by Erin Marissa Russell
Many different bacteria and fungi can cause bulb rot diseases to take hold in the garden. We’ve developed this guide to fighting bulb rot to teach you what you can do to prevent and treat bulb rot. You shouldn’t just accept bulb rot and the losses of your beautiful plants that can accompany these diseases as part of gardening. There’s lots that you can do to prevent and treat bulb rot diseases so you can minimize their effect on your plants.
Identifying the Different Types of Bulb Rot
There are several different diseases that can cause the bulbs of your plants to rot, and identifying which disease you are dealing with can help guide you as you work to treat the problem and put measures in place to prevent it coming back in future seasons. Here we’ve outlined the plant diseases that can cause bulb rot, the symptoms caused by each disease, and which plants they tend to affect. The next section will discuss ways to prevent bulb rot from becoming a problem in your garden and how to treat bulb rot if you are already struggling with it.
Bacterial Streak and Bulb Rot of Onion (Pseudomonas viridiflava)
There is more than one disease that can cause bulb rot in onions, but bacterial streak and bulb rot is a result of colonization by the bacterium Pseudomonas viridiflava. This pathogen not only affects onion but also is known to colonize the leaves of certain other crops and weeds as well. When the weather is cool and wet, the bacteria will move downward from the leaves of the plants they colonize into the bulb, resulting in decay of the bulb’s inner scales.
The first symptoms of bacterial streak and bulb rot tend to appear on the foliage of the plant, as streaks of discoloration or oval-shaped lesions with a water-soaked appearance. As time goes on and the disease progresses, the streaks on leaves will become darker, and eventually affected leaves will wither, finally collapsing as they succumb to the disease.
On the bulb, the symptoms of bacterial streak and bulb rot only appear on the inner scales as areas of discoloration in shades of red and brown or spots of decay. It is common for plants infected with bacterial streak and bulb rot to have secondary infections as well, as the initial disease makes the plant vulnerable to other pathogens.
Botrytis Bulb and Neck Rot of Onion and Garlic (Botrytis allii and Botrytis aclada for onion, Botrytis porri for garlic)
Botrytis bulb and neck rot of onions is caused by the fungi Botrytis allii and Botrytis aclada, while Botrytis bulb and neck rot of garlic is caused by Botrytis porri. These diseases affect crops of onions and garlic that are grown in temperate regions. The disease causes bulb rot in the field, and symptoms continue throughout storage and can become quite severe, developing into neck rot. Crops are more likely to become infected with Botrytis bulb and neck rot during spring seasons when the weather is especially moist and cool, specifically when temperatures are between 50 and 75 degrees Fahrenheit (10 to 24 degrees Celsius).
The first symptoms of Botrytis bulb and neck rot are small, sunken areas of brown or gray discoloration that appear either on the side of onion bulbs or near their base. In some cases, the lesions will have visible gray fungi growing inside them. As the disease progresses, expanding areas of brown discoloration develop around the initial lesions, which also grow in size. The brown discoloration appears on the two or three outermost scales of the onion and may be accompanied by hard, dry formations of hardened fungal mycelium called sclerotia forming on the scales. These sclerotia can initially be white but will darken to black as time goes on.
Botrytis bulb and neck rot is due to a soil-borne fungus, and it is often introduced to a garden with infected bulbs or infected seeds, though the disease can also spread via the movement of spores dispersed on the wind. Over the winter, the fungus hides in onion debris left in the field in the form of either spurs or sclerotia. Then, when spring comes and temperatures rise, the sclerotia germinate and emit spores. The spores then travel on the air to infect more plants when they land. Often, onions will contract the disease in the field, but it will not become apparent until the onions are in storage.
The symptoms of Botrytis neck rot generally begin with mushy, water-soaked areas of tissue around the neck of the onion that become discolored to brown. When it is especially humid, this may be accompanied by a gray substance similar to felt on the rotting scales. These gray areas are where the spores of the fungus are produced. Between the scales, mycelia may also begin to form. Finally, the hard, dry formations of sclerotia may appear between the scales or around the neck of the onion.
When Botrytis bulb and neck rot affects garlic crops, signs of the disease tend to emerge either in the field near the end of the growth period or while the garlic is in storage. When the disease appears in the field, plants may display dying or dead outer leaves and overall stunted growth. Areas of affected tissue appear water-soaked at first but will later become necrotic and dried-out. The hard, dry formation of sclerotia may attach to the rotted outer scales of the garlic bulbs or the neck, where the bulb meets the stalk. It is common for initial signs of the disease to go unnoticed until foliage begins to deteriorate and show signs of necrosis.
