What to Do With Potatoes and Tomatoes Infected With Late Blight

Every gardener knows that no plant, vegetable, fruit, or flower is completely immune to disease or infestations. All you can do is take the proper preventative measures to keep your plant safe, and know how to deal with a diseased plant if you end up having one.

Two well-loved garden heroes, tomatoes and potatoes, are especially susceptible to an affliction known as late blight. Characterized by large, irregularly-shaped, greasy gray spots (and even soft rot), late blight is a serious plant affliction that led to the Irish Potato Famine in 1845. Although plants infected with late blight tend to die quickly, you can often salvage some of your tomatoes and potatoes from the plant before they meet their fate thanks to these tips for dealing with late blight.

Identifying Late Blight

Late blight is a disease caused by the fungus Phytophthora that can impact both tomatoes and potatoes. While it typically occurs late in the season (as the name implies), it can actually strike your garden at any time. Not only does the pathogen destroy the fruits of the plant it infects, but it can also spread very quickly and widely, wreaking havoc in your garden if not dealt with promptly and properly. The worst part: Late blight can overwinter, meaning that, even after a cold frost, it can return the following year if the plants and soil containing the pathogen are not removed.

Late blight first presents itself as a soggy or water-soaked appearance on the older leaves or stems of a plant, eventually spreading into white and dark spots that cover the entire infected plant and its crop. Because it's carried by spores, late blight can travel long distances, often blowing nearby infections to your garden or spreading between your plants. If you do determine that your plants have been infected, it's a good idea to contact your Cooperative Extension Service to let them know. There's a chance that your infection came from another local source, or that your plants' infection will affect others in close proximity.

Treating and Preventing Late Blight

If you suspect that your tomatoes or potatoes have become infected by late blight, your best course of action is to treat the plants as promptly as possible in order to prevent the disease from spreading.

Start by tending to the infected plants, taking care to remove any diseased leaves, stems, or fruit. Be sure to dispose of the scraps—composting may only further spread the infection throughout your garden. Once you've removed all visual signs of the infection, apply a copper-based fungicide to the plants weekly in an attempt to dissuade the late blight from spreading or returning. At the end of your harvest, completely remove the plants, their scraps, and the surrounding soil from your garden. Start anew with fresh seeds and soil the following year to guarantee the health of your next crop.

There are also several steps you can follow to prevent late blight from taking ahold of your garden in the first place. First, look for varietals of plants that are resistant to late blight—Crimson Crush and Mountain Magic are just two types of tomatoes known to resist the disease. Additionally, blight thrives in a humid, wet environment. Water your plants early on in the day so they have time to dry out in the warm sun more quickly. Take care to help your tomato plates grow off the ground by using stakes or cages, and, if possible, choose a spot in your garden for the plants that are sheltered from the wind, which can help protect them from blight spores that have blown from neighboring gardens.

Saving Effected Foods to Eat

The good news: Late blight cannot infect humans, so depending on when you're able to salvage your tomatoes or potatoes, they are safe to eat. If blight lesions are evident, you can simply cut those parts off the tomato or potato and use them as normal. However, keep in mind that the late blight infection may reach the plant before it has had the opportunity to completely ripen and, oftentimes, it will not progress once infected. Therefore, you may be unable to let the plant ripen further as it will likely just rot instead, making it unusable.

Canning or Saving Seed from Effected Plants

A little-known fact: Late blight can raise the pH levels of otherwise acidic tomatoes high enough to allow bacteria or other spoilers to grow, in turn affecting the safety and flavor of canned tomatoes. Take care to use only your freshest, healthiest tomatoes when canning and preserving.

Tomato diseases like to stick around from year-to-year, but late blight needs living tissue to survive. Drying, saving, and using seeds from tomatoes infected with late blight is fine. However, it’s still smart to plant your tomatoes in a different section of your garden and to throw away (not compost) all affected foliage and leftover fruits in the fall.

Potatoes are another story, however. Since potatoes are still alive when you store them, they could continue to harbor the disease. Feel free to eat and enjoy your potatoes this winter—just be sure you don’t leave any potatoes in the garden over winter and start with fresh seed potatoes in the spring.