Diagnosing Hemp and Cannabis Crop Diseases

Shouhua Wang

• Editorial: CAB International
• Año: 2021
• ISBN:
  9781789246094
• Categoría: Biowissenschaften
• Categoría: Botanik

Hemp and cannabis, both belonging to Cannabis sativa, have emerged as some of the most valuable crops because of their multiple functionalities - industrial, medicinal, and recreational uses. Like all other crops, they are at risk of diseases and pests. In certain cases, an entire hemp field can fail due to unexpected disease. As a new and highly regulated crop, research on Cannabis crop diseases is scarce, and the science of plant diagnostics is not well covered in the literature. Taking hemp/cannabis as a model crop, the book illustrates how to diagnose a disease problem and how to manage it effectively. It presents real disease cases encountered during crop production, and explains methods of diagnosis, both in the field and in the lab, in order to find out the cause(s). The book provides: ·A field and laboratory guide to diagnosing hemp and cannabis diseases and pest problems·Ready-to-adopt skills, methods and protocols in plant diagnosis, which can be applied to other crops·Over 300 colour photographs accompanied by a wealth of disease information, including field observations, unique symptoms, microscopic details, and molecular data. This book is essential for anyone who is interested in learning about Cannabis crop diseases, for crops grown in the field, and in indoor production facilities.

1 The Cannabis Plant

Cannabis, a genus name referring to both hemp and marijuana (or hereafter cannabis), has emerged as a global cash crop with diverse applications such as medicinal and recreational use, human consumption of seed and oil and the industrial use of fibre and other products. Cannabis plants have been cultivated and used for several thousand years (Schluttenhofer and Yuan, 2017; Brand and Zhao, 2017), but only recently has hemp and cannabis cultivation dramatically increased in the USA, Canada and other countries. Because Cannabis is considered a multi-functional crop, it has captured the attention of a broad range of scientists, including agriculturists, chemists, biomedical researchers and clinical scientists. At the time of writing, there are 22,424 research articles or books related to Cannabis sativa in the PubMed database (https://pubmed.ncbi.nlm.nih.gov/), compared with 35,704 for rice, 27,346 for wheat, 35,789 for corn, 27,346 for soybean and 10,341 for potato. A draft haploid genome sequence of 534 Mb and a transcriptome of 30,000 genes for C. sativa is also available (van Bakel et al., 2011), which provides a base for further research in Cannabis plant biology and pathology.

Classification

The nomenclature of Cannabis can be traced back to 1753 when Carl Linnaeus first described Cannabis sativa as a single species in the genus Cannabis in his book Species Plantarum (Linnaeus, 1753; Pollio, 2016). This book is considered the starting point for giving every plant species a binomial name comprised of two Latin words. For example, hemp is named Cannabis sativa, where Cannabis is the genus name and sativa is the species name. The binomial nomenclature has been widely used in naming living organisms such as plants, animals and microorganisms. Cannabis is a genus of flowering plants in the family Cannabaceae and it contains the most known species, Cannabis sativa (Table 1.1). In 1785, the French biologist Jean-Baptiste Lamarck proposed a new species C. indica based on plant samples he received from India (Erkelens and Hazekamp, 2014). When compared with the European C. sativa, the Cannabis plants from India appeared to have smaller and narrower leaves, as well as much firmer stem. In Lamarck’s view, C. indica was a psychoactive non-fibre producing species of Cannabis, different from the European C. sativa in terms of morphological characteristics and physiological effects. Thus, the genus of Cannabis temporarily contained C. sativa, the species mostly cultivated in the western continents, and C. indica, a wild species mainly growing in India (Erkelens and Hazekamp, 2014). However, this taxonomic treatment by Lamarck only remained intact for about 50 years. In 1838, Lindley rejected the two-species classification and restored C. sativa as the only species in the Cannabis genus (Lindley, 1838) and since then Cannabis had been considered a monospecific genus. In 1924, a new species, Cannabis ruderalis, was identified in wild areas of south-eastern Russia (Janischevsky, 1924) and 50 years later Schultes et al. (1974) reinstated the species C. indica. Thus, C. sativa, C. indica and C. ruderalis are commonly seen in literature. However, these proposed species may not have solid taxonomic foundations (Pollio, 2016). There are still debates and disagreements on the classification of many types of Cannabis plants. One study compared 157 Cannabis accessions of diverse geographical origins for allozyme variations at 17 gene loci and the results support a polytypic concept for Cannabis genus, which recognizes species of C. sativa, C. indica and C. ruderalis as well as seven other putative taxa (Hilig, 2005). Others consider that the genus Cannabis comprises only C. sativa L. with highly polymorphic subspecies sativa, indica and ruderalis.

