The Pareto principle tells us that 80% of consequences are the result of 20% of the potential causes. But what about those 1-in-5 oddballs that can be encountered in cannabis production?
While infrequent, these lesser-encountered problems can negatively affect growth when they do occur. Therefore, simply knowing what they are can help you identify them and possibly avoid them in the future.
Water stress symptomology initially develops along the leaf tips before damage expands inward toward the leaf midrib and base (Fig. 1). Initially, the leaf discoloration is a light brown. Knowing the irrigation history (frequency and duration) will help diagnose the problem. High levels of fertility (electrical conductivity (EC)) can also result in similar symptoms. Checking the irrigation water and substrate EC levels will provide the needed data to confirm the symptomology’s cause.
Sunscald of cuttings can occur when the young plants are placed in propagation (Fig. 2). During periods of intensive solar radiation or horticultural lighting, a light shade will help ease the transition and avoid the leaf burn. Shading should be provided to acclimate young plant material to environmental changes that occur from cutting stick to toning. With that said, heavy shading may result in extension growth and weak-rooted cuttings. Therefore, managing and manipulating the propagation environment to elicit favorable plant responses is required.
Epinasty of cuttings occurs when the rooting hormone IBA is applied at too high of a rate to cuttings (clones; Fig. 3). Cannabis is very sensitive to foliar applications of IBA, and the new growth will twist when the rates are too high. Fewer problems occur when the IBA is applied to the plant base.
Cannabis is extremely reactive to changes in photoperiod and light quality. Photoperiod sensitivity can occur on the shady side of a large mother stock plant (Fig. 4), even during the longest day of the year. Ample horticultural photoperiodic lighting used to provide long days will help avoid the problem.
Changes in leaf coloration can be caused by viruses, nutritional disorders, and mutations. Figure 5 highlights a yellow and green leaf coloration mutation of cannabis. Yellow leaves do not produce chlorophyll for plant growth, so usually these mutations are not an advantage to a plant, unless there is ornamental value.
Dodder is a parasitic weed that was listed in Ernest Small’s “Cannabis: A Complete Guide” (Fig. 6). The orange to yellow weed intertwines with the stems and leaves and looks like orange spaghetti. For greenhouse production, the introduction of the problem typically occurs when the substrate contains seeds, so discarding the infected plant is the best option. Dodder-infected plants should not be transplanted into production fields.
Soil nematodes are microscopic, worm-like organisms (roundworms). Some nematodes are beneficial in the soil ecosystem, while other types are parasitic cannabis feeders and, in some ecosystems, over one million individuals can inhabit a square meter in contaminated fields. The roots form large bulbs when infected, which is the easiest method to identify the problem (Fig. 7). Soil nematodes have been reported as a disorder of cannabis.
Termites are not a common pest of agronomic crops (Fig. 8), yet they were reported in “Cannabis: A Complete Guide,” as having occurred in Africa. They were also found in cannabis plants in North Carolina. The feeding resulted in a girdling of the stem, leaving the upper plant to wilt and die.
Through observations and investigation, we have found several unique and infrequent disorders of cannabis. While these are uncommon problems, we were able to observe them in commercial operations. Nearly every pest seems to like cannabis. It is unlikely you, too, will experience them—but it is advantageous to know what some of these problems are.
Dr. Brian E. Whipker, Ph.D, is a professor of floriculture at North Carolina State University specializing in plant nutrition, plant growth regulators and diagnostics.
Patrick Veazie is an undergraduate researcher in the Department of Horticultural Science at North Carolina State University.
David Logan is an undergrad research assistant in the Department of Horticultural Science at North Carolina State University.
Paul Cockson is a Ph.D. student at the University of Kentucky’s Horticulture department. He is a part of the controlled environment horticulture (CEH) lab and is conducting research on plant nutrition and abiotic stress impacts on greenhouse vegetable quality and fruit development.
Dr. W. Garrett Owen is an assistant professor and extension specialist of floriculture, greenhouse, and controlled-environment crop production in the Department of Horticulture at the University of Kentucky.