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Tomatoes are one of the most
commonly grown vegetables in home gardens. While tomatoes are
relatively easy to grow, there are a few diseases you should keep
your eye out for. Three of the most common diseases people encounter
are early blight, Septoria leaf spot, and anthracnose. All of these
diseases are caused by fungi, and consistently wet conditions are
needed for these diseases to develop.
Early blight (Alternaria solani) primarily infects the foliage of
plants, but it can also infect the stem and fruit. It appears as
irregularly shaped brown spots that have concentric rings
(resembling a bull's-eye or target), commonly on older leaves. The
spots are often surrounded by yellow tissue. The spot can grow to be
¼ to ½ inch in diameter and will often grow together (coalesce),
forming large brown areas. Eventually, leaves will drop off plants.
Septoria leaf spot (Septoria lycopersici) is also primarily a leaf
infection, but it will also infect the stems (rarely infects fruit).
It forms small (1/16 to 1/8 of an inch) circular spots on leaves.
These spots have a tan or light-colored center with dark purple or
brown margins. Spots will grow to be around ¼ of an inch in
diameter. Like early blight, the individual leaf spots will often
coalesce, forming large areas of diseased tissue. Heavily infected
leaves will turn yellow and fall off the plant. Unlike early blight,
it does not form concentric rings. But, if you look closely (you may
need to use a magnifying glass), you can often see small black
pimple-like fruiting bodies in the center of the leaf spots.
Anthracnose (Colletotrichum spp.) is primarily a fruit disease.
Circular sunken lesions develop on fruit. Over time, these spots
will enlarge and darken. Often, pink to orange masses of spores form
in concentric rings on the surface (especially when it’s humid).
Anthracnose may occasionally be found on leaves and stems, where it
will cause irregularly shaped brown spots with dark brown edges.
Early blight causes spots with a
bull's-eye or target pattern on the leaves and stems of plants.
Photo by University of Illinois Plant Clinic.
Tomato leaves infected with Septoria leaf spot will have small
circular spots on leaves with light-colored centers and dark purple
or brown margins. Photo by University of Illinois Plant Clinic.
Tomato fruit infected with anthracnose will have circular sunken
lesions that may produce pink to orange masses of spores. Photo by
Ken Johnson, University of Illinois Extension
Managing tomato diseases
There are several things you can do to manage these diseases.
Remove and destroy any diseased foliage and fruit to help prevent
disease from spreading further. When removing it, don’t just throw
it on the ground; get it out of the garden!
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Make sure plants are dry before
working with them; if leaves are wet, you may end up spreading the
disease.
When watering plants, avoid watering late in the day and try not to
get the foliage wet. The longer the leaves remain wet, the greater
the chance that disease will develop.
Early blight and anthracnose can
survive in the soil, on seeds, or in infected plant debris. While
Septoria can survive for up to three years on infected plant debris,
as well as weedy hosts, it does not survive in the soil on its own.
Because these diseases can survive on plant debris, it is important
to remove any diseased plant tissues from your garden.
Using mulch around plants can help create a barrier between the soil
and the plants, helping prevent spores that may be in the soil from
getting onto plants.
Controlling susceptible weeds, like nightshade, and volunteer tomato
plants that can act as a source of infection, are also important
parts of keeping your garden clean.
Fungicides can also be applied to plants. This will not get rid of
the disease on infected leaves, but will protect healthy leaves from
infection. When using pesticides, make sure to read and follow all
label directions. Contact your local extension office to get a list
of recommended chemicals.
Preventing tomato diseases in the future
There are also several different things you can do to manage these
diseases in your garden for next year.
Make sure you are practicing crop rotation. Ideally, you wouldn’t
grow any solanaceous (tomato family; includes peppers, eggplants,
and potatoes) plants in the same area (this is often easier said
than done in backyard gardens).
If you are growing tomatoes from seed, look for disease-free seed.
If you save seeds, make sure you are saving seeds from healthy
fruit.
If you are purchasing transplants, inspect plants for any leaf spots
before purchasing.
Regardless of whether you are growing from seed or transplants, look
for disease-resistant varieties.
Providing adequate spacing can help reduce disease in tomatoes. This
will allow airflow between plants, and it will allow them to dry out
faster. It is also a good idea to stake or cage your tomatoes. This
helps speed up the drying of plants and keeps them off of the
ground.
[Ken Johnson, Extension
Horticulture Educator, University of Illinois]
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