White mold is a plant disease caused by the sclerot fungus (Sclerotinia sclerotiorum). A virus knows how to tame sclerot, and even make it beneficial to plants.
White mold is known by more than 60 names, including cottony rot and blossom blight. Most of the names have “rot” in them, though lettuce “drop” is a classic, dating to the late 19th century.
Sclerot penetrates the cuticle of a plant by sheer mechanical pressure. The first signs are often watersoaked spots on leaves or young stems. In other hosts, the fungus creates relatively dry lesions on stems, showing a sharp demarcation between healthy and diseased tissues.
Once sclerot has secured its infection, the fungus produces black resting structures, known as sclerotia, and white fuzzy growths of mycelium. Mycelium are the mass of branching thread-like hyphae that is the vegetative part of a fungal colony.
Continued consumption by the fungus extensively destroys parenchymatous tissues. The remaining structural and vascular elements have a characteristic shredded look.
In the spring, sclerotia give rise to a fruiting body that produces a sac of spores: the next generation of white mold.
Sclerot proliferates in moist and cool environments. Common hosts of sclerot are the vegetables and flowers of herbaceous succulents. Some 450 species in 64 genera of plants are susceptible to the fungal pathogen, including such high-value crops as soybeans.
Sclerot can affect their host at any stage of growth. White mold is usually found on tissues with high water content and close to the soil. The fungus can also go dormant in soil for years, seeding new infections that can raze entire fields of crops.
Considered a pest by mushroom growers, the mushroom fly is one of many fungus-eating gnats in the sciarid family. The mushroom fly is formally known as Lycoriella ingenua.
The mushroom fly is subject to being used for transport by a sclerot-infecting virus, which is a DNA mycovirus (fungi-infecting virus) called SsHADV-1. DNA mycoviruses were unknown prior to the sclerot virus. All other known fungal viruses take RNA form.
Mushroom flies, including larvae, eat a virus-infected batch of sclerot and carry the virus on to new sclerot establishments.
The virus successfully hijacks a host sclerot’s cells on a colonial scale, completely changing sclerot’s effect on a plant. Virus-infected fungi no longer flood plants with tissue-macerating juices. While sclerot still forces its way into a plant host, virus-infected fungi are far more gracious tenants. “A mean pathogen turns into a gentle microbe,” observed plant pathologist Aurelie Rakotondrafara. This turn of temper toward the tame is known as hypovirulence.
Viral puppeteering makes sclerot beneficent, as rapeseed illustrates. Virus-infected sclerot somehow has plants sprouting heftier leaves and more robust roots. Buoyed rapeseed plants also became resistant to damaging fungi, including pathogenic sclerot. The sclerot virus acts as a fungal vaccine.
Once tamed by the virus, sclerot does not grow as enthusiastically on rapeseed, possibly because the newly bolstered plant can keep its guest in check.
Half of the world’s species are parasitic or pathogenic. As sclerot illustrates by way of a virus, that is a lifestyle choice. Further, as the sclerot virus exemplifies, viruses have scant physical presence, but tremendous intelligence.
Xiao Yu et al, “A geminivirus-related DNA mycovirus that confers hypovirulence to a plant pathogenic fungus,” PNAS (4 May 2020).
Hongxiang Zhang, “A 2-kb mycovirus converts a pathogenic fungus into a beneficial endophyte for Brassica protection and yield enhancement,” Molecular Plant (29 September 2020).
Katherine J. Wu, “Infected by a virus, a killer fungus turns into a friend,” The New York Times (29 September 2020).
Si Liu et al, “Fungal DNA virus infects a mycophagous insect and utilizes it as a transmission vector,” PNAS (24 October 2016).
Maria Mazin et al, “Mushroom sciarid fly, Lycoriella ingenua (Diptera: Sciaridae) adults and larvae vector mushroom green mold (Trichoderma aggressivum ft. aggressivum) spores,” Applied Entomology and Zoology (1 November 2019).
L.H. Purdy, “Sclerotinia sclerotiorum: history, diseases and symptomatology, host range, geographic distribution, and impact,” The American Phytopathological Society (1979).
Loren J. Giesler, “Sclerotinia stem rot,” University of Nebraska – Lincoln.