Nothing beats a homegrown tomato. Even when in season, store-bought tomatoes just cannot compare to a fully ripe fruit harvested at its peak from your own garden. For this reason, countless gardeners across America choose to plant tomatoes every year, making it the most popular garden crop in the U.S. However, as anyone who has planted this garden staple knows, it can be plagued with problems if proper attention is not given to the common issues many growers experience.
On the bright side of things, insects are rarely the main problem. The tomato hornworm is the most damaging insect pest I have dealt with, requiring attention almost every year. These defoliating larva emerge from pupa in July after overwintering in the soil. They are native to North America with a large home range spanning sea to sea.
It is often very easy to identify the damage that ensues after they hatch. The little critters are big consumers of leaves, fruits and smaller stems on tomato plants, leaving very noticeable damage in as little as a day. I have always had excellent control from timely hand removal of these caterpillars. It does take a watchful eye to notice damage promptly and remove the culprit. During daytime, these guys hang out in the shady lower portion of the plant, waiting for their favorite time to feed, which is after dark. At night, they are easily found feeding in the tip-tops of plants. I make the rounds each evening to easily pluck them off the branch tips.
Fungal pathogens are probably the worst of the problems I have dealt with on tomatoes, requiring attention every year. Early blight is a foliar disease caused by the fungus Alternaria solani. It appears as yellow spots on lower leaves which enlarge into dark brown spots, eventually withering and killing infected leaves. The disease always originates on lower, older leaves and moves up, creating a noticeable pattern. As the infection worsens, it is known to attack petioles, stems and fruits. Alternaria solani overwinters in infected plant debris at or near the soil surface. During favorable spring conditions, the fungi produce spores that are spread by splashing rain drops or wind to infect leaves.
Since the pathogen moves from soil to leaves, a mulch barrier is one of the most effective control measures. In fact, I’ve had excellent control of this condition simply by mulching immediately after planting my tomatoes, offering no opportunity for rain drops to splash the fungi up onto leaves. Watering plants with drip irrigation can also help immensely.
Septoria leafspot is another fungal pathogen that infects leaf tissues. It is characterized by small black spots on leaves, with centers that later turn white and develop tiny black dots. This disease also begins on lower leaves, thriving in wet weather and spreading up the plant. It is transmitted from infected plants or plant debris near the soil, much like early blight, with control recommendations identical for each pathogen.
Providing good air circulation from staking and pruning can limit its spread by reducing favorable conditions, such as wet leaves. Preventative fungicides (cholorothalonil or mancozeb) may also be used to limit infection and spread, although I have never had to use any, accomplishing adequate control from cultural measures such as mulching, staking and pruning.
Both of these fungal diseases can also be addressed by keeping plants healthy to limit susceptibility. Good fertilization (but not over fertilization) is part of that strategy, including a starter fertilizer at the time of planting, a side-dress application when fruits are about golf ball sized, followed by two more applications (spaced out three to four weeks) throughout the growing season. Follow product instructions for the recommended rate for each application.
If your tomato plants are looking bad this time of year, with dying lower leaves spreading up the plant, don’t fret. There is still time to turn things around. Aggressive pruning to remove diseased leaves is a very effective method to limit spread and focus growth on the top, healthy portion of the plant, which will produce more flowers and fruits in the coming weeks.
Don’t be afraid to prune away! Properly fertilized tomato plants are vigorous growers and can certainly “grow out of” any of the damage mentioned in this article. There is still time to produce a bumper crop of one of our best fresh garden crops we can grow in Illinois.
Ryan Pankau is horticulture extension educator with University of Illinois Extension serving Champaign, Ford, Iroquois and Vermilion counties.