Lessons from our year of botrytis
Last year on our farm, Moonshot Farm in East Windsor, New Jersey, we experienced a devastating year for a relatively common cut flower disease, botrytis (aka gray mold). The disease first appeared on ranunculus and freesia in our winter greenhouses, and later a different strain took hold of our lily crop. All told, we lost over $15,000 in product due to botrytis in 2023 and learned a lot of tough lessons along the way.
Botrytis is a fungal disease that eats away at flower petals and leaf tissue, causing telltale spots that make stems unmarketable. Eventually those spots develop a fuzzy gray mold. The most common form we have seen is Botrytis cinera, which can infect many different varieties of flowers. There are also species-specific strains, including Botrytis tulipae, also called “tulip fire,” which can devastate tulip crops. Botrytis paeoniae is on peonies, and Botrytis elliptica infects lilies.
Botrytis growing on white ranunculus.
In our experience, Botrytis cinera tends to show up as spots on petals late in the growing cycle (i.e., around harvest time) and is relatively easy to eliminate. The species-specific forms tend to cause more devastating issues, like deformed plants and aborted buds, and can be much harder to knock out. Regardless of the botrytis type, they have a lot in common.
Humidity is the enemy
Botrytis requires high humidity to infect plants. If you are able to effectively manage humidity, keep plants dry, and enable good airflow, botrytis simply won’t develop. Humidity control applies mostly to growing in greenhouses and high tunnels. We utilize Govee temperature monitors in all of our greenhouses and have them set to alert us if humidity rises above 85 percent, the danger zone for botrytis.
We’re able to control humidity through vents and side walls that open and close, plus HAF fans to keep the air moving. During times of overcast, wet weather we will pay special attention to humidity control. We’ll sometimes run heaters at dusk with our greenhouse vents open to help draw out moisture. We’re also careful to irrigate in the morning and only on sunny days when water will have ample time to evaporate.
In the field, managing humidity is more difficult. Increasing plant spacing can make a big difference in improving how air moves through the crop and help minimize botrytis. We are also prioritizing using greenhouse space for botrytis-prone flowers.
We learned this lesson the hard way in 2023 when we decided to remove the plastic from our lily tunnel. We had hoped that growing lilies in the open air (utilizing shade cloth only), they would have lots of air flow and thrive. We then experienced a long stretch of unusually cold, wet weather in late spring. The lilies got wet and never dried out again. Botrytis destroyed the entire crop. In the future, we will always grow lilies in a high tunnel under plastic, and we’ll work harder to keep humidity low.
In most pest- and disease-related articles, authors advise regular scouting. Catching issues early is key to eliminating them. Unfortunately, botrytis can be so aggressive that scouting is less helpful. We have seen it take hold overnight. One day a crop will look beautiful and then 12 or so hours later, every flower in a greenhouse will be trash. Instead, we have taken the approach that botrytis is inevitable in humid conditions.
Hygiene
Botrytis infests and rapidly reproduces on decaying plant matter, so hygiene is critical. We now strip leaves into crates to keep dead plant matter away from growing crops. We regularly sweep out aisles. If we find any actively growing mold spores, we carefully throw them away. Once a crop is done blooming, we try to flip beds quickly and remove any spent crates of growing medium.
Botrytis on a lily bud. All photos courtesy of the author.
We also now sanitize our greenhouses and high tunnels several times a season using a product like Zerotol. This hydrogen peroxide-based product can be applied to the walls and floors of the greenhouse, as well as to the soil and even to plants. We find spraying it can make a dramatic difference in Botrytis pressure as it kills the spores.
During times of high pressure, we have even misted it onto blooming plants and it has worked to prevent botrytis. Zerotol can cause some damage on sunny days to blossoms, so be sure to do a test patch. If we are applying biofungicides (more on that below), we will wait a day after spraying Zerotol as it can render them ineffective.
Post-harvest botrytis
More scary news about botrytis: sometimes it appears after flowers have been harvested. We experienced this phenomenon for the first time on our freesia crop last year. Freesia is a very slow growing flower and can take six or more months from planting until bloom. A lot can go wrong in that time, and we were thrilled when we finally started harvesting flowers that appeared perfect.
We cut several thousand stems and did our normal post-harvest freesia routine: bunched in 10s, wrapped bunches in paper, and popped them into our cooler for several hours. When we took them out of the cooler, the freesia was covered in botrytis spots. We have since seen this post-harvest botrytis appear in other flower varieties, including lilies and chrysanthemums. The culprit? Botrytis has already colonized on the flowers, and the high humidity of the cooler makes it much worse.
