Late Blight on Tomatoes

 Late Blight Is On The Rampage On The Tomatoes


Late Blight (Phytophthora infestans ) has been troublesome to tomatoes off and on for many years.  However, 2009 is the year when late blight (LB) went crazy.  The reason was that we had just the right amount of rain and cool days and nights which is perfect for LB to reproduce.  LB spores travel through the air (sometimes many miles) and on plants sold at the big boxes.  Once on board it is soil borne and spread by splashing rain water.  Normally it doesn’t show up until August but this year it started killing thousands of tomato plants in June.


Description of symptoms 

When it shows up you see lesions on leaves and stalks; the plant loses its leaves; all its leaves. LB starts out on tomato leaves as pale green, water-soaked spots, often beginning at leaf tips or edges. The circular or irregular leaf lesions are often surrounded by a pale yellowish-green border that merges with healthy tissue. Lesions enlarge rapidly and turn dark brown to purplish-black.


Brown spots (lesions) on stems are one of the most visible early symptoms. They begin small and firm, then quickly enlarge. Under moist conditions, a white fungal growth develops and a soft rot collapses the stem of the tomato plant, which turns black.


The first appearance of lesions commonly occurs after periods of wet weather. Black lesions appear within 3-7 days of infection of leaves. On tomato fruit, late blight causes a firm, dark, greasy looking lesion from which the pathogen spore producing structures emerge under humid conditions.


Late blight can also develop on green tomato fruit, resulting in large, firm, brown, leathery-appearing lesions, often concentrated on the sides or upper fruit surfaces. If conditions remain moist, abundant white mold growth will develop on the lesions and secondary soft-rot bacteria may follow, resulting in a slimy, wet rot of the entire fruit.


All Spots On Tomato Leaves Are Not Always Late Blight

For instance, a very common and mild soil-borne wilt makes the bottom leaves of tomato plants grown in the same area year after year turn yellow; this is not late blight.  Spots the size of a pencil eraser on tomato leaves aren't late blight either-the spots this nasty spider mite critter is hard to spot.


Monitoring techniques and timing in season

This year in NE the LB hit in June while normal time is late July and August.  The weather has to be wet, cool (60 to 80), and humid for LB to thrive and spread. Check your tomato plants twice each week; more often if there is wet weather.  Look inside the foliage to the stems especially near the soil level.  LB moves quickly so if you see a small lesion and its bigger the next day, that tomato goes into the trash.  


If there are spots on just a few leaves, remove them and those around them and watch for a day or two.  Remove all infected plants and place in the trash; not in the compost pile.


Spray unaffected plants with fungicide

If you have found LB on one or more plants and have removed them, you may prevent the spread of the disease if you immediately spray the remaining plants with a fungicide.  Your organic choices include Copper Sulfate or Actinovate.  On the chemical side you have Daconil,  a product that has 29.6% chlorothalonil in the concentrate. Ortho Disease Control and Bonide's Fung-Onil contain the same active ingredient. With both of these one can spray up to the day of harvest for tomatoes.  Remember these products are only effective if used before the disease appears on the plant and should be reapplied every 5-7 days if wet weather persists.


If you choose the organic route we advise using alternately the Actinovate one week and the copper fungicide the next week; repeating this alternating approach until first frost.  


Prevent in first place

The best way to avoid problems of LB is to start your prevention program after the first frost and your plants are dead.  The critical issue here is to have zero tomatoes left on the ground through the winter.  LB spores can over winter on those innocent looking tomatoes.  The spores do not overwinter on the roots or the stems and leaves.  


You should then leave a 3 inch layer of organic mulch (straw or chopped leaves) over the tomato beds for the entire winter.  If you had mulch under your tomatoes this year, remove that mulch and use it under shrubs or perennials that are not bothered by LB.  The overwintering mulch is going to prevent any spores that do make it to late spring from splashing up on the plants from heavy rains.  


If you don’t start your tomatoes from seed, avoid buying your tomato seedlings from the big boxes.  Independent garden centers usually grow their own seedlings.  


There are no tomato varieties resistant to LB.  There will be some coming out in 2011, 


Determinate varieties should grow on cages and indeterminate varieties should be on a trellis.  You want your tomato plants to allow as much air move through the plant so it dries out faster after a rain.  Mature plants should be at least a foot from the next plant.  A soaker hose is the best way to water tomatoes so water does not unnecessarily get on the leaves.


Preventive spray program (to be completed later)






PGA from Organica


Compost tea



see all questions...

Do you have a gardening question? Ask Nancy