Fungal and bacterial diseases are common causes of tomato loss for the home gardener. The combination of high temperatures and humidity are breeding grounds for bacteria and fungus. Garden debris and weeds provide a safe harbor and a moist environment for these diseases. Prevention is the predominant control, which includes planting disease-resistant varieties, cleaning garden areas and rotating crops. Recognizing the most common diseases that affect tomato plants will help you achieve a bountiful crop.
Bacterial wilt, also known as Southern bacterial blight, is caused by the bacterium Ralstonia solanacearum. The soil-borne bacterium enters the root of the tomato plant where an injury has occurred. It multiplies inside the plant, causing the interior stems to turn to yellow slime and the plant to wilt.
There are no chemicals for the treatment of bacterial wilt, so prevention is necessary. Control measures include planting disease-resistant varieties, rotating crops every three years and destroying infected plants.
Fusarium wilt, caused by the fungus Fusarium oxysporum, lives in the soil and invades the tomato plant at the roots where injuries may have occurred. This disease blocks the conduction of water throughout the plant. Leaves yellow and wilt at the base of the tomato plant, initially on one side. As the wilting from the fungus climbs the plant, the leaves drop and the tomato plant dies.
There are no chemical treatments for fusarium wilt but raising the soil PH level to a range of 6.5 to 7.0 will suppress the development of the disease. Using nitrate nitrogen rather than ammoniacal nitrogen is also recommended, reports Clemson University Extension. Other measures include buying disease-resistant varieties and rotating tomato crops every four years to eliminate spread of the disease.
Early blight is also known as Alternaria leaf spot or target spot. It appears after the tomato plant begins to bear fruit. High humidity and warm weather can accelerate the disease. Early blight is caused by the fungus Alternaria solani, which first exhibits as small black spots on older leaves. Eventual loss of leaves occur. As the disease progresses, bulls-eye spots appear on fruit.
Control measures for early blight include buying disease-resistant varieties, crop rotation, destroying garden debris and fungicides. A few fungicides that are recommended for treatment of early blight are mancozeb, chlorothalonil, maneb and fixed copper, reports Clemson University Extension.
Late blight is caused by the fungus Phytophthora infestans. Spread by wind and rain, late blight thrives in cool wet weather. The disease is recognized by water-soaked spots on top of the leaves, with a white moldy growth on the underside. Tomatoes have dark olive marks that may cover large areas. The tomato plant usually loses it foliage completely within a two-week period.
Treatments for late blight include allowing air to flow between plants, avoiding overhead watering, removing diseased plants and the use of fungicides. The same fungicides that are effective for early blight are also effective for late blight. They are mancozeb, chlorothalonil, maneb and fixed copper.