As warmer temperatures return to most growing areas, so do occurrences 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.

Identifying and scouting

Common fungal pathogens responsible for leaf spot diseases include:

• Alternaria spp.
• Cercospora spp.
• Colletotrichum spp.
• Myrothecium spp.
• Entomosporium spp.
• Septoria spp.

Susceptibility to these agents can vary by the host plant. When scouting for leaf spots, inspect foliage in the lower and interior canopy of the plant, as they may initially develop there due to wet foliage, reduced air movement and higher relative humidity. 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. Under certain environmental conditions, some may form small black fruiting structures within the leaf spot. Cultural practices such as reducing leaf wetness and spacing plants for increased air flow are important for prevention, as well as proper sanitation practices.

Leaf spots on hydrangea

Leaf spot solutions

Fungicide resistance can occur more quickly in pathogens that produce a lot of spores, like foliar leaf spots. To avoid resistance, include at least two to three products with different modes of action in a rotation program. Programs should incorporate a combination of products with systemic and contact or translaminar activity that maximizes their strengths and leverages modes of action when they are most beneficial.

Syngenta offers both greenhouse and nursery programs for leaf spot prevention to help mitigate fungicide resistance. These programs include a rotation of:

• Daconil WeatherStik^® or Daconil Ultrex^® fungicides
• Palladium^® fungicide
• Mural^® fungicide
• Concert^® II fungicide (outdoor nursery operations only)

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 of Daconil WeatherStik or Daconil Ultrex in your rotation as this can damage flowers.

To incorporate different modes of action into your rotation program, Palladium is a combination of two active ingredients, fludioxonil (FRAC 12) and cyprodinil (FRAC 9), for systemic and contact activity on leaf spot pathogens. Mural is powered by SOLATENOL^® technology (FRAC 7), one of the most broad-spectrum SDHI fungicides, combined with a reliable industry standard, azoxystrobin (FRAC 11), for control of a wide range of foliar and root pathogens.

For outdoor nursery programs, Concert II can be used early in production, prior to blooms. Concert II features a convenient pre-mix combination of propiconazole (FRAC 3) and chlorothalonil (M5). With this combination of active ingredients, it has a unique suspo-emulsion formulation that results in improved coverage and mix stability, helping the active ingredients work better together for extended disease control. 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 and your operation by visiting GreenCastOnline.com/Solutions.

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

©2021 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^®, SOLATENOL^® and the Syngenta logo are trademarks of a Syngenta Group Company.