Growing garlic from seed to harvest requires careful planning, as well as proper and ongoing production management practices to ensure a healthy crop. By paying attention to factors like soil health, watering, and pest and disease management, the risks associated with garlic production can be minimised.

Speaking at the South African Garlic Growers’ Association’s annual conference in Pretoria recently, Elsie Cruywagen, specialist researcher in mycology and plant pathology at the Agricultural Research Council Vegetable, Industrial and Medicinal Plant (ARC-VIMP) Institute, pointed out that garlic is susceptible to several diseases.

“Garlic is prone to mainly fungal and viral and to a lesser extent bacterial diseases,” she said.

She added that most of the major fungal diseases are soilborne.

Cruywagen said that although there aren’t any active research projects on at present, ad hoc samples are being received at the ARC-VIMP Diagnostic Centre at Roodeplaat.

“In the past we received samples for diagnosis of black mould, blue mould and Fusarium basal rot.”

Cruywagen pointed out that there are about 10 groups of bacterial diseases that affect garlic and over 35 fungal diseases can impair garlic production. Some of these are listed below.

Leaf diseases

• Leaf blight

Leaf blight is caused by Stemphylium vesicarium. The pathogen’s spores are dispersed by wind and are present throughout the garlic growing season, with spore counts increasing as the season progresses. Stemphylium leaf blight thrives in warm and humid climates.

Lesions generally develop during warm temperatures with adequate rainfall or irrigation until harvest. S. vesicarium is also found in onions and asparagus.

The first symptoms that will be observed can include small, water-soaked spots that will gradually enlarge, turn yellow and develop a pale tan colour in the centre. Older lesions will turn brown or purple, in an oblong shape. Dieback of foliage can occur.

• Purple leaf spot

Purple leaf spot or blotch is caused by Alternaria porri and causes purple spots on garlic plants. It is a concern throughout the growing season. Infection occurs when fungal spores from nearby plants are blown by wind into garlic fields where they infect wounded plants.

The pathogen overwinters in crop residue on or near the soil surface.

It is an important disease in warm, humid allium-growing regions around the world. This disease results in significant losses for farmers. Yield losses caused by their infection can reach 50% to 57%, even causing crop failure. Garlic, onions, and leeks can be affected.

• Downy mildew

Downy mildew is caused by Peronospora destructor and is a common fungal disease that affects garlic foliage, causing yellow or chlorotic patches on leaves. It also produces a purplish-grey fuzzy growth on the undersides of leaves.

Adequate spacing between plants, good airflow, and regular inspection for early signs of the disease are important for control.

• Rust – Puccinia porri (previously known as P. allii)

When it comes to rust, it is usually easy to diagnose and it often won’t kill a plant. The fungus that causes it, P. porri, shows up as raised, reddish-orange, oval-shaped or circular pustules. As the disease progresses, the pustules darken and the leaves might turn yellow.

Rust will start on leaves as small, irregular-shaped specks that are white to light tan in colour. As the disease advances, the lesions enlarge and typically turn into bright-orange pustules on both sides of the leaf. Severely infected leaves will turn chlorotic and then necrotic, with severe stunting of plants and bulbs. It is an airborne disease.

P. porri needs high humidity as well as high temperatures to reproduce. It can infect any allium, especially garlic.

Root and bulb diseases

• Basal rot

Basal rot is the most serious threat to commercial garlic production in the world. It is caused by various Fusarium species, such as F. oxysporum f. sp. cepae. It can cause yellowing foliage and dying leaves as well as rot below ground.

Fusarium basal rot is a soilborne fungal disease that affects the basal plate and roots of garlic bulbs. The pathogen lives in the soil and can survive for decades, even if a host isn’t present. It can also be spread by infected bulbs.

Early infection can cause roots to turn pink and leaves to turn yellow, with stunted growth. The tips of the leaves will turn yellow first, and then the disease will work its way down the rest of the leaves to their base.

Because the rot prevents water from moving from the roots to the foliage, the leaves will wilt and the parts that are furthest away will die first.

