Late blight, early blight, septoria blight and southern blight are all diseases that fall under the category of tomato plant blight, and each can wipe out your entire crop. Blight is caused by pathogens that thrive in wet conditions and produce spores that spread in the wind, water or soil.
Causes of Blight
All forms of tomato blight are caused by fungal or fungal-like pathogens that spread quickly when conditions are favorable. Rain, heavy dew or mist, and even fog can increase the odds of your garden becoming infected with blight, especially when temperatures do not fall below 50 degrees F while humidity levels stay above 75 percent for two or more days.
Sometimes even the best garden practices are not enough to prevent the spread of blight, as spores are carried on the wind and can settle anywhere. Fungal spores live in water droplets, and unless the water dries within a couple of hours, they attach to the foliage or stem of the tomato plant to spread infections.
Moving through the garden while plants are wet, splashing when you water, and failing to practice clean gardening techniques like removing dead or decaying debris and allowing weeds in and around the garden, create good host environments for blight to grow.
Fungicide sprays, as well as the organic copper sprays, are best considered as preventative measures, as they will not usually control blight once it has set in. When conditions are ripe for infection, applications are made on a weekly basis and after rainfall. These sprays need to be diluted with water per manufacturer recommendations, and applied with a sprayer to the entire plant, including under the leaves and down the stems to the soil, until the plant is saturated.
To decrease the chances of blight taking over your garden, plant tomatoes where the morning sun will be able to quickly dry heavy morning dew and fog. Water the soil under the plant instead of spraying water on the leaves and fruit of the plant. Use a softer stream of water to prevent splashing. Plant tomatoes further apart then normal or prune plants back to increase air flow and drying times. Because fungal spores can live over winter, the spores stay in the ground waiting to reinfect next year's crop. Utilize three-year crop rotation, changing where you plant tomatoes each year, returning to the initial planting area only after at least three years have passed. Remove plants at the end of the growing season, being sure to bag up diseased plants for burial or burning. Do not compost leftover plants if blight has affected your garden.