By Millie Reeves
There are about 50 species of Botrytis affecting a wide range of plants. Botrytis cinerea has the largest host range and is the likely cause of Botrytis flower and cane blight that we see in our rose gardens. This fungal disease affects many different plants and can damage many plant parts including flowers, pedicels, stems, leaves, buds, fruits, bulbs, corms, tubers and roots. Botrytis blight usually attacks tender tissues such as flower petals, buds or seedlings; weakened or injured tissues (such as pruning cuts) or aging and dead tissues. Actively growing tissue is seldom affected with the exception of flower petals.
Botrytis cinerea can persist in greenhouses year round as mycelium, conidia or sclerotia on living or dead tissue and as sclerotia or conidia in infested soil. Outdoors the fungus overwinters on decaying plant material and in infested soil. Sclerotia are the main structures for surviving outdoors and can survive a temperature range of 39 to 131 degrees F. Anything that moves soil or plant debris can spread the overwintering stages. When the sclerotia germinate they give rise to conidia (gray-brown powdery spores) and occasionally to infection hyphae. The conidia are then dispersed in large numbers by air currents or splashing water to new plants. The fungus can produce 60,000 or more spores on a piece plant tissue the size of your small finger nail, and it only takes one spore to infect your plant. Infection occurs when the conidia penetrates the plant tissue. The optimal temperature for germination of the conidia is 72 to 77 degrees F with free water or high relative humidity of 90 to 100 percent. The fungus requires an external nutrient source such as aging flower petals or dying leaves and therefore rarely penetrates actively growing tissue directly, except thru pruning cuts or other damage. A source of even greater inoculum potential than the conidia is a diseased flower petal or other infected debris that comes in contact with otherwise healthy tissue. The infected tissue provides the food base for the fungus to penetrate green tissue directly on contact. While Botrytis is usually considered a cool weather pathogen that grows and sporulates best from 66 to 74 degrees F, it is still active at low temps and can cause considerable damage at 32 to 50 degrees F. Once infection by the fungus occurs it can grow over a range of 30 to 96 degrees F.
On roses Botrytis can affect blooms, buds, stems and leaves. On the bloom you may notice small water-soaked spots with red borders on light colored roses. As flowers age they become more susceptible to infection. Infected petals turn tan-brown, wither and fall off. If weather conditions are right, severe infections can occur and the petals become matted and stick together before buds even open. Once this happens the fungus causes the bud to turn brown-black (often covered with light tan to gray mycelium) and can even invade the pedicel, which then rots. Infection can spread from the blossom, leaf or petiole into the stem or enter through injuries during pruning. Lesions on rose canes tend to be black, sunken and elongated with a definite outline. Infections can weaken and girdle the stem which leads to wilting and death of the foliage above the lesion. Botrytis leaf spot appears when infected petals or other debris falls on the leaf and the fungus can then invade the healthy tissue. The leaf spot will take the shape of the debris.
Controlling Botrytis requires attention to both cultural practice and chemical use. Strict sanitation with removal of old blooms, canes showing dieback and other infected plant material can’t be overemphasized since they can produce a tremendous quantity of airborne spores. These materials should be placed in the trash or burned and never placed in a compost pile. Water in the morning and avoid splashing water on foliage when watering to reduce spread of the spores. It takes four to six hours of leaf wetness for Botrytis to germinate. Space plants to provide for maximum air circulation. Avoid over-fertilization, especially with nitrogen, and unnecessary wounding of plants. Since all activities in the garden (irrigation, pruning and fertilizing) will result in the spreading of spores you should time your fungicide spray program to follow these activities.
There are many fungicides that will control Botrytis, but as with all fungal diseases, rotation of chemicals will be required to prevent resistance from developing. No single fungicide should be used more than two consecutive times without rotating to another effective chemical. These chemicals should be from different classes since different chemicals in the same class will have similar resistance profiles. Some of the most effective chemicals are from the following four different classes: Chloronitrile class – chlorothalonil (Daconil Ultrex); Dicarboximide class – iprodione (Chipco 26019) and vinclozolin (Curalan); Hydroxyanilide class- fenhexamid (Decree) and Phenylpyrol class – fludioxinil (Medallion). Many populations of Botrytis in the East and Midwest already have documented resistance to the Benzimidazoles (thiophanate methyl) and the Dicarboximides (iprodione and vinclozolin), so these may not work in your garden. Fungicides should be applied at an interval of five to seven days in rainy, overcast weather and every seven to 10 days in warm weather. Generally these fungicides should be applied prior to the development of disease but some fungicides such as fenhexamid (Decree) will reduce sporulation in active infections. When trying to keep blooms free of Botrytis, mist with a solution of ½ to 1/3 the normal rate of fungicide to the open flower to prevent injury to the bloom.
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