Green mold rot of onion by Ninjatacoshell (CC BY-SA 3.0)
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One of the most serious diseases to affect onion crops. It can be very difficult to get rid of once it is established in the soil as the fungus can wait in the soil for many years. It will kill onions and similar plants at any stage in their life and so it is very important to not allow for the fungus to infect the soil.
Leaves turn yellow and dieback, sometimes can cause the plant to keel over and rot the roots. Keeling over is most common among seedlings. Cottony patches of white fungi appear around the bulb that have small black bodies attached. The disease often appears in patches throughout a crop, these patches can often join up to damage large areas of plants.
Previous infection of Onion White Rot
Yellowing of leaves.
Leaves dying back
Onions toppling over
White cottony growths around the bulbs that have small black bodies attached.
There are no biological controls for this fungus. However, some success has been made using onion based compost on soils that are infected to trick the fungus into germinating and then dying off as it needs living onions to survive.
There is no chemical control for this fungus
The black bodies in the soil wait for susceptible plants to be in their vicinity and then germinate and infect the roots. They can spread through the soil on their own but only for a limited distance.
Not composting onions that have been infected with white rot.