By: Amy Grant
Have you ever seen purple blotches on your onions? This is actually a disease called ‘purple blotch.’ What is onion purple blotch? Is it a disease, pest infestation, or an environmental causal? The following article discusses purple blotch on onions, including what causes it and how to manage it.
Purple blotch in onion is caused by the fungus Alternaria porri. A fairly common disease of onions, it first manifests as small, water-soaked lesions that rapidly develop white centers. As the lesions progress, they turn from brown to purple with a halo of yellow. Often the lesions merge and girdle the leaf, resulting in tip dieback. Less commonly, the bulb becomes infected through the neck or from wounds.
Fungal growth of spores of A. porri is fostered by temperatures of 43-93 F. (6-34 C.) with the most optimal temperature of 77 F. (25 C.). Cycles of high and low relative humidity encourage spore growth, which can form after 15 hours of relative humidity greater than or equal to 90%. These spores are then spread by wind, rain, and/or irrigation.
Both young and mature leaves affected by thrip feeding are more susceptible to purple blotch in onions.
Onions with purple blotch present symptoms 1-4 days after infection. Onions infected with purple blotch become defoliated prematurely which compromises bulb quality, and lead to storage rot caused by secondary bacterial pathogens.
When possible, use pathogen free seeds/sets. Ensure that plants are properly spaced and keep the area around the onions weed free to increase circulation, which will allow the plants to dry from dew or irrigation more rapidly. Avoid fertilizing with food that is high in nitrogen. Control onion thrips, whose feeding makes the plants more susceptible to infection.
Purple blotch can overwinter as mycelium (fungal threads) in onion debris, so it is important to remove any debris prior to planting in successive years. Also, remove any volunteer onions which may be infected. Rotate your onion crops for at least three years.
Harvest onions when conditions are dry to avoid neck injury, which may act as a vector for infection. Let the onions cure before removing the leaves. Store the onions at 34-38 F. (1-3 C.) with a humidity of 65-70% in a well aerated, cool, dry area.
If need be, apply a fungicide according to the manufacturer’s instructions. Your local extension office may be of assistance steering you to the correct fungicide for use controlling purple blotch in onion crops.
This article was last updated on
Don't forget to practice something that never goes out of fashion вЂ” crop rotation. That's right, be up with the best, rotate your onions with other vegetable crops to prevent a buildup of disease and insect pests.
A good suggestion is onions, then carrots the next season, then potatoes, then onions again. These vegetables all have totally different pests and diseases so can't cross infect each other.
Lesser bulb fly: These prefer narcissus bulbs but occasionally can be an onion pest.
They have a similar lifestyle pattern as the onion fly, but are slightly smaller at about 5mm.
With onion fly and lesser bulb fly, say goodbye to infected onions, but future plants can be protected a couple of ways:
1. Pull up and destroy all plants infected with these onion pests at the time you know they are in the bulbs and before they have burrowed back into the soil. The fact that these onion pests work in patches or bulb by bulb down a row, makes it easier to spot plants in trouble and get them out.
Scattering onion planting throughout the garden helps, and removing the soil is also a good idea.
2. Press down the soil around seedlings well, cover with paper, cloth or other insect-proof material to stop the onion fly from laying its eggs into the soil.
3. Spreading a fine layer of sand or wood ash around the onion plants is also a good deterrent to adult onion flies from laying their eggs at the base of the plants.
So far impossible to eradicate, this disease is caused by the fungus Sclerotium cepivorum, which produces sclerotia — tiny dot sized black fungal bodies that can infect all members of the Allium or Onion family.
Sclerotia can hang around, seemingly dormant, in soil for up to 15 years, so it's important to take serious steps, such as:
вЂў If possible check your source if you are planting onion sets or seedlings, and if you are in any doubt, grow onions from seeds only.
вЂў If you've identified onion white rot, remove the infected plants and burn or seal tightly and dispose of — certainly don't compost.
вЂў Keep any future onions a safe distance, at least 5m (16ft) away from any previous infected spots.
вЂў Watch your shoes — don't walk on previously affected areas and then onto fresh soil because it's easy to transfer the dormant sclerotia.
The first symptoms of onion white rot are the green foliage turning yellow then dying. When the plant is pulled out there are no roots, they have been destroyed and there is nasty white mould in their place. Within that mould lurk the sclerotia.
Stemphylium blight: In Stemphylium leaf blight, the symptoms, causes and remedy are similar to Purple Blotch with the difference that the blight lesions are light yellow to brown.
Remember, it's important for onion pests and diseases, and in particular harmful pathogens in the soil such as onion white rot and pink root, to rotate crops.
It might not be possible to completely rid the soil of some diseases, but they will not build up to dangerous levels if you constantly grow your onions in the same area year after year.
For all the onion pests and diseases above, hop on over to Organic garden pest control to find some handy natural solutions you can use.
Growing Onions — This is the main information page all about growing onions.
Extra Handy Tips on How to Grow Onions Get the edge for super successful onion growing.
List of Vegetables — Now you are an expert on growing onions, check out more vegetables to grow.
If you’re still not sure that growing onions in containers is worth it then let’s go over some reasons as to why it’s a good idea:
Now that we have some reasons why growing onions in containers can be a good idea let’s go over the different types that work well in pots.
Another member of the notorious Alternaria gang, this is one of the most common garlic, onion and leek diseases. It likes warm (25 degrees C), wet weather and first manifests itself as small, purplish, watery lesions on the leaves. These eventually enlarge and turn brown (and may girdle and kill the leaf). There may also be a yellow halo around the lesion.
The best way to deal with this fungus is to keep your plants well fed and watered, as healthy plants are rarely seriously affected. In ideal warm, wet conditions it produces wind borne spores and can spread to neighboring plants, so if any plants show signs of infection, remove and destroy them. The spores overwinter on crop debris so clean up the beds at the end of the summer, to reduce sources of infection for the following year. Keep the soil well drained and provide good air circulation. The feeding of thrips may make plants more vulnerable to purple blotch disease.
Image: Howard F. Schwartz, Colorado State University, Bugwood.org
Learn how to plant and grow onions, leeks, and shallots.
Copyright © 2019 Green Living Solution, Inc. Smart Gardener ® is a registered trademark of Green Living Solution, Inc. All rights reserved.
Older leaves tend to be more susceptible than younger leaves. Symptoms begin as water-soaked lesions that usually have a white center. Edges of lesions become brown to purple and the leaf turns yellow above and below the lesions. With time, dark brown to black concentric rings form throughout the lesions. These are areas of sporulation of the fungus. As the disease progresses, lesions may girdle the leaf causing it to collapse and die. Similar symptoms occur on seed stalks and infected stalks can collapse resulting in shriveled seed development. When bulb infection occurs, it is normally through the neck. If the fungus invades the bulb, the infected area is initially bright yellow, but eventually turns a characteristic red wine color.
Expanding brownish-purple lesion showing concentric rings of sporulation.
Brownish-purple leaf lesion.
The fungus over-winters as mycelium in leaf debris and cull piles. Spores are formed during humid nights and leaf wetness periods greater than 12 hours. As the morning dew dries, spores become air-borne and are disseminated to susceptible onion tissue. 1-4 days are needed for symptoms to develop after infection. Disease development is greatest during prolonged periods of leaf wetness.
A fungicide spray program with broad spectrum protective fungicides applied prior to infection can provide good protection. Minimizing leaf wetness by using surface rather than sprinkler irrigation, good field drainage and correct plant spacing can reduce disease development. A rotation out of Allium to unrelated crops for several years can reduce disease as well.