INTRODUCTION

The hemp plant, scientifically known as Cannabis sativa, possesses a wide array of uses such as fibers, seeds, oil, and cannabinoids. It is important to note that hemp differs from its closelyrelated counterpart, marijuana, as it contains negligible amounts of the psychoactive substance THC (tetrahydrocannabinol). This plant has a rich history of cultivation spanning millennia due to its ecological advantages and significant economic promise.

The management of diseases in hemp (Cannabis sativa) cultivation is of paramount importance for the promotion of robust growth and optimal yields. Numerous diseases have the potential to adversely impact hemp, thus necessitating the implementation of effective management protocols. Presented are essential strategies for the disease management in hemp cultivation.

Cultivation and Processing

The cultivation and processing of hemp can be traced back to Central Asia. Historical records indicate that hemp was cultivated for its fiber in China as early as 2800 BCE and was also practiced in the Mediterranean regions of Europe during the early Christian era. Its cultivation expanded across Europe during the Middle Ages and was eventually introduced to Chile in the 1500s and North America a century later.

Hemp is cultivated in temperate regions as an annual plant sown from seed and can attain a maximum height of 5 meters (16 feet). The most favorable growing conditions for the crop are found in sandy loam soils with efficient drainage, and necessitate an average monthly precipitation of at least 65 mm (2.5 inches) over the entire cultivation period. When hemp is grown for fiber production, it is densely planted and generates plants averaging 2–3 meters (6–10 feet) in height with minimal branching. On the other hand, plants cultivated for oilseed are spaced farther from each other, resulting in shorter plants with multiple branches. To achieve the highest yield and quality in fiber production, it is imperative to harvest the crop soon after the plants attain maturity, which is indicated by the full blossoms and the release of pollen by the male plants. While the plants are at times uprooted manually, they are more frequently severed about 2.5 cm (1 inch) above ground level. 

CAUSES

Hemp diseases can arise as a result of diverse pathogens, environmental factors, and agricultural methods. Profound comprehension of the etiology of hemp diseases is imperative for their efficient prevention and control. Below we outline the principal causes of hemp diseases:

Fungal Pathogens

Gray Mold (Botrytis cinerea): 

Etiology: High humidity, inadequate air circulation, and moist conditions. 

Manifestations: Bud rot, grayish-brown mold on flowers and stems.

Powdery Mildew (Golovinomyces cichoracearum, Leveillula taurica):

Etiology: Arid, warm conditions coupled with high humidity. 

Manifestations: White, powdery coating on leaves, stems, and buds.

Bacterial Pathogens

Bacterial Blight (Pseudomonas syringae):

Cause: Wet and humid environmental conditions, along with injuries to plant tissues.

Symptoms: Development of water-soaked lesions on leaves, followed by browning and necrosis.

Bacterial Wilt (Ralstonia solanacearum):

Cause: Elevated temperatures and excessive soil moisture.

Symptoms: Manifestation of wilting, followed by leaf browning and eventual death of the plant.

Viral Pathogens Hemp Mosaic Virus (HMV):

Cause: Transmission through infected seeds, direct plant-to-plant contact, and insect vectors. 

Symptoms: Appearance of mottled, yellowed, and distorted leaves, accompanied by stunted growth.

Environmental Conditions Humidity and Temperature:

Elevated humidity and inadequate ventilation facilitate proliferation of fungal pathogens.

Extreme temperature variations impose stress on plants, heightening their vulnerability to diseases. 

Cultural Practices Crop Density:

Excessive crowding diminishes air circulation, leading to heightened humidity and increased susceptibility to fungal infections. 

Sanitation:Poor hygiene practices, including the retention of infected plant remnants in fields, can serve as reservoirs for pathogens, facilitating the spread of diseases.

Insect Vectors

Aphids, Whiteflies, and Thrips:These insects can transmit viral diseases by feeding on infected plants and subsequently moving to uninfected ones.

TYPES OF DISEASES 

Fungal Diseases Gray Mold (Botrytis cinerea)

Symptoms: Grayish-brown mold on buds, stems, and leaves; bud rot.

Causes: High humidity, poor air circulation, wet conditions. 

Management: Improve air circulation, reduce humidity, remove infected plant material, and use fungicides if necessary.

Powdery Mildew (Golovinomyces cichoracearum, Leveillula taurica)

Symptoms: White, powdery coating on leaves, stems, and buds.

Causes: Warm, dry conditions with high humidity.

Management: Ensure good air circulation, reduce humidity, use sulfur or potassium bicarbonate-based fungicides, and remove affected plant parts.

