White mold, caused by the fungus Sclerotinia sclerotiorum, is a specific fungal pathogen that infects several hundred species of plants from more than 75 different families, resulting in death of the infected plant (Fig. 1). Many of the most popular annuals and perennial plants have been reported to be susceptible to white mold, from aster (Aster spp.) to zinnia (Zinnia spp.), and even a few woody ornamentals, too (Table 1).
Infection by the fungus often begins early in the season when overwintering structures called sclerotia [not Latin for ‘fungus ball that looks like a rat poop’ (Fig. 2)] germinate in the spring, forming either:
- a specialized infection structure called an apothecium. After apothecia have formed, they mature to produce sacks (asci) containing ascospores. Ascospores are discharged and inoculate flowers of the host plant and germinate to form hyphae, or
- hyphae that directly infect the plant.
Option one occurs more commonly in early spring, and option two occurs throughout the growing season. Regardless of how the infection starts, the fungus continues to produce oxalic acid, which kills plant tissue. This is followed by hyphae that grow and invade the plant and feed upon it. One of the most frustrating aspects of this disease is the fact that plant death from white mold isn’t noticed until later in the summer (Fig. 3). By this point, infestation of a garden bed with S. sclerotiorum often results in a long-term problem, because the fungus produces numerous sclerotia. Once established and left untreated, white mold can persist with repeated infection each year, particularly in those beds where plants are closely spaced, mulched, and watered overhead.
Although controlling when and where ascospores land is not possible, there are things that can be done to reduce your risk of acquiring sclerotia, and white mold. Bird seed from feeders are a significant source of sclerotia, particularly when black oil sunflower seed is used (Fig. 4). Sclerotia are of similar size, and often get included in seed mixes. This is the primary source of inoculum for most landscapes.
Managing this disease is challenging, but not impossible, and involves cultural, biological and chemical management.
Cultural: Good sanitation and removal of debris and of infected plants is necessary—along with good spacing and airflow of neighboring plants to allow the lower leaves to dry out. To do this effectively requires ruthlessness. In my personal experience, removing all my lovely mulch and humus, in addition to seemingly healthy plants was probably the toughest part of management. Weeds can also become infected with this pathogen, so good weed control is an important component of disease management.
Some plants are less susceptible than others, and should be used to replace any gaps in the landscapes, or used in future plantings. These include pentas, impatiens, sweet flag (Acorus spp.), purple millet grass (Pennisteum glaucum) and other ornamental grasses.
Biological controls containing Coniothyrium minitans (Contans) and Trichoderma spp. (PlantShield, SoilGard,) have reduced the incidence and severity of white mold infections. These mycoparasites colonize and feed on the sclerotia, reducing to preventing germination of hyphae or apothecia development.
Chemical: To protect neighboring plants prior to infection, the FRAC 7-11 fungicides (Pageant, Orkestra, Mural) are labeled. Any of these should be rotated with Terrachlor (FRAC 14) to provide different modes of action and minimize the risk of resistance.
Table 1. Plants Susceptible to Sclerotinia sclerotiorum. Modified from: https://pnwhandbooks.org/plantdisease/pathogen-articles/common/fungi/plants-susceptible-sclerotinia-sclerotiorum