Should I Spray?

The jury is still out as to whether we will have problems with the latest strain of late blight (Phytophthora infestens) on tomato and potato crops in Maryland.  Our hope is that we will not, thanks to sunny weather that the fungus dislikes for dispersing spores.  Still, there is a lingering sense of dread about this new disease strain, which did so much damage in the Northeast and Mid-Atlantic area of the US last year.  So far this year, there is only one confirmed outbreak in Maryland and all measures have been taken to contain it.

Because this blight is so devastating when it appears on tomato plants, we take it more seriously than most garden pest problems that we encounter each year.  Once it appears, with dark colored lesions on leaves, stems and fruit, it cannot be suppressed before it kills the whole host tomato plant.  Prevention - spraying tomato plants with an appropriate fungicide prior to seeing the signs of fungal infection - is the only possible means of stopping severe damage to the crop.

So, you may be asking:  Should I spray now?  None of us really wants to spray a pesticide unless the plants are actually threatened.  But right now, we're not sure whether the threat will become a reality here or not.   But if we wait too long and our tomato plants start to show signs of the disease, we may be too late to do anything about it.

For preventing late blight disease, the experts' fungicides of choice for home vegetable gardens are two chemical fungicides which have been used in gardens for a number of years.  Chlorothalonil is the generic chemical name of the old trademarked fungicide Daconil or Daconil 2787.  It is now sold under a variety of brand names, so you need to check the ingredients on the label to find out if it is Chlorothalonil.  The other fungicide recommended is Mancozeb, a general plant fungicide based on active ingredients that contain manganese and zinc compounds.  Copper-based fungicide may have a limited affect on preventing the blight, but still may be used by gardeners looking for a product that's a more naturally based formulation than Chlorothalonil or Mancozeb.


If you would not like to take any chances of acquiring this disease on your tomato plants, you should start to spray soon, covering the entire plant with the spray.  The spraying regimen requires that you spray these plants in 2-week intervals through the summer, using those specific fungicides mentioned.  Use all due precautions in using these products by reading AND following all the instructions on the product label.  I recommend that when using concentrates you add a spreader sticker, a harmless chemical surfactant which is added to the water as a wetting agent, to allow the spray to spread and stick to the surface of foliage, giving the spray more effective coverage.  Most premixed, ready-to-use pesticides have this surfactant as part of the spray mix.

If you do get the disease on any of your tomato plants, be sure to pull them up, and put the diseased plants and all the plant litter in sealed plastic bags for disposal as trash.  Do not compost them or recycle them in any way.

Photo by Texas A&M

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