Bulb and Basal Rot of Lilies (Fusarium oxysporum f. sp. lilii)
The fungus responsible for bulb and basal rot of lilies, Fusarium oxysporum f. sp. lilii, can persist in garden soil indefinitely and is normally carried on the bulbs of susceptible plants. More often than not, bulb and basal rot of lilies is introduced to a garden by bulbs infected with the Fusarium oxysporum f. sp. lilii fungus.
The infection most often begins at the tips of the plant’s roots, and the initial sign of the disease is normally yellow foliage during the growing season. Lilies are most likely to contract bulb and basal rot during the cold of the winter season, especially when the weather has been particularly wet. Bulb and basal rot of lilies often appears in conjunction with other strains of Fusarium, with Cylindrocarpon, or with other fungal infections.
Lilies that are infected with bulb and basal rot will display discoloration of the rotted tissue in shades of brown or blue-gray that can spread from the basal plate near the root up to the scales near the stem. Scales that have been infected with rot will retain their smooth texture until they begin to disintegrate. Infected lilies may have scales that detach at the base of the bulb, or the bulbs may simply fall apart. Signs of bulb and basal rot of lilies above the ground can include foliage that turns yellow too early, stunted growth, and early deterioration and death. Flowers of infected plants may wilt while they are still in the bud or buds may never open.
Gray Bulb Rot of Tulips (Rhizoctonia tuliparum)
Gray bulb rot is associated with a disease called tulip crown rot, and both are caused by the fungus Rhizoctonia tuliparum. In addition to tulips, gray bulb rot can also infect bulbous iris, cochicums, crocuses, daffodils, gladioli, hyacinths, ixias, lilies, and narcissus flowers. Plants are most susceptible to gray bulb rot immediately after being planted and at the very beginning of the spring season.
Both diseases result in the bulbs of tulip plants being discolored to gray and withering, sometimes before they have even sprouted. When bulbs are infected with gray bulb rot, it is common for most of the plants to fail to sprout. Plants infected with gray bulb rot that do sprout will normally mature more slowly than normal and tend to shrivel and succumb to the disease before they have a chance to blossom. Patches of infected bulbs tend to appear in the midst of healthy plants in the garden.
When the bulbs are dug up and examined, those infected with gray bulb rot will be coated with soil stuck to the bulbs, and from the neck portion down, dry rot will be visible. Felt-textured masses of fungus can develop between the scales, and as the name suggests, infected bulbs and roots are often discolored to gray. Infected bulbs will wither and dry out beginning at the site of infection at the top of the bulb moving downward. Some infected bulbs may have a white substance resembling mold on the bulbs themselves as well as in the soil that surrounds the infected bulbs.
Gardeners may also see sclerotia, which are deposits of hardened fungal mycelium that can be either round or flat and reach sizes of up to 80 millimeters wide (3.15 inches) but are normally much smaller, between one and 10 millimeters across. The sclerotia start out white and darken in color as time goes on, eventually turning almost black. The disease can remain in infected soil for a minimum of three to five years in these sclerotia. Even after a decade, up to 10 percent of the sclerotia may still be infectious.
Tulip Fire (Botrytis tulipae or Botrytis cinerea)
In addition to gray rot, tulips can become infected with the fungi Botrytis tulipae or Botrytis cinerea. When Botrytis tulipae is the culprit, the resulting disease is known as tulip fire. Blooms from plants infected with tulip fire are called fireheads. The fungi behind these diseases are prevalent, but infection results from injury to bulbs either due to insect damage or careless handling of the bulbs. Symptoms become apparent when leaves appear and remain visible until folaige dies. Tulips are the only plants affected by these fungi.
Plants infected with these diseases will have brown discolored areas of dead tissue on their foliage or petals. In severe cases, the spots expand until large areas of the foliage and petals look shriveled and brown, as if they were singed or burned (which is why the disease is called tulip fire). The foliage of infected plants may also appear distorted in shape or twisted. In addition, the stems of the plants can weaken and eventually collapse completely. The bulbs of infected plants will eventually be covered in lesions.
When conditions are damp, the dead areas of the foliage may be covered with a fuzzy gray mold. Dead areas of tissue may develop small black structures that resemble seeds. These are hardened collections of fungal mycelium called sclerotia that can go on to contaminate the outer bulb scales as well remaining in the soil for several years and making it infectious. Airborne spores will also be produced from the gray moldy spots.
Preventing and Treating Bulb Rot
Although, as we’ve discussed, there are several different types of bulb rot that can strike in the garden, the prevention and treatment measures are very similar for all the various types. And most of the time, the measures that help keep bulb rot at bay are just good practices in general for the health of your plants. We recommend implementing as many of these environmental and cultural controls as possible to keep your garden healthy. If bulb rot does take hold in your garden, follow the treatment recommendations here to quickly put a stop to the disease’s spread and reduce the chances of it recurring in subsequent seasons.
