Late Season Botrytis

Diseases in bedding plants are a problem throughout the growing season, however, the types and frequency of diseases vary depending on the growth stage of the plant and the environmental conditions in the greenhouse or production area.

Early in the season there is plenty of space on the bench, the plants are smaller, and there is adequate spacing between plants. This spacing allows for good air flow around and under the plants. Consequently a disease like Botrytis Blight is less of a problem. As the season progresses the plant canopy fills and the spacing between plants becomes less and less. As this process takes place, the environment becomes more favorable for a fungus like Botrytis to become established. Also, as plants begin to flower and the flowers age they become very susceptible to Botrytis attack. For Botrytis to thrive it needs a good food source ( senescing flowers and leaves) and the proper environmental conditions (relative humidity at or above 85%). These are the exact conditions that exist late in the growing season.

Flowers on bedding plants such as geranium, petunia, and impatiens are very susceptible to Botrytis attack, particularly once they have passed their prime (Figure 1) (Figure 2) (Figure 3). The area under the leaf canopy is also an ideal environment for Botrytis to develop (Figure 4). Here the disease becomes established because the humidity is high and the older leaves are a very good food source for the fungus. With plants such as vinca the Botrytis can move from the infected leaf into the stem and cause cankering (Figure 5) (Figure 6)

Although Botrytis is very difficult to avoid in the late season there are ways to cut down on it's incidence. Keep as much space as you can between plants. Keep the air circulating in the greenhouse. This will help reduce the relative humidity in the plant canopy and discourage Botrytis from gaining a foothold. Remove spent flowers and leaves. This removes the food source. If all else fails, you may have to resort to fungicides. However, in order for fungicides to be effective they must get down into the canopy. Because the canopy is so thick, high volume sprays are the choice of application method.