What Tomato Disease is this?

A tomato disease hit our garden hard starting in the damp month of August. You’d hear talk of “blight” if you conversed with other gardeners but is it really the same disease that killed our tomatoes in July of 2009? That disease struck farms and gardens throughout New York and was Phytophthora infestans, or late blight, the same fungus that was responsible for the Irish potato famine. This disease is clearly not the same. For one it does not affect the fruit, whereas blight does. In addition its lesions have a different shape and color from late blight symptons (see tomato blight for pictures). Finally farmers and gardeners and scientists are not talking about blight this year in Rockland county, and it’s unlikely that there would be a late blight infestation that’s only affecting our garden given how devastating the blight was to all the local farms in 2009.

I’ve looked at various Web sites and concluded that it might be Gray Leaf Spot, or Stemphylium. It could also be Septoria Leaf Spot. I’m thinking this because of the affected tissues, leaves only (not fruit), and the appearance of the lesions. Take a look at the pictures below, these show how the disease progresses:

• Small, dry brown or gray-brown spots appear first on the underside of the leaves
• Dry yellow or brown spots appear on the top of the leaves once the infection is established underneath
• Sections of the leaf turn brown and die

Here’s some common-sense advice:

• Keep your tomatoes dry by watering them at ground level since the fungus flourishes on wet leaves
• The disease starts at the bottom of the plant so check the undersides of those lower leaves
• Remove any leaf that has the characteristic gray or brown spots
• Take any and all infected tissue out of the garden

A useful Web site is the Vegetable MD Online, from Cornell.

Brian O