I feel like we are so fortunate to have gotten some significant rains that I almost welcome this topic… Heavy rains can generally cause disease issues for tomatoes in several ways. Knowing and understanding the causes can go a ways towards reducing crop losses due to these diseases. Tomato plants can suffer from a number of afflictions. If the information presented below does not answer your questions, there are a number of OSU Fact Sheets dedicated to this important garden plant.
HLA-6012 Growing Tomatoes in the Home Garden is a good all-purpose document. For specific information and helpful photographs, see EPP-7625, 7626, and 7627 Common Diseases of Tomatoes parts 1, 2, and 3.
Now, back to the issues we may encounter from the rainfall. With moisture readily available, you can expect fungal issues to be a problem. Fungicide treatments can be helpful if they begin when leaf damage is minimal. If the majority of a leaf (or leaves) are affected, it is usually better to remove the leaves completely. Be careful to discard them away from all desirable plants. Depending on the disease and the diligence in your composting efforts, these pathogens could easily remain in your compost and re-infect healthy plants next season.
Fungal borne diseases can also move onto the plant from the soil, especially after a heavy rain where soil may have been splashed onto the plants. Once again, lower leaves that contact the soil should be completely removed. Remember to avoid touching a healthy plant after you’ve handled diseased one until you’ve had a chance to wash your hands well. If you are wearing gloves, keeping a can of disinfectant spray like Lysol close by is a good idea.
Another very common problem with tomatoes, while not directly related to the rain, also involves moisture. That is the fluctuation of water availability to your plant. Tomatoes perform best when the moisture level remains fairly consistent within the soil. While this can be terribly difficult to do some years, try to keep your moisture levels as even as possible.
Once again, you can find much more detail on the fact sheets. These are always available in our office at for no charge or you can find them on-line at osufacts.okstate.edu.
For more information of this or any other horticultural topic, you can contact Keith Reed, the Horticulturist in the Payne County Extension office. Keith can be reached via email at firstname.lastname@example.org, phone at 405-747-8320, or in person at the Payne County Extension office, located at 315 W. 6th in Stillwater.
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