Garlic Disease Identification and Integrated Control Techniques

In recent years, with the expansion of garlic planting areas, continuous changes in cropping systems and cultivation ecological environments, the disease resistance of garlic plants has also decreased year by year, and the losses caused by garlic diseases have become increasingly severe. In particular, the continuous emergence of new diseases, with some diseases even showing a sharp upward trend, has further intensified the harm to garlic. According to investigation and analysis, the damage caused by garlic diseases generally results in a loss rate of 20%-30%, and in severe years and plots, it can reach over 50%, or even higher. For example, in a 1993 survey, garlic rust disease in Zhongmu County reached a diseased plant rate of up to 80% in severely affected garlic fields, with the affected area exceeding 65% of the planted area. The affected garlic was severely downgraded, affecting export earnings and greatly dampening the enthusiasm of garlic farmers. At present, more than 20 known garlic crop diseases exist, caused by fungi, bacteria, viruses and nematodes respectively. In addition, physiological diseases are also attracting increasing attention. The occurrence and damage of diseases has become a major obstacle in garlic production. The main garlic diseases and their integrated control techniques are introduced as follows.

I. Main Garlic Diseases and Their Occurrence Patterns

1. Garlic virus disease. Garlic virus disease occurs to varying degrees every year in all major garlic-producing areas. This disease can cause garlic bulbs to become smaller, with yield reductions between 20%-50%. It is currently the most important disease causing hidden yield losses in production.

When virus-carrying garlic cloves are planted, the disease can occur after emergence. It generally occurs more severely under conditions of high temperature and drought, large aphid populations, or in plots with dry soil, lack of fertilizer, and overgrown weeds. Fields that are continuously cropped or adjacent to other crops of the genus Allium or the family Liliaceae are also prone to the disease.

2. Garlic rust disease. Garlic rust disease is a major epidemic disease in garlic areas of China, occurring every year. The annual disease rate is over 10%, and in severe years it can reach over 80%. Generally, yield is reduced by about 10%, and in severely affected plots by over 30%. This disease mainly damages leaves, and secondly damages the false stem; diseased plants have noticeably smaller bulbs that are prone to splitting and loose cloves, and their commercial value is significantly reduced. According to surveys in Zhongmu County, from 1993 to 1995, this disease caused leaves to wither and die 15-20 days early.

The garlic rust pathogen prefers warm, cool and highly humid conditions. When spring temperatures rise quickly, with frequent rain, especially abundant rainfall from March to May, and with gentle breezes, it favors the spread and infection of the pathogen. Disease is severe in plots with high groundwater levels, heavy clay soil, and low-lying areas.

3. Garlic leaf blight. Garlic leaf blight is one of the most serious diseases of garlic in garlic areas. It is widely distributed and causes severe damage in Henan garlic areas; in normal years, losses exceed 10%, and in severe cases can reach about 60%. This disease mainly damages leaves and garlic scapes. In severe cases, diseased leaves wither and dry up, and scapes cannot emerge, or break off at the diseased part when emerging, with a yellowish-white color.

The pathogen has a wide temperature range for development and requires high humidity. The timing of disease onset depends on temperature, and the severity depends on humidity. There are 2 peak periods of disease occurrence: November-December before the year ends, and April-May in spring. Continuous cropping fields, especially those where diseased plant residues are piled in the field or at the field edges, experience severe occurrence.

4. Garlic leaf spot diseases. Garlic leaf spot diseases include purple blotch, black spot, blight, gray leaf spot, and sooty blotch, etc., distributed in all major garlic production areas. They occur severely in years with abundant rainfall from March to May. The annual disease rate is about 5%, and in severe years the diseased plant rate can reach about 15%. These diseases mainly damage leaves, causing leaf death, and in severe cases cause the entire garlic plant to die.

The pathogens of these diseases have a wide temperature adaptation range and require relatively high air humidity. If a late spring cold spell occurs in spring, combined with abundant rainfall in April, the disease occurs severely. Along the Yellow River basin, from November to December, weather with frequent rain, heavy fog, and little sunlight often occurs, which can also produce peak periods of disease occurrence. Fields with high planting density, lack of soil organic matter, and excessive nitrogen fertilizer application often experience severe occurrence; generally, late-maturing varieties and purple-skin garlic are less affected.

5. Garlic sclerotinia rot and white rot. Both diseases occur in Henan garlic areas. The former occurs intermittently, while the latter occurs year-round. Both diseases have low annual incidence rates, but in severe years individual plots can have incidence rates over 30%. Generally, yield is reduced by 30%-50%, and in severe cases, total crop failure occurs. Garlic sclerotinia rot damages the false stem, causing yellowish-brown rot, lodging, or slow aboveground growth, with plants withering and dying prematurely. Garlic white rot first affects the tips of outer leaves, showing yellowish-brown longitudinal streaks, with plants growing slowly and becoming chlorotic, then spreading to the false stem, becoming soft and rotted under high humidity. Underground bulbs are small, and harvested bulbs lack wrapper leaves and lose commercial value.

The occurrence of both diseases is mainly related to the climate in April-May. During this period, with frequent rain, little sunlight, and temperatures of 20-25°C, the disease occurs severely, even causing disaster. Disease is severe in plots with heavy soil texture, low-lying fields, or fields at forest edges or under forest canopy. Continuous cropping, waterlogging, high density, or excessive nitrogen fertilizer application lead to severe occurrence; fields with relatively complete plastic film mulching have milder occurrence.

6. Other relatively important diseases. Garlic bacterial soft rot, blue mold, etc. are also important diseases that cannot be ignored. Garlic bacterial soft rot generally occurs sporadically in small patches in normal years, but has shown an increasing trend in recent years. Blue mold is a disease that very easily occurs after harvest and during storage. If rain occurs at harvest, if drying after harvest is not timely, or if storage conditions are humid, the disease occurs severely with a high infection rate.

