Environmental Conditions that Increase the Risk of Fall Diseases

Fall is just around the corner, which means garden mum and poinsettia production has begun. Both crops are susceptible to a variety of ornamental diseases such as root and stem rots, botrytis and more. The first step toward growing healthy crops is to understand how environmental conditions in your operation impact disease development. With this knowledge, you can mitigate disease threats by maintaining proper growing conditions for your crops.


Outdoor production during the summer can be stressful for garden mums and provide conditions favorable for Fusarium, including high temperatures, direct sun on the container and over-saturated soil conditions caused by summer rains. Affected crops can show one-sided wilt, light-colored foliage and root discoloration. Symptoms of infections often show in late summer, but infections begin much earlier, so it is important to protect crops throughout the season.

Early signs of Fusarium on garden mums

Early signs of Fusarium on garden mums
Syngenta photo, 2011


Botrytis blight, or gray mold, is the most common disease that affects many ornamental plants. Symptoms appear as light brown spots or tiny flecks on flowers, or V-shaped, tan-brown lesions on foliage. It is more likely to affect poinsettias, especially during shipping when they will be boxed, stored or transported, as humidity and ethylene levels contribute to plant stress and susceptibility. Botrytis is more likely to develop when there is an extended period of cloudy, damp weather, temperatures are between 62 – 75°F and leaves are wet for four or more hours.

Powdery Mildew

Powdery mildew fungi form white, talcum-like spots called “colonies” on leaves, stems and flowers. Over time, these colonies increase in size and number to cover the plant’s surface. If not controlled, severe infections will cause leaves to turn yellow, brown and then drop, resulting in reduced plant vigor and growth. Powdery mildew is most likely to develop in the fall when temperatures are between 62° – 72°F, relative humidity levels are 70% or greater and there is reduced light intensity.

Early signs of Fusarium on garden mums

Powdery mildew on poinsettia bract
Margery Daughtrey, Cornell University, Bugwood.org


Rusts caused by Phragmidium spp., Puccinia spp., Coleosporium spp. or Uromyces spp. pathogens are more likely to affect garden mums during production. Pale white chlorotic spots appear on the upper surface of the leaves. Raised, blister-like pustules typically form on the undersides of leaves directly under these spots or on the stems. Mums are susceptible to infection during cool, humid conditions and when foliage has been wet for extended periods.


Root and stem diseases caused by Pythium spp. pathogens are likely to emerge under high temperatures and moist conditions. Roots can be compromised by repeated wetting and drying resulting in wounds, which increase the risk of infection. Growing media with high soluble salts can also cause root burn and make plants more susceptible to attack. Affected plants will appear pale green and show signs of wilt during the warm times of the day and recover in the evening. As the disease progresses, plants become stunted and fail to recover.


There are numerous Phytophthora spp. pathogens that can affect ornamental crops. In poinsettia, Phytophthora spp. is responsible for causing root and stem rots as well as stem cankers that result in plant collapse and death. Dispersal of the pathogen can occur through the splashing of irrigation water, use of infested potting media and soils, as well as contaminated plant material. Plants grown in saturated soil conditions due to overwatering and areas prone to flooding are at risk for infection.

Put a Plan in Place

Ensuring proper environmental conditions and cultural practices are the first steps toward preventing issues during production, however diseases can be tenacious. Plan ahead and prepare for the unexpected with a reliable agronomic program. Agronomic programs provide the framework for properly using different products to control primary disease problems, infestations and proactively manage against resistance.

Postiva® and Mural® fungicides are key components of many Syngenta agronomic programs as they are both broad-spectrum and extremely effective against difficult-to-control ornamental diseases. Powered by ADEPIDYN® technology and difenoconazole, Postiva provides strong protection against Fusarium, powdery mildew and more, while Mural controls key foliar, stem and root diseases including botrytis.

Visit GreenCastOnline.com/Solutions for agronomic programs from Syngenta.

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

© 2022 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. ADEPIDYN®, GreenCast®, Mural®, Postiva® and the Syngenta logo are trademarks of a Syngenta Group Company. All other trademarks are the property of their respective third-party owners.

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