NC State Univeristy  
College of Life Sciences
Plant Pathology Departmenthttp://www.cals.ncsu.edu/plantpath/

Sclerotia
Courtesy F. J. Crowe. Reproduced from Compendium of Onion and Garlic Diseases and Pests, 2nd ed., 2008, American Phytopathological Society, St. Paul, MN.

Sclerotium cepivorum

By Kathryn Cherry
PP728 Soil-borne Pathogens
S. cepivorum on onion bulb
Courtesy F. J. Crowe. Reproduced from Compendium of Onion and Garlic Diseases and Pests, 2nd ed., 2008, American Phytopathological Society, St. Paul, MN.

Introduction

Sclerotium cepivorum is the causal agent of the disease commonly known as Allium root rot. It is a problem found all over the world on Allium spp. that can be very devastating since it can results in large crop losses. Once a field has S. cepivorum it is difficult and costly to continue growing Allium spp. there, if possible at all. The fungus is favored by cool weather and survives in the soil as small, round structures known as sclerotia.  These sclerotia can survive in the soil for decades (1). 


Host Range and Distribution

This soil-borne plant pathogen causes disease on plants in the Allium genus, which includes onion, garlic, and leeks. The disease can occur in any location where Allium spp. are grown, as long as part of the crop growth occurs during a cool season. 


Symptoms

The first symptoms that will likely draw attention are the foliar symptoms.  Plants may be stunted or there will be yellowing and wilting of the leaves.  Eventually the leaves will die, beginning with the oldest leaves. Foliar symptoms
Courtesy E. A. Kurtz. Reproduced from Compendium of Onion and Garlic Diseases and Pests, 2nd ed., 2008, American Phytopathological Society, St. Paul, MN.
During cool weather there may be white, fluffy mycelial growth at the base of the stem plate when the leaves are yellowing. Mycelial growth is favored by cool and moist soil, and in the right conditions the mycelia can cover the whole bulb. On these mats of mycelium black sclerotia will form that will be about the size of a poppy seed.  Hyphae on bulb
Courtesy Paul Koepsell. Oregon State University Extension. White Rot.
If there are low amounts of S. cepivorum in the soil then small clusters of plants will appear that have died suddenly. If there are high amounts of the pathogen present in the soil then large areas of dead plants will occur. Field losses
Courtesy F. J. Crowe. Reproduced from Compendium of Onion and Garlic Diseases and Pests, 2nd ed., 2008, American Phytopathological Society, St. Paul, MN.

Isolation

To isolate the sclerotia from a dry soil sample of known volume, start by rinsing the soil on an 80-mesh sieve with running tap water (4). Rinse the remaining residue into a 250-ml beaker. Swirl the beaker to suspend the sclerotia in the water and then pour the suspended particles and organic matter back onto the 80-mesh sieve. Repeat the process of suspending the sclerotia in water and pouring the suspension onto the sieve until there are only sand particles left in the beaker. To grow the fungus from a sclerotium, simply place a sclerotium on a plate of 1% water agar. To isolate the fungus from diseased tissue simply place a small piece of the disased tissue onto PDA media and allow to grow in a cool environment.
Sclerotia germinating in culture
Courtesy Mike Davis, Plant Pathology Dept, University of California at Davis.

Identification

Identifying the fungus is possible by considering the combination of symptoms and signs observed in the field. During a cool season, or right after one, if there is white mycelium at the base of an Allium plant in the field which is white and fluffy then that’s one clue the fungus is S. cepivorum. This mycelial growth can usually be found when foliar symptoms first appear. This fungus can form black, near-spherical sclerotia that are 200-500 µm in diameter. It can also form large sclerotial bodies of irregular shape with lengths varying between 0.5 and 1.5 cm (6). The sclerotia can be found on the mycelium. In addition, PCR can be used to detect the presence of S. cepivorum. Thanks to work done in the United Kingdom there are specific primers available (3).

Lots of sclerotia
Courtesy F. J. Crowe. Reproduced from Compendium of Onion and Garlic Diseases and Pests, 2nd ed., 2008, American Phytopathological Society, St. Paul, MN.


Ecology and Life Cycle

Growth in the soil
Courtesy F. J. Crowe. Reproduced from Compendium of Onion and Garlic Diseases and Pests, 2nd ed., 2008, American Phytopathological Society, St. Paul, MN.
The sclerotia that form on the decaying host will lay dormant until a host plant’s root exudates stimulate germination, specifically root exudates that are unique to Allium spp.  Cool weather is also needed for germination of sclerotia and hyphal growth.  The soil moisture levels optimal for host root growth are also optimal for sclerotia germination. Mycelium will grow through the soil, and once it encounters a host root the fungus will form appresoria, structures whose purpose is to aid in the attachment and penetration of the host. Mycelium can grow outwards from the roots of one plant to the roots of a neighboring plant, and it is by this method that the pathogen can move down a planted row. Sclerotia are formed on the decaying host tissue, and once the host tissue completely decays the sclerotia are free in soil. If the bulbs survive long enought to be placed into storage, the pathogen may continue to decay the bulbs if there is high humidity and low temperatures.  If the bulbs are stored dry then the disease may not spread but bulbs infected in the field will continue to decay (5).



References and Useful Links

1. UC IPM: UC Management Guidelines for White Rot on Onion and Garlic.

2. White rot, An Online Guide to Plant Disease Control. Oregon State University Extension Service.

3. Anwar Haq, M., et al. 2003. Detection of Sclerotium cepivorum within onion plants using PCR primers. Physio. and Mol. Plant Path. 62: 185-189. 

4. Adams, P.B. 1979. A rapid method for quantitative isolation of sclerotia of Sclerotinia minor and Sclerotium cepivorum from soil. Plant Disease Reporter. 63: 349-351.

5. Crowe, F.J. 2008. White Rot. Pages 22-26 in: Compendium of Onion and Garlic Diseases and Pests. Schwartz, J.F. ed. APS Press, St. Paul, MN.

6. Georgy, N.I. and Coley-Smith, J.R. 1982. Variation in morphology of Sclerotium cepivorum sclerotia. Trans. of the British Mycol. Soc. 79: 534-536.