Root Rot (Pythium spp., Rhizoctonia spp., Fusarium spp.)

Manifestations: Desiccation, chlorosis of foliage, and disintegration of roots.

Origins: Excessive irrigation, inadequate soil drainage, and elevated soil moisture levels.

Mitigation: Enhance soil drainage, refrain from overwatering, employ well-draining soil, and utilize biological agents such as Trichoderma spp.

Downy Mildew (Pseudoperonospora cubensis) 

Manifestations: Yellow lesions on upper leaf surfaces, and grayish-purple mildew on lower leaf surfaces.

Origins: Chilly, damp conditions accompanied by heightened humidity.

Mitigation: Decrease humidity, enhance air circulation, administer fungicides, and excise affected foliage. 

Bacterial Pathogens Bacterial Blight (Pseudomonas syringae)

Clinical Manifestations: The appearance of water-soaked lesions on leaves, followed by browning and necrosis.

Etiology: Favorable conditions such as moisture and humidity, as well as plant injuries.

Therapeutic Measures: Implementation of ground-level irrigation, enhancement of plant spacing to facilitate air circulation, elimination of infected plant matter, and application of copper-based bactericides.

Bacterial Wilt (Ralstonia solanacearum)

Clinical Manifestations: Manifestation of wilting and leaf browning, frequently culminating in plant mortality.

Etiology: Elevated temperatures and excessive soil moisture.

Therapeutic Measures: Utilization of disease-free planting material, avoidance of planting in contaminated soil, enhancement of drainage, and implementation of crop rotation.

Viral Disease

Hemp Mosaic Virus (HMV)

Manifestations: Faint patterns, discoloration, and abnormal shaping of leaves; inhibited development. 

Origins: Contaminated seeds, direct contact between plants, transmission by insects.

Control measures: Employ virus-free seeds, manage insect carriers (such as aphids and thrips), and eliminate and dispose of afflicted plants.

Nematode Disease 

Root-Knot Nematodes (Meloidogyne spp.) 

Manifestations: Protuberances or nodules on roots, dwarfed growth, chlorotic foliage. 

Origins: Infestation by root-knot nematodes in the soil.

Control measures: Implement crop rotation with non-host plants, utilize nematode-resistant cultivars, incorporate organic enhancements like neem cake, and consider soil solarization.

DISEASE MANAGEMENT

Cultural Practices 

Crop Rotation: Rotate crops to prevent planting hemp in the same field consecutively, thereby reducing soil-borne diseases.

Sanitation: Eliminate infected plant debris and weeds to prevent pathogen harborage.

Proper Spacing: Maintain adequate spacing between plants to enhance air circulation and lower humidity.

Environmental Control 

Humidity Management: Utilize dehumidifiers for indoor cultivation and ensure proper ventilation in greenhouses to keep humidity levels low. 

Water Management: Prevent overwatering and ensure adequate drainage to avoid root diseases.

Resistant Varieties

Use Resistant Cultivars: Choose hemp varieties that are resistant to specific prevalent diseases in your region.

Biological Control 

Beneficial Microorganisms: Apply biocontrol agents such as Trichoderma spp. and Bacillus subtilis to control pathogenic fungi.

Predatory Insects: Introduce beneficial insects that target pests responsible for disease transmission.

Chemical Control

Fungicides and Bactericides: Use appropriate chemical treatments sparingly and adhere to integrated pest management (IPM) guidelines to prevent resistance development.

Monitoring and Early Detection

Regular Inspections: Frequently inspect plants to identify early disease symptoms.

Diagnostic Testing: Utilize lab tests to accurately identify pathogens and customize management strategies.

Integrated Pest Management (IPM)

Combine Strategies: Implement a mix of cultural, biological, and chemical methods for comprehensive disease management. 

Conclusion

Effective disease management in hemp cultivation is crucial for ensuring healthy growth and maximizing yield. By adopting an integrated approach that includes selecting resistant varieties, maintaining proper soil health, and implementing regular monitoring and timely interventions, farmers can significantly reduce the impact of diseases. Additionally, staying informed about the latest research and advancements in hemp disease management will equip growers with the tools necessary to combat emerging threats. At Khethari Agri Tech Private Limited, we are dedicated to providing innovative solutions and expert guidance to help you achieve a thriving, disease-free hemp crop. Embrace these practices to safeguard your hemp plants and ensure a successful harvest season after season.

Education and Training: Keep up-to-date with new diseases and management practices through ongoing education and training.