Taxonomy and life cycles matter when managing Sclerotinia sclerotiorum diseases

Estimated reading time: 6 minutes

  • Diagnosing soya bean stem rot and sunflower head rot, caused by Sclerotinia sclerotiorum, is relatively simple due to characteristic signs and symptoms such as wilting, water-soaked lesions and white fluffy fungal growth.
  • Pathogen identification is the process of determining the organism responsible for the disease.
  • Assigning a name to an organism requires an understanding of how an organism‚Äôs DNA aligns with the taxonomic hierarchical system, from largest to smallest, i.e., kingdom, phylum, class, order, family, genus and species.
  • Sclerotinia libertiana was declared inconsistent with the International Rules of Botanical Nomenclature; as a result, S. sclerotiorum (Lib) was proposed and retained (Buchwald and Neergaard 1976; Dennis 1974).
  • In managing diseases caused by S. sclerotiorum, it is critical to consider the reduction of the population of sclerotia, reducing the risk for high inoculum loads in future seasons.

We all drive motor vehicles and several, for instance, come from Italy, but there is a fundamental difference between a Ferrari and a Fiat. Knowing exactly what type of organism is causing disease is equally important (Crous, 2005).

The development of an efficient plant disease management strategy is dependent on accurate disease diagnosis and pathogen identification. Knowing what the host plant is and recognising its healthy appearance is key to making comparisons to identify abnormalities in the host and patterns in the field.

Diagnosing soya bean stem rot and sunflower head rot, caused by Sclerotinia sclerotiorum, is relatively simple due to characteristic signs and symptoms such as wilting, water-soaked lesions (symptoms of the disease), and white fluffy fungal growth (a sign of the pathogen known as mycelia). In severe or progressive stages of the sclerotia pathogen life cycle, black survival structures can be found in and on the stems and heads of the host crops.

Pathogen identification

In contrast, pathogen identification is the process of determining the organism responsible for the disease, i.e., giving a name to our pathogenic enemy. Taxonomy provides a framework to describe, classify, name and communicate information about organisms. Traditionally, describing and naming organisms relied on observing the pathogen on the plant (in situ), their growth on artificial media in culture (morphology), and structures under a microscope (microscopy).

However, through molecular techniques, the untangling of an organism’s DNA is possible, and we can capture robust data on the organism. Consequently, as new data about organisms is gathered, a revision may be required of the description, classification and name assigned.

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Naming an organism

Assigning a name to an organism requires an understanding of how an organism’s DNA aligns with the taxonomic hierarchical system, from largest to smallest, i.e., kingdom, phylum, class, order, family, genus and species. Fungi belong in their own kingdom, and their names are determined by a set of rules in the International Code of Nomenclature for algae, fungi and plants (ICNafp).

Taxonomy is critical to understanding the diversity and distribution of plant pathogens, their evolutionary relationships, mechanisms by which they cause disease, and how disease can be prevented, mitigated and managed. Most plant diseases are caused by fungi found in the phylum of Ascomycota, the taxonomic home of S. sclerotiorum. Globally, Sclerotinia stem rot is considered the second most yield-limiting disease of soya beans (Savary et al., 2019), and the history of naming S. sclerotiorum is fraught with confusion.

History of S. sclerotiorum

S. sclerotiorum (Lib) de Bary was first described by the Belgian botanist, mycologist and plant pathologist Marie-Anne Libert as Peziza sclerotiorum (Ekins 1999; Phillips 1979). Over the decades, the pathogens’ scientific name has undergone multiple changes, with a key change leading to what we now know as the genus of Sclerotinia, although still honouring Libert as S. libertiana (Merrick Ekins 1999; Phillips 1979; Willbur et al. 2019).

However, Sclerotinia libertiana was declared inconsistent with the International Rules of Botanical Nomenclature; as a result, S. sclerotiorum (Lib) was proposed and retained (Buchwald and Neergaard 1976; Dennis 1974). Sclerotiniaceae, the family of fungi to which S. sclerotiorum belongs, contains 33 other recognised fungal genera. However, within the genus of Sclerotinia there are three universally accepted species, S. minor, S. sclerotiorum and S. trifoliorum (Kohn et al., 1988).

The pathogen, Sclerotinia homoeocarpa, causing dollar spots on turfgrasses is thought to be more accurately classified within the genera of Lanzia or Moellerodiscus (Dernoeden, 1995 cited in Raina et al., 1997).

Understanding a pathogen

Naming the enemy is one component required for developing disease management programmes, although understanding a pathogen’s life cycle and manner of reproduction is equally important. S. sclerotiorum is a monocyclic pathogen, although it can produce two different forms of inoculum. Ascospores, associated with sexual reproduction, are produced in apothecia due to carpogenic germination, whereas mycelium arises directly from sclerotia and is responsible for asexual reproduction (Kohn, 1995; Amselem et al., 2011).

As a monocyclic pathogen, sclerotia overwinter and germinate to produce primary inoculum, i.e., ascospores and/or mycelium, and may initiate primary infections in the growing season. No secondary inoculum types arise from lesions, i.e., points of infection. Once an infection has been initiated by either the successful germination of an ascospore or hypha/e (thread/s of mycelium), disease establishes and colonisation occurs, and no further spore forms are generated from the lesion.

However, secondary infections are possible in the same season when infectious material (tissue successfully colonised by the pathogen) comes into contact with susceptible host tissue (Link and Johnson, 2007). Once the season ends, or conditions are not favourable for further disease development, the pathogen starts to melanise the mycelium forming sclerotia.

Managing S. sclerotiorum

In managing diseases caused by S. sclerotiorum, it is critical to consider the reduction of the population of sclerotia, reducing the risk for high inoculum loads in future seasons. Monitoring for apothecia is critical in the growing season, as environmental conditions will regulate the germination of sclerotia, resulting in asynchronous events of germination, resulting in flushes of ascospores available to initiate disease.

Not all sclerotia will germinate at the same time, not all apothecia will release ascopores at the same time, and not all ascospores will successfully germinate. Staggering planting dates can reduce the risk of ascospores being present in multiple fields where susceptible host tissue is available, although one must consider the yield losses which may be incurred due to plantings outside optimum planting dates.

Positive impacts in managing S. sclerotiorum rely upon converting raw, unorganised facts about the pathogen to contextualised and meaningful information to gain an understanding of the host-pathogen interaction. This knowledge will guide the discovery of patterns, relationships and unapparent aspects of epidemiology, population biology and genomic data to make wise decisions on disease management, securing the development of sustainable crop protection programmes. – Dr Lisa Rothmann, Department of Plant Sciences, Plant Pathology, University of the Free State

For more information, contact Dr Lisa Rothman on 051 401 3666.

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