Cut Flower Updates: July 11, 2025

Our phones (and email chimes) have been ringing off the hook this season with reports of fungal diseases, not just in the cut flower field but in nurseries, garden centers, and landscapes. Weeks of humid, soggy weather have created great conditions for fungal and fungus-like pathogens to thrive, from those that cause root and crown rot (pythium, fusarium, rhizoctonia), to those that affect foliage and flowers (botrytis, powdery mildew, leaf spots, rusts). We thought this week would be a good time to offer a quick primer on managing fungal diseases in cut flowers.

For starters, it's always best to have a handle on which common diseases might affect your most important crops, because identifying the disease is the first step towards taking action. Do your phlox and zinnias often get overwhelmed with powdery mildew? Does rust appear on your snapdragons? Do your dahlia blossoms wither with botrytis before you can harvest them? If you don't know the identity of the pathogen causing crop damage, fungicide applications are less likely to be effective and could be a waste of money (not to mention the potential environmental impact). When in doubt, contact an Extension Educator for help. Sometimes we can identify a disease based on photos or a site visit. If not, we can help you send a sample to the Penn State Plant Disease Clinic for diagnosis.

Some fungal issues can be reduced or prevented by changing the cultural conditions in which the plant is growing. For example, switching from overhead watering to drip irrigation helps deliver water directly to the root zone, keeping plant foliage dry and preventing some fungal spore germination. Root rot pathogens like pythium and fusarium are more likely to cause damage to plants in wet soils with poor drainage. Avoiding planting in poorly drained soils, or taking steps to improve drainage, can help prevent these diseases. Sanitizing tools, greenhouse benches, buckets, and anything else that comes into contact with crops will also help to reduce disease spread. Choosing to plant disease-resistant varieties is another great way to give your plants a leg up.

Some cultural control methods, like increasing plant spacing to improve air flow, are not always feasible in a commercial cut flower setting, as many growers prefer close spacing to encourage tall, straight stems. And in wet seasons like this one, cultural control is not always enough to prevent fungal problems. Growers considering the use of fungicides should keep a few key things in mind.

The first is that most fungicides work to prevent, rather than cure, diseases. Some fungicides, such as root dips used to prevent damping off of seedlings, can be used prophylactically before symptoms are observed. For plants in the field, regular scouting is necessary to identify fungal diseases as soon as they are visible so that applications can be made in a timely manner. Foliar symptoms often appear first on lower or inner stems or on the undersides of leaves, which can be difficult to see without getting up close and personal with your plants on a regular basis.

Once a disease is observed and identified, it's up to the grower to choose a product that fits into their style of production. Conventional fungicides can be very effective, but it's important to rotate modes of action (indicated by the FRAC code on the label) to prevent fungicide resistance. Some commonly used active ingredients in conventional fungicides include etridiazole (damping off), mefenoxam (root and stem rot diseases like pythium and phytophthora, and downy mildew), mancozeb (leaf spots, rusts, botrytis), and chlorothalonil (leaf spots, rusts, powdery mildew, and bulb rots).

Biological fungicides can also be very effective in cut flower production. Most are organic (OMRI) certified, have shorter reentry intervals, and pose less risk to applicators and the environment. They can also be used in rotation with conventional fungicides to prevent resistance. Some active ingredients in biological and organic fungicides include Trichoderma spp. (root dip to prevent damping off), Streptomyces spp. (root and stem rots, fusarium, alternaria, botrytis, powdery mildew), neem oil extract (alternaria, botrytis, downy mildew, powdery mildew, rust), potassium bicarbonate (leaf spots, rusts, powdery mildew, downy mildew), and copper hydroxide (leaf spots, botrytis).

Growers should consult the product label for rates and application instructions, including how often the product needs to be applied to be effective. Some fungicides perform best with the addition of a surfactant, like a spreader-sticker, which will also be indicated on the label. All growers should apply fungicides with care, but cut flower growers should be particularly mindful of the impact of certain fungicides on pollinators and other insects when managing diseases.

In other news, thrips and Japanese beetles are out in full force and starting to cause damage to crops. We'll dive more deeply into these two flower foes next week.