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Purple Blotch of Onion Symptoms and Treatment

What is purple blotch disease? Can I use purple onion instead of brown? How to control purple blotch in an onion? Let’s find out about purple blotch of onion symptoms and treatment guidelines!

Purple Blotch of Onion Disease Definition

A purple blotch of onion means – What is a purple blotch? Purple blotch is found worldwide, but it is most dangerous in places with warm, humid weather. It is a big concern for onion, garlic, and leek. Purple blotch is caused by two fungal pathogens that can infect the plant together or on their own.

purple blotch of onion symptoms and treatment - chemical control of purple blotch of onion

Purple Blotch of Onion Causes

What causes purple blotch of onion? What is the most common cause of purple blotch of onion?

Purple blotch is caused by two types of pathogens in the fungi imperfecti group. Either fungus or fungi can cause purple blotch lesions. Alternaria porri makes brown conidia a golden brown. The main spore body is ellipsoidal with 8–12 transverse septa and usually only a few longitudinal septa. Conidia have a long, tapering beak and are born one at a time. Overall conidial dimensions are 100–300 x 15–20 μm.

The other thing that causes purple blotches is Stemphylium vesicarium. This fungus makes conidia that are olive-brown to gold-brown, oblong to oval, and have up to four longitudinal septa and varying numbers of transverse septa. Conidia are 25–48 x 12–22 m long and have a length-to-width ratio of 1.5–3.0. Conidia are carried by single conidiophores, which have a distinctly swollen tip. In some places, Pleospora allii’s sexual stage pseudothecia and ascospores can be found with crop debris.

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Onion Purple Blotch Symptoms

Signs and symptoms of a purple blotch of onion – What are some symptoms of a purple blotch of onion?

The first signs are small, wet spots on the leaves, which slowly get bigger, turn yellow, and get a pale tan center. Older lesions turn brown or purple, have an oblong shape, and may be several centimeters long. In humid conditions, dark fungus spores grow in the center of the lesion. Finally, large, dark purple lesions form on the flower stalks.

When many spots appear on leaves, the leaves can die back. Purple blotch can only be confirmed with a microscopic look because the symptoms can be confused with those of Clado­sporium leaf blotch, white tip, and other leaf diseases.

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Purple Blotch of Onion Disease Cycle

Both pathogens need the same things to spread, and diseases are worse when it’s warm and humid or wet. Spores travel through the air and infect the leaves through the stomata or the epidermis. Inoculum comes from infected crop residue in the field.

The best temperature for A.porri is 18–25 C for germination and 15–25 C for infection. On the other hand, the pathogen can still be active at lower temperatures (4–13 C), allowing diseases to develop in autumn and winter. Conidia are made at night and released early in the morning as the humidity drops. The infection needs more than 8 hours of leaf wetness at 15–25 C.

When there is dew for 16 hours or more, A. porri conidia infect leaves and cause typical lesions. If there are less than 12 hours of dew, infections only cause small leaf flecks. The older leaves are more likely to get sick than the young ones, but damage from thrips may make the young leaves more likely to get sick. As symptoms show up in 1–4 days and new spores are made after only 5 days, epidemics spread quickly when conditions are right.

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Purple Blotch of Onion Treatment Guidelines

Treatment of purple blotch onion – What is the best treatment for a purple blotch onion?

Plant-resistant or tolerant varieties. For example, don’t use sweet Spanish onions because they are said to be very sensitive to A. porri. Instead, after harvesting a crop, plow under the crop residues to stop the spread and survival of the inoculum.

Rotate crops with those that aren’t hosts. Choose sites and water them so that the leaves dry faster. Fungicides should be used, especially as the crop canopy ages and becomes denser and when leaf wetness makes it easier for infections to spread.

I hope you understand about purple blotch of onion symptoms and treatment guidelines.

About Micel Ortega

Dr. Micel Ortega, MD, PhD, is a highly respected medical practitioner with over 15 years of experience in the field of internal medicine. As a practicing physician, Dr. Micel has built a reputation for providing compassionate and evidence-based care to his patients. He specializes in the diagnosis and management of chronic conditions, including diabetes, hypertension, and heart disease. In addition to his clinical work, Dr. Micel has published extensively in top-tier medical journals on the latest advancements in internal medicine and has played an instrumental role in the development of innovative treatment options.

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