Cheap and simple to use, copper sulfate is one of the oldest and most powerful fungicides still in use today. Originally used by vintners in the Bordeaux region of France to keep mold and other fungi off of their grapes, copper sulfate remains ideal for just that purpose when applied to certain fruits and vegetables, including tomatoes. However, this blue crystal is also incredibly toxic in certain concentrations, and it isn't appropriate for use on any plant aside from ones it is specifically approved for. Copper sulfate can prevent and/or treat common tomato blight, as well as fusarium wilt, crown rot and many others.
Things You'll Need
Copper sulfate crystals
Chemical-resistant gloves, jacket, pants, mask and goggles
Don your protective gear before opening the bag of copper sulfate. Copper sulfate toxicity by ingestion is fairly unlikely, since it's a gastrointestinal irritant that will immediately induce vomiting. However, long-term exposure through the skin or mucus membranes can cause liver and kidney disease, cancer, sterility and birth defects.
Measure out 6 ounces of copper sulfate per gallon of water, and pour it into the applicator container. Fill the container the rest of the way with the appropriate amount of water. Stand on the side of the container facing away from the output line, or angle the container away from you if it has a top-mounted line. This will keep the copper sulfate from spraying into your face should the line come off. Pump the handle to pressurize the container.
Hold the sprayer tip 8 to 12 inches from the tomato plant and trigger the sprayer. Work the sprayer back and forth across the plant, coating it as evenly as possible with the solution. The copper sulfate kills by contact, so any area left untouched by the fungicide is a potential growth zone. Avoid spraying the chemical on the ground, because it can leach into the soil and, thus, the water table.
Mark your plants with a green ribbon or warning label to ensure that no one eats any of the fruit directly from the vine. You'll need to wash the fruit before eating it to rid it of any residual sulfate.
Check with your local regulatory body or chemical retailer before applying copper sulfate, because they may require certification that you've attempted other means of fungi control or may prohibit its usage outright.