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Be on the lookout for foliar diseases

As warmer temperatures return to most growing areas, so does the occurrence of foliar diseases. Leaf spot diseases can be particularly challenging on ornamentals grown in outdoor production areas since rainfall and nightly dew provide the perfect conditions for fungal spore germination and infection. Although most pathogens causing leaf spots do not lead to plant mortality, they can greatly reduce the appearance and marketability of the plants they infect.

Common fungal pathogens responsible for leaf spot diseases include Alternaria spp., Cercospora spp., Colletotrichum spp., Myrothecium spp., Entomosporium spp. and Septoria spp.; with susceptibility to these agents varying by the host plant. Leaf spots can vary in shape and size but tend to be more rounded in appearance and typically have distinct margins with a colored border. When scouting, be sure to inspect foliage in the lower and interior canopy of the plant, as leaf spots may initially develop there because of reduced air movement and higher relative humidity. Leaf spots caused by fungal agents can be identified by the way they sporulate on infected tissue. Under the right environmental conditions, some may form small black fruiting structures within the leaf spot that can be examined more closely under a light microscope.

Septoria leaf spot on rudbeckia

Other fungal diseases such as Botrytis and powdery and downy mildews produce masses of fuzzy spores directly on the infected tissue. Infections by Botrytis cinerea can be more of a problem on tender new growth and blooms.

Spores from Botrytis and other fungal pathogens are easily spread through air movement. Conditions of high relative humidity and free moisture on the leaves for 4-6 hours will promote foliar diseases. Cultural tactics that help reduce the duration of leaf wetness, along with proper sanitation practices and preventive fungicides, are all important tactics for controlling foliar diseases caused by Botrytis and fungal leaf spot pathogens.

Conditions that favor leaf spot diseases:

  • Wet foliage
  • High relative humidity
  • Poor air movement

Recommended fungicides for foliar disease control:

Preventive applications can be made on a 14-day interval. For control of active infections, reduce the application frequency to a 7-day interval. Once blooms are present, discontinue the use Daconil Ultrex in your rotation as this can damage flowers. Mural and Palladium fungicides can be used to control Botrytis on blooms. Consult the product labels for additional instructions.

Click here to view leaf spot rotation program for greenhouses

Click here to view leaf spot rotation program for nurseries

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