Hemp Disease Management 101: Back to Basics

 PREVENTION VS CURE

Diseases are managed first and foremost by prevention of infection by pathogens*.  Once host plants are infected, disease** is not curable.  Although some measures can suppress pathogens or slow symptom development, disease cannot be reversed or cured.  Thus, it is recommended that preventative measures become priority in every system.

Introduction of diseased plant material into a field or greenhouse puts all healthy plants at risk for infection.  Careful selection of plant material and adoption of production guidelines is critical.  For example, new plants should remain separated for two to three weeks until they are confirmed to be disease (and insect) free.  This period of quarantine helps protect existing healthy plant material. 

On occasion, pathogens are introduced to production systems.  Immediately upon identification/confirmation, protect healthy plants from infection and spread.  Destroy, quarantine, and/or apply chemical treatments to infected plants.  Nearby plants should be treated as infected and infective (even if symptoms have not yet developed).  All diseased plant material is a threat to healthy plants, and thereby should be removed from production areas.  As mentioned previously, disease cannot be cured or eradicated.  Elimination of pathogens is only possible by extreme measures (e.g. crop destruction or greenhouse disinfestation). 

Note:  Fungal biology is different for each pathogen and specific methods for treatment and disease management varies from one disease to another. 

















 
DISEASE TRIANGLE

Plant disease can be represented by the “disease triangle.”  The premise behind this visual is that by breaking just one of the sides of the triangle, plant disease cannot exist.  The three sides of the disease triangle are:

Host – a susceptible host plant must be present in order for disease to occur.  A number of pathogens are host specific, meaning that particular hosts are needed for infection.  Some plant genera or species are not susceptible to infections by certain pathogens, while in other instances, certain plant varieties or cultivars may be resistant or tolerant.
Pathogen – a disease-causing pathogen (e.g. fungus or water mold), must be present in order for disease to occur.  Pathogens may be actively growing, or they may survive or overwinter in a number of ways (e.g. survival structures or alternate hosts).
Environment – specific environmental conditions must be present for disease to occur.  In particular, moderate temperatures (not too hot, not too cold) and moisture (humidity, fog, irrigation, or rain) are required for infection, sporulation, and dissemination.  While each pathogen species has its own set of optimal conditions, most of them can cause disease under a similar range of temperatures and wetness levels.   

Removal of just one side of the disease triangle can break the disease cycle.  Cultural practices take advantage of one or more of these components, while fungicide use and sanitation eliminate the pathogen from the disease cycle.  Growers can utilize concepts portrayed in the disease triangle to eliminate components of disease, thereby managing it.

















 
START WITH CULTURAL PRACTICES

It is often impossible to eliminate disease factors completely, but growers can consider a variety of approaches in combination to significantly reduce disease.  Use of cultural practices requires consistent implementation, and disease management will continue to improve each season.  Cultural practices include adjustment of crop management practices such as

  • use of resistant varieties
  • crop rotation
  • installation of disease-free plant material
  • removal of infective plants
  • weed management
  • destruction of clippings and diseased plants/plant parts
  • sanitation in greenhouses and fields, as well as tools, containers, and soil
  • elimination of plant stress
  • increased plant vigor
  • manipulation of excess water and humidity
  • increased plant spacing for air circulation


Fungicides should never be utilized without adoption of cultural practices.  Organic and no-spray growers rely more heavily on cultural practices than conventional growers.  However, fungicide efficacy is reliant on amounts of pathogen inoculum present and its ability to infect.  Cultural practices reduce this inoculum load before fungicides are applied***. 
















 

MANIPULATE COMPONENTS OF THE TRIANGLE TO MANAGE DISEASE

Consider each of the factors for disease development, and manipulate each for disease management.  These strategies can function as components of a preventative disease management program, and they can also serve as a means to manage disease once it establishes.

Selection of Host Plant

Some varieties/cultivars may exhibit more tolerance to infection than others.  Ideally, selection of resistant cultivars would be the one-and-done approach to managing disease.  However, only minor resistance or tolerance may be available.  Utilize any available resistance, and combine this approach with other cultural practices.

