The above onion sample consisted of two mature bulbs that were suffering from an
unknown soft rot, infested with small maggots, and also showing signs
of black fungal growth. All contributing factors were identified;
however none of these pests or pathogens are the primary cause for the
damage noted. All insect, fungal, and bacterial pests found are post
harvest problems that only appear after injury or improper curing.
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)
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