Organic Small Fruit Disease Management Guidelines Integrated Management of Grape Diseases Prepared by Mike A. Ellis, Professor and Mizuho Nita, Graduate Research Associate Department of Plant Pathology at the Ohio State University/OARDC This PDF file is provided as a part of “OSU Fruit Disease Resources” website (http://www.oardc.ohio-state.edu/fruitpathology/). Introduction Disease management strategies are very similar for both organic and conventional small fruit production systems in the Midwest. In both systems it is important to develop and use an integrated disease management program that integrates as many disease control methods as possible, the more the better. Major components of the disease management program include: use of specific cultural practices; developing knowledge of the pathogen and disease biology, use of disease resistant cultivars, and timely application of organically approved fungicides or biological control agents or products when needed. These guidelines have been written for caneberries (raspberry and blackberry), strawberry, blueberry and grape. Specific information is provided for each crop in its respective chapter. Most disease control methods or strategies are identical for both conventional and organic production systems. Perhaps the greatest difference between organic and conventional production systems is that organic growers are not permitted to use synthetic "conventional" fungicides. If disease control materials are required in the organic system, growers are limited to the use of "inorganic" fungicides such as sulfur (elemental sulfur and lime-sulfur) or copper fungicides (Bordeaux mixture and fixed copper products). In addition, there are several new "alternative" disease control materials and biological control products that are currently available and are cleared for use in organic production. There are several problems associated with the use of these inorganic fungicides and "alternative" products in small fruit disease control programs. Among the most important are 1) Phytoxicity, which is the potential to cause damage to foliage, fruit set and fruit finish (this is a concern primarily with copper and sulfur fungicides); and 2) their limited spectrum of fungicide activity, which means they may not be capable of providing simultaneous control of the wide range of fungal pathogens that can cause economic damage to the crop. For example, sulfur is highly effective for controlling powdery mildew on most fruit crops, but provides little or no control of most other diseases. In a climate like the Midwest, environmental conditions during the growing season are generally very conducive (warm and wet) to the development of several important diseases, insect pests and weeds. Limitations in relation to which pesticides may or may not be used, present the organic grower with some unique and very demanding challenges. Whereas the use of various cultural practices and disease resistance will be the "back bone" of the organic disease management program, the limited use of organically approved pesticides or biocontrol agents will probably be required at times. Integrated Management of Grape Diseases Developing a disease management program that successfully controls all of the important grape diseases simultaneously presents a unique challenge. In order to accomplish this, all available control methods must be integrated into one overall disease management program. The disease management program should emphasize the integrated use of disease resistance, various cultural practices, a knowledge of disease biology, and the use of organically approved fungicides or biological control agents or products when necessary. Identifying and Understanding the Major Grape Diseases It is important for growers to be able to recognize the major grape diseases. Proper disease identification is critical to making the correct disease management decisions. In addition, growers should develop a basic understanding of pathogen biology and disease cycles for the major grape diseases. The more one knows about the disease, the better equipped they are to make sound and effective management decisions. The following publications contain color photographs of disease symptoms on grapes, as well as in- depth information on pathogen biology and disease development. Compendium of Grape Diseases - Published by the American Phytopathological Society, 3340 Pilot Knob Rd., St. Paul, MN 55121. Phone: 612-454-7250 (1-800-328-7560). This is the most comprehensive book on grape diseases available. All commercial growers should have a copy. Grapes: Production, Management and Marketing - Bulletin 815 from Ohio State University Extension can be obtained from: Ohio State University Extension Publications, 2120 Fyffe Rd., Columbus, OH 43210. Phone 614-292-1607. The following information gives a description of symptoms and causal organisms for the most common grape diseases in the Midwest. Identifying and Understanding the Major Grape Diseases Black Rot The causal fungus (Guignardia bidwellii) overwinters in mummified fruit on the vine or on the ground. Spring rains trigger the release of airborne ascospores and/or rain splashed conidia from the mummies. Primary infections occur on green tissues if temperatures and duration of leaf wetness are conducive (Table 5). Recent research indicates that the majority of ascospores from mummies on the ground are discharged within a time period from one inch shoot growth to 2 to 3 weeks after bloom. If mummies are allowed to hang on the vines, they can discharge ascospores and conidia throughout the growing season. Black Rot Disease Cycle (Figure 58) Figure 58: Black rot disease cycle. We wish to thank the New York State Agriculture Experiment Station for use of this figure. Figure taken from Grape IPM Disease Identification Sheet No. 4. In conventional production systems, black rot is controlled primarily through the use of effective fungicides combined with various cultural practices. Black rot may be particularly important in organic production systems because the organically approved fungicides (copper and sulfur) are not very effective for black rot control. Therefore, growers should develop a thorough understanding of the black rot disease cycle and the cultural practices used to control it. Lesions on canes from the previous season can also produce conidia for a period of at least one month starting at budbreak. Cane lesions are probably most important in mechanically pruned or hedged vineyards that have an abundance of canes in the canopy. All green tissues of the vine are susceptible to infection. Leaves are susceptible for about one week after they reach full size. Brown circular lesions develop on infected leaves about 9 to 11 days after infection (Figure 59). Within a few days, black spherical fruiting bodies (pycnidia) form within the lesions (Figure 60). Each one of these pycnidia can produce a second type of spore (conidia). These conidia are spread by rain splash and can cause secondary infections of leaves and fruit throughout the growing season. It is important to emphasize that a single ascospore can cause a primary infection (leaf lesion). Within each leaf lesion (primary infection), many pycnidia form. Each pycnidium can produce hundreds of thousands of conidia, each of which can cause another infection (secondary infection) later in the season. Therefore, it is extremely important to control the early season primary infections caused by ascospores. Remember, infection by one ascospore can result in the development of millions of secondary conidia in the vineyard. Figure 59: Black rot lesions on grape leaf. Figure 60: Close-up of black rot leaf lesion showing fungal fruit bodies (pycnidia). Figure 61: Black rot on berries. Figure 62: Close-up of black rot mummy. The fruit infection phase of the disease can result in serious economic loss. Berries are susceptible to the infection from bloom until shortly after bloom. Older literature reports that berries become resistant when they reach 5 to 8 percent sugar. Recent research in New York indicates that berries become resistant to black rot much earlier (2 to 3 weeks after bloom). Therefore, the most critical time to control black rot fruit infections with fungicide is from immediately prior to bloom through 2 to 4 weeks after bloom. An infected berry first appears light brown in color. Soon the entire berry turns dark brown, and then black pycnidia develop on its surface. Infected berries eventually turn into shriveled, hard, black mummies (Figure 61, 62). These mummies also serve as a source of secondary inoculum later in the growing season and are the primary means by which the fungus overwinters. Table 5. Grape Black Rot. Leaf wetness duration-temperature combinations necessary for grape foliar infection by black rot. Temperature °F and minimum leaf wetness duration (hr) for light infection Temperature Leaf wetness duration 50 24 55 12 60 9 65 8 70 7 75 7 80 6 85 9 90 12 *Data represent a compilation from several experiments with the cultivars Concord, Catawba, Aurora and Baco noir. The bottom line for black rot control Sanitation is critical to successful black rot control. Mummies are the most important overwintering source of the black rot fungus. If all mummies and infected canes are removed from the vineyard, there is no source of primary inoculum in the spring and, thus, the disease is controlled. Any practice that removes mummies and other infected material from the vineyard will be beneficial to the disease management