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Maverick Sanchez
Maverick Sanchez

Black Root Rot Fungus

Black root rot impacts a range of woody and herbaceous ornamental plant species primarily in greenhouse ornamental plant production, but also in home and commercial landscapes and nurseries. This disease causes decay of the root system and leads to yellowing, wilting, and necrosis of foliage. It is widely distributed and has been described on approximately 30 plant families in many parts of the world. Some of the most frequently impacted ornamentals are pansy, viola, Calibrachoa, annual vinca (periwinkle), Salvia, petunia, Persian cyclamen, snapdragon, Begonia, Verbena, Phlox, and Gerbera daisy. In addition to ornamental plants, some vegetable and other agricultural crops are also susceptible to infection.

black root rot fungus

The symptoms are not necessarily unique to Berkeleyomyces spp. and can be confused with other root rot pathogens and general nutrient deficiencies. The decaying root system is not able to uptake adequate amounts of water and nutrients to support the above-ground tissues. Plants may appear to be stunted and slow-growing initially. In most cases, the foliage begins yellowing, wilting, or completely dying as the disease progresses (Figure 1). Dark brown to black lesions form on the middle portion of the root and extend outward in both directions to form cankers. The diseased areas are visibly different from the white, healthy portions of the root system and can be used to distinguish this disease from other root rot diseases. Berkeleyomyces spp. produce an abundance of black-colored spores (chlamydospores) that form within and on the root tissues, causing the dark coloration (Figure 2). In advanced cases of the disease the entire root system will appear black and necrotic.

The most effective means of managing this pathogen focus on preventative sanitation and control measures that can be employed before and throughout the growing season. Once black root rot is introduced to an area with conducive environmental conditions, it is difficult to manage, and impractical to eradicate. Implementation of the following techniques can help prevent and reduce losses due to black root rot:

There are several fungicides labeled for use on ornamental plants to prevent and manage black root rot (Table 1). Fungicide treatments are most effective when applied preventatively, before the appearance of symptoms, and when used along with on-going sanitation practices.

Black root rot (BRR) caused by the fungus Thielaviopsis basicola is a very persistent and damaging disease for growers of bedding plants, herbaceous perennials and some woody species including poinsettias. This disease is seen most often on pansy, viola, vinca, calibrachoa, petunia and poinsettias. It causes little to no symptoms on some cultivars and species, so it can easily be passed from greenhouse to greenhouse on healthy-appearing plants. On a highly susceptible cultivar, however, Thielaviopsis causes serious growth reduction and crop losses that are quite dramatic. Some crops, like poinsettias, can go for years without any outbreaks of black root rot.

One way to avoid favoring black root rot is to adjust the pH of your growing media. Thielaviopsis grows best (and is most harmful to plants) at a high growing pH, 6.2 and above. Adjusting the pH to 5.5 can reduce the impact of the fungus. Mixes that drain well are desirable, as the fungus is also favored by saturated soil conditions.

It is also especially important to keep the disease from cycling back into your crops year after year. This can happen easily if flats and pots are not scrupulously cleaned and disinfested after an outbreak. Reusing plastic containers after a bout of black root rot will ensure that the disease comes back to haunt you.

Finally, research has also shown that certain common greenhouse insects can carry spores of Thielaviopsis. This is just one more reason to keep fungus gnats, shore flies and moth flies under control: fungus gnats, in particular, are able to injure a diseased root system further, and to spread the fungus within the greenhouse.

Black root rot is also called Thielaviopsis root rot. Plants are stunted and grow poorly. Infected roots may initially have small dark brown to black bands where infection has taken place. As the disease progresses, roots can become badly rotted. Stems below ground may enlarge and develop black, rough, longitudinal cracks. Characteristic dark brown to black, thick-walled, barrel-shaped chlamydospores form in infected tissues and may be visible under magnification.

The fungus has a wide host range: 120 species in 15 families are known to be susceptible. Strains of the fungus are known that differ in pathogenicity and virulence. Important ornamental hosts include begonia, cyclamen, geranium, gerbera, kalanchoe, pansy, petunia, poinsettia, primula, snapdragon, sweet pea, verbena, and viola. The disease is favored by wet, cool soil and any condition that weakens plants; it is most severe from 55 to 61F, while only a trace of disease develops at 86F. Alkaline soil favors the disease, which can be prevented at pH 4.8 and greatly reduced at pH 5.5 or below. However, many plants do not grow well under such acidic conditions.

