Gray mold is caused by the fungus Botrytis spp, a fungus with over 200 hosts. Among those hosts are plants including fruits (ex. strawberry, brambles, grape), vegetables (ex. lettuce, onion, bean) and ornamentals (ex. peony, geranium, petunia). Gray mold can result in severe losses of potential flowers (blossom blight) and mature fruits. The disease is severe during periods of extended moisture and rainfall (or over watering) and cool temperatures.
Signs and symptoms
The symptoms present will vary depending on the type of plant and the plant part infected, but common symptoms caused by Botrytis spp infection are water soaking and discoloration that ranges from gray to brown. Signs of the fungus can be identified by a fuzzy white to gray mold, which are the mycelia (vegetative propagules) and spores of the fungus. The fungus can form sclerotia (minute, hard, black, irregularly shaped, survival structures) for overwintering as it grows.
Not to be confused with stone fruit brown rot.
Botrytis spp can overwinter as hyphae (filamentous, vegetative growth propagules) or spores in plant debris or the soil in temperatures as low as 39ºF or lower temperatures as sclerotia. These asexual fungal propagules can germinate under favorable conditions to form more spores for dissemination. Spores d infect weak or injured tissues, such as flower petals, fruits, or wounds. Spores need free moisture for germination and infection, making disease development more prolific in periods of high moisture and humidity. As the disease develops, more spores can form, providing a source of inoculum for secondary infection in a single growing season.
Type of sample needed for diagnosis and confirmation
Collect 3 to 5 twigs/stems with leaves attached and fruits/flowers. For further information go to sample collection page. The Iowa State University Plant & Insect Diagnostic Clinic can help you to investigate and confirm if your plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each states diagnostic laboratory for U.S. residents can be located at the NPDN website. If you have a sample from outside of Iowa, please DO NOT submit it to the Plant & Insect Diagnostic Clinic without contacting us.
Plant health maintenance
One of the best ways to prevent development of this disease is to maintain sufficient air circulation for plants, which involves proper plant spacing when planting or transplanting, and pruning to avoid overcrowding in raspberries. This will help to minimize the moisture retained in plant canopies and reducing the ability of the spores to germinate and infect. Minimize wounding of the plants when moisture or dew is present in the plants, injuries can be an opportune area for the fungus to begin colonization.
If planting from seed or transplant, planting certified disease-free seeds could help ensure that gray mold does not appear in your garden. If one plant, fruit, or vegetable appears to be infected by Botrytis spp, be sure to remove affected plant parts promptly to prevent the development of more spores that will make the disease more prolific.
Scouting for disease
Symptoms can be first observed as water-soaking or browning lesions. Keep an eye on plant parts exhibiting such symptoms to monitor disease development. Be especially vigilant when plants are flowering, and fruits are developing. Ideally, the disease will be caught before spores form on infected plant parts, but spores may be present as a gray, fuzzy growth. If suspected to be Botrytis spp infection, remove plant parts promptly and if you desire a diagnosis, submit to the Plant and Insect Diagnostic Clinic.
Sanitation is the most important tactic to gray mold prevention. Since the fungus primarily overwinters in debris and soil, removing infected debris at the end of the season will reduce a source of primary inoculum. Additionally, it is crucial to remove any plant parts that are suspect of infection by B. cinerea to remove further sources of inoculum that can perpetuate the disease.
Using fungicides is not warranted in the home garden. Commercial producer of raspberries and other susceptible plants should focus preventative spray programs. In raspberries, application, when plants are flowering, can be a very effective way to prevent disease development on fruits and increase yield. Spray applications can be repeated every 7-10 days through fruit harvest though be cognizant of the components of the fungicide spray. However be very mindful of the plant and timing, to protect pollinators. See How to protect and increase pollinators in your landscape
Did you know that Botrytis cinerea is used in the cultivation of a particular type of dessert wine called Botrytis wine? The disease, in this case, is called “Noble rot” and is favored. Grapes are harvested at a specific time after initial infection in the grape bunch when the sugars are condensed, which results in the wine that is extra sweet! Botrytized wine is made in some vineyards by spraying inoculum in the form of spores onto healthy grapevines.
By Chelsea Harbach (former ISU Plant Pathology Graduate student) and Lina Rodriguez Salamanca