How to Identify and Treat Common Onion Pests and Diseases

If you plan on growing onions, you will be dealing with onion pests and diseases at some point. You may get lucky and have some very successful years, but they are likely to be followed by a crop of fuzzy, slimy, or buggy bulbs.

Read on to discover some of the most common insect, scorching and fungal issues that can damage your onions (and other plants in the Allium fam). And of course, we’ll show you how to deal with them when they do appear.

Common Onion Pests (and Repellent)

Although alliums repel many insects and pathogens, some stubborn species are very fond of them. Here are some setbacks you may encounter, and how to deal with them.

Spider mites

Oh these people. You know, generally, alliums are planted to repel spider mites. But there is one species that really likes them. The two-spotted spider mite — also known as the red mite — is ridiculously fond of onions. To drive away the onion, apply cilantro around it and treat the infected plants with neem oil.

bulb mites

These mites are less than 1 mm long and cause considerable damage to the bulbs during the growing process. Once they do appear there is no way to treat them, you can try to prevent them.

Rotate your crops regularly, so you don’t plant onions in the same spot for two years in a row. Aim for a rotation gap of 5-6 years. Also avoid planting large clusters of onions together. Instead use intercropping and companion planting methods throughout your garden.

stag beetle

Are your onion leaves transparent and silvery? If they are, there’s a good chance you’re dealing with thrips. You can treat them with various insecticides, but once again, prevention is better than cure. Avoid planting your onions anywhere near grain, as grain plants attract and encourage insects.

Onion Flies / Maggots

Did you know that onions can carry insects? I didn’t until I found some of my beloved bulbs and ended up swearing in several different languages. These flies lay their eggs on the basis of onions. When the eggs hatch, the maggots burrow into the bulbs and eat their way through them as they mature.

The only way to prevent this from happening is to cover your young onion plants with agricultural wool. If you dig up any maggot onions, burn them to prevent the cycle from happening again.

leaf miners

These donkeys eat any plant they land on. They leave behind yellow, winding trails: proof of where they have chewed their way onto your plants. Remove and burn infested leaves as soon as they are found. Spray nearby plants with neem oil, and use floating row covers to prevent any further eggs from being laid on your plants.

Oh, and till your soil in autumn to snuggle up in the earth to completely take out the pupae.

Onion diseases you may encounter, and how to deal with them

All plants are prone to various health problems. Then again, which is not a life form? Onions are quite hardy but can also suffer from many different diseases and pathogens. Below are the most common ones you may encounter and how to combat and fix them.

damping-off disease

This can be a problem when you start your seedlings. This is common when onions are planted outside too early and exposed to cold, moist conditions. You can’t fix it once it’s set, but you can try to prevent it from happening.

Start your seeds in a soilless starting medium, such as perlite or vermiculite, and coconut coir. If you start them in soil, make sure it is a well-aerated potting mix. Avoid any soil that can be compacted easily, and hydrate the seeds only with warm water.

Additionally, you can try treating the seeds with a fungicide before planting. You can either look for commercial fungicides that contain metalaxyl or mefexonum, or dissolve your seeds in some diluted apple cider vinegar before planting.

onion white rot

As far as pests and diseases of onions are concerned, sclerotium sepivorum One of the most insidious. This fungal disease can live in your soil for 20 years! It is devastating to all allium crops and is responsible for crop failures worldwide. You can recognize it by discolored, yellow leaves along with bloated white fungal growth around the base of the bulb and stalk.

There is no real cure for this issue. Your only option is to destroy the crop, dig up the soil and treat it with hardcore fungicide and solarization, and avoid planting any alliums there for at least two decades. When you decide to try planting onions again, try to find rot-resistant varieties and soak the seeds in warm water before planting.

Fusarium Basal Plate Rot

This fungal disease manifests as yellow, twisted, necrotic leaves and brown, mottled bulbs. The fungus thrives in warm, moist conditions and there is no cure.

Prevention is best, once again. Be sure to plant in soil with good drainage and rotate the area with non-allium crops for at least 5 years between onion plantings. You can also try planting fusarium resistant varieties if you can find them.

