Featured Article | GreenCast | Syngenta

Fight off Foliar Leaf Spots

Temperatures are rising, and with that comes increasing disease pressure in your greenhouse and nursery operations. Most pathogens that cause leaf spots do not typically lead to plant mortality, but they can impact the appearance and marketability of plants they infect.

Be aware of common pathogens

Every plant species is susceptible to different foliar leaf-spotting pathogens, so being familiar with the ones that could affect your crops is crucial. Some of the most common pathogens are: 
Myrothecium fruiting structures
  • Alternaria spp.: Aquilegia, Calendula, Coreopsis, Dianthus, Geranium, Impatiens, Marigold, Pansy, Shasta Daisy, Stock, Vinca, Zinnia
  • Cercospora spp.: Coreopsis, Hydrangea, Liatris, Monarda, Pansy, Phlox, Rudbeckia, Snapdragon, Zinnia
  • Anthracnose/Colletotrichum: Daylily, Gomphrena, Heuchera, Hosta, Lupine, Snapdragon, Stokesia
  • Corynespora spp.: Crossandra, Foxglove, Hydrangea, Ornamental Pepper, Sage/Salvia
  • Myrothecium spp.: Ajuga, Begonia, Echinaceae, Gerbera Daisy, Lantana, Salvia, Stock, Verbena
  • Septoria spp.: Gaillardia, Shasta Daisy, Phlox

How to avoid favorable conditions

Leaf spots on ornamentals grown outdoors are particularly challenging during this time of year, since rainfall and nightly dew provide ideal conditions for fungal spore germination and infection. Leaf spot development favors conditions such as:
  • Wet foliage
  • High relative humidity
  • Poor air movement
Cultural practices such as reducing leaf wetness by watering early in the day and spacing plants for increased air flow are important for prevention, as well as proper sanitation practices.

Symptoms to scout for

When taking the time to scout for leaf spots, it is important to inspect the foliage in the lower and interior canopy of the plant. Leaf spots may initially develop there before spreading due to the reduced air movement and higher relative humidity. Although they vary in shape and size depending on the pathogen, most leaf spots will appear as round, dead areas with defined margins and a colored border scattered over leaf surfaces. Under certain 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

What can you do to prevent leaf spots?

Fungicide resistance can develop quickly with leaf spotting pathogens, so it is important to have a robust rotation program that includes two to three fungicides with diffident modes of action. A rotation program should incorporate products with both systemic and translaminar activity in order to maximize their strengths and leverage different modes of action.

For nursery programs, we suggest applying Concert® II fungicide early in production, prior to blooms, followed by Palladium® and Mural® fungicide applied on a 14-day interval. Our greenhouse program for leaf spot prevention includes a rotation of Daconil WeatherStik® or Daconil Ultrex® fungicides because Concert II is not for use in greenhouses, followed by Palladium and Mural fungicide. If an infection is active, apply products on a 7-day interval. With this product rotation, you have access to five differing modes of action with strong activity on foliar leaf spots.

Find a foliar leaf spot program that will work for you by visiting GreenCastOnline.com/Solutions.

All photos are either the property of Syngenta or are used with permission.

©2020 Syngenta. Important: Always read and follow label instructions. Some products may not be registered for sale or use in all states or counties and/or may have state-specific use requirements. Please check with your local extension service to ensure registration and proper use. Concert®, Daconil Ultrex®, Daconil WeatherStik®, GreenCast®, Mural®, Palladium® and the Syngenta logo are trademarks of a Syngenta Group Company.

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