Early Blight

Early blight or is a disease that most commonly affects tomatoes and potatoes but can also occur on peppers and eggplants. Very common to the New England states of the US this disease untreated can devastate a crop of tomatoes or potatoes. Because the fungus overwinters in the soil, it can be difficult to control without the use of fungicides. Management and control of early blight depends on several factors such as spraying, plant waste management, watering and plant rotation. Most commercial and home gardeners are well familiar with the signs of early blight, which first start to appear near the end of the season, usually sometime in July. Normally it starts with small brown lesions on the lower leaves of the plant; these lesions will develop into dark concentric rings of raised tissue with the area surrounding them often turning yellow. As the disease begins to progress the entire leaf will become yellow and then brown and die. Tomato plants that are severely affected can become completely defoliated and the exposed fruit may become sun scalded. The organism that causes early blight is called alternaria solani, it is a fungal pathogen that attaches itself and then penetrates the leaves of the affected plant. This produces the raised lesions which in turn will produce more spores.

These spores are spread by the wind, water, or by gardeners and their tools. This cycle under favorable conditions takes about one week. Because of this quick advancing cycle of the early blight, plants that are not treated can be defoliated in a matter of a few weeks. Early blight thrives in humid warm conditions since the leaf must be wet for spores to germinate and the growth of lesions increases with greater amounts of leaf moisture. Temperature will speed up this process with the most rapid progression occurring at temperatures between 65 and 85 degrees. Long periods of rain followed by warm humid conditions are the ideal breeding ground for this disease.

Management of early blight is difficult but possible and starts with clean disease free seed or healthy plants. Crop rotation will also help as the fungus will not survive more than three years in the soil without a host. Proper disposal of infected leaves as well as turning under tomato and potato crops debris can be crucial in stopping the spread of this disease. Making sure your plants are properly fed can help as well, plants that have a higher nitrogen supply are more resistant to the early blight. Avoid over head watering and never water at night can help slow the growth and spread of the spores. There are fungicides available that will help to inhibit the growth of early blight; most fungicides will need to be applied every 7 to 14 days to coincide with the growth cycle of the early blight spores. While many of the fungicides are chemicals, there are now more and more plant based fungicides that are highly effective in the control of early blight. Marrone Bio Innovations is a leading researcher and producer of natural products for pest and disease management. If you need to control early blight on your crops they produces a fungicide that uses the giant knot weed plant as its active ingredient to turn on a plants own natural defenses against disease.