Vegetables and fruit – Gardening rescue

Yellow tomato leaves can be really alarming for gardeners (myself included). The conclusion people jump to is...could it be blight, perhaps overwatering, or is there a frustrating nuance of tomato growing I have missed?! The annoying thing about tomato growing is that there are around 6 completely different reasons that result in almost identical leaf yellowing. The good news, however, is that the reasons the leaves turn yellow are not random as such. Plants in general follow what I call "biological logic", and once you are able to decipher their logic, the yellowing tomato leaves give you clues as to what is wrong. I have written in this article the diagnostic framework I use and 2 simple tests you can do to identify the problem. Read the Leaves Before You Do Anything The first and most important step in diagnosing yellowing tomato leaves is identifying where on the tomato it is happening (lower or upper leaves, for example) and what pattern it is following. Noticing the difference can help you identify whether the cause is a nitrogen deficiency or an iron chlorosis problem, which require very different interventions to remedy. The reason the location is so important is that, in plant biology, nutrients behave differently inside a plant depending on whether they are mobile or immobile. Together, we need to understand the distinction to unlock almost all the diagnoses for yellow tomato leaves. Mobile nutrients...nitrogen, magnesium and potassium are the big three (tomatoes need the nutrients more than any other), and they can all be actively drawn out of the older leaf tissue and the resources are redirected to newer growth when the tomato is running low on these nutrients. The tomato is using up the energy from the lower leaves to keep the new growth (which is closer to the light) growing. Immobile nutrients...Iron and calcium, however, are different as they cannot be essentially moved from older leaves to new leaves once the plant has deposited them in the plant's tissue. When you have a deficiency in these nutrients, the tomato cannot resupply them to new growth. This means that the deficiencies in immobile nutrients always show up in the newer, uppermost leaves first. This framework of mobile nutrients (affecting lower older leaves) and immobile nutrients (affecting newer leaves) is the key to diagnosing the cause before you have even looked at the soil or spent money on treatments. The Diagnostic Map WhereWhat You SeeLikely CauseFixBottom / older leavesUniform fading, pale yellow overallNitrogen deficiencyFish emulsion or blood mealBottom / older leavesYellow patches between veins, veins stay greenMagnesium deficiencyEpsom salt drenchTop / new leavesSharp yellow or bleached white, veins stay greenIron chlorosisTest and correct soil pHWhole plantGeneral drooping, dull yellowing, lower leaf dropLack of air around the rootsImprove drainage immediately When the Plant Is Feeding Its Fruit, Not You Around three summers ago, my Cherokee purple tomatoes had problems, just at the wrong time! The first lovely clump of green tomatoes began growing, but the bottom leaves turned a uniform yellow practically overnight. I thought they had blight, but when I looked closely, there weren't any dark spots, lesions or spreading patterns, so the plant wasn't diseased; it was just low on nutrients. The tomato had started to redirect its nutrients stored in its lower leaves and transport nutrients (nitrogen in this case) to the developing fruit. The tomatoes' priorities change when they flower and begin to fruit. Rather than directing energy into foliage growth, they put their nutrients into developing the fruit. How to tell the difference...If the lowest leaves are turning uniformly yellow whilst the fruit is set, but the leaves do not have spots or patterns and the upper leaves remain green, and the plant looks okay overall, it is a lack of nitrogen. The fix...A dressing of organic nitrogen or a tomato feed (I use fish emulsion or blood meal or tomato feed, and tey all work well) applied to the soil gives the tomato enough nutrients to fuel the growth of fruit and the lower leaves. However, I must caution against overfeeding. More is not more when it comes to feeding tomatoes, as too much nitrogen causes excess foliage growth and decreases the plants' resistance to disease and pests. I use a half strength and apply it consistently when it comes to tomatoes. The Healthy Soil, yet a Starving Plant Problem This is the diagnosis that most gardeners miss, but it is more common than you'd think. Years ago, I once