Foliar Diseases of Outdoor Hemp (Cannabis sativa) and Their Management 2021

As canopies begin to close in hemp fields and dew periods lengthen, the risk of foliar diseases of hemp increases. The recent wet weather in much of southern Ontario also increases the risk of most diseases in the field. There are three main foliar diseases of hemp that have been identified in Ontario: Septoria leaf spot, downy mildew and powdery mildew.

Powdery Mildew

Powdery mildew is the most common disease on both indoor and outdoor grown hemp. It can cause severe damage if not controlled, but this is more common indoors than outdoors. Several species have been identified as causing powdery mildew on hemp with Sphaerotheca macularis (=Podosphaera macularis) and Golovinomyces cichoracearum reported as the most common causes. These species have a fairly wide host range, so hemp does not have to be the original source of the disease. Powdery mildews can be caused by many different species on different hosts, so powdery mildew of one crop will not necessarily affect another.

Powdery mildew first appears as faint white tufts of fungal growth, mainly on the upper leaf surface (Figure 1). The fungus grows primarily on the leaf surface, penetrating the leaf to feed. Numerous conidia (asexual spores) are produced on leaf surface and are spread by wind. Over time the patches coalesce, and the entire surface can be covered. Eventually the leaf is weakened and will turn yellow and fall off the plant. Plant vigour will be reduced over time and yield can be impacted. The fungus can also grow on flower buds, affecting quality.

A hemp leaf with white tufts of fungal growth on the surface

Figure 1. Initial symptoms of powdery mildew on a hemp leaf. Note the faint white tufts of fungal growth scattered over the leaf surface. Photo credit: Amy Fang Shi, OGGA

Powdery mildew is favoured by high humidity, but not rainfall. It is often worst in dry years due to added moisture stress on the plants, which make them more susceptible to the disease. Dew periods usually provide sufficient moisture, especially once canopies close in July. The disease then continues to spread for the remainder of the year.

Over the past year, three biofungicides have been registered for suppression of the disease in the field: Regalia Maxx, Actinovate/CannaPM, and Milstop. The products are also best used preventatively before symptoms appear in the field, since they cannot cure ongoing infections. Because these products only provide suppression and not control of the disease, they are best used in combination with practices that reduce humidity within the canopy and lower plant stresses. Humidity can be reduced by promoting good airflow through the canopy through wider plant spacing and pruning. Adequate irrigation and fertility are important for minimizing plant stress.

Septoria Leaf Spot

Septoria leaf spot is caused by two species of hemp (Cannabis)-specific fungi, Septoria cannabis and Septoria cannabina. Both have been identified in Canada in the past, but Septoria cannabis is cited as causing the disease most commonly in the field. The disease has been reported in field-grown hemp in southern Ontario over the past week.

Septoria leaf spot is favoured by warm weather with high humidity and prolonged leaf wetness. It is often reported as a disease in the southern US where these conditions are more common. The conditions over the past few weeks in southern Ontario have been ideal for disease development. Humidity and leaf wetness tend to be greatest within a dense canopy and the disease usually begins in these areas once plant canopies begin to close.

Symptoms appear as greyish-brown spots with a distinct yellow halo (Figure 2). As the spots expand, tiny black fruiting bodies called pycnidia appear within the lesion that makes the lesion look peppery. The centre of older lesions may fall out resulting in a shot-hole appearance (Figure 3). The presence of pycnidia is the best way to distinguish this disease from downy mildew. These pycnidia release numerous airborne and water-splashed asexual spores called conidia. Just like powdery mildew, spores can be spread by wind to infect other areas of the field and other fields many kilometres downwind. Septoria appears to require moister conditions for this spread to occur than either downy or powdery mildew. Unlike powdery mildew, Septoria cannabis is specific to hemp and cannabis and there are no other known hosts, so must spread from these crops to new fields.

A hemp leaf with circular brown lesions with a yellow halo and black specks within the lesions

Figure 2. Septoria leaf spot lesions on hemp. Note the yellow halo around the lesions and the tiny black specks (pycnidia) within the lesion. Photo credit: Amy Fang Shi, OGGA.

The tip of a hemp leaf showing one large lesion with a hole in the centre and several smaller lesions with black specks.

Figure 3. The centre of this older Septoria leaf spot lesion has fallen out, leaving a hole. Black pycnidia are clearly present on the lesions near the tip of the leaf.

There are no products currently registered for control or suppression of this disease in outdoor hemp in Canada. Use of Regalia Maxx and Milstop for suppression of powdery mildew may also provide some suppression of Septoria leaf spot if applied before symptoms appear.

Downy Mildew

Despite the similarities in name between powdery and downy mildew, downy mildew is very different. It is not caused by a fungus, but an oomycete (water mould). Oomycetes are more closely related to algae and green plants than to fungi. Oomycetes cause some of the most destructive diseases of crops due to their ability to spread and defoliate plants rapidly.

Downy mildew of hemp was first identified in the field in Ontario in 2020. It is caused by a hemp-specific species, Pseudoperonospora cannabina. It is favoured by humid conditions and extended periods of leaf wetness. Just like the other two foliar diseases, it usually begins once canopies begin to close and there is a humid environment in the centre of the plant.

Symptoms begin as small reddish-brown irregular spots that expand into roughly circular patches (Figure 4). As they expand, they tend to become confined by the leaf veins and elongate (Figure 5). Just like Septoria leaf spot, spots are usually surrounded by a yellow halo, but this halo is more irregular in downy mildew. Once lesions are present, spores are often produced on the underside of the leaf below the lesion each morning. This usually appears as a purplish/grey fuzz. The spores are spread by the first winds of the day and will not be present by midday. Downy mildew lesions lack the black pycnidia that are present in Septoria leaf spot lesions.

A hemp leaf with small, irregular brown lesions with a yellow halo.

Figure 4. Initial symptoms of downy mildew on a hemp leaf. Lesions are more reddish-brown and irregular compared to Septoria leaf spot, with the absence of any black specks (pycnidia). Photo credit: Amy Fang Shi, OGGA.

A hemp leaf with elongated brown lesions confined by the veins

Figure 5. As downy mildew lesions advance, they become confined by the veins and elongate. Photo credit: Amy Fang Shi, OGGA.

There are no fungicides currently registered for management of this disease in Canada. Application of Regalia Maxx for powdery mildew suppression may also suppress downy mildew, since Regalia Maxx is registered for suppression of cucumber downy mildew, which is caused by a closely related species, Pseudoperonospora cubensis. Just like the other two diseases, promoting good airflow through the canopy is the best way to manage the disease.

About Sean Westerveld

Ginseng and Medicinal Herbs Specialist, OMAFRA
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