Two of about six plants—when like this—the plant continued to fruit but all the leaves started to curl. I’ve been growing hydroponically for many years and I haven’t encountered this before. I was using an A&B mix with with an EC of 2.3 and pH of 6.5 and growing in clay balls. The problem is affecting two new plants that went in three weeks earlier and they are looking very sad. Thanks, Tony V.

There can be several different reasons why tomato leaves may curl like this and it’s not an uncommon problem, however, your photo is showing quite a severe case with leaf roll right to the top of a large, mature plant. Leaf curl, which more typically occurs more on the older, lower leaves can be caused by factors such as: high EC/salinity (always check the EC in the drainage solution which flows from the base of the growing pots to ensure this is not getting too high under warm growing conditions), high light, very low humidity and/or high temperature levels, starch accumulation in the leaves due to high rates of photosynthesis or low fruit loading, excessive pruning, over- or under-watering, and other plant stresses. Usually these stress related leaf curl problems start on the lower leaves and slowly develop further up the plant, often the very top leaves may not curl at all. This can also be a symptom of toxicity such as hormone herbicide drift or contamination which tomatoes are extremely sensitive to; or other toxicities from certain plastics or other chemicals/compounds in the growing environment or water supply.

However since you only have two out of six plants showing the leaf curling symptoms and you haven’t seen this problem before despite years of growing, it is less likely to be an environmental or plant-stress issue as all plants would be affected at the same time if this was the case. When only a few plants show quite significant leaf curl symptoms such as this while the rest do not, we would suspect a disease and, in this case, the most likely one would be a virus which has now spread to your new, younger plants. Virus diseases of tomatoes (such as the `tomato leaf curl virus’) — of which there are many — can cause several symptoms with leaf curl only being one of them, and they can easily spread from plant to plant via sucking insect pests and on hands, tools, etc. as the plants are pruned and trained. Viruses can come into a growing area in several ways and generally start with one or two isolated plants before spreading to others. Some viruses are seed borne, others are introduced via sucking insect pests (whitefly is a common vector) that carry virus into crops and spread it from plant to plant. Selecting tomato hybrids which have inbred resistance to the common tomato viruses is always advisable to help prevent some of these issues.

Unfortunately while virused plants may continue to produce fruit, the virus is not curable, and infected plants and their growing substrate should be removed and destroyed away from the planting area to prevent infection spread, particularly to any other new seedlings which are brought into the growing area. A clean down of tools and floors with a strong disinfectant after plant removal would also be advised as viruses can be carried on old plant debris and on surfaces.

Kind Regards,
Dr. Lynette Morgan
Suntec International Hydroponic Consultants