At this time of the year it is common to see leaf spots on tomato leaves. These spots can be caused by fungal and bacterial pathogens, but does it matter which one? Yes, knowing which disease is affecting your tomatoes may be important to control the disease and prevent severe infections in future.
In some cases, leaf spots can look very similar and disease identification can be tricky. Recently, we have seen Septoria leaf spot and bacterial speck and spot of tomato. These diseases cause dark brown spots, surrounded by yellow halos, and severe infections can cause entire leaves to turn yellow. However, there are some characteristics that can help homeowners tell them apart in the field.
Septoria leaf spot
Septoria leaf spot is caused by a fungus and infection usually begins on lower leaves. This is a good way to tell it apart from bacterial speck and spot. Additionally, spots have a dark well-defined margin and a gray area in the center. Fungal pimple-like structures can be seen in the center of the spots with the help of a magnifying glass. This disease affects leaves and stems, but not fruit.
Bacterial spot is caused by Xanthomonas campestris pv. vesicatoria, and it can affect both tomato and pepper. The infection starts off as small circular to irregularly shaped spots that are greasy in appearance. The disease affects leaves, fruit, and stems. Spots on fruit will first appear as raised black spots that will later become large sunken scab-like areas.
Bacterial speck is caused by Pseudomonas syringae pv. tomato. This disease does not affect peppers. The disease affects leaves, fruit, and stems. In contrast to bacterial spot, this disease will cause tiny black specks on fruit that are slightly raised but do not rupture the skin of the tomato.
Control of leaf spot diseases
A proper diagnosis is critical before spraying chemicals on your plants; otherwise you may waste your time, money, and the disease will still be an issue. You may find several fungicides effective against Septoria in your nearest garden center; however, not all of them will be effective against bacterial speck or spot. Sprays of fixed copper sprays can help reduce bacterial speck and spot, but applications will be effective if started as soon as symptoms are observed.
Cultural practices are critical to control and avoid these three diseases. Always start off with healthy transplants or clean seed if growing your own. In general, any practice that that reduces leaf wetness or that promotes rapid drying of leaves (e.g. increased planting distance, de-suckering, and soil level watering) will help avoid heavy infections. Avoid working handling your plants when foliage is wet, this could enhance infection and promote pathogen spread. These pathogens can survive the winter in infected plant tissues, so dispose of plant debris in the fall and avoid planting tomato, pepper, or eggplants in the same location every year.