Gray mold (Botrytis cinerea) and its many strains cause death on flower parts, leaves, buds, shoots, seedlings and fruits. Herbaceous plants and woody plants can be attacked. The disease is not host specific. The number of plants that Gray mold (Botrytis) will attack is in the hundreds, maybe even the thousands. The disease needs moisture as one of its criteria for infection. The wetter the plant is, the more likley the gray mold will show up on plants. Not only are the numbers of infected areas increased but also so are the numbers of plants attacked as well as the severity of the infections (quicker growth of the disease and death of tissue).
Botrytis is sometimes confused with old age and thrip damage. To help separate browning on petals caused by old age and the disease, check the petals closely. Browning from old age should occur on the outer petals first and along the edges or at the tip of the petals. When inner petals or the middle of the petals develop brown lesions first, gray mold is probably the reason. On leaves that are relatively thin such as redbud, lesions are light brown to brown and irregular. On fleshier leaves such as rhododendrons concentric rings of different shades of brown may be visible.
Symptoms may vary depending on the plants attacked, plant parts attacked and growing conditions. Under moist humid conditions a grayish thin mass of webbing (mycelium) may be visible. A structure called conidiophore that contain spores often develop in the "webbing." When the infected plants are disturbed, large dusts like clouds of released disease spores may occur. Misty or rapidly rising humidity may also trigger the release of spores. Poor air circulation adds to the survival and growth of the disease. When temperatures are between 68 and 76 degrees and humidity is high, it takes about 20 hours for the disease to infect. Warm to hot dry weather tends to reduce or stop the growth and spread of the disease.
The disease often over-winters on infected dead plant material. Sanitation is important. Dead head dying flowers and remove all infected plant tissue. Burning is the desired method of destroying infected tissue. However, burning is banned in most areas of the Chicagoland area. Therefore, infected plant material may be buried. Dig a hole at least a foot deep. Cover the infected Botrytis plant material with at least a foot of soil.
Since Gray mold (Botrytis) often attacks flower petals, fungicides are seldom suggested. Sanitation is the main control. Sanitation is not perfect since the disease can blow in from a long distance. Some house plants that are susceptible to gray mold include African violets, amaryllis, Amazon-lily, azaleas, begonias, cacti, caladium, calla lily, camellias, castorbeans, chrysanthemums, cinerarias, coleus, corn flowers, dalias, dracaenas, dusty millers, ferns, fig, fuchsias, ardenias, gloxinias, heliotrope, orchids, passion flower, and poinsettias.
Some herbaceous annuals that are susceptible to Botrytis are ageratum, begonia, caldium, carnation, celosia, chicory, geranium, gerbera, gladiola, impatiens, marigold, nasturtium, pansy, petunia, snapdragon, statice, stock, sunflower, sweet pea, verbena, and zinnia.
Some herbaceous perennials that are susceptible to Botrytis are anemone, aster, baby's-breath, bellflower, bleeding heart, bloodroot, bluebell, buttercup, calendula, candytuft, carnation, chrysanthemum, chicory, coralberry, cranesbill, dandelion, daylily, delphinium, Dutchman's- pipe, foxglove, globe-amaranth, hyacinth, iris, Jack-in-the-pulpit, liy, lily-of-the- valley, lupine, Maltese cross, narcissus, peony, phlox, pinks, plantain lily, poppy, primrose, purple coneflower, rose, snowdrop, tulip, vinca, and violet.
Some trees and shrubs that are susceptible are alder, apple, apricot, ash, azalea, barberry, cherry, Douglas fir, flowering almond, hawthorn, holy, honeysuckle, hydrangea, larch, lilac, magnolia, mockorange, peach, pear, persimmon, plum, rhododendron, tree-of-Heaven, and viburnum.
Some vegetables and small fruit plants that are susceptible to gray mold include artichoke, asparagus, bean, beet, blackbery, black-eyed pea, blackberry, blueberry, broccoli, Brussels sprouts, cabbage, carrot, cauliflower, celery, chives, cucumber, currant, eggplant, endive, gooseberry, grape, kale, kohlrabi, leek, lentil, lettuce, okra, onion, parsnip, pea, peanut, pepper, pumpkin, quince, raspberrie, rhubarb, rutabaga, shallot, squashe, strawberry, sunflower, sweet potato, tomato, and turnip.
Written by James Schuster, Extension Educator, Horticulture, and reviewed by Bruce Paulsrud, Extension Specialist, Pesticide Applicator Training and Plant Pathology, University of Illinois at Urbana-Champaign.