Seeing spots?

For many gardeners, a big challenge to growing healthy plants is keeping them free of disease, especially blackspot, rust and anthracnose.  Caused by fungi, this trio of diseases are typical in wet spring and fall months but can infect gardens during our dry summers given the right conditions.

Blackspot produces characteristic round black spots with fringed or feathery margins on the upper surface of leaves or stems, generally seen first on leaves closest to the ground. Infected leaves drop off, and the plant may drop all its foliage.

Rust is fairly easy to identify with its orange, powdery pustules on the undersides of leaves and other plant parts that turn dark orange to red brown with age. 

Anthracnose, also known as shot-hole fungus produces red or sometimes brown to purple circular spots that are mostly on the leaves and occasionally on the stem. As the disease progresses, the spots may enlarge with the centers of the spots turning gray, tan or white with a dark red margin. The spot itself drops out of the leaf, leaving a circular hole, thus creating a “shot hole” effect, while most of the leaf remains green.    

Prevention is the most effective means of managing these nasty diseases.  Some options include:

  • Cultural methods
    • Purchase plants that are resistant to disease when possible.
    • Plant in locations that have good drainage and get plenty of sun.
    • Avoid shady spots and dense plantings.
    • Ensure the plants get lots of air movement to dry any moisture off the leaves. 
    • Maintain a “clean” garden (pick up and dispose of any infected leaf material).
  • Mechanical and physical methods
  • Take care when doing any overhead watering; do it early in the day so plant surfaces have time to dry before temperatures cool in the evening. 
  • If possible, switch to alternate methods of irrigation that don’t wet leaf surfaces.

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