Weeds and volunteer plant material can serve as host reservoirs for some pathogens.  Weed removal can reduce carryover sources of infection.

Manipulation of Environment

Fungal pathogens require two environmental conditions in order to infect and cause disease – moderate temperatures and available moisture.  Manipulation of these environmental factors can suppress pathogens and reduce infection and spread. 

Moderate temperatures are required for infection and disease development.  In fact, optimal temperatures for pathogen sporulation, dispersal, and infection result in maximum spore production and fungal growth.  Often a range of adequate temperatures are sufficient for limited infections to occur.  Manipulation of temperatures (e.g. using greenhouse fans and field orientation) can reduce disease incidence and spread.  Each pathogen has its own set of optimal temperatures; specific environmental conditions are determined by university-conducted research and reported in resulting grower publications. 

Moisture is also required for pathogens to infect and develop.  Free water such as rain or irrigation creates a high risk for disease development.  Thus, during rainy seasons, disease is often higher than in dry seasons.  High humidity (>95%) can also provide sufficient moisture for many fungal pathogens.  Greenhouse environments are thereby ideal sites for disease development.  Even in controlled environments, fungal spores can take advantage of damp tissue and microclimates (pockets of high humidity such as between plants and inside plant canopies).  These microclimates also provide conditions conducive for infection. Sufficient air movement such as fans help dry leaves and circulate air.  Space plants to improve air circulation, thus, considering mature height and spread.  Reduce amounts of irrigation water, when possible, and avoid overhead watering.  In outdoor settings, orient rows to take advantage of natural air movement.  Avoid planting in shaded locations.

Elimination of Pathogens

Disease-causing fungi can be reduced or eliminated by means of fungicide or by sanitation.  Sanitation is the act of cleaning and/or maintaining a pathogen-free environment.  Prune infected plants, quarantine infected or susceptible plants, and discard severely affected plants in efforts to limit or reduce amounts of pathogens and their infective spores.  Sweep or rake fallen leaves and other debris, as they can harbor infective propagules, as well.  Clippings and debris should be hauled away from growing sites and burned or buried, as appropriate.  (ref Sanitation pub)

Fungicides remain a contentious issue, especially for growers who strive to maintain organic or non-sprayed products.  There are no commercial fungicides that are labeled for Cannabis sp.  Unfortunately, there are many reports of illegal use of fungicides and other pesticides.  Although there are no registered fungicides that include Cannabis sp. on their labels, there are some broadly-labeled products that contain language that does not exclude them from use.  These products are primarily botanicals or other biological products (reference).  Read product labels carefully and follow instructions before use; the label is the law.  Many “approved” products include biological and botanical products.  Efficacy of these products (such as oils and antagonistic microorganisms) has proven poor to fair, at best.  Many of them are not effective at all (fungicide trials being conducted at University of Kentucky in 2016).  Combinations of cultural practices (air movement, plant spacing, sanitation) and botanical products, however, may provide adequate control.


















SUMMARY

Hemp diseases, like all plant diseases, are managed by a combination of practices.  The disease triangle makes it easy to visualize the components that must be available for disease to occur.  Removal of just one side of the triangle results in breakage of the cycle.  Unfortunately, it is often difficult to eliminate a component completely.  Thus, growers should utilize a variety of practices (cultural practices and if available fungicides) to manage diseases of hemp.

 
*Plant pathogenic microorganisms may include fungi, water molds, bacteria, nematodes, viruses, phytoplasmas, and parasitic plants.  Hemp pathogens are primarily restricted to fungi and water molds, although exceptions may exist.

**Diseases are symptoms produced by plants infected with plant pathogens.  Some diseases develop in a few hours or a few days after infection, but some disease requires weeks or months before symptoms are observed.

***There are no fungicides specifically labeled for hemp, but some states reference “open label” products for use on hemp.  A few fungicides and botanical products are available for application.  See Fungicides for hemp.

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A Scientific Look at Diseases of Kentucky Hemp