This time of year, two common leaf-spot diseases appear on tomato plants.
Septoria leaf spot and early blight are both characterized by brown spots on
Septoria leaf spot usually appears earlier in the season than early blight
and produces small dark spots. Spots made by early blight are much larger
and often have a distorted “target” pattern of concentric circles. Heavily
infected leaves eventually turn yellow and drop. Older leaves are more
susceptible than younger ones, so these diseases often start at the bottom
of the plant and work up.
Mulching, caging, or staking keeps plants off the ground, making them less
vulnerable. Better air circulation allows foliage to dry quicker than in
plants allowed to sprawl. Mulching also helps prevent water from splashing
and carrying disease spores to the plant.
In situations where these diseases have been a problem in the past, rotation
is a good strategy. It is too late for that now, but keep it in mind for
next year. Actually, rotation is a good idea even if you have not had
problems in the past. But many gardens are too small to make it practical.
If you have room, rotate the location of the tomatoes each year to an area
that has not had tomatoes or related crops (peppers, potatoes, eggplant) for
If rotation is not feasible, fungicides are often helpful. Be sure to cover
both upper and lower leaf surfaces, and reapply fungicide if rainfall
removes it. Plants usually become susceptible when the tomato fruit is about
the size of a walnut. Chlorothalonil is a good choice for fruiting plants
because it has a 0-day waiting period, meaning that fruit can be harvested
once the spray is dry. Chlorothalonil can be found in numerous products
including Fertilome Broad-Spectrum Landscape and Garden Fungicide, Ortho
Garden Disease Control, GardenTech Daconil and others. Be sure to start
protecting plants when the disease is first seen. It is virtually impossible
to stop it on heavily infected plants.
If chlorothalonil doesn’t seem to be effective, try mancozeb (Bonide
Mancozeb Flowable). Note that there is a five-day waiting period between
application and when the fruit can be harvested. You may wish to pick some
tomatoes green just before you spray if you use Mancozeb as they will ripen
By: Ward Upham