Most Prevalent Diseases
How to use this page: This gallery includes common diseases seen in hemp within the North Central region. As production increases, so will disease observations.
Root Diseases
Fusarium wilt and fusarium root rot
Fusarium wilt is caused by Fusarium oxysporum in the soil. Fusarium rootrots are caused by many species within the genus Fusarium and are widespread in the soil. Yellow leaves are often observed first as the vascular tissue is damaged, followed by more severe wilting. Dark and sunken cankers may be visible on the stem at the soil line. Infected roots may appear pink to brown and have internal staining when cut
crosswise. Warm soils with poor drainage are conducive to disease, with damaged roots and girdling being more prone to infection.
crosswise. Warm soils with poor drainage are conducive to disease, with damaged roots and girdling being more prone to infection.
Pythium
Multiple Pythium species can infect hemp. Infected plants will be stunted, yellow, and eventually wilt. Brown lesions on the stem may be visible. Excessive rainfall and poorly drained fields are conducive to disease.
Foliar Diseases
Cercospora leaf spot
Cercospora leaf spot is caused by Cercospora cannabis and possibly otherspecies. Lesions begin as small yellow spots that become tan to brown with a darkmargin. Lesions may merge and cause larger spots. It is sometimes confused with other leaf spots. Humid and warm weather are conducive to disease.
Nancy Gregory, University of Delaware, Bugwood.org 
Downy Mildew
Downy mildew is caused by the pathogen Pseudoperonospora cannabina. Lesions first appear as yellow-green spots on the upper leaf surface. As lesions progress, they become angular and brown. Leaves with many lesions can become distorted andplants may defoliate. Lesions may look like chemical damage or other types of leafspots. However, the underside of the leaves can provide a crucial sign of downy mildew.During wet periods, gray to black down-like fuzz can be observed. High humidity and free water on the leaves are conducive to disease.
Hemp leaf spot
Hemp leaf spot sometimes called Bipolaris leaf spot is caused by Drechslera gigantea. Lesions begin as light green spots on the leaves that progress to round, whiteto tan spots. It may be confused with other leaf spots. Non-hemp plants in or near the field may be infected as well. Warm and wet conditions are conducive to disease.
Powdery Mildew
Powdery mildew in hemp can be caused by multiple species. A powdery white appearance can be patchy or widespread on leaves and flowers. Warm to high daytime temperatures and humid nights, shade or low light intensity is conducive to disease.
Rust
Rust is caused by the pathogen Uredo kriegeriana. Lesions appear as light to dark brown with a yellow halo on the topside of leaves. Orange pustules with visible spores are found on the underside of the leaves. Moderate temperatures and high humidity appear conducive to disease; however, exact conditions are unknown
for this species.
for this species.
Septoria leaf spot
Septoria leaf spot is caused by Septoria cannabis. Spots begin as yellow flecks and progress to darker brown lesions with yellow halos. Disease is often observed in older leaves first. Warm, humid, and rainy conditions are conducive to disease.
Crown and Bud Diseases
Gray Mold
Gray mold is caused by Botrytis cinerea. Infection causes browning and dieback of grain or flower tissue and can also affect leaves, especially on plants with injury. Gray mycelia may be visible and overwintering sclerotia look like rat droppings. Cloudy conditions, high humidity, and moderate temperatures are conducive to disease.
Fusarium head blight
Fusarium head blight is caused by multiple Fusarium species. Brown, dead tissue can be observed in both grain and flower structures. White mycelial growth is often observed when heads are broken open. Warm and humid conditions are conducive to disease.
White Mold
White mold is caused by Sclerotinia sclerotiorum. Portions of branches or whole plants can brown and wilt. White, fluffy mycelial growth is often observed. Black sclerotia may also be observed and are overwintering structures. Cool to moderate temperatures, high humidity and moist soil are conducive to disease.
