Tomato Troubleshooting

By Bryan Fischer, Curator of Plant Collections and Horticulturist

As temperatures climb, healthy tomatoes grow with vigor. While they won’t yet have ripe fruits (or maybe even flowers!), you should see new growth weekly on your plants. If this doesn’t seem to be the case, you may be experiencing “tomato troubles.”

Broadly speaking, tomato issues fall into two categories: early season problems with the growing environment or, as often occurs later in the season, trouble with pests or pathogens. 

The most common early-season tomato environmental issue in our area is simply underwatering. How can you tell? Plants that are underwatered will not put on mentionable new growth and will look thin or lack vigor. It is relatively difficult to overwater in our climate, so if in doubt, try putting down a soaker hose and running it 3-4 nights weekly for 30 or more minutes.  Watering evenly, regularly, and deeply is key. Wetting only the soil — not the leaves — will also reduce the chances of problems later in the season. Less commonly, soil fertility is the culprit. Soil fertility issues appear as yellowing lower leaves on plants, purpling foliage, and lack of plant vigor. If you suspect fertility is the issue, a soil test through Colorado State University (around $35) is your best bet. Nitrogen is most commonly the limiting nutrient in Front Range garden soils, though dozens of other macro and micronutrients can cause problems, many with overlapping or inconclusive symptom sets. 

Later in the year, it’s common that I begin to get texts or emails with tomato plants experiencing other issues. Typically, these result from pests or pathogens. Identification and management of the former are easier than the latter. Psyllids and aphids are common field pests of tomatoes, and both can be seen with the naked eye; look for winged or unwinged, grey, black, green or orange insects about the size of a comma congregating into herd-like clusters on plant tissues. Both sit on the plant’s surface, usually in branch crotches, new growth points, or sheltered under leaves, where they tap into the plant’s vascular system and drink its juices. A sharp jet of water over the plant, especially into their preferred feeding locations can make quick work of most infestations. Generally, one or two of such treatments are enough to reduce the population to a point where beneficial insects (think lacewing larvae and parasitoid wasps) will manage the population for the rest of the year. 

Things become more challenging if your plants exhibit signs or symptoms of disease. In some cases, such pathogens can be soil-borne (a reason to always rotate your vegetable patch with a four-or-greater year rotation), or they can be vectored by insects or inanimate features of the environment. Hail, for example, can open plant vasculature and allow disease to enter, which is one reason I counsel gardeners to wait a day or two after a hailstorm to begin cleanup. Symptoms of disease include leaves that cup upwards, fruit with splotches or discolored rings, black or yellow spots on foliage, or stems that have dark streaking when cut. In the home garden, most diseased tomatoes are best pulled promptly and discarded, not composted. Be sure to wash your hands and avoid brushing healthy plants on your way through the garden with the sick plant, and always wash your hands before entering the garden if you smoke – a virus borne in dried tobacco vectors easily to tomatoes. 

While most tomato troubles fall into one of the above categories, some cases remain hard to diagnose. If you still question what vexes your vigorless plants, consider contacting Colorado State University Extension. As a land grant institution, a crucial part of the CSU mission is to provide direct feedback to community members for horticultural questions of all kinds, and it’s generally free or very cheap! You can find their contact info with a quick internet search and can easily submit your questions through their online form.