Protecting potatoes and tomatoes from garden pests, such as the tomato hornworm caterpillar, and fungal diseases requires some work. These plants are in the same genus (Solanum), so they have a lot of the same maladies. Their foliage of these plants can become infected with early blight, late blight, leaf spot and other fungal problems. (Late blight was a large factor in the Irish Potato Famine in the 1850s.) These problems are quite common in balcony container gardens, but they are easy to avoid.
Preventing Fungal Problems. Fungal problems start when spores in the potting soil come into contact with your potato or tomato’s foliage. This occurs when water splashes up onto the plant's leaves during watering or when low-hanging leaves touch the ground (you will notice that the fungus affects the bottom leaves first, then moves up the plant). To avoid these garden problems, trim off lower tomato or potato branches. The pruning will keep the leaves off of the soil, as well as improving air circulation, which will also help ward off any fungal disease, as fungus thrives on moist plant leaves in poorly circulated air. When watering your garden plants, make sure water doesn’t contact the foliage, especially water that has touched the ground and splashes up. Mulch can help avoid splashback. Water your plants early in the morning so the soil has time to dry before night brings cooler temperatures. Do not overwater.
Identifying Fungal Problems. If you find yellow plant leaves with black spots, you probably have early blight, which is caused by the fungus Alternaria solani. If foliage turns gray and the container plant wilts, it means it is late blight. The entire plant should be removed from the balcony garden and thrown away.
Treating Fungal Problems. Remove infected container plant foliage without letting the leaves touch any other part of the container plant. If the plant is too infected, remove the entire plant and burn it or throw it away. Fungal spores travel in potting soil and through the air, so you or your neighbors’ tomatoes and potatoes may become infected even after the infected plants are disposed of. Dispose of infected soil, and do not plant tomatoes or potatoes in the same garden soil year after year. You can also use fungicides (Daconil) to kill the fungal culprits, but this will also harm beneficial soil fungus (yes, beneficial fungus exists!).