managed a community garden bed where the Brandywine tomatoes looked very chlorotic (low in iron) despite being planted in the best compost I could find. A well-meaning colleague suggested using a nitrogen fertiliser; however, I did some research and used a cheaper pH probe I bought online. The soil was at 7.6 pH, which is highly alkaline for a garden. I had been informed that someone had amended the soil with wood ash beforehand (which is highly alkaline), which explains the reading. Iron (which is often the cause of chlorosis) was present in the soil, but the high alkalinity had effectively locked it in the soil so that the tomato plants' roots could not draw it up. High alkalinity essentially makes it impossible for certain nutrients to be accessed by plants. Therefore, adding fertiliser wouldn't have done anything in this case. So I treated the soil pH, which brought back the green colour in about a week! The signs of iron chlorosis... The newest leaves at the top of the plant emerge pale yellow, whilst the veins of the plant remain green in contrast. This intervienal chlorosis on the new growth rather than the old growth is a classic symptom of an immobile nutrient problem. This is different from the nitrogen deficiency, which produces uniform yellowing of older leaves. If you suspect it's a pH problem, then... A cheap soil meter (I got mine from Amazon) is a useful addition to your gardening tool kit. Tomatoes prefer a pH of 6 to around 6.8. If the pH is above 7, then it is more difficult for the iron and manganese to be accessible by the roots. You can correct it with sulfur, but the way I do it is with ericaceous fertiliser. Ericaceous just means acidic. I used some of this at half strength, applied twice a month, and the tomatoes recovered quickly. To be on the safe side, I also used a 1-inch mulch around the base of the tomatoes with ericaceous compost to rescue my tomatoes. Two Tests You Can Do Today Test number 1: A 48-hour foliar feed isolate... If you suspect a nitrogen deficient problem but you cannot be certain, then this can help to confirm it without committing to feeding the whole plant. Choose one very clearly yellowing lower branch of your tomato plant and mix a half-strength solution of a water-soluble nitrogen fertiliser (fish emulsion works) and mist only that branch diligently, ideally early in the morning and leave the rest of the plant untreated. Then you need to check in around 48 to 72 hours time. If the branch that has been sprayed has a deeper colour whilst the rest of the plant has yellow leaves, then you have confirmed it's a nitrogen deficit problem. However, if noting changes, then the cause is usually environmental rather than nitrogen, so I would look at drainage and pH instead. Test number 2...Do a Drainage Assessment You can dig a small-sized 6 inch hole next to your tomato plant (or look at the base of the container if it's in a pot or planter) and fill it with water and time how long it takes to drain. If the water sits there for more than 10 minutes and it looks boggy, then the yellowing is almost certainly as a result of the roots not having enough oxygen for root respiration. Boggy soil excludes oxygen from around the roots, and without oxygen, the roots cannot respire and draw up nutrients properly. The consequence is that this causes the tomato plant to yellow and droop, even though the nutrients are there...the plant cannot draw them up. I like this test as it takes around 5 minutes and rules out one of the most common causes of yellowing leaves, so you don't have to spend money on treatments. When Yellow Leaves Are Simply Normal One final thing I want to say is that some lower leaf yellowing on a tall, mature tomato plant around midsummer is normal. As your tomato plant grows taller and its canopy increases, the lower leaves receive less direct sunlight. The tomato plant can then often redirect the nutrients and energy, as we discussed, away from these leaves, especially if they're not receiving enough light. My rule is that if the yellowing is just restricted to 1 or 2 of the lowest leaves on a healthy, productive tomato plant, then this is not a problem. Yellowing leaves that need attention is when it spreads, there is a pattern or other symptoms such as drooping, poor growth or spots. Most of the time, the yellowing on the lower is just a change in energy. Are you dealing with yellow tomato leaves and not sure which category they fall into? Drop a comment with the details, such as where on the plant, what pattern, and what your recent care has looked like, and I'll help you work through it.