II. Integrated Control Techniques for Garlic Diseases

(I) Before and After Sowing

Control strategy: Based on agricultural control, with the aim of cultivating strong seedlings, and optimizing suitable matching varieties.

1. Agricultural control

(1) Before sowing, optimize and carefully select garlic seeds, using virus-free garlic, disease-resistant garlic, and pest-free garlic as much as possible; and select large-clove garlic. Obtaining seeds from other locations is a good method, for example, Zhongmu County annually brings in large quantities of Cangshan garlic from Shandong.

(2) Choose sandy loam soil with a dry and flat terrain, convenient drainage and irrigation, rich soil organic matter, and strong fertilizer and water retention capacity for planting garlic. Avoid continuous cropping or successive planting with Allium crops.

(3) Carefully level the land, water sufficiently to establish soil moisture, and plow and harrow in a timely manner to create a soil environment favorable for root growth and development, in order to improve disease resistance.

(4) Sow at the appropriate time to minimize the occurrence of frost damage. For early-maturing bolting garlic, strengthen seedling vigor before winter to reduce the degree of frost damage.

(5) Apply fertilizer scientifically and reasonably, apply sufficient base fertilizer, and organic fertilizer must be fully decomposed. Increasing the application of phosphorus and potassium fertilizers and cake fertilizer can promote healthy seedling growth and improve the disease resistance of green garlic before the year ends. Use formula fertilization or garlic-specific fertilizer as much as possible to reduce the occurrence of physiological diseases.

(6) Adopt plastic film mulching cultivation, which can effectively reduce opportunities for pathogens on diseased residues in soil to spread through rain, irrigation water, and furrow irrigation, increase soil temperature, preserve moisture, reduce the number of irrigations, and play a role in preventing disease occurrence.

(7) The pathogens of garlic leaf blight, leaf spot diseases, white rot, sclerotinia rot and other diseases all overwinter and oversummer in soil with diseased plant residues. Diseased plant residues should be thoroughly cleaned up, and a rotation of more than 3 years should be implemented.

2. Chemical control

For leaf blight, leaf spot diseases, gray mold, white rot, sclerotinia rot, etc., 50% thiophanate-methyl wettable powder or 50% carbendazim wettable powder or 70% mancozeb 0.5-1kg can be mixed with 5-7.5kg water, evenly sprayed on garlic seeds, then dried before sowing, which can achieve good control effects.

(II) During the Growth Period

This period spans a long time, from seedling emergence after sowing to harvest in late May. It is the period when leaf blight, leaf spot diseases, gray mold, rust, blight and other diseases occur. The following measures are mainly taken.

1. Agricultural control

Strictly prohibit flooding irrigation, drain water promptly after rain, and clean up diseased plant residues in conjunction with field management, carrying them out of the field. During the garlic mother bulb shrinkage stage, scape elongation stage and bulbil enlargement stage, topdressing should be carried out. Each time, apply 5-10kg urea per 667 sq m, or 10kg ammonium nitrate, and apply 10kg ammonium sulfate as "bulb-promoting fertilizer"; potassium dihydrogen phosphate can also be sprayed to improve disease resistance. For garlic white rot and sclerotinia rot, plant ash or gypsum powder can be spread at the stem base to reduce opportunities for pathogen invasion. Harvest scapes promptly, avoiding excessive wounding as much as possible.

2. Chemical control

(1) For controlling garlic rust, in years with abundant rainfall, check for disease centers promptly in early spring and carry out timely prevention. Generally, use 15% triadimefon wettable powder at 1000 times dilution, or 15% triadimefon wettable powder at 1500 times dilution plus 70% mancozeb wettable powder at 1000 times dilution for the first control in early April, and a second control after 10-15 days. Generally, 2-3 applications are made depending on disease severity.

(2) For controlling garlic leaf blight, leaf spot diseases, gray mold, etc., use 75% chlorothalonil wettable powder at 800 times dilution, or 64% oxadixyl-mancozeb wettable powder at 500 times dilution, or 50% copper bis(dodecylbenzenesulfonate) wettable powder at 500 times dilution, or 50% iprodione wettable powder at 1500 times dilution. Depending on field disease conditions, begin control in late November or late March, with 2 applications in spring.

(3) For controlling virus diseases, using virus-free garlic gives the best control effect. Field plants are strong with dark leaf color, and generally no spraying is needed. Secondly, in the early stage of disease, spray 1.5% plant virus inhibitor at 1000 times dilution, or 20% virus A at 500 times dilution, or 5% junduqing at 300 times dilution.

(4) For controlling bacterial soft rot, spray 72% agricultural streptomycin soluble powder at 4000 times dilution, or 77% copper hydroxide wettable micro-particles at 500-800 times dilution in the early stage of disease on leaves, once every about 7 days, with 2-3 consecutive applications depending on disease conditions.

(III) After Harvest and During Storage

This mainly involves controlling storage-period diseases such as blue mold, gray mold, and purple blotch. The following measures are currently mainly taken:

(1) Stop irrigation 5 days before harvest to prevent garlic skin from rotting and being unsuitable for storage.

(2) Avoid mechanical damage as much as possible during harvest, promptly clean up diseased plant residues, carry them out of the field, and conduct high-temperature composting and retting centrally, which helps reduce disease sources and lower opportunities for disease occurrence.

(3) Harvested garlic should be dried promptly, and the outer skin removed according to commercial garlic requirements, with rotten garlic and green garlic sorted out.

(4) Radiation treatment and cold storage preservation can effectively prevent further damage to garlic bulbs from late-stage and storage diseases.