We are a little over a month into the processing tomato season. I have received a couple of farm calls regarding herbicide damage or symptoms originally thought to be diseases. The first was trifluralin injury to young tomato plants. These samples were brought in to confirm that it was in fact trifluralin injury and not corky root rot. Corky root rot in tomato presents as corky bands on the roots with brown lesions leading to cracking of the cortex on larger roots. The tips of older roots may be pinched off. Small roots may be decayed and the tomato plants may be stunted. Trifluralin injury to tomatoes presents as root stunting, callused stems, swollen crowns, leaf distortion, stunted plants. It can look very similar to corky root, due to the corky appearance of the roots but is generally not banded and the swollen crown is distinct.
The second issue encountered in the fields was in month-old tomato plants showing uniform symptoms across the field of leaf distortion on newer growth. The new growth was twisted and cupped upward, but the plants looked otherwise healthy. The concern was that a virus was present in the field, because similar symptoms were found in a second field of the same variety. Due to the uniformity of the problem and viruses having patchier distributions, we guessed it was not a virus but rather herbicide injury due to a potential overspray of rimsulfuron that had been applied 7-10 days before.
In addition to this uniform problem, a small number of plants exhibited symptoms of yellow rings and mosaic. Samples were tested by the Gilbertson lab at UC-Davis. Plants without the viral symptoms but still showcasing the distorted new growth were also tested. Only plants with the yellow rings and mosaic were confirmed to a have a virus and this virus was at a very low level in the field. The plants without these symptoms were negative for the virus, eliminating the concern that the uniform distorted new growth was caused by this virus.
It has been over 2 weeks since the rimsulfruon application and the plants are recovering well. Fortunately, this particular injury did not cause severe damage to the tomato field. Diagnosing herbicide injury can be challenging when so many diseases present similar symptoms. When in doubt, contact your local UCCE Advisor.