Botrytis spots on freesia, which did not appear until after several hours in the cooler.
During times of high disease pressure, we now test flowers for 12 hours in the cooler before putting whole buckets of blooms in. If botrytis develops on the test stems, we’ll avoid using the cooler for the remainder of the flowers. Flowers can instead be stored somewhere cool, dry and dark, like an air conditioned room or closet, where botrytis will develop less rapidly. We have saved entire crops of flowers by avoiding the cooler.
We have also noticed post-harvest botrytis can develop inside humid boxes when shipping flowers. Testing stems and choosing less susceptible varieties seems to be key in preventing this.
Finally, we have learned only to put dry flowers into the cooler. On wet, rainy days, we let flowers dry off in front of box fans for several hours before putting them into the cooler. We also run a box fan in our cooler to help keep air flowing and reduce humidity slightly. High humidity in the cooler is important for flower quality, but even a reduction from 90 percent to 85 percent seems to reduce botrytis.
Varieties and colors matter
We find there is a huge difference in botrytis susceptibility depending on species, variety, and color of flower. Any flowers with thin, delicate, or waxy petals seem susceptible. For us this has included lilies, lisianthus, chrysanthemums, ranunculus, anemones, tulips, and freesia. But within these species, some varieties and colors perform differently than others.
We have found white freesia to be less susceptible to botrytis than colored varieties.
White ranunculus seem far more likely to show botrytis than colored varieties — so much so that we have essentially stopped growing white ranunculus. Freesia are the opposite. Colored freesia varieties seem to show botrytis much worse, so now we grow only white freesia.
Choosing double-flowering varieties has also helped. Because botrytis tends to show up on the outside of flowers (where condensation develops), the external petals on double flowers can be peeled off during times of high disease pressure. During our terrible lily botrytis outbreak, we were able to salvage our double-flowered roselilies by removing the outermost petals.
Several species of lisianthus seem less susceptible to botrytis, like the Celeb series.
Certain lisianthus varieties seem to be less prone to botrytis than others. We have had success with the Voyage and Celeb series, which have sturdy petals that hold up to botrytis pressure. The plant breeder Evanthia is now focusing on breeding botrytis-resistant varieties, and we are trialing some of these varieties, like LisAdora, this year.
Botrytis is a major issue across the global floriculture industry, so hopefully breeders will continue to focus on developing resistant varieties.
Chemical and biological controls
Our farm is not certified organic, but we generally choose biofungicides and organic-allowed solutions. Botrytis is resistant to many systemic chemical pesticides, so this organic approach may make sense even for conventional growers.
We have had some success in managing Botrytis cinera using a biofungicide called BotryStop. Sprayed regularly and early in the crop growing cycle, it will inoculate plants and out-compete the botrytis. Unfortunately, we have not found it to be effective against species-specific strains of botrytis like the form attacking our lilies. Actinovate is another biofungicide that we are finding may be helpful against the lily-specific strain.
Perhaps the most promising botrytis control is surprisingly simple: Calcium. This discovery was first made by the landscaping and nursery industry when a few studies showed calcium greatly improved botrytis severity in bedding plants like petunias. After reading these studies, we started applying foliar calcium on a weekly basis to all of our plants grown under cover.
Calcium increases the strength of plant tissues, making them less susceptible to botrytis. We have found it to be extremely effective and think it will be the backbone of our control going forward. Calcium also seems effective at preventing powdery mildew and strengthens the stems of plants.
Some newer studies have even suggested that calcium can be applied as a post-harvest spray or dip on flowers to prevent botrytis from developing in the cooler. We will definitely be testing that this year on our lilies and freesia.
After losing so much of our spring lily crop to botrytis, the year ended on a better note. Before planting fall lilies, we cleaned up the farm, threw all diseased plant matter into the garbage (not our on-farm compost). We sanitized our greenhouse thoroughly and added more HAF fans.
We spaced our lily crates apart for better airflow and were religious about only watering in the morning, never wetting foliage. We also sprayed the plants weekly with calcium and Actinovate. The year ended in a much better place: a gorgeous crop of thousands of botrytis-free lily blooms, right in time for the holidays. Although this disease is scary, I hope these simple tips help you keep it under control.
Rebecca Kutzer-Rice owns Moonshot Farm, a specialty cut flower farm in East Windsor, NJ. She grows flowers year-round including in a geothermal greenhouse, for retail markets in and around NYC.
