Photo 1. Cupping of tomato leaf
Photo 2. Twisting of leaves
Photo 3. Strapping of leaves
Photo 4. Characteristic frilling of leaf edges
Photo 5. Severe curling of leaves
Each of the past several years, we have received samples of tomato plants from small growers and homeowners that have severe leaf deformity. The leaves of affected plants have the following symptoms: the leaves are distorted with cupping and twisting (Photos 1 & 2), as well as strapping or shoe string symptoms (Photo 3); the leaves can also have distinctive flaring along the edges (Photo 4). In addition, the leaves can be tightly curled (Photo 5). We examined these samples for the presence of viruses, but have never found viruses associated with these symptoms. In addition, the symptoms do not fit classic virus symptoms such as mottling (blending of dark green and yellowing). The symptoms on these tomatoes most closely fit with issues caused by plant growth regulator herbicides such as 2,4D, dicamba, and others. We worked with the growers to determine source(s) of contamination. In all cases, accidental application or drift from a neighboring operation seemed highly unlikely. In all the recent cases that we have dealt with, the most likely source of contamination of a plant growth regulator herbicide was from compost added to the soil mix. In an incident that occurred last year, the potting mix used by the grower was made with compost made from municipal yard waste.
Photo 6. Plant on left grown in affected soil; plant on right grown in affected soil with activated charcoal
We confirmed that the herbicide originated in the soil by planting small pots with the affected potting mix with and without activated charcoal. Photo 6 show the difference in growth between pots with and without activated charcoal (plants grew more or less normal where activated charcoal was added to the soil). In the case of municipal yard waste, there is a large range of possible sources of herbicides: turf clippings, weeds or woody plants. Herbicide residue carryover from clopyralid applied to turf was shown to survive the composting process and damage subsequent crops grown on the composed residue; label restrictions were added to the clopyralid label that restricted the use of lawn clippings from clopyralid treated turf for use in making compost or as a mulch. It is unclear what chemical is the cause of the current set of issues that we have been dealing with here on the Central Coast. The situation that I am describing is not a huge issue, but it is interesting that herbicide carryover shows up with regularity and can cause severe problems in some situations.