Managing Tomato Blights
Many home gardeners struggle with tomato blights and other disease problems. We get several calls at our office on how to control these problems. Whether you are using conventional or organic methods, control is very difficult or impossible to achieve. The best we can hope for is to suppress or slow diseases down enough (management) so we can enjoy some vine-ripened garden tomatoes! We will cover some cultural practices, organic and conventional fungicides. There are commercial guides available to farmers that gardeners can use for some products, but they mainly help in the timing of application. Most of the products listed on the guide are not available to gardeners or are too expensive for just a few plants. NC State also has information on Managing Diseases in the Home Vegetable Garden.
Cultural Practices: Cultural methods should be incorporated no matter if you are growing organically or not. In managing plant diseases, the old cliché sums it up best, “an ounce prevention is better than a pound of cure.” For plant diseases to occur, three things have to be present; a pathogen, a host and favorable environmental conditions. With foliar diseases, the favorable environmental conditions usually means moisture. Take away any of those three factors and disease does not occur. Not much a gardener can do about preventing the pathogen from coming in and you have to plant tomatoes to get tomatoes, but you can control the moisture getting on the plant to some extent. Some ways to help keep moisture off the plants are: mulching around the plants to keep splashing of soil containing disease spores from getting on the plants, using some kind of cover over the plants (high tunnel, low tunnel or if container gardening, place the plant under cover during rain and at night to keep due off) and never water the top part of the plant, always just water the roots when needed. Other cultural methods are to rotate every year from solanaceous plants (Irish potatoes, eggplant, tomatoes) and space plants out to promote air movement.
Probably the number one problem gardeners have with tomatoes is early and late blight. The blights can only start when moisture is on the leaves and a spore lands there and infects the plant. At this point, the only thing you can do is to slow the disease down. The only cultural controls available after infection is to keep the top part of the plant as dry as possible, prune out infected leaves and plant resistant varieties. Keep in mind, “resistant” does not mean immune. Resistant varieties can still contract diseases, they just won’t be affected as much as nonresistant varieties.
Organic Fungicides: Most calls we get from gardeners are that they want to only use organic methods or organically approved products. The good news is there are some products on the market that do have some efficacy against the blights. The bad news is, most products work very little if any. All a company needs to do to get a product labeled for disease management is to show any improvement to the crop against the disease. In most cases, the control you are hoping to get will be disappointing. With any garden labeled fungicide, conventional or organic, you have to think prevention, not cure. The product has to be on the plant before the pathogen infects the plant. Coverage is key, so applying the product over the entire upper part of the plant is necessary.
Cornell University does an annual study on the efficacy of organic approved and conventional fungicides on vegetable diseases. The report covers different crops with different products used on them and comments on how each product(s) worked. The studies in the report were conducted in the northeastern part of the country, so results may and probably do vary by location. For early blight, it states, “Sporatec AG 3pt/A effective (others not).” I have talked with other agronomists and farmers that have used Serenade Max and/or Regalia with some success.
For late blight, it states, “Double Nickel 1 qt/A + Cueva 2 qt/A somewhat effective. Preventive. Conventional treatments all more effective.” A major problem for organic gardeners is finding some of these products and if they do, most are fairly expensive due to being sold in containers that can cover many acres and the added expense of being organically approved.
Copper fungicides can also help some in preventing blights. Mix in with some of the other fungicides but not every week. It is good to rotate the fungicides you use, but do not overuse copper. Excessive amounts of copper can build up toxicity in the plant and over a period of years, toxic levels in the soil.
Conventional fungicides: For the most part if applied correctly, the conventional fungicides will give better results for blight management. Products with the active ingredient Mancozeb will give gardeners the best results on early blight. It can be applied from transplanting until fruit starts ripening. At that point, you should switch to a product containing the active ingredient Chlorothalonil. This product is more effective on late blight and does not have a harvest restriction waiting period like Mancozeb. Like the organic fungicides, it is good to occasionally mix in a copper fungicide. Copper also helps with bacterial spots and speck. Many farmers and gardeners will alternate organic and conventional products. Alternating different modes of action helps in disease management and delays pathogens developing resistance to one product.
As stated before, most all fungicide products available to home gardeners are preventative and how any fungicide is applied is very important to get best results. Coverage is key and you have to use a sprayer with enough pressure to move leaves around to coat stems and the underside of the leaves. Most gardeners make the mistake of waiting until after a rain to apply a fungicide. Again, these are preventative and they are not helping you still in the jug. You just need time for the product to dry before rain comes. When using any pesticide, organic or conventional, always read and follow the label directions for use, rates and safe use of the product.