Effective Strategies for Managing Garlic Leaf Pests and Diseases

Garlic, like many other crops, is susceptible to a range of pests and diseases that can affect its leaves, bulbs, and overall vigor. Here are some common pests and diseases that specifically target garlic leaves:

Diseases affecting the vegetative system in the field:

1- Downy Mildew (Peronospora destructor):

This fungus infects onions, causing a severe shortage of bulbs (onion wicks) and seeds (onion heads) due to budded and infected onions. The disease spreads in all areas of onion and garlic cultivation. However, it is severe in humid regions, where it causes significant losses. The severity of infection in some fields reaches 100%. Still, the average annual losses caused by the disease to the crop in both onions and garlic are about 1–10%, depending on the severity of infection in different years and regions, and sometimes losses increase in acute epidemic years.

The fungi that cause the disease live within the host's tissues (residues of the previous crop). Their hyphae grow between the rows of cells in the plant’s tissues and send their tiny pipettes into the cells to absorb food. From these soft tumors, thick bacterial vectors branch out to the surface of the host through the stomata.

Symptoms of Infection:

Symptoms appear on the leaf blades and flower spikes as pale yellow spots at first, with the appearance of growth covering the surface of the leaves in a whitish-gray color, followed by bluish paint on the surface of the older leaves. This bluish color appears more clearly when water drops (high humidity) are on the leaves. 

It helps the downy growth to occur; when dry, the downy growth does not appear. The color of the down can be seen in the early morning when there is dew or rain. If the disease intensifies, infection of the leaves leads to yellowing, wilting, and death, resulting in small-sized bulbs and garlic heads. 

Thus, the resulting crop decreases, and the plants' leaves become soft and vulnerable to storage diseases. This also leads to the formation of spongy bulbs and heads of garlic that violate export specifications, negatively affecting export operations. Here, it must be ensured that bulbs and heads taken for export are healthy and free from downy mildew.

Chemical Control:

Control begins as soon as the infection appears.

• Approved recommendations by the Ministry of Agriculture for 2017 include:
• Mancosib Acrobat 69% WP at a rate of 250 g/100 liters of water.
• Intracol 70% WG 200 g/100 liters of water.
• Amistar Top 32.5% SC 300 cm³/fed.
• Tazolin 72% WP at 250 g/100 liters of water.
• Delta cup 23.5% liquid at a rate of 100 liters/feddan (4200 m²).
• Ridomil Gold Plus 71.5% WP at a rate of 200 g/100 liters of water.
• DB shampoo is 57.6% WG at 180 g/100 liters of water.
• FolioGold (53.75%): 100 liters per feddan (4200 m²).
• Cupox 84% WP at a rate of 250 g/100 liters of water.
• Manzitob 70.56% WP at 225 g/100 liters of water.

2- Purple Blotch (Alternaria porri):

The disease spreads in all regions of the Republic and affects onion and garlic crops. It involves the vegetative system and leads to their drying out in the case of severe infection. This results in decreased quantity and quality of the bulbs or garlic heads and the drying of the flower spikes of onions, where the severity of the disease causes them to break, causing a significant decrease in the quantity of the crop.

Symptoms of Infection:

Infected leaves of onions and garlic show faint, hollow spots. These spots enlarge and become round or oval, with intersecting rings with a purple or blackish center. Infection of the flowering pedicels causes them to break, ultimately reducing seed yield. Eventually, the affected tissue dries up and turns crimson and then brown. Environmental conditions play a significant role in the disease’s severity.

Chemical Control:

Control should begin as soon as the disease appears.

• Approved recommendations by the Ministry of Agriculture for 2017 include:
• Mancosib Acrobat 69% WP at a rate of 250 g/100 liters of water.
• Intracol 70% WG 200 g/100 liters of water.
• Amistar Top 32.5% SC 300 cm³/fed.
• Tazolin 72% WP at 250 g/100 liters of water.
• Delta cup 23.5% liquid at a rate of 100 liters/feddan (4200 m²).
• Ridomil Gold Plus 71.5% WP at a rate of 200 g/100 liters of water.
• DB shampoo is 57.6% WG at 180 g/100 liters of water.
• FolioGold (53.75%): 100 liters per feddan (4200 m²).
• Cupox 84% WP at a rate of 250 g/100 liters of water.
• Manzitob 70.56% WP at 225 g/100 liters of water.

3. Rust on garlic leaves

Symptoms and Signs of Rust on garlic

Rust is a disease that primarily affects garlic, although onions, leeks, shallots, and wild garlic species are hosts. Symptoms begin as small spots (0.04 to 0.08 inches, or 1 to 2 mm), which expand into slightly larger (0.12 to 0.2 inches, or 3 to 5 mm), oval to diamond-shaped, red to pale orange pustules that develop on the leaf blades. Lesions typically form first on older leaves and then spread to younger leaves. Airborne red spores (urediniospores) are produced profusely within the lesions. Later in the growing season, lesions may appear dark as black survival spores (teliospores) develop within the pustules. Severely infected leaves may be covered entirely with pustules, turn yellow, and then dry and die. When the infection is severe, bulb size and quality are reduced.

Comments on the disease

Previously, rust was a sporadic disease that caused little or no economic damage. However, since 1998, rust has caused significant damage in some garlic-growing areas of California. In addition to garlic, onions and chives can be severely affected. However, the disease only harms onions when planted next to a severely infected garlic field. Shallots, leeks, and shallots are more resistant.

Weedy garlic plants can host the fungus and serve as a source of inoculum. The fungus can survive the winter on volunteer garlic and chive crops.

The pathogen is active during excellent weather conditions, between 57°F and 75°F (14°C and 24°C). Optimal conditions for infection occur at about 59°F (15°C) with 100% relative humidity for at least 4 hours.

The pathogen causing rust, Puccinia allii, is a group of several species and may be referred to elsewhere as P. porri, P. mixta, P. blasdalei, Uromyces ambiguus, and U. gurus.

Management of Rust on garlic

Rotate crops away from garlic for two to three years, separate garlic fields to prevent movement, and destroy volunteer garlic plants during this period. There are currently no completely resistant industrial garlic cultivars, but some early California cultivars show tolerance to the disease; plant seeds in well-drained soil.

If more than a few spores appear on plants, fungicide applications may be necessary. Repeated applications are needed to maintain effective control.

For best results, use the fungicide as a preventative treatment. Begin application when environmental conditions favor disease development (see comments on disease).