfected, or perhaps separate ones kept for working the affected soil.

Growing allium crops in containers with brought in compost will enable some to be produced. A couple of garlic cloves raised under cover in the winter, then moved outdoors in the spring will produce better crops than if they were grown in the open and avoid the rot. Leeks for show are usually grown in pots, so can also be raised this way.

Commercial growers sterilize affected areas using methyl bromide fumigation, but eradication is not complete. Some work has been done using a petroleum based stimulant, diallyl disulphide, which has a similar sulphurous odour to alliums that causes the dormant sclerotia of the fungus to grow and since there are no roots present as a food source, it dies away. Other trials have used garlic powder which is cultivated into the ground and results are said to be similar to the petroleum-based compound in exhausting the numbers of sclerotia in the treated area. Composted onion waste and Eucalyptus leaf mulch have been used in laboratory conditions and they reduced the viability of sclerotia. When used in the field these types of treatments produce a increase in the yield of an onion crop.
Garlic powder would probably be the easiest treatment for the gardener to use, but it is difficult to get it down into the soil evenly. It could be used in a four year crop rotation, applying it each year while the non-Allium crops are growing, so when it is time to grow onions again the dormant sclerotia should be greatly reduced.


There is a home remedy which appears to work made from lime and soot. The soot is allowed to weather for about six months to leach out dangerous chemicals. A tablespoonful of each is applied per onion set or garlic clove at planting time. The combination of the two probably releases a similar sulphurous ordour to fool the sclerotia.


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You can prevent the spread of these infections by planting the cast iron plant in the appropriate conditions and with proper care. When watering the plant, do not allow leaves to become wet. Water at the base of the plant. The potting medium should drain well and should be kept moist but not waterlogged. Remove infected leaves when problems are noted.

Leslie Rose has been a freelance writer publishing with Demand Studios since 2008. In addition to her work as a writer, she is an accomplished painter and experienced art teacher. She has a Bachelor of Arts degree in art with a minor in English.


Management

Currently, there is no way to eradicate white rot from a field where it has become established. Some varieties of allium crops may be less susceptible to white rot than others but no truly resistant varieties exist. Therefore, the best way to manage this disease is to avoid introducing it into your fields.

  • Plant disease-free sets and cloves obtained from reliable sources.
  • Contaminated soil may be carried on cultivating equipment, tools, or shoes. Avoid moving soil from infested fields to uninfested areas. Keep accurate records of disease occurrence.
  • Do not plant Alliums in infected fields. Other crops may be planted, but keep in mind the risks of moving soil from field to field.
  • Remove affected plants, inasmuch as this is practical, and destroy off-farm. Do not compost infected plant material.


Controlling White Rot

White rot grows most rapidly in wet, boggy soil. The process can be slowed or stopped by removing soil from around the base of the tree and allowing the base to dry. Find a way to drain the soil. You may have to re-route small channels that funnel rainwater away from the base of your oak trees. Whether attempting treatment or prevention, reducing moisture in the soil around the base of the tree is job one. Fungus can also enter the tree through wounds, so any bark damage you find near the base should be sealed and treated. By the time basidiocarps appear, it may be too late. Sometimes, all you can do is remove the tree to prevent it from infecting other nearby trees with the disease.


Watch the video: This is What Happens When You Put Earthworm in Your garden soil


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