This article teaches you how to treat the leaf blotch disease of onion. You can also learn more about the causes and symptoms of the disease. This management will be easy for you to use as it results from tested research.

This disease is a significant economic threat to onion growers worldwide. Studies have estimated that this disease alone causes approximately $500 million in losses. It destroys the crop and makes onions unmarketable due to unsightly brown spots on the leaves and stems of the affected plants.

What Causes Leaf Blotch of Onion?

This disease is caused by a fungus called Alternaria porri belonging to the Pleosporaceae family. The disease survives in the soil over winter in addition to infected crop debris; Wet weather causes its spores to start their life cycle anew. Spores can also spread to healthy plants and soil through wind, irrigation water, or rain showers. 80-90% relative humidity and 20-30°C temperature are favorable conditions for this disease. 

What Are The Symptoms?

You can easily identify this disease by seeing two main symptoms.

• • When this disease attacks, long spots are seen along the veins of the leaves.
  • A dark brown coating of the fungus is seen on the affected leaves.

Affected Onion Leaf.

How To Treat Leaf Blotch Disease of Onion? Its an Integrated Suppression Management System

1. 1. It would be best if you kept the land clean. 
   2. Cultivate a crop other than onion/garlic several times in the ground and then cultivate garlic/onion again. That is, you should do farming according to the crop stage. Do not cultivate onion/garlic again and again in the same land.
   3. It’s best to avoid planting onions close to each other.
   4. Infected leaves and seeds should be pruned and destroyed.
   5. Balanced fertilizer application and inter-cares should be done.
   6. It would help to irrigate onion land regularly every 12-15 days according to soil requirements.
   7. Fungicides of propiconazole group (eg: Tilt) at 0.5 ml per liter of water. Mix and spray twice every 12 days.
   8. If humid and warm weather prevails, you should mix Mencozeb-64% +Metalaxyl-8% fungicide (Ridomin Gold MZ-68WP) with 2 grams per liter of water and spray every seven days.

Conclusion: 

Integrated Suppression Management is the most effective way to prevent and treat onion leaf blotch. If you have any questions about this disease, you can contact us. We will continue to bring articles about symptoms, causes, and prevention of more than nine important onion diseases. 

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Rebecca Vittetoe
I’m Rebecca Vittetoe, a field agronomist working with farmers through Iowa State University Extension.

Most of my time is not spent in an office—it’s spent in the field. I work directly with farmers, crop scouts, and ag professionals to solve real problems they face every season. From pest pressure to nutrient issues, I focus on what is actually happening in the field—not just what is written in books.

Over the years, I’ve learned that good farming decisions come from a mix of research and real-world experience. That’s what I try to bring into everything I do.

At toagriculture.com, I share simple, practical insights from the field:

What I see in crops during the season
Common mistakes farmers make
What works—and what doesn’t

My focus areas include crop management, pest management, soil health, and cover crops. I’m especially interested in helping farmers improve productivity while keeping their farming systems sustainable.

Agriculture is always changing. My goal is to make that change easier to understand—and easier to apply in the field.