My tomato plant gets full sun most of the day and is mulched with straw. The leaves began to yellow and get some brown spots (pictured). It started at the bottom but quickly spread to 2/3 up the plant. So far the fruit is doing fine, with a couple cherry tomatoes already ripening.;I have not seen any spots on the stems, but I have seen some fungus gnats. Should I be concerned?
Thank you for contacting the Toronto Master Gardeners. Looking at your plant overall, there are quite a few yellowing leaves that have no brown spots. I think the reason for this might be chlorosis. Plants with chlorotic leaves might not produce flowers or fruit, and chlorotic leaves are more prone to scorching and leaf diseases. Our weather this summer could have contributed to the development of chlorosis on your plant (temperature extremes, minimal rainfall). Other possible causes for chlorosis include poor drainage, too much water, high soil pH and nutrient deficiencies.
Tomatoes prefer soil with lots of organic matter that is well-drained and slightly acidic. If the pH is too high, the plant will not be able absorb nutrients. For your plant, it looks like there is general yellowing of the leaves (chlorosis), as opposed to yellowing tissue between the veins which remain green (interveinal chlorosis). Chlorosis (not interveinal chlorosis) is often the result of nitrogen deficiency, and it affects leaves beginning with the older (lower) ones on the plant. It is a good idea to get your soil tested so that you know exactly what needs to be amended for optimal plant growth. In addition to information about the level of nutrients in your soil, a soil test will also provide information about pH and organic matter. Basic soil testing kits can be purchased at any garden centre. For more complete soil testing, the Ontario Ministry of Agriculture and Rural Affairs (OMAFRA) has a list of accredited soil testing labs.
It is good that you have mulched with straw to help prevent moisture loss. As a general rule, it’s best to water tomatoes only when the soil looks and feels dry when you stick your finger in the soil. When you water, water deeply. And avoid watering your plant from overhead to keep water away from the leaves. Water the roots instead. More about watering here.
Regarding the brown spots on your leaves, I think there are a couple of possibilities. Early blight and Septoria leaf spot are common fungal diseases in tomatoes. One of the trademarks of Early blight is dark concentric rings (like a target) which appear as the spots get bigger, and I don’t see this in your picture (although it does get blurred as I zoomed in on it). I suggest that you check your plant for this symptom. I think the brown spots on the leaves of your plant could be caused by Septoria leaf spot which starts on the lower leaves after the first fruit sets. One of the trademarks of this fungal disease is tiny black fruiting structures (where spores are produced) inside the tan to grey centre of the spot. I suggest that you check your plant for these black specks. The spores spread easily by wind and rain and especially splashing water. High humidity, temperatures of 20 – 25 Celsius and extended periods of leaf wetness (which could be caused by overhead irrigation) are optimal conditions for this disease. As the disease progresses, leaves will drop from the plant, which weakens it, reduces the size and quality of the fruit, and exposes the fruit to sunscald.
Steps you can take to manage Septoria leaf spot include removing and destroying infected leaves below the fruit as soon as possible, improving air circulation around the plant (don’t crowd it, for staked tomatoes allow at least 2 feet of space from any other plant), and don’t do overhead watering. It is also important to collect and destroy all foliage from infected plants at the end of the season (because spores can overwinter on this foliage), and to use crop rotation (don’t plant tomatoes in the same spot where there was a diseased plant for 1-2 years).
These links provide more information about growing tomatoes and tomato foliage and other problems :
Best of luck with your tomato plant !