|Common beet showing signs of southern blight (Athelia rolfsii)|
Southern blight (Athelia rolfsii) is a corticioid fungus in the family Atheliaceae. It is a facultative plant pathogen.
S. rolfsii primarily attacks host stems, although it may infect any part of a plant under favorable environmental conditions including roots, fruits, petioles, leaves, and flowers. The first signs of infection, though usually undetectable, are dark-brown lesions on the stem at or just beneath the soil level; the first visible symptoms are progressive yellowing and wilting of the leaves. Following this, the fungus produces abundant white, fluffy mycelium on infected tissues and the soil. Sclerotia of relative uniform size are produced on the mycelium: roundish and white when immature then becoming dark brown to black. Mature sclerotia resemble mustard seed. The fungus occasionally produces basidiospores (the sexual stage of reproduction) at the margins of lesions and under humid conditions, though this form is not common.
Seedlings are very susceptible and die quickly once they become infected. Older plants that have formed woody tissue are gradually girdled by lesions and eventually die. Invaded tissues are pale brown and soft, but not watery.
Initially light brown spots apear on carpels. Spots enlarge rapidly over whole carpels, peduncles and leaf petioles become brown and cause all tissues to dry up. Eventually, white feather-like mycelium is produced on surface of bolls. Later, numerous round to irregular-shaped white to light brown sclerotia are produced in the mycelium. When a diseased boll splits, white cottony mycelial growth and sclerotia are seen in locules, lint and seeds. Inner tissues of bolls dry up and become brown and lint becomes discoloured and eventually disintegrates. Frequently seeds within a diseased boll decay.
Control of Sclerotium diseases is difficult and depends on a combination of cultural, biological and/or chemical methods. Good cultural practices include roguing, eliminating weed hosts, and avoiding crop injury during cultivation. A dense canopy increases disease incidence, thus increasing plant spacings can help keep infection down. A delayed planting date may also help reduce disease incidence if planting is timed so that the dense canopy forms after temperatures fall so that infection is not as likely. Also, keeping plant bases free of dead leaves (and weeds) will deny the pathogen a food source, helping to keep disease incidence down.
Because S. rolfsii has such a broad host range, crop rotation has less of a chance of being successful as there are few resistant crops. There are some grasses and grains that are not as susceptible to the fungus that help in reducing soil inoculum levels. Onion is susceptible to S. rolfsii, however, some cultivars have been shown to reduce the viability of sclerotia when cultivars are planted in winter when temperatures are too low for disease development. A significant increase in yield and reduction in disease incidence was reported for summer peanut crops when appropriate onion cultivars were planted the previous winter. It has been postulated that onion exudates cause the pathogen to become susceptible to antagonistic microflora in the soil.
Deep plowing to at least 20cm (8in) with a moldboard extention inverts soil so that organic matter, sclerotia, and plant debris are buried at least 10cm (4in) beneath the surface. This helps to eliminate inoculum when plowing occurs just prior to planting. Buried soil must not be re-surfaced during the growing season.
Compost, oat, or straw added to the soil has been shown to limit disease incidence. The addition of an amendment may increase populations of antagonistic soil microorganisms (see biocontrol section). This method may be reasonable for small-scale farms and greenhouses, but is probably not practical for large farms unless it is combined with crop rotation.
Soil solarization or solar heating is a relatively recent method for controlling S. rolfsii inoculum. Sclerotia grown in vitro are still viable after 12 hours at 45°C (113°F), but are killed in 4-6 hours at 50°C (122°F) and in 3 hours at 55°C (113°F). Covering soil with transparent polyethylene sheets during the hot season increases soil temperatures and kills sclerotia when the temperature under the sheets get hot enough for an appropriate length of time. Most field trials have achieved sclerotia degradation at 1cm (⅓in), but eradication at greater depths usually did not occur. In addition, this method requires immediate planting, which excludes crops that are planted in spring because temperatures are not high enough to affect sclerotia. Soil solarization combined with the addition of Trichoderma harzianum (a mycoparasite, see biocontrol section) has been shown to decrease disease incidence more than either treatment alone. However, the practicality of soil solarization is questionable. First, the length of time of solarization may be limited; a trial in Arizona reported that the tarps disentigrated after 6 weeks. Second, it is not known what affect solarization has on the existing soil microflora and what affect any microflora change would have on the crop. Third, it is not known what affect solarization has on non-target and/or temperature-tolerant pathogens.
Black plastic mulch
Mulching with black plastic has been shown to reduce disease incidence and perhaps provide greater crop yields. Black plastic mulch (BPM) prevents or reduces the "bridge" of dead tissue between the soil and plant and may increase temperatures, conserve soil moisture, and help control weeds for a higher crop yield. BPM alone and BPM with floating row covers both provide better control than no treatment. Disease incidence can still be high, but significantly lower than no treatment. BPM alone and BPM with floating row covers combined with a chemical treatment (PCNB) provides even better control.
A number of antangonistic fungi have been shown to provide control against S. rolfsii in controlled experiments, though field trial results vary. Some of the commonly used organisms are: Trichoderma harzianum, T. viride, Bacillus subtilis, Penicillium spp., and Gliocladium virens.
Gliocladium virens have been shown to rapidly degrade S. rolfsii strain SR-1 in soil. G. virens will colonize S. rolfsii strain SR-3 but sclerotia can germinate under good conditions. The different reactions between S. rolfsii strains may be due to the size of the sclerotia (SR-3 sclerotia are up to 15-20 times larger than SR-1 sclerotia) and the amount of melanin in the sclerotial rind (SR-3 sclerotia has more melanin). In vitro studies show that varying concentrations of G. virens provide a corresponding variation in the germination of S. rolfsii (SR-1) sclerotia, but all concentrations result in a low percentage of S. rolfsii infection. In most cases there were always sclerotia that germinated but the pathogen did not infect plant tissues. G. virens had little effect on the germinability and infectivity of S. rolfsii (SR-3). Data such as these suggest considerable specificity in biocontrol due to differences in susceptibility of strains of the same pathogen to a single biocontrol strain, in addition to specificity due to various strains of a biocontrol agent.
Control measures include chemical disinfection of vegetative propagation material, adjustment of soil pH by liming, adjustment of fertilizer regime, and use of herbicides for weed control. Formalin, chlorobromopropene and methyl bromide are among the most promising fumigants for treatment of seed beds or fields for valuable crops.
Pre-plant chemicals and application techniques: fumigants such as metamsodium (Vapam), Vorlex, methyl bromide, and chloropicrin, when applied to soil, reduce southern blight incidence.
- Ferreira, S.A and Boley, R.A. (1992). Sclerotium rolfsii. University of Hawaii at Manoa, Department of Plant Pathology, CTAHR. Retrieved: 2011-02-03.
- Nyvall, R.F. (1999). Field Crop Diseases, "Diseases of Cotton - Sclerotium boll rot". Wiley-Blackwell; 3rd Edition edition. p.209. ISBN 0813820790