Tuesday, October 8, 2013
Got Rot in Your Onions after Harvest?
One of the fungal, postharvest pathogens recovered from this sample was Rhizopus spp. It is the cause of mushy rot. This pathogen appears on onions stored with high moisture content and humidity. Onions should be grown with good management practices in order to reduce foliar and bulb diseases. Be sure to handle bulbs carefully during harvest, transport, and storage to prevent sunscald, bruising, cutting, or freezing injury. After the bulbs have cured, bulbs should be stored below 15 degrees C with a relative humidity of 50%. These management suggestions were taken from the Compendium of Onion and Garlic Diseases and Pests.
Bacterial soft rot was also found on the onion sample. It is often caused by a combination of many bacterial pathogens. The most important step to controlling this disease is proper curing and storage of mature onion bulbs after harvesting. Fertilization and irrigation should cease early in the season to allow for bulb maturity. Onion tops should be allowed to mature (more that 90% lodged) before harvest. No moist or green tissue should be left on the necks of the bulbs so that they are properly cured. Avoid bruising and handling during harvest. Onions should only be stored after they have been well dried. Storage at 0 degrees C and less than 70% relative humidity with good ventilation will prevent condensation of moisture on the surface of bulbs and therefore; helps to reduce the rate of development and the spread of soft rots. These management suggestions were taken from the Compendium of Onion and Garlic Diseases and Pests.
Onion maggot pupae and adults were identified within the onion sample. Later in the season, damage can occur by second and third generation larvae, but it is less common of a problem on healthy onions, because the developing bulb is difficult for the larvae to penetrate. We suspect that the problem, in this case, was that the onion maggot larvae were able to enter through a preexisting wounds due to disease or injury of the bulb. Onion maggot injuries are almost always associated with secondary rotting organisms; therefore, further decay can follow wounding by third generation onion maggots infection in the field and continue during storage. For additional information, you can refer to the following link: http://urbanext.illinois.edu/vegproblems/rootmaggots.cfm
To help manage onion maggots, a rotation from onions is recommended, remove cull onions after harvest, remove volunteer onions in spring, minimize herbicide and cultivation damage, and destroy nearby weeds. Insecticides may not be available to homeowners.
It is very, likely that several other secondary, postharvest pathogens could have been isolated from this sample. Overall, to avoid most bulb rots, you need to: control leaf diseases, let onion tops ripen (fall over) naturally, dry bulbs before storage, and avoid storing improperly cured or injured bulbs. Store bulbs under cool, dry conditions. Do not expose cured bulbs to sunscald, water, or high humidity.
(Sean Mullahy and Stephanie Porter)