Powdery Mildew of Hemp

Powdery mildew can affect numerous plants, including fruit, vegetable, and agronomic crops, as well as woody and herbaceous ornamentals. Kentucky hemp is also susceptible to this disease. Plants infected early in the season can be seriously damaged, while those affected later in the season may not be significantly harmed. Powdery mildew may adversely affect flowering, plant vigor, and yields, although this disease rarely kills plants outright. As hemp is a new crop, no fungicides are available to treat powdery mildew, thus management is dependent upon cultural methods.

Powdery Mildew of Hemp Facts

• Powdery mildew is primarily a greenhouse hemp disease; it is rarely problematic in field situations.
• Symptoms include the presence of white, tan, or gray powdery fungal growth (mycelia and spores) on surfaces of infected plant parts (Figure 1).

Figure 1: Symptoms of powdery mildew on hemp include the presence of white, tan, or gray powdery fungal growth on the surface of leaves. (Photo: Nicole Ward Gauthier, UK)

• Infection of young leaves can result in distortion, stunting, marginal leaf curl, discoloration, and/or the development of irregular spots or blotches. Severely infected leaves may drop prematurely.
• Little is known about the host range of the powdery mildew that affects hemp. However, most powdery mildew species have limited host ranges, so infection of hemp does not necessarily mean that other species nearby are in danger of infection.
• Primary infection occurs in spring when spores are splashed by rain/irrigation or carried by wind currents to susceptible tissues.
• Subsequent infections result from spores produced throughout the growing season.
• Powdery mildew is favored by temperatures between 68° and 77°F and humidity of 40% or more. This pathogen does not require free moisture to infect.
• Caused by the fungus Golovinomyces sp.
• The powdery mildew fungus overwinters on plant debris and on live greenhouse plant material.

Management Options

• Select varieties that are tolerant to powdery mildew.
• Maintain plant health with proper nutrition and irrigation practices.
• Select planting sites with adequate light.
• Keep plants well-spaced and properly thinned to promote air movement, light penetration, and rapid leaf drying.
• Remove and destroy infected plant tissues and debris.
• There are currently no fungicides labeled for the management of powdery mildew of hemp.

Additional Information


By Kim Leonberger, Extension Associate and Nicole Ward Gauthier, Extension Specialist


Posted in Hemp