Iris Borers and Bacterial Soft Rot 

Iris borers (Macronoctua onusta), are a species of cutworms that bore through iris leaves down into the rhizomes, causing browning and wilting. Their feeding damage can also cause a disease known as bacterial soft rot, aka. root rot. This bacterial disease, caused by the bacteria Erwinia carotovora, causes the rhizome to rot and become mushy, emitting a foul odour. Iris rhizomes infected with this disease must be dug up and destroyed.

Iris borer Identification and Life Cycle

Iris borer adults: are moths with dark brown heads, a dark brown hairy thorax and abdomens that are a bit lighter brown in colour. They have a wing span of about 5cm (2″) with the two forewings being brown and patterned and the two hind wings being a bit lighter brown in colour, often with a yellowish tinge. They are not often spotted in the garden, as they, like most moths, are nocturnal and fly at night. However they are attracted to lights and you may be able to spot one that way. The adults emerge from pupae in the first half of September and fly September through October, feeding on nectar and laying their eggs in small clusters on iris leaves, where they over winter.

Iris borer larvae: are pinkish coloured caterpillars with a reddish/brown head capsule. When the eggs hatch in spring (early May), the tiny larvae borer their way into the newly emerging foliage. The larvae feed inside the leaves tunneling their way down towards the rhizome and growing in size. By the time they hit the rhizome they are fully grown, measuring about 5cm (2″) in length. They continue feeding in the rhizomes until the beginning of August then move into the soil to pupate.

Recognizing Iris Borer Damage and How to Manage This Pest

In April and May begin examining Iris leaves for signs of young larvae, which include pinholes and discoloured lines, which may appear water soaked. At this point leaves can be cut off below the point of infection and destroyed. As the larvae tunnel down the leaf towards the rhizome, leaf tips turn brown and entire leaves and flowers can begin wilting and browning and the base of the stalk rots. At this point in the infection dig up rhizomes and inspect for iris borer damage. If damage is minor dispose of dead and damaged foliage and cut away the damaged areas of the rhizome, allowing cuts to air dry before replanting. If the damaged area is extensive, dispose of the entire rhizome. To prevent future infections remove iris leaf debris in late fall, after first frost. Siberian irises tend to be less affected as they are planted more deeply in the soil.

There are two species of beneficial nematodes (Heterorhabditis and Steinernema) that can be applied to the soil to help control iris bores. They are usually applied once the leaves have fully expanded, but before flowering.

Iris Borer Distribution

You will find Iris borers in eastern North America from the southern tip of Canada down to Georgia in the south mostly east of the Mississippi river.

Bacterial Soft Rot 

Bacterial soft rot aka. root rot is caused by the bacterial pathogen, Erwinia carotovora. It enters iris plants through wounds. Commonly those wounds are created by the iris borer but other injuries such as slug and snail feeding, mechanical injuries, etc. can also provide entry points for this bacteria. Once inside the plant the bacteria colonize this damaged tissue and produce enzymes that break down the tissue, causing it to turn watery and soft and produce a slimy foul smelling ooze. The bacteria can spread from plant to plant and through a soil water. Infected plants must be removed and disposed of if the rot has spread and is extensive. For minor infections the plants can be removed and the diseased area cut out, using sterilized tools, that are sterilized also between each cut and at the end.

Some Good Cultural Practices For Healthy Iris Plants

  • Plant your iris in a sunny location. The more sun they receive the better the blooming.
  • Iris require good drainage to prevent their rhizomes from rotting. The prefer sandy, gravely soil over clay.
  • When introducing new iris plants to the garden inspect them for any signs of iris bore or soft rot prior to planting.
  • Plant the iris rhizomes shallowly, leaving their tops exposed to the air. They do not like to be planted to deeply. (An exception to this is Siberian iris which are planted 2.5-5cm (1″-2″) deep.
  • Provide adequate spacing between plants to help prevent disease. Thin over crowed patches in late summer to fall. Patches will likely have to be thinned every 3-5 years.
  • Do not cultivate around iris patches. Any injuries to the rhizomes can provide entry points for disease (like soft rot). If you are digging to thin or move the rhizomes use a garden fork rather than shovel to loosen the soil, being careful not to spear a rhizome.
  • Check you iris regularly for sign of iris bore and remove infected plant material promptly.
  • Manage slug and snail populations by hand picking and/or setting out traps or lures.

Summary

Iris bores and bacterial root rot are two serious pests of iris plants. They can cause extensive damage to an iris patch. Monitoring and early detection are key to managing these pests along with providing ideal growing conditions.

References:

Cooper J. (2006). Bacterial Plant Pathogens And Symptomology. WSU County Extension. Retrieved from: https://s3.wp.wsu.edu/uploads/sites/2054/2014/04/BacterialPlantPathogens_001.pdf

Cheung D. K. B., Llewellyn J. and Marshall S.A. Macronoctua onusta. Canadian Journal of Arthropod Identification. Retrieved from: https://dkbdigitaldesigns.com/clm/content/macronoctua-onusta

Hahn J. and Fetzer J. (n.d., rev. 2018). Iris Borers. University of Minnesota Extension. Retrieved from: extension.umn.edu/yard-and-garden-insects/iris-borers

Mukherjee J. (2007). Rot And Remedies. DVIS Newsletter. retrieved from: http://dvis-ais.org/iris-rot_remedies.html

https://garden.org/ideas/view/TBGDN/1588/Growing-Irises-in-Spite-of-Borers/

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