Table 1.1. Classification from Kingdom Plantae down to species Cannabis sativa L. (adapted from USDA, 2020).

Kingdom Plantae – Plants

Subkingdom Tracheobionta – Vascular plants

Superdivision Spermatophyta – Seed plants

Division Magnoliophyta – Flowering plants

Class Magnoliopsida – Dicotyledons

Subclass Hamamelididae

Order Urticales

Family Cannabaceae – Hemp family

Genus Cannabis L. – hemp

Species Cannabis sativa L. – hemp or marijuana (cannabis)

Characteristics

Hemp versus marijuana

Botanically, both hemp and marijuana belong to C. sativa, but they differ by use and chemical compositions (Table 1.2). Marijuana or cannabis generally refers to a group of distinct cultivars or varieties within the C. sativa species that are cultivated and used as psychotropic drugs, for either medicinal or recreational purposes, while hemp refers to another set of cultivars or varieties cultivated mainly for fibre, seed or oil (Fig. 1.1). Industrial hemp is bred to maximize fibre, seed and oil with very low levels of THC (delta-9 tetrahydrocannabinol) while marijuana varieties are bred for high levels of THC. Hemp plants and their products can be used for food and beverages, nutritional supplements, personal care products, fabrics and textiles, paper, construction materials and other industrial goods. Because of the significant difference in their uses, hemp and marijuana are cultivated differently. The majority of hemp crops are planted in regular farmlands ranging from a few acres to a thousand acres, while marijuana plants are mostly cultivated in secured indoor facilities. Hemp and marijuana also have separate statutory definitions in US laws (Congressional Research Service, 2019).

Table 1.2. Differences between hemp and marijuana (adapted and modified from Congressional Research Service, 2019)

	Hemp	Marijuana
Scientific name	Cannabis sativa	Cannabis sativa
Statutory definition	The term ‘hemp’ means ‘the plant Cannabis sativa L. and any part of that plant, including the seeds thereof and all derivatives, extracts, cannabinoids, isomers, acids, salts, and salts of isomers, whether growing or not, with a delta-9 tetrahydrocannabinol [delta-9 THC] concentration of not more than 0.3 percent on a dry weight basis’. – Agricultural Marketing Act of 1946, Subtitle G, SEC. 297A	The term ‘marijuana’ means ‘all parts of the plant Cannabis sativa L., whether growing or not; the seeds thereof; the resin extracted from any part of such plant;and every compound, manufacture, salt, derivative, mixture, or preparation of such plant, its seeds or resin’. Such term does not include the mature stalks of such plant, fiber produced from such stalks, oil or cake made from the seeds of such plant, any other compound, manufacture, salt, derivative, mixture, or preparation of such mature stalks (except the resin extracted therefrom), fiber, oil, or cake, or the sterilized seed of such plant which is incapable of germination. – 21 U.S.C. § 802 (16), Title 21 – Food and Drugs
US Laws governing Cannabis sativa	Agricultural Marketing Act of 1946 (AMA, 7 U.S.C. 1621 et seq.) Federal Food, Drug, and Cosmetic Act (FFDCA; 21 U.S.C. §§ 301 et seq.)	Controlled Substances Act (CSA, 21 U.S.C. §§ 801 et seq.) Federal Food, Drug, and Cosmetic Act (FFDCA; 21 U.S.C. §§ 301 et seq.)
Primary Federal Regulatory Agencies	US Department of Agriculture (USDA), Food and Drug Administration (FDA)	US Drug Enforcement Administration (DEA), Food and Drug Administration (FDA)
THC threshold defined	No more than 0.3% delta-9 THC on a dry-weight basis	Not specified
Phytocannabinoids	More than 90 different cannabinoids have been reported in the literature (Andre et al., 2016)	More than 90 different cannabinoids have been reported in the literature (Andre et al., 2016)
Physiological effect	Not psychoactive	Psychoactive
Plant part harvested	Fibre, seed and flower	Flower
Major use of products	Regular or functional foods, cosmetics, fabrics and industrial products	Recreational and medicinal products
Common growing conditions	Farmlands and greenhouses	Secured indoor facilities
Size of mature plants	3.0–4.6 m for fibre crops, 1.8–2.7 m for seed crops and 1.2–2.4 m for flower or oil crops	1.2–2.4 m

Fig. 1.1. A seedling plant of hemp (Cannabis sativa).

Sex

Cannabis is a genus of annual, dioecious and flowering plants, some of which can attain heights up to 8 m (Edwards and Whittington, 1992). In most populations, there are more female plants than male plants. Plants undergo vegetative growth in early season, then turn to flower production when days are shortened. During the vegetative stag...