Cut Flower Updates: June 27, 2025

Temperatures soared into the upper 90s this week across much of Pennsylvania, making field work difficult and potentially dangerous. It's unlikely to be the last hot spell of the summer, so it's a good time to review safe working habits for very hot days (Rutgers has a great factsheet on the basics of heat safety) as well as the best practices for harvesting and storing flowers when high heat is expected.

Harvest in the morning when temperatures are cool and plants have a higher water content. Remove most stem foliage to decrease transpiration, and stick stems in buckets of cool water in the shade or a cooler as soon as possible. Be sure you are harvesting at the correct stage to ensure a longer vase life (learn more).

Despite the high temperatures this week, we are still seeing the reminders of our cool, humid spring with the emergence of some early fungal diseases on spring crops. Iris leaf spot, shown below, is one such example that was observed in Southeast PA this week.

Iris leaf spot is a common disease of irises and narcissus that often becomes visible in the weeks after harvest. The disease, caused by the pathogenic fungus Didymellina macroscopa, is most severe in German bearded irises. Small brown spots on the foliage have water-soaked margins in the early stages, and later develop yellow haloes. The spots grow larger as the season progresses, often causing leaves to die back from the tips down. Premature leaf dieback can impact photosynthesis, leading to reduced carbohydrate stores in the rhizome and less vigorous plants the following season.

The pathogen overwinters on dead tissue, so inoculum for the next season can be reduced by removing old, diseased leaf debris in the fall. If severe infection is expected based on the site history, preventative fungicides can be used with a spreader-sticker to reduce disease pressure starting when leaves are 4 inches high.

Educators have also received calls about other fungal and fungus-like pathogens this week, including pythium and fusarium wilt, which both impact crop roots. Both of these diseases were reported on lisianthus.

Photo: Ask Extension

Plants infected with fusarium wilt will exhibit yellowing leaves and wilting during the hottest parts of the day, even in well-watered beds, perking back up as temperatures cool at night. Eventually, the plants will die. This disease spreads on seeds and in soil, and can survive on plant debris. Be sure to use certified disease-free seed, or if you are using plugs, be sure to use a trusted source. If you have plants that you suspect may be infected with fusarium wilt, send a sample to the Penn State Plant Disease Clinic. 

Pythium is a fungus-like organism that causes root rot, particularly in wet and water-logged soils and greenhouse media. Similar to fusarium, plants infected with pythium can appear stunted, yellow, and may wilt in midday. The roots of infected plants are brown, and the outer layer of the root (cortex) will slough off easily.

Photo: Mary Burrows, Montana State University, IPMImages.org

It is easier to prevent pythium infection than to treat it. Keep all greenhouse and seeding benches, tools, and equipment clean and avoid allowing water to collect where seedlings are kept. Seedlings and plugs can be treated with root dips containing Trichoderma to protect roots where infections are likely.

Stay cool out there! Remember that you can always email Margaret Pickoff at mfp5667@psu.edu if you're experiencing a pest, disease, or production issue you’d like us to feature in this update. We’re always looking for new issues to explore in our weekly post. If you haven't signed up for our email reminders, you can manage your communication preferences and select "Commercial Cut Flower" as one of your preferred Areas of Interest.