- Do everything you can to keep the bulbs of your plants dry while they are in storage. Do not store bulbs in a location with high humidity or a location where there is a chance of the bulbs getting wet.
- When you harvest a batch of onions, make sure the onions are cured thoroughly before moving them into storage. Onions that get wet or are not cured properly are much more likely to develop fungal or bacterial rot than onions that are well cured and kept dry. You can confirm that onions are well cured by checking to make sure the neck is dry and tight. Get in-depth instructions on curing onions in our article How Do You Store Onions for a Long Time? [https://www.gardeningchannel.com/store-onions-for-a-long-time/] and learn about storing onions properly in our article How to Braid Onions for Long-Term Storage and Other Storing Techniques [https://www.gardeningchannel.com/how-to-braid-onions-storage/].
- Onions should be kept cool while in storage, ideally between 32 and 36 degrees Fahrenheit (0 to 2.2 degrees Celsius). The humidity should also be kept ideally between 65 and 70 percent.
- If it is warm when your onions will be removed from cold storage, there is a potential for condensation to form on the onions, which can lead to decay. For this reason, onions being removed from cold storage should gradually be warmed over a period of two or three days.
- If cold storage is not possible and you need to keep onions long term, store them in a well ventilated shed.
- Onions that show any signs of disease, that are bruised, or have been sunburned should be discarded (not added to compost) as soon as they are harvested. Including these onions in storage can cause the entire batch to be ruined. The best way to dispose of these onions is by burning or burying them immediately after harvest.
- When you are harvesting onions and preparing them for storage, handle them carefully to avoid bruising or nicking the onions, which can provide an entry point for the pathogens that cause bulb rot.
- When you use nitrogen fertilizer, make sure to use only the recommended amount, as too much nitrogen fertilizer can increase the risk of bulb rot. You may also consider using low-nitrogen fertilizer on your vulnerable plants.
- All tools and equipment that you use in the garden should be regularly cleaned and sanitized [https://www.gardeningchannel.com/garden-shears-care-maintenance/]. From small tools like spades or clippers up to large equipment such as tractors and tillers, gardening equipment can easily spread disease from plant to plant or from field to field if not sanitized frequently. When harvesting, disinfect clippers frequently by dunking them in a bucket of chlorinated water.
- As many types of bulb rot are spread through the soil, it is not recommended to plant in an area where infected plants were growing for a period of five years. Because these diseases are most prevalent in wet soil that has poor drainage, find a new location for your vulnerable plants that has loamy soil offering plenty of drainage.
- You can grow plants that are not vulnerable to bulb rot in parts of the garden that experienced the disease in previous seasons, but you may consider improving drainage where it is poor by amending the soil with lots of organic material. Do not plant pulses (such as dry beans, soybeans, and alfalfa) in locations where plants have been infected with bulb rot. Pulse crops can be alternate hosts for the pathogen responsible for bacterial leaf streak.
- Working in the garden when plants are wet from rainfall, irrigation, or morning dew can help to spread the bacteria and fungi that lead to bulb rot. Try not to do any work among your vulnerable plants when it is wet in the garden.
- Because the fungi and bacteria behind bulb rot can remain in plant debris to return the next season, carefully clean the garden to remove any remaining debris after harvest. The best way to dispose of the debris you collect from infected parts of the garden is to bury it deeply.
- Weeds can serve as alternate hosts for the pathogens that cause bulb rot, so the more meticulously and frequently you weed the garden, the less the chances are of the pathogens infecting your plants. You should weed a zone around the perimeter of the garden in addition to the area of the garden itself.
- In areas where excess moisture is a concern, spread the rows of your garden out farther and plant the rows in the same direction as prevailing winds so the air circulation among the plants is increased. The boost to air circulation will help reduce the amount of time that foliage remains wet, reducing the likelihood of bulb rot for your plants.
- Examine bulbs carefully before you plant them, discarding any bulbs that show signs of disease, such as dark spots, mold, or spongy areas. You can also drop the bulbs you will be planting in water, as healthy bulbs will sink to the bottom while bulbs infected with rot will float to the surface.
- As soon as you notice symptoms of bulb rot, pull up the affected plants and discard them, ideally by burning them or burying them deeply. Under no circumstances should diseased plants be used in compost, or you run the risk of re-introducing the disease when the compost is used in the garden. Also remove and dispose of the soil surrounding infected plants, as it is likely to be contaminated as well.
Bulb rot can be a daunting disease to face just because of how severely it can strike when it does take hold and how long the pathogens can persist in soil—up to 10 years! However, as long as you take quick action to address the disease if it rears its head in your garden, bulb rot can be overcome. Best of all, by incorporating the prevention measures we’ve discussed, with some luck you’ll never need to battle bulb rot in your garden at all.