Photos courtesy of Marguerite Bolt

A lot can go wrong when growing plants outdoors. Drought, extreme rainfall, frost, hail, wildfires, tornadoes (you get the point) are outside of our control. Even when weather conditions are ideal for plant growth, they can also be ideal for pests. So many factors can impact the health of a hemp crop, and it can be challenging to determine the root cause of an issue.

When trying to identify a problem with your hemp, it’s helpful to start by thinking of whether the cause could be biotic or abiotic. Because many stressors manifest in similar ways, this can help you narrow down what the issue may be.

Biotic stressors, caused by biological organisms such as insects, weeds, nematodes and pathogens, have identification traits that differentiate them from abiotic stressors. Abiotic stressors are non-living factors, like nutrients, weather conditions, mechanical damage and chemical damage. It is crucial for growers to know the key differences to create effective crop management plans and mitigate the issue.

The Basics of Identifying Biotic and Abiotic Stress in Hemp

Symptoms of biotic stressors can look very similar to abiotic stressors. And even with a single stressor, growers have many things to consider when determining the cause. When multiple stressors occur at once, diagnosis becomes significantly more challenging. It is important to consider both plant-level and field- or regional-level observations.

Growers can use the following tactics to help differentiate biotic and abiotic stressors.

1. Consider the Plant

At the plant level, knowing about the cultivars you’re growing can help uncover the issue. Some cultivars are more susceptible to biotic disease or insect damage. In addition to the cultivar, looking at the age of the plant can also help with diagnosis. For example, plants often drop their lower leaves or start to yellow toward the end of their lifecycles. This is normal but can sometimes look like a nutrient deficiency or water issue. (In the latter cases, leaf yellowing or dropping would occur earlier in the plant's lifecycle.)

2. Search for Signs of the Stressor

Biotic stressors will often show signs of the causal agent. This could mean bacterial ooze (exudate full of bacteria seeping out of the plant tissue) or fungal growth on the plants. Or, in the case of insect or mite pests, sometimes the arthropod will still be on the plant after causing damage. The damage caused by biotic stressors can have an irregular appearance and sometimes affect just one portion of the entire hemp plant. Meanwhile, abiotic stressors tend to have a more uniform effect throughout the whole plant.

3. Look for Patterns in the Field

Biotic stressors tend to appear in patches in the field. These stressors spread from plant to plant, leading to clumped or even random distributions. On the other hand, abiotic stressors cannot spread from plant to plant, so damage from these stressors may look more uniform.

The distribution of the problem is important for both diagnosis and understanding what plants in the field may be afflicted next. Clumped areas of unhealthy plants may signal a biotic disease. Meanwhile, patterns may show parts of the field that correspond with low-lying areas or areas with lower fertility. Taking the whole field into consideration can save a lot of time when trying to narrow down what the stressor is.

4. Observe Plants in the Margin or Nearby Fields

Sometimes non-crop plants surrounding hemp in a field can be a huge clue to solving the mystery of a sick crop. Weeds may show the same symptoms as the hemp crop, which can indicate whether the issue is abiotic or biotic. If all the plants (of multiple species) in an area show the same symptoms, it is a pretty good indication that an abiotic force is at play. It is important to note if only some of the plant species observed show the same symptoms as the primary crop because some diseases and insect pests can attack multiple plant species.

5. Understand the History of the Field and Its Management

Field selection plays a huge role in plant health. Many things can go wrong due to the specific area where the crop is planted. Fields with low areas are prone to standing water during heavy rain events. The previous crop planted there could increase the risk of disease or insect pressure. Herbicide carryover could damage the current crop. Taking notes on the history of the field, learning how it was managed, and collecting soil samples for analysis can help diagnose problems—or, better yet, prevent certain issues.

6. Consider Recent Weather Conditions

You should consider weather events and temperature when evaluating plant health. Keeping an eye on what the weather has been doing can help you record events leading up to the observed damage. Weather events can directly damage plants. This could mean a single event, like a hailstorm, or prolonged events, like drought. Weather events can also increase the likelihood of infection by pathogens or increase the movement of insects into the field.

Both local and statewide extension educators or crop consultants often scout or receive information on many different fields and will be aware of drought events or other stressors that may be affecting larger areas.

Pinpointing the Problem

Once you’ve identified whether the stressor is likely biotic or abiotic, you can then begin to take a closer look at what the stressor may be.

Symptoms of stressors may be difficult or impossible to identify just by looking at the plant. Because biotic and abiotic stressors can look similar in some cases, appropriate diagnosis may require sending tissue to a lab.

Having an idea of what the stressor(s) may be reduces the need to send samples to multiple labs. For example, growers often work with university-based plant diagnostic labs or private labs, depending on their locations and services. Some facilities may be limited in the services they can provide, so samples may need to be sent to multiple locations for a complete evaluation.

Diagnosing issues in hemp takes a lot of practice. Understand that symptoms of stressors are still being investigated, and stressors in some plants may look different in hemp. Knowing what a healthy hemp plant looks like will make it easier to spot an unhealthy plant. Doing good detective work takes patience, observation and an understanding of the crop.

Marguerite Bolt is the hemp extension specialist at Purdue University’s Department of Agronomy. She received her M.S. in entomology from Purdue University and her B.S. in entomology from Michigan State University. Bolt’s research has focused on hemp-insect interactions and plant chemistry.