Iris Disease Management: Comprehensive Guide to Rhizome and Bulb Diseases

Managing Diseases in Iris Production

Iris species (both rhizomatous and bulbous) face a complex array of bacterial, fungal, nematode, and viral diseases. Effective management requires understanding the specific diseases affecting each type and implementing targeted cultural practices.

Bacterial Leaf Blight

Xanthomonas campestris pv. tardicrescens causes faint water-soaked leaf spots enlarging along leaves in wet weather, ceasing in dry conditions.

Management: Remove infected leaves. Avoid overhead irrigation.

Botrytis Rhizome Rot

Botrytis convoluta causes few spring leaves, yellowing/browning/death, gray spore masses on leaf tissue, dry mealy rhizome rot, large shiny black sclerotia on rhizomes.

Management: Don't plant infected rhizomes. Destroy infected plants.

Crown Rot

Corticium rolfsii causes slow leaf tip death, white fungal threads at leaf bases, tan spherical sclerotia between rotting leaves.

Management: Discard infected bulbs.

Ink Spot (Bulbous Iris)

Drechslera iridis causes tiny leaf spots/streaks enlarging to dark reddish-brown, gray centers in wet weather with dark spore masses; premature yellowing/death; heavy bulb rot pre-flowering.

Management: Destroy leaf debris. Remove/destroy infected bulbs. Apply fungicides.

Leaf Spot

Mycosphaerella macrospora (Didymellina) causes small brown spots with water-soaked margins turning yellow, enlarging with reddish-brown borders; leaf death; dark spores in spots.

Management: Remove/destroy infected leaves and debris. Apply fungicides.

Nematodes

Ditylenchus destructor causes black streaks along outer bulb leaf veins; gray basal plate separated from outer scales by dark sunken groove.

Management: Destroy infected bulbs. Plant in nematode-free soil.

Soft Rot

Erwinia carotovora pv. carotovora causes sudden leaf collapse or gradual tip death; foul-smelling soft rot at leaf bases and rhizomes.

Management: Destroy infected rhizomes. Control iris borers (create entry wounds).

Viral Diseases

Multiple viruses (iris mild/severe mosaic, cucumber mosaic, broad bean wilt, tobacco ringspot/rattle, bean yellow mosaic, narcissus latent, bearded/beardless iris mosaic) cause mild mosaic on stalks/spathes, yellow-green stippling/stunting, flower color breaking/puckering. Most cultivars tolerate viruses.

Management: Destroy severely affected plants.

Integrated Iris Health Program

Critical management points: separate management for rhizomatous vs. bulbous types; strict sanitation (debris removal, infected plant destruction); overhead irrigation avoidance; borer control for soft rot prevention; nematode-free soil for bulbous iris; fungicide applications for leaf spot and ink spot; and tolerance of low-level viral infections in most cultivars. The diverse iris groups require type-specific disease management approaches.