Black Size

Although black mold can develop on onions during the maturation process, it appears more often when onions are stored. The symptoms live up to its name: You’ll see dark spots around the neck of the onion, and it will spread to the black/brown, watery flesh in the center of the bulb. Avoid placing your bulbs in moist, hot conditions to prevent this stuff from showing up.

downy mildew

You’ve undoubtedly seen it on many species before. It looks like a fuzzy coating on one’s tongue after a week-long bender.

Powdery mildew is a fungal problem caused by a combination of wet foliage and a celebration of cool temperatures. It also likes to hide in the soil for a long time. You can avoid this problem by treating both the soil and the plants with organic fungicides and watering at soil level rather than overhead. Oh, and rotate crops regularly (noticing a trend?)

purple spot

This fungal problem is fairly self-explanatory, as it causes purple spots to appear on your onion stems. It is common in areas where there is a lot of humidity and high rainfall.

There is no cure, so if you live in a hot, wet climate, try planting your onions in well-draining soil. Make sure there is enough space between plants, and eliminate any dead or decaying leaf matter.


If your cheeks or green onions have ever looked twisted and twisted, they probably have smut. This fungal disease can also cause powdery-looking black “bleh”-filled sores. There’s no cure: just destroy the onions and disinfect the soil.

The pathogen can live in that area for years, so never re-plant the onion anywhere near it.

yellow onion dwarf

Are your onions small with yellow leaves? So you are probably dealing with yellow onion dwarf disease. Like many other onion pests and diseases listed here, there is no “cure” for it. Try to find disease-resistant seeds whenever possible, and spread your onions throughout your garden.


Do your onion leaves look like they rusted up like that truck chassis in your neighbor’s yard? Yes, that rust is rot. It thrives in hot, humid conditions where there is not much rainfall.

There’s no way to treat it once it appears, so it’s best to try and avoid it. Be sure to plant your onions in well-draining soil, with plenty of space between them. Rust cannot survive on dead plants, so remove fallen leaves, tall grass and any other plant debris from your onions, leeks and garlic.

pink root

While it sounds like a festive, even fun thing to discover, it isn’t. With a pink root, onion roots are actually pink, and the bulbs are shriveled and undersized. This is what happens when you don’t rotate your crops! Allow the area to be fallow every few years, and be sure to solarize and then refill the soil. Aim for a gap of at least 5 years between planting onions.

botrytis rot

It appears only once when the onion has been harvested and has been in storage for some time. It begins at the neck or bottom of the onion and creates a brown, translucent discoloration throughout the bulb.

The way to prevent this is to make sure you cut off all the roots and leaves of the onion as you harvest it. Then dry them for a few days before putting them in cool, dry storage.

Other Potential Issues When Growing Onions

In addition to the above onion pests and diseases, you may have to deal with a few other allium issues. For example, bolting is a pain in the back but may be accompanied by sudden heat waves.

There’s really no way to prevent onions from bolting. If it’s gonna bolt, it’s gonna bolt. The only thing you can do at that point is to quickly cut off the flower stem. You will still be able to eat the bulb, but it will taste a little different. You won’t be able to store it for long, as it will decompose quickly.

Try to cut off bolted bulbs and cook with them as soon as possible. If too many of them have bolted and you can only eat a lot of them at once, cut up the portions and freeze, or make onion syrup out of them.

Onions grown in groups share diseases. To prevent this from happening, “socially distance” your onions. Do your research on companion planting to learn what other crops will benefit from nearby alliums. Plant them as perimeter plants to protect against deer and rabbits. Use them to create fruit tree guilds, and plant them around your roses to protect against pests.

Most importantly, enjoy your harvest! There are so many species of onions to enjoy, so diversify your garden and enjoy every delicious bite.

Was this article helpful?

yes no


We appreciate your helpful feedback!

Your answer will be used to improve our content. The more feedback you give us, the better our pages can be.

Follow us on Social Media:

Facebook Pinterest

Idea Source:

Leave a Reply

Your email address will not be published. Required fields are marked *