Managing cool-season diseases in ornamental greenhouse production

Winter greenhouse conditions are marked by lower light levels, cooler temperatures and higher humidity, creating an environment where many pathogens thrive. While many crops, such as geraniums and pansies, prefer cooler temperatures, cool-season conditions also favor destructive diseases such as Botrytis blight and root rots caused by Pythium spp. and Phytophthora spp.  While all ornamental crops are susceptible, certain crops require more frequent scouting, especially high-value crops like bedding plants, perennials and woody ornamentals. 

BOTRYTIS 

Botrytis blight, commonly known as gray mold, is the most prevalent disease threatening ornamental crops. Caused by the fungus Botrytis cinerea, this pathogen can colonize both living and dead plant tissue, and thrives in cool, humid environments that are typical from fall through winter.  Young plant material such as seedlings and cuttings are especially vulnerable, as are crops in postharvest conditions where high humidity and excess moisture allow infections to spread rapidly. This is a critical consideration for finished plants that have been or will be boxed, stored or transported. Protective treatments prior to shipping can help minimize disease outbreaks. Disease risk increases during periods of extended cloudy, damp weather or when humidity levels exceed 85%, exposing leaves to prolonged wetness for four or more hours per day.  When scouting, look for the early signs of infection: 

• Small, light brown spots or tiny flecks on flowers 
• V-shaped, tan-brown lesions on foliage 
• Sunken, discolored cankers on stems 
• Brown flower buds that appear water-soaked 
• Fuzzy brown or gray spores 
• Plant wilting due to stem rot. 

(Shown above) Progressive botrytis development on geranium plants, from a small lesion with yellowing on a leaf (1) to advanced stages with fuzzy, gray mold visible (3). Botrytis has a high potential to develop fungicide resistance, so it is important to follow an effective management program featuring both cultural practices and a carefully designed fungicide rotation. Key prevention measures include maintaining airflow and spacing, removing diseased debris, irrigating early and sanitizing between crops.  Postiva fungicide from Syngenta is one solution with two modes of action that provides long-lasting protection. Rapidly absorbed into plant tissue, it creates a rainfast barrier that prevents infection and disease progression.  It also serves as an excellent rotation partner with Palladium fungicide, another product with two modes of action, broadening spectrum control and strengthening resistance management strategies. 

PYTHIUM SPP. 

Pythium is a persistent threat, spreading through greenhouses during the fall and winter months. Cooler temperatures and overcast weather create ideal conditions for infections, especially when growing media stays wet for longer periods.  Under these conditions, oospores germinate and invade plant wounds and tender tissue, quickly compromising root systems and stems. Both unrooted cuttings and young seedlings in propagation are vulnerable, but established plants can also suffer significant root damage. During scouting, examine plants carefully and be aware of the warning signs: 

• Seedlings that collapse shortly after germination or appear nutrient deficient and stunted 
• Transplants and unrooted cuttings developing lower stem rot 
• Yellowing of lower leaves 
• Plants showing occasional wilt even when the media feels moist 
• Roots that appear soft, brown or water-soaked, allowing for the outer portion of the root to be easily removed, leaving behind the stringy vascular core of the root. 

Preventing Pythium spp. begins with proper sanitation, spacing, reducing humidity, elevating pots to avoid standing water and promptly removing infected plants.  Mural fungicide from Syngenta delivers systemic, protective activity against pathogens like Pythium, and complements well with other products in an agronomic program, such as Subdue Maxx fungicide, for broader activity against common root and stem pathogens while also providing plant health benefits. Together, Subdue Maxx and Mural deliver robust, multi-spectrum protection. 

PHYTOPHTHORA SPP. 

Phytophthora is a soil-borne pathogen spread through contaminated water or infected plant material. While some Phytophthora species favor warm conditions, others thrive in cooler temperatures. This versatility makes the pathogen a year-round threat in greenhouse production.  Phytophthora spp. can affect a wide range of herbaceous and woody ornamentals, including azalea, boxwood, magnolia, rhododendron and many others. Infections often follow extended periods of soil or foliar wetness. The pathogen also survives in soil or debris as oospores or chlamydospores, producing new infections each spring.  Early detection is critical for preventing widespread damage. Symptoms of Phytophthora spp. infection include: 

• Slow plant growth, stunting and smaller-than- normal foliage 
• Yellowing leaves that roll, wilt and drop due to poor water absorption 
• One or more branches wilting, then turning brown and necrotic 
• Plant collapse when infection reaches the lower stem 
• Dark brown to black roots with a musty odor 
• Progressive decline over weeks to months as roots and stems are destroyed. 

Effective management starts with cultural practices that reduce favorable conditions for Phytophthora, including using clean planting media, switching from overhead to drip irrigation if foliar Phytophthora spp. is of concern, avoiding overwatering, irrigating early and most importantly, elevating plants to reduce exposure to standing water.  A proactive program combining cultural practices with a rotation of systemic fungicide products, such as Segovis and Subdue Maxx fungicides from Syngenta, offers growers reliable protection against Phytophthora spp. and long- term peace of mind.  Botrytis, Pythium spp. and Phytophthora spp. remain top threats to greenhouse ornamentals during cool- season production. A robust agronomic program including vigilant scouting, sound cultural practices and a preventive fungicide rotation is the most effective strategy for protecting crop quality and potential marketability.  Photos by Nancy Rechcigl. © 2025 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. Mural®, Palladium®, PostivaTM, Segovis®, Subdue Maxx® and the Syngenta logo are trademarks of a Syngenta Group Company.

Nancy Rechcigl is technical field manager for Syngenta Ornamentals.