Controlling Early Blight in Tomatoes

 

Our monsoon season has brought lots of rain and humid conditions for both July and August.  While early blight is not usually an issue because of our dry conditions this year is the exception and many people have lost their tomatoes.  Early blight can affect both potatoes and tomatoes.  The key to not losing your crop is to recognize it early, understand this pathogen, and use cultural methods to control and prevent, and use fungicides when necessary.

What is Early Blight?

Early Blight is a fungal disease that occurs on mature vines.  Don't be confused by the term early; it usually shows up in mid to late summer if rainy, humid conditions exist.  It can affect the leaves, stems, and fruit of both potatoes and tomatoes.

Early blight is caused by the fungus Alternaria solani. The fungus overwinters on debris from previously diseased plants. Airborne spores will cause infection on  new tomato and potato plants.  The spores are  produced on infected debris and on volunteer tomato and potato plants. The disease is more serious under wet or humid conditions. The first lesions occur in the lower part of the plant where humidity is highest.  (USU Extension)

Blight on tomato fruit.

Symptoms of Early Blight (USU Extension)

• Early leaf symptoms begin as pinpoint brown or black spots, usually on the lower (older) leaves.
• Leaf spots are frequently surround by a yellow halo.
• Concentric rings can be seen within the enlarged lesions.
• Irregularly shaped lesions that may be somewhat angular because they are often limited by large leaf veins.
• Sunken and silvery stem lesions ranging from 1/4 to 1/2 inch in size.
• Stem lesions may join to produce large infected areas.
• Stem lesions are an indication of a serious early blight outbreak.
• Tomato fruit symptoms are not common in Utah, but include greenish brown to black sunken lesions (usually on the stem-end of the fruit) with concentric rings.
• Irregularly shaped potato tuber lesions that look dark colored at first and then become sunken.
• Dry and leathery tuber tissue that ordinarily remain hard and dry, but wet rot will occur if the lesion is invaded by other fungi or bacteria
• Tuber infections are infrequent in Utah.

Notice the lesions on the stems

Treatment

• Frequently check for early signs of blight in wet conditions especially on the lower leaves.  Brown spots will be the first indication.
• As the disease spreads and progresses, leaves will yellow, lesions will cause dark dead spots, and it will progress up the plant.
• As you monitor your plants remove any diseased leaves by pruning them off.  There is no cure for a fungus so pruning off diseased leaves is important.  Fungicides only prevent the fungal spores from colonizing on new leaves.
• Do not throw pruned leaves on the ground or leave in the garden.  Throw them in the trash.
• If the disease occurs during the beginning of August you will need to treat with a fungicide to prevent the spread.
• I use a copper fungicide when I have blight.  Neem is also a fungicide but with continued humidity and rain you will want to use the copper fungicide.  
• If you know moist, wet conditions are coming your way you can use Neem as a preventative spray before the moisture comes but it is not as affective when you already have early blight.

Cultural Methods to prevent fungal disease

• Remove all diseased plant material at the end of the season and put it in the trash or burn it.
• Rotate where you plant tomatoes and potatoes.  Use a 2 year rotation.
• Do not use an overhead water system.
• Give plants adequate space to allow good air circulation.
• Buy disease free starts.
• Destroy all volunteer tomato and potato plants.