Tomato Blights

Hot Springs, Ark. – Each year many of us look forward to harvesting that first tomato from our garden. The last two years have been a challenge for many of us, due to the rainy weather and the cool nights. These conditions are perfect for the development for Tomato blights.

The blight most common is early blight, caused by Alternaria solani.   Early blight symptoms usually begin on leaves at the bottom of the plant; it will start with small brownish-black lesions then turn yellow and the leaves fall off the plant.  In severe cases the entire plant may be defoliated.  Fruit and stems may also develop lesions with concentric rings.  Fruit becomes infected, usually through the calyx or stem attachment.  This can happen at any stage of fruit development.  Losses of 30-50% of fruit may occur.  Stem lesions may completely encircle a stem, killing the plant entirely.  Proper sanitation, crop rotation, mulching, reducing overhead watering and the use of fungicides are necessary for good control.  Homeowners may use fungicides labeled for tomatoes containing chlorothalonil (Daconil) or mancozeb or maneb. 

Another blight we are seeing this year is late blight.  Late blight is not as common as early blight, but conditions seem to be ideal this year for it to develop.  Late blight, caused by Phytophthora infestans, is perhaps the most destructive disease affecting tomato and potato crops.  Plants are particularly vulnerable when grown under cool temperatures, sprinkler irrigation, or prolonged periods of rain and heavy dew.  When the humidity is 100% and temperatures are between 18-24 °C, the disease can run rapidly through a field.  Leaves, flowers, stems, and fruit may all be infected.  Lesions first appear on the leaves, petioles, or stems as small water-soaked spots which grow rapidly into large pale-green to brown lesions.  A grayish-white fuzzy mold grows on the lesions.  Initially the mold can be observed on both sides of the leaf, but later is found on the underside.  Affected tissues become brown, shriveled, and dies. The lesions on the fruit begin as olive-colored greasy spots.  These may enlarge to engulf the entire fruit. Whitish-gray fuzzy mold can also occur on the fruit, followed by fruit rot.  Rotted fruit and tomato vines let off a nasty odor.  Ideally, tomatoes should be grown on raised beds in well-drained soil.  Fruit should be prevented from touching the ground by staking or mulching.  Avoid over-watering.  Home gardeners may use the fungicides Maneb or Mancozeb.  If late blight becomes a problem in your garden, the use of resistant varieties is the best strategy for managing late blight.  Here is a list very good resistant cultivars for late blight:

• Defiant – Determinate (bush) plants produce round, medium size red fruits, rated at 70 days to maturity.

• Iron Lady – Determinate (bush) plants produce round, medium size red fruits, rated at 75 days to maturity.

• Jasper – Tall indeterminate (cordon) plants bear trusses of red cherry tomatoes starting 60 days after planting. All America Selections winner and RHS Award of Merit.

• Lemon Drop – Indeterminate (cordon) plants bear hundreds of small yellow-green tomatoes in 80 to 90 days. Open-pollinated heirloom variety, a sport of 'Snow White' cherry.

• Matt's Wild Cherry – Sprawling indeterminate (cordon) plants bear scads of tiny red cherry tomatoes starting 55 to 60 days after planting. Open-pollinated heirloom from Mexico.

• Mountain Magic – Vigorous indeterminate (cordon) large red cherry tomatoes, rated at 75 days to maturity.

• Mountain Merit – Determinate plants produce large red round fruits about 75 days after planting. All-America Selection winner.

• Mr. Stripey – Indeterminate (cordon) plants produce medium size round fruits marbled with red and yellow in about 80 days. Open pollinated heirloom.

• Plum Regal – Determinate plants produce red plum tomatoes weighing 3 to 4 ounces each, rated at 80 days to maturity.

Remember, fungicides work best as a preventative or at the first signs of the disease.  Always read and follow label directions.  For more information contact the University of Arkansas Cooperative Extension office at 236 Woodbine or call 501-623-6841.  

By Allen Bates
County Extension Agent - Agriculture
The Cooperative Extension Service
U of A System Division of Agriculture

Media Contact: Allen Bates
County Extension Agent - Agriculture
U of A Division of Agriculture
Cooperative Extension Service
236 Woodbine Hot Springs AR 71901
(501) 623-6841
abates@uada.edu

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