Diseases caused by the fungal pathogen Botrytis spp. can be very common in onions, garlic and other plants in the allium family. These pathogens are common in most parts of the world where onions are grown commercially. There are a few different species in the Botrytis family that can affect onions, with distinct symptoms on affected plant tissues. Both leaves and onion bulbs can be affected by botrytis spp. related diseases, and management strategies will be different for these different infection sites
Botrytis Leaf Blight On Onions
Identification:
Botrytis Leaf Blight (BLB) on Onions is caused by the fungal pathogen Botrytis squamosa. In some producing areas, this disease is referred to as “Blast”. Infection results in whitish lesions on leaves that can, in severe cases, cause leaves to wilt and die. Lesions begin as small water soaked marks on the leaf tissue. As the infection develops, lesions will be sunken in the leaf tissue and will take on a straw color. They will be surrounded by a lighter green “halo” effect.
Older onion leaves are more susceptible and are where infections will usually occur. Conidial spores are often visible on older, dead, or blighted leaves. Younger infections on green leaves will often not produce conidia.
Conditions For Infection
Botrytis Leaf Blight is more common under cool and wet conditions. Optimum conditions for infection are considered to be: temperatures of 59°F to 68°F, with 7 or more hours of leaf wetness. Temperatures outside of this optimum range can still produce infections, however, if leaf wetness duration is long enough.
Conidia (asexually produced spores) are usually spread by wind. Older fields, culled plants, or debris piles can be sources of inoculum. This fungus can also produce sclerotia which are able to survive in long periods of dormancy, making this disease particularly challenging to control and manage.
Botrytis Leaf Blight Management on Onions
The primary strategies for managing Botrytis Leaf Blight in Onion are eliminating potential sources of inoculum and preventively spraying fungicides. Culls and infected older leaves should be removed completely from the field, so that the conidia cannot be wind-spread into newer leaves. Onions should also be planted in a crop rotation, with other allium species avoided. Because the Botrytis squamosa sclerotia can survive so long in a dormant state, rotations longer than 3 years are often recommended for onions, at least in areas where this disease is very common, or severe.
There are many classes of fungicides that also can be used to manage Botrytis Leaf Blight on Onions. Most of these fungicides work best when applied as a preventative spray, before symptoms appear. The weather conditions should be monitored to determine optimum times to spray fungicides, as well as to predict the correct intervals between fungicide sprays. The Pest Prophet app can provide growers guidance in this area, by using weather data to model optimum conditions.
Preventative sprays should also be applied after lesions are observed in a field. This will help to prevent the disease from spreading. Orienting plant rows and spacing to increase air movement will also help by reducing leaf wetness duration, and improving spray coverage. Deep plowing and tilling can also be helpful to disrupt Sclerotia in soil.
Botrytis Leaf Fleck on Onions
Similar to Botrytis Leaf Blight, Botrytis Leaf Fleck is a disease on Onions caused by Botrytis cinerea. This species of Botrytis is very common, with a wide variety of hosts. The symptoms of this disease are close to Botrytis Leaf Blight, and can cause white flecks on Onion leaf surfaces. However, with this disease, these lesions will remain superficial and will not penetrate the surface of the leaf tissue.
Botrytis Brown Stain on Onions
Botrytis cinerea, in addition to causing Botrytis Leaf Fleck, can also infect onion bulbs. When infection occurs, outer tissues of the bulb and neck can have brown stains. Stained tissues can often dry out during storage, post harvest, and can easily be removed. This disease is rarely an economic concern and most fungicide spray plans for Botrytis Leaf Blight or Downy Mildew will also control Botrytis Brown Stain.
Botrytis Neck Rot on Onions
This disease can be very destructive on onions and can be challenging to control, because symptoms often are not visible until harvest. Neck Rot on Onions is most commonly cause by the fungal pathogen Botrytis allii, but can also be cause by Botrytis acclada or other similar fungi.
Infection occurs in younger, health leaf tissues and usually moves into the neck through a wound or water-soaked lesion. Once the fungi is in the neck, it begins to produce white or gray mycelia that spread into the onion bulb. Infected tissues of the bulb become brown, soft and spongy. Infected bulbs typically appear somewhat sunken and are spongy around the neck area, and in severe instances, black sclerotia may also develop on the outer bulb scales. As the plant tissues are destroyed, secondary fungal pathogens may also enter, making the effects of the disease even more severe. The disease can continue to progress in storage if not bulbs are not dried out.
Conditions for Infection
Optimum conditions for this disease are cool, wet conditions. Conidia can be spread by wind around infected fields and from infected tissues. Disease spread is most rapid during periods of moderate temperatures with high humidity, heavy rainfall, excessive overhead irrigation, or poor drainage or flooding. Weather conditions at harvest are especially important, because severe infections can occur when the onions necks have not dried yet. Storage conditions also play in role in the progression of this disease, with higher humidity and moderate to higher temperatures allowing the disease to continue to grow, reproduce and infect more tissue within infected bulbs.
Management of Botrytis Onion Neck Rot
There are a number of strategies used to prevent or reduce occurrence of Botrytis Neck Rot in onions. While there are no resistant onion varieties, it important to choose varieties that are wells suited to a give growing region to ensure that onions are at full maturity before harvest time. Poorly timed fertilizer applications can also delay maturity. Harvesting onions at less than full maturity will increase susceptibility to Botrytis neck rot.
Many of the same general practices that help with Botrytis Leaf Blight apply to Neck Rot as well. Removing culled onions completely from the field and eliminating any debris or cull piles will help decrease the amount of conidia present. Deep tilling or cultivating can help break up sclerotia. Crop rotations of at least 3 years are also recommended. Preventative fungicides should be used before harvest, especially if environmental conditions are favorable to disease. Any practices that help reduce injuries or wounds to onion necks or bulbs should be implemented, especially at harvest time. After harvest, bulbs should be stored in a area where they are allowed to dry as quickly as possible. Artificial drying through heating may be needed in some cases.
Learn More: Complete Guide to Botrytis (Gray Mold)
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