3/17/2023 - By Dr. Aaron Palmateer

Another new year often brings exciting opportunities to take advantage of and challenges to overcome. One challenge that has been part of the ornamental plant world from what seems like the very beginning is the fungal pathogen Botrytis. In this article, I will discuss what Botrytis is, where you may see it, and how you can counteract it going into the new year.

What is Botrytis?

Botrytis cinerea is a fungal plant pathogen that causes one of the most destructive and challenging diseases of plants grown in greenhouses. Gray mold or Botrytis blight can be an issue but is less common on outdoor plantings including shade houses and open field nurseries. The disease occurs mostly during or following cool, damp, and cloudy conditions meaning that going into spring is a high risk for Botrytis to develop. In addition to plaguing ornamentals, Botrytis attacks most greenhouse vegetable and fruit crops and is common on plants and plant products, during shipping and post-harvest storage even occurring on plant products in cold storage.

What Makes Botrytis Such a Challenging Plant Pathogen to Control?

Botrytis cinerea is a saprophytic plant parasite meaning that it has the capability of living on both dead and live plant material which allows for this pathogen to build up a large population on your plants, but also on any crop debris (i.e., senescent leaves and flowers) where it can easily go undetected and be the next source for a disease outbreak. The fungus can invade and damage just about any part of the plant, but blight mostly occurs on more tender plant tissues including the flowers, buds, and leaves with weakened and damaged tissue being the most susceptible. Botrytis is notorious for invading freshly cut tissue during pruning and propagation, which frequently results in stem lesions, dieback and has been reported to inhibit rooting on infected cuttings.

When Are Plants Most Vulnerable to Botrytis?

Botrytis likes cool weather and disease development is most often favored by cool and damp conditions with temperatures in the 70s and relative humidity at 90% or greater. The fungal spores need free moisture or high relative humidity to germinate, so allowing the leaves to dry will help reduce disease. Botrytis can be active at very low temperatures even causing losses on plant material being held in cold storage ranging from 32̊ to 50̊ F. A challenge is that most cold storage units are maintained with high relative humidity to prevent shrinking or shriveling of plant material, but consequently these conditions are favorable for Botrytis. Thus, the preventative use of fungicides and effective sanitation is essential for protecting plants and plant products during transportation and storage.

How Can I Manage Botrytis?

The most cost-effective way to manage Botrytis is disease prevention. Routinely scouting plants for early disease detection is important to prevent outbreaks. Maintain a clean operation by properly sanitizing pots, benches, tools, and floors to minimize contamination. Botrytis can easily survive on dead plant tissue, so it’s important to remove and discard any fallen or dead plant debris. A preventative fungicide rotation is especially important when growing Botrytis susceptible plants in a greenhouse. Botrytis cinerea is reported as a high-risk pathogen for its potential to develop fungicide resistance by the Fungicide Resistance Action Committee (FRAC). The high-risk category includes fungal plant pathogens that have evolved resistance to fungicides in a sufficiently short time frame (e.g. within a few cropping seasons) and where there are documented cases of resistance to more than one class of fungicide.

Although Botrytis is labeled as high-risk for developing resistance to fungicides growers should never assume they have a resistant population. Fungicide resistance is not likely to occur when following a preventative rotation and maintaining a sanitary operation. Be sure to follow the fungicide label and especially the recommendations for rates and application intervals. Note most fungicides have specific rates for Botrytis that may differ based on production stages (i.e., plant propagation vs finishing) or environmental conditions. For example, the recommended application rate and interval may be higher and more frequent when conditions for disease are favorable.

Botrytis Prevention

Fungicides are one of the most important tools for effective disease management and individuals making fungicide application decisions must follow the manufacturer’s recommendation for proper rates and application intervals. The proper use of fungicides and recognizing the importance of rotating FRAC groups for control of economically important plant diseases such as Botrytis blight will ensure long-term success and future availability of fungicides for disease management.

Another mode of action to consider as a tank mix or rotation includes Chlorothalonil® (FRAC MO5).

Other modes of action to consider as a tank mix or rotation include Chlorothalonil® (FRAC MO5) or the all-new ProtectMAX Fluoxastrobin SC (FRAC 11).

Reach Out to Your Harrell's Rep for More on Botrytis

For more information on Botrytis, prevention or any of the products mentioned, reach out to your local Harrell's Rep. We're here to help.