This is the first in a series of posts on tomato disease imposters, or problems that present symptoms similar to diseases, but actually have an environmental or nonliving cause.
Tomatoes are one of the most profitable crops to grow, particularly in high tunnels, but they are also very prone to atypical appearances. These may have a biotic cause, such as an insect pest infestation or microbial pathogen infection (which are what entomologists and plant pathologists, respectively, focus on). Frequently, the cause of an abnormal appearance in tomato may be abiotic, or caused by a nonliving entity. Here are a few of the more recent cases of “not a pest problem” we have encountered, with more to come as the season continues.
Chimera (pronounced ki-mare-a), is a genetic mutation producing color variegation in plants. The leaf in Figure 1 came from a cherry tomato plant, and only the lower leaves displayed this symptom of color-blocking, which is starkly delimited by the leaf veins.
This plant was symptomatic when purchased, but symptoms of variegation may also appear spontaneously in plants at a later growth stage. Plants with chimera symptoms typically still produce fruit; however, the fruit may be smaller, fewer, or of lower quality, due to the yellow areas of leaves photosynthesizing poorly, so it is advisable not to buy plants with an appearance as in Figure 1. Due to their genetic basis (and non-infectivity), symptoms of chimera will remain restricted to this plant, and often to only part of the plant. Chimeras occur in a number of different plants that undergo extensive plant breeding but overall are fairly rare.
Tomato leