Control of Common Diseases and Pests in Garlic [Part 1]

The major diseases and pests affecting garlic production include leaf blight, rust, garlic maggots, and onion thrips. Below is an overview of the prevention and control measures for these common garlic pests and diseases:

1. Leaf blight disease

Symptoms: It primarily damages leaves and flower stalks. On leaves, infection typically begins at the leaf tip or other areas; initial lesions appear as small, white, circular spots that later expand into irregular or elliptical gray‑white to gray‑brown patches, bearing black, moldy growths. In severe cases, infected leaves wither and die. On flower stalks, disease often causes breakage at the affected site, with numerous tiny black dots scattered along the lesion. When the damage is extensive, garlic may fail to bolt.

Control methods: Strengthen field management by applying additional fertilizer, practicing appropriate planting density, and draining standing water to enhance the plants’ disease resistance. After harvest, burn infected plants and promptly remove damaged leaves and flower stalks. At the onset of the disease, spray with a 600‑fold dilution of 75% chlorothalonil wettable powder, or with a 1,500‑fold dilution of 50% procymidone wettable powder, a 500‑fold dilution of 64% mancozeb wettable powder, a 500‑fold dilution of 60% copper‑zinc ethylphosphonate wettable powder, or a 500‑fold dilution of 50% copper‑based ammonium phosphite wettable powder, repeating every 7–10 days for 3–4 consecutive applications.

2. Rust disease

Symptoms: It primarily infects leaves and pseudostems. At first, the lesions appear as fusiform chlorotic spots; later, beneath the epidermis, round or elliptical, slightly raised uredinia develop. Upon rupture of the epidermis, an orange‑yellow powdery mass—consisting of urediniospores—is released. A yellow halo surrounds the lesions, and in severe cases, the lesions coalesce, causing the entire leaf to turn yellow and wither, leading to premature plant death. In the late growth stage, on unruptured uredinia, black telia form without breaking through the epidermis.

Control methods: Practice rational crop rotation, avoid intercropping onions and garlic, and promptly clean the fields to reduce primary inoculum sources. Sow late at the appropriate time, apply fertilizer judiciously, minimize irrigation frequency, and eliminate flood‑irrigation practices. In years with heavy rainfall, conduct timely inspections in early spring to identify disease foci and apply fungicides for preventive control. At the onset of the disease, spray with one of the following formulations: a 15% triadimefon wettable powder diluted 1,500 times; a 20% triadimefon emulsifiable concentrate diluted 2,000 times; a 15% rust‑control WP diluted 300–500 times; a 25% difenoconazole emulsifiable concentrate diluted 3,000 times; or a 25% difenoconazole emulsifiable concentrate diluted 4,000 times combined with a 15% triadimefon wettable powder diluted 2,000 times. Alternatively, use a mixture of 70% mancozeb wettable powder diluted 1,000 times plus a 15% triadimefon wettable powder diluted 2,000 times. Repeat applications every 10–15 days, for a total of 1–2 consecutive treatments.

3. Gray Leaf Spot Disease

Symptoms: It primarily damages the leaves. Lesions are oblong, initially light brown and later turning grayish-white. Fine, gray‑black moldy structures—fungal fruiting bodies—develop on both sides of the lesions. In severe cases, the lesions coalesce, leading to localized leaf necrosis.

Control methods: After harvest, promptly remove and destroy diseased or dead plant parts by burning them on-site or burying them deeply. Strengthen cultural practices, apply formulated fertilizers to enhance plant resistance to disease, and ensure timely drainage following rainfall. At the onset of the disease, spray with a 600‑fold dilution of 75% chlorothalonil wettable powder, or with a 500‑fold dilution of 50% copper‑based wettable powder, a 1,000–1,500‑fold dilution of 50% prochloraz wettable powder, a 500‑fold dilution of 77% Kasugamycin wettable microgranules, or a 500‑fold dilution of 50% mancozeb + sulfur suspension. Repeat applications every 10 days, for a total of 1–2 sprays.

4. Coal Spot Disease

Symptoms: It primarily damages the leaves. At first, lesions appear as pale white small spots, which gradually expand into elliptical or spindle-shaped lesions with long axes parallel to the leaf veins; the centers turn yellowish‑brown, the margins are reddish‑brown, and a yellow halo surrounds them. The lesions rapidly extend toward both ends of the leaf, with the fastest progression toward the leaf tip, causing the tip to twist and eventually die. The lesion centers are dark olive, becoming velvety in humid conditions and powdery when dry. During severe outbreaks, a single leaf may bear several lesions, leading to the entire plant’s death. A thick, dark olive‑colored, velvety growth develops on these lesions, distinguishing this pathogen from the leaf‑blight–causing species of *Alternaria* that infects garlic.

Control methods: Sow at the appropriate time, plant at a reasonable density, apply sufficient base fertilizer, and topdress promptly. Increase the application of potassium fertilizer and humic‑acid fertilizers to promote robust plant growth and enhance disease resistance. Strengthen field management by promptly cleaning the fields, burning or deeply burying diseased plant residues, and using well‑decomposed organic fertilizers to prevent pathogen introduction through the fertilizer. For chemical control, at the onset of the disease, spray with a 600‑fold dilution of 75% chlorothalonil wettable powder or a 400–600‑fold dilution of 65% zineb wettable powder, repeating every 7–10 days for 2–3 consecutive applications.