With advanced infection, the bulb will start decaying and the whole plant will die. What makes it worse is that it can continue to spread even after you harvest and store your garlic bulbs, which means they might look disease-free when you harvest them, but will eventually rot away.

• Black mould

Black mould is caused by the Aspergillus niger species complex, resulting in a dusty, black fungal growth on and between garlic and onion bulb scales. High temperatures favour development, with secondary bacterial infections contributing to soft rot.

The pathogens are seed-, soil- and airborne and can occur in dead plant foliage. Cruywagen said most losses occur in storage.

• Blue mould

Blue mould is caused by the Penicillium species, mainly P. hirsutum, and can result in death of planted cloves prior to emergence. Water-soaked, irregular-shaped lesions can be seen on infected cloves.

These can become tan to light brown, and later lead to extensive rot, with blue-green fungal growth and sporulation. The spores are soil- and airborne and originate mainly from infected bulbs.

• Pink root

Pink root, caused by Setophoma terrestris, is a fungal disease that affects garlic roots, causing them to turn pink or reddish-brown. Infected plants may exhibit stunted growth and the yellowing of leaves. The disease is soilborne but can also survive in crop residues.

Infected roots turn pink, after which it may turn into a more intense dark purple. Affected roots will be water-soaked and rotten. The plants will appear stunted, with symptoms similar to nutrient deficiencies and stress.

Outer bulb scales may become infected and develop pink to red blemishes or water-soaked rotting.

Maintaining proper soil pH and drainage, using disease-free seed cloves, and avoiding excessive nitrogen fertilisation can help manage pink root.

• White bulb rot

Sclerotium cepivorum, the fungus that causes the disease, can live in the soil for decades, even if there are no alliums planted there for it to infect. In the soil, it survives as sclerotia that will germinate and cause infection when suitable hosts are planted. The sclerotia can persist in soil for over 20 years, according to Cruywagen.

“White bulb rot is a devastating soilborne fungal disease. It will affect the roots and crowns of the garlic plant, after which the foliage will turn yellow, wilt, and eventually die.

“In advanced stages, roots and bulbs become soft and rotten due to secondary infections. Secondary infection occurs through mycelial growth from plant to plant.

“Proper crop rotation, maintaining well-drained soil, and avoiding excessive irrigation can help prevent white rot,” said Cruywagen.

Disease management

Plant diseases can be analysed using the disease triangle concept. In the figure to the left, three factors are identified that must interact to cause plant disease. These are:

• A susceptible host;
• A disease-causing
  organism (the pathogen); and
• A favourable environment for disease.
  The plant is the host. All plants have a range of susceptibilities to a range of diseases.

The pathogen is the disease-causing agent. Diseases of plants can be caused by fungi, bacteria, and viruses. Some pathogens are specific to only one or a few host plants, while others have broad abilities to attack almost everything.

Without the right host under the right conditions, pathogens cannot cause any harm. A favourable environment refers to optimum weather conditions needed for a pathogen to thrive.

The figure illustrates that a disease caused by a biotic agent requires the interaction of a susceptible host, a virulent pathogen, and an environment conducive to disease development. A plant disease can be prevented by eliminating any one of the three causal components.

To ensure a good crop, Cruywagen said the following aspects were important:

• Use disease-free and chemically treated seed or planting material. Although good-quality, disease-free seed is expensive, farmers should invest in it rather than trying to cut costs by buying inferior garlic seed.
• Plant tolerant varieties.
• Irrigate using drip instead of overhead systems.
• Plough in crop residues after harvest.
• Crop rotation with non-hosts in three-cycles is advisable.
• Avoid over-fertilising with nitrogen.
• Use registered fungicides.
• Avoid causing damage to garlic bulbs during harvesting and packaging.
• Keep relative humidity and temperature low during storage.
  Registered chemicals, good cultural practices, and field sanitation are the three mechanisms that should be used in an integrated control programme for all the pests and diseases that affect garlic, advised Cruywagen.

Email Elsie Cruywagen at [email protected].