Seeing Canna Rust?

Cannas are a popular herbaceous perennial that can be grown in containers, as a boarder background or as a specimen in gardens and landscapes throughout Harris County. It did not take long after the wet spring for fungal problems to start rearing their ugly head. We have recently received several emails and calls regarding orange pustules on Canna leaves. Yes, your plant has Canna Rust (Puccinia thaliae).

The Puccinia fungus requires long periods of humid weather, poor air circulation and waterlogged soil to grow and flourish. That sounds just like the month of May.

The symptoms of Canna Rust include spore-like orange spots on the plant’s leaves and stems. In advanced stages of infection, the upper leaf surface spots will join, turn dark brown to black and become dry and cause the leaf to drop prematurely. When Canna Rust first appears, the affected foliage should be removed and discarded. Leaving the affected foliage on the plant will result in continued growth and spread of the fungus.

Canna plant showing various stages of rust development.

Canna plant showing various stages of rust development.

The fungal infection can be managed, but is difficult to eradicate. The best approach is incorporating good management practices when establishing the landscape.

  • Cannas prefer at least 6 hours of sun per day
  • Provide adequate space to insure good air circulation
  • Avoid planting Cannas in shady or poorly drained locations
  • At the first sign of rust, prune the foliage and discard.
  • Prune plants periodically to reduce crowding and humidity within the canopy.

During heavy infestations, a rotation with contact and systemic fungicides can be implemented. Contact materials for rust management include mancozeb, chlorothalonil or coppers. Systemic materials include myclobutanil, kresoxim-methyl, triadimefon, triflumizole, azoxystrobin, trifloxystrobin, or pyraclostrobin. Always check labels and follow restrictions and recommendations.

Canna rust pustules on the leaf surface.

Canna rust pustules on the leaf surface.

It will be an interesting summer for diseases, we will keep you up to date on what we are seeing and hearing.

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