What is late blight (on tomatoes) and are there any organic controls for it?
A.Z.PennsylvaniaAnswer: Late blight is caused by the fungus Phytophthora infestans. A fact sheet from Ohio State University (see Resources, below) provides a succinct description of late blight in potatoes and tomatoes. The essential points are that late blight is spread through overwintering inoculum such as tomato vines and leaf debris, through transplants, and through climate related conditions. Thus, sanitation is an important first step. Secondly, when weather conditions are prime for late blight development a protective spray is needed during the growing season.You mentioned in your inquiry that a spray mixture of hydrogen peroxide, copper sulfate, and foliar fertilization was fairly effective. In fact, these kinds of mixtures are what many other organic growers are using, as well. However, there is a lack of efficacy data on what combinations of foliar materials are effective for tomato diseases.The following materials provide a series of helpful resources for organic disease control in tomatoes. Your local library can provide computer access to the Web-based items.Biorationals (e.g., horticultural oils, plant extracts, insecticidal soaps), and more broadly biopesticides (e.g., biorationals, biological controls, microbial antagonists, entomopathogenic fungi) are members of a new generation of pest control products that are also known as least-toxic, soft, or reduced-risk pesticides. They have minimal effect on non-target organisms and short-term persistence in the environment.The American Phytopathological Society and Ohio State University co-host the most significant resource list of commercial biocontrol products for control of plant diseases, Commercial Biocontrol Products Available for Use Against Plant Pathogens. These biocontrol products include antagonistic fungi and bacteria.The Integrated Plant Protection Center at Oregon State University hosts the Database of Microbial Biopesticides. The first part contains entries for products, including the biocontrol organism, a description, target pests, trade names, and mode of action. The second part contains a list of manufacturers and suppliers of biopesticides.Biorational products include hydrogen peroxide, plant essential oils, and related natural products. OMRI — the Organic Materials Review Institute — compiles a list of organically approved fertilizers and pest control products. You can find biorational products among the OMRI Brand Name Products List. Look for disease control products such as Oxidate, Sporan, Trilogy, and Serenade.Reduced Risk Pesticides and Biopesticides (PDF 49 kb), is a list provided by the New York State Department of Environmental Conservation, Division of Solid & Hazardous Materials, Bureau of Pesticides Management.Biological Control of Plant Pathogens: Research, Commercialization, and Application in the USA, an article by Brian B. McSpadden Gardener, Department of Plant Pathology, The Ohio State University-OARDC, Wooster, OH 44691; and Deborah R. Fravel, Vegetable Laboratory, USDA, ARS, appeared in Plant Health Progress. May 2002.Farm-made biological teas and foliar sprays include Compost Teas (CT) and Effective Microorganisms (EM). Compost teas and EM influence the biologically active zones surrounding the plant leaf (phyllosphere) and root surfaces (rhizosphere). The beneficial microorganisms in CT and EM compete with pathogenic organisms that cause plant disease problems, and through various biocontrol mechanisms. ATTRA has additional informationon these two topics, available on request.Serenade, a biocontrol product containing Bacillus subtilis (QST 713 Strain), is manufactured by AgraQuest, Inc. in Davis, California. The company Web page offers a fact sheet on Serenade (PDF, 162 kb). It states that Serenade is labeled for tomatoes, and it has the late blight pathogen, Phytophthora infestans, listed as a target pest.Excerpts: “Serenade contains a unique, patented strain of Bacillus subtilis (QST 713 Strain) which produces over 30 different lipopeptides that work synergistically to destroy disease pathogens and provide superior antimicrobial activity.” “It stops harmful spores from germinating, it disrupts the germ tubes and mycelial growth and it inhibits attachment of the plant pathogen to the leaf by producing a zone of inhibition which restricts the growth of the disease.” “Serenade is a flexible tank mix option and complements standard disease control programs. It is compatible with registered products such as copper, sulfur and other foliar-applied micronutrients, insecticides and fungicides. Serenade is also compatible with use of non-penetrating, non-ionic products such as silicon surfactants.”AgraQuest is also selling a complementary product called Biotune. This is a “unique surfactant system, formulated to enhance the efficiency of biological disease control products, particularly Serenade.”Excerpts: Adding Biotune to Serenade: *Provides superior coverage and enhances efficacy for greater disease control *Increases the solubility of the disease-fighting lipopeptides that are produced by Serenade’s active ingredient: Bacillus subtilis *Extends the interval between spray applications Vegetable Crop Advisory Team AlertMichigan State UniversityVol. 19, No. 1, August 18, 2004 * Testing disease predictive systems for timing sprays for management of foliar blights * Evaluation of new fungicides and biofungicides for management of late blight * Table 1. Evaluation of fungicides and biopesticides for managing late blight, 2003. * Evaluation of registered fungicides for management of early blight * Table 2. Evaluation of registered fungicides for managing early blight, 2003.ResourcesAn Eco-Farming Approach to Foliar FertilizationSteve Diver, ATTRA ? National Sustainable Agriculture Information Service800-346-9140Late Blight of Potato and Tomato, HYG-3102-95Ohio State University Extension ServiceNotes on Hydrogen Peroxide in AgricultureSteve Diver, ATTRA ? National Sustainable Agriculture Information Service800-346-9140Serenade Fact Sheet (PDF, 162 kb)Vegetable Crop Advisory Team AlertMichigan State UniversityVol. 19, No. 1, August 18, 2004 * Testing disease predictive systems for timing sprays for management of foliar blights * Evaluation of new fungicides and biofungicides for management of late blight * Table 1. Evaluation of fungicides and biopesticides for managing late blight, 2003. * Evaluation of registered fungicides for management of early blight * Table 2. Evaluation of registered fungicides for managing early blight, 2003.