I recently encountered some lesions of Tomato Spotted Wilt Virus (TSWV) that appeared relatively large and had a ring-like structure. I was able to confirm the presence of the virus, but at first glance, the lesions could be mistaken for a very common tomato disease: early blight. This article will describe how the foliar lesions of these two diseases might be distinguished. First, a bit about the symptoms of these diseases.
Figure 1. The early blight lesions on the top of this tomato leaf have a concentric ring structure. While the lesions on the leaf bottom lack a ring pattern, the lesions are prominent.
Early blight is perhaps the most common foliar disease of tomato in Indiana. One might first notice that the older leaves turn necrotic. If left uncontrolled, the diseased lesions appear to ‘move’ up the plant. A closer look at early blight lesions may reveal the bull’s-eye lesions of this disease (Figure 1). These lesions may also be described as having concentric rings similar to a target. (Early blight is not the only disease to have concentric rings.) I usually don’t take photos of the underside of lesions, but Figure 1 also has a lesion that can be seen on the bottom of the leaf. Although it lacks, concentric rings, the difference between this photo and later photos of TSWV is clear. Most foliar disease lesions are obvious on the top and bottom of leaves, such as with early blight. But read on for an exception.
Figure 2. Lesions of Tomato Spotted Wilt Virus on the top of tomato leaves often have a ring pattern.
The lesions of TSWV on the top of the tomato leaf in Figure 2 have a similar ring pattern to the early blight lesions in Figure 1. Now, let’s flip the leaf over and look at the bottom of the leaf (Figure 3). The lesions of TSWV on the bottom of the same leaf range from almost invisible to a light gray, somewhat circular pattern. Thus, one way in which early blight lesions can be differentiated from TSWV lesions is by looking at the underside of the lesions. Early blight lesions will normally show up clearly on both sides of a leaf. Lesions of TSWV do not necessarily show up clearly on both sides of a leaf. In general, disease lesions show on both sides of leaves. TSWV is an exception.
Figure 3. The lesions of Tomato Spotted Wilt Virus on the bottom of the same tomato leaf in Figure 2. The lesions shown here consist mostly of collapsed tissue and are not distinctive.
There are other important differences in biology that helps to distinguish early blight and TSWV. Early blight is a fungal disease that requires leaf moisture for infection. For this reason, early blight is not usually observed in greenhouses or high tunnels. TSWV is a virus disease that is transmitted by thrips. Often, TSWV is observed in greenhouses or high tunnels that are used to grow vegetables and flowers side-by-side.
It is very difficult to distinguish foliar diseases by the symptoms. When in doubt, it is always best to send a sample to a diagnostic laboratory such as our own Plant and Pest Diagnostic Laboratory.