With the intense period of rain we just had, and with MORE rain coming on Friday, it’s time to think about Botrytis control and prevention. One of the most common and destructive diseases of greenhouse crops, outbreaks usually follow periods of cool, damp, cloudy weather. Unfortunately, I can’t order up more sun for you, but I CAN suggest some management tactics.
What is Botrytis?
Estimated to cause more economic loss in ornamentals than any other disease, this fungus has many common names, including blossom blight, bud rot, stem canker, stem and crown rot, cutting rot, leaf blight, and damping-off or seedling blight. Although there are over 50 different pathogenic species of Botrytis, in floriculture, Botrytis cinerea has the largest host range, and is most likely the one you’re dealing with.
Unfortunately, Botrytis cinerea has an extremely wide host range. Flowers with thick succulent petals, such as begonias, peonies and geraniums, are particularly susceptible. The disease also commonly affects African violet, dahlias, pansy, snapdragon, zinnia, chrysanthemum, and many others.
How Do I know I Have it?
Botrytis can cause different symptoms on different kinds of plants, so you may need to confirm your diagnosis by sending samples to Guelph Lab Services. However, Botrytis infection usually begins as brown to gray circular spots that appear water-soaked regardless of the tissue affected. Later, tan to gray fuzzy mold develops on rotted tissue under humid conditions.
Generally, Botrytis attacks tender tissues (flower petals, buds, or seedlings), weakened or injured tissues, and aging and dead tissues. Actively growing tissues (e.g. healthy leaves) are seldom invaded, unless dead, infected tissue touches them directly.
How Does it Spread in the Greenhouse?
Botrytis cinerea persists in the greenhouse year as resting spores on living or dead plants, or in infested soil (which is one reason it’s important to always use new potting mix and sterilize pots appropriately). When the environment is right (i.e. temperatures around 15 C with >90% RH), the resting spores will germinate into active spores, called “conidia”. Conidia are dust-like, and can be easily dispersed in large numbers by air currents to new plants or overhead watering.
How Do I Control It?
Strict sanitation is your FIRST and BEST line of defense. Old, infected tissue provides a food base, allowing the mycelia of Botrytis to penetrate nearby healthy tissue, and also produces the conidia that travel through your greenhouse by air or water splash. Inspect plants regularly for signs of infection and carry a paper bag: place faded or blighted flowers, blighted leaves, or entire plants in the paper bag and discard. It’s best not to do any sanitation when plants are wet since this could spread fungal spores during conditions which favor infection.
Because Botrytis thrives on humidity, also maximize air circulation in your greenhouse. Maintain the humidity below 85 to 90 percent by (a) forced circulation or (b) an increased amount of heat. Proper bench spacing is also essential in reducing humidity in the canopy.
Unfortunately, most chemicals for Botrytis control are preventative. They’ll help control the spread of the disease, but are unlikely to cure it once it happens (which is why cultural controls, like air circulation, are so important here). However, if you have a particularly susceptible crop, or have struggled with Botrytis before, consider a fungicide application before an upcoming period of extreme cloudy/wet weather.
Note that many of the products for Botrytis can cause phytotoxicity, especially on open blooms, so it’s EXTREMELY important to read the label before applying. You can search for the newest copy of the label for any fungicide here.
Some of the products registered for Botrytis in Canada are the following *. Check out OMAFRA Floriculture Production Guide for other options. (Remember to always rotate chemical classes to avoid resistance!):
- Decree (fenhexamid) and Heritage (azoxystrobin): Both are Group 17 fungicides, have systemic activity and low REIs (4hrs). Both also performed well in efficacy tests for Botrytis control compiled by the United States IR4 program. When conditions favor severe disease incidence, however, you may need to spray at 7 day intervals. Although neither chemical have reported phytotoxicity issues on the label, ALWAYS check a portion of your crop for potential damage.
- Rhapsody ASO and Cease Biological Fungicides: You might be surprised that biological agents can control of foliar diseases – not just root rots. Two independent studies have demonstrated that microbial products with Bacillius subtilis have good efficacy against Botrytis. Like other products, you’ll likely need to spray at 7 day intervals for good control and need to rotate with other chemical classes.
- Compass (trifloxystrobin, Group 11): also has systemic activity (a plus) and provided good control in IR4 tests on geranium and poinsettia, but has known phytotoxicty to petunia, pansy and New Guinea impatiens. It has a 12 h REI in potted crops but this goes up to 48 in cut flowers.
- Medallion (fludioxonil; Group 12): also provided good to excellent Botrytis control in IR4 tests, although it showed some phytotoxicity on geranium in these tests (not on the label). It has a 12 hr REI.
- Daconil (chlorothalonil; Group M5): Although this chemical also has good activity according to IR4 tests, it has a long re-entry time (48h). It also only has contact activity, and is also known to discolor blooms of certain plants if sprayed during flowering. Check the label for details.
- The copper-based product Phyton 27 (Group M) is labeled as a post-harvest dip for cut flowers and buds to prevent Botrytis during storage/shipping.
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