What are tomato diseases?
Tomato diseases refer to infections that can result in a damaged crop, inedible fruit, or even plant death. Like any plant, tomatoes are susceptible to infections, both bacterial and viral. Lots of diseases originate from previously-contaminated soil where the tomatoes get their nutrients, while some form due to damp or humid conditions.
What are some common tomato diseases to look for?
Septoria leaf spot is recognized by its circular spots up to 1/8 inch in diameter. It usually starts at the bottom leaves of the plant and works its way up. It can cause serious defoliation and plant death.
Early blight causes bigger spots with concentric rings on the leaves. It can lead to inedible fruit and defoliation.
Bacterial spots or specks can appear in various formations on the leaves or fruit. The spots and specks are smaller than those from Septoria. The infection can lead to inedible fruit, and plant death.
When does disease strike?
Chelsea Harbach is a Plant Disease Diagnostician at Iowa State University. She says diseases start to show themselves in tomato plants once they grow to their full size. This is because larger plants have a more expansive canopy and therefore trap more humidity. That humidity lends itself to disease propagation.
Harbach also says rain events and humid climates are also troublesome for keeping tomatoes disease free.
If you have a diseased tomato plant but are unsure of the cause of the disease, send a picture of your plant to the Iowa State University Plant and Insect Diagnostic Clinic.
What should you do when you see disease?
If you see diseases forming on the lower canopy of the plant, water at the base and avoid overhead watering - pathogens easily travel through the canopy from water splash.
Caging tomatoes can help ensure the plant grows in an upright pattern. Tomato plants that are lodging (when the stem buckles) increase humidity in the canopy, which creates an environment that fosters pathogen growth.
Some fungicides are available for fungal pathogens. For bacterial and viral infections, Harbach says there's not much to be done once disease presents.
It's also important to note that a lot of pathogens have a latent period, where spores or bacterium will land and will live on the leaf before disease symptoms present themselves. Even if you remove infected leaves, it’s likely other parts of the plant are already infected.
How concerned should you be?
Most tomato diseases are pretty serious and can cause irreversible damage to the plant and/or the fruit. However, Harbach says if you catch bacterial spots on the tomato fruit early enough, they can be cut out and safely consumed. She also says some issues that arise in tomato plants appear to be disease, but are actually physiological issues:
- Blossom end rot is a common problem that is a result of a lack of calcium. Harbach says that in this instance, watering can help.
- Sunscald occurs when the tomato fruit gets too much direct sunlight, often a result of over pruning.
- Cracking fruit is a result of rapid fluctuations in moisture and temperature. And while cracking isn’t an issue itself, it makes tomatoes more prone for serious issues to arise - so it's best to eat or use cracked fruit sooner than later.
Disease management techniques
The best time to prevent disease is outside of the growing season. Remove any infected or dead plant material from the garden. Mulching, (effectively creating a barrier between the plant and the soil) also helps limit disease. Straw and untreated grass clippings will do the trick. Harbach also recommends rotating the location of your tomatoes each year, which helps balance nutrient usage and prevent disease.
Chelsea Harbach's #1 tip? Start with disease-free seeds or disease-free transplants.