Use appropriate sanitation measures to prevent spread of the pathogen via diseased plant material, contaminated soil mixes and containers, and contaminated water runoff. The use of pathogen-free plants, along with improved sanitation and cultural practices, has reduced the importance of this disease, which at one time was widespread, especially in poinsettias. The fungus can still be troublesome in field-grown flowers. The benzimidazole fungicides such as thiophanate-methyl are very active against the fungus and are used as soil treatments to control it.

Scouting crews that note uneven or irregular growth among plants in the same tray or container, or spotty chlorotic plant tissues randomly distributed throughout the greenhouse, have likely uncovered black root rot, which is caused by the fungus Thielaviopsis basicola.

Many growers will often misdiagnose black root rot as a nitrogen or other nutrient deficiency. The key to getting an accurate diagnosis (prior to sending off plant tissue samples for lab confirmation) is to observe whether all plants on a single bench are equally sick looking (likely a nutrient deficiency), or if random groups of plants throughout the bench or tray are affected.

Soil temperatures between 55 and 65F generally favor this fungus, particularly if the soil is wet. The fungus can be found in field soil but has also been found associated with commercial peat moss.

Black Root Rot is caused by the fungus Thielaviopsis basicola. Plants affected by black root rot may show above-ground symptoms that are not diagnostic in of themselves and can be confused with other root rot diseases or general nutrient deficiencies. These may include yellowing of leaves, plant stunting, wilting or even plant death. As the name would suggest, a diagnostic feature of black root rot is the presence of black lesions on the roots, which are visibly very different from healthy white roots. These lesions occur in the middle of the root and expand to form cankers. Root discoloration initially is brown and becomes dark black as abundant black-colored spores (chlamydospores) form on and in the root. Sometimes black lesions can extend into the crown and on the plant stem near the soil line causing stems to be necrotic and soft and leaves to wilt.

A disease that is commonly confused with black root rot is Pythium root rot. However, Pythium initially attacks the root tips, causing a soft brown rot as it progresses down the root. Wash infected roots and study the lesions with a hand lens. If black root rot is suspected, a plant diagnostic lab can confirm the presence of Thielaviopsis basicola if black, barrel-shaped spores are seen on the lesions under a microscope.

Black root rot is a common fungal disease of strawberries in Utah. Several fungi attack the roots of strawberries and eventually cause the death of some or all of the plants in the patch. Often, above ground symptoms are not apparent until after the roots are severely damaged by rot.

Two types of roots compose the root system of a healthy strawberry plant. The first are structural roots. These are hard and woody, and are covered by a bark-like tissue. The structural roots support the plant and the feeder rootlets. As their name suggests, these roots "feed" the plant by absorbing water and nutrients from the soil. A glistening white or cream color and extensive growth indicate healthy feeder rootlets. A healthy root system is necessary to produce a good yield of strawberry fruit.

Plants infected with black root rot are often stunted and appear wilted: the fruit is small and the plants initiate few runners. As the severity of the disease increases, the leaves begin to turn brown and die from the edges inward. Affected structural roots are entirely rotted and dark or have dark brown, sunken areas along their length. The small feeder roots are usually absent or few in number. Affected plants usually die within the year, especially during hot weather.

Black root rot is caused by a complex of soil inhabiting fungi, including Pythium sp. and Rhizoctonia fragariaea, and possibly plant parasitic nematodes. The symptoms may be caused by a single agent or a combination of several pathogens. Adverse environmental conditions add to the problem by stressing strawberry plants and making them more susceptible to infection by the pathogens. Conditions that may contribute to black root rot are waterlogged soils, drought, nutrient deficient soils, winter injury, and chemical or salt damage. Black root rot may also be compounded by Verticillium wilt (Verticillium albo-atrum) which causes general decline and death.

While root rots caused by Pythium and Rhizoctonia are the most common among greenhouse crops, black root rot is a serious threat to pansies, viola, petunias and vinca. Black root rot is caused by the fungus Thielaviopsis and may also infect cyclamen, poinsettia, primula, impatiens, snapdragon, verbena, phlox, begonia and nicotiana. Plants with black root rot often show symptoms that mimic nutrient deficiencies such as stunting with older leaves shriveling. Leaves may turn yellow and the youngest leaves become stunted and tinged with red. In mild infections, older leaves are yellow-green with the veins retaining their green color. Black root rot may also affect the lower stem on crops such as poinsettia, causing cracks that appear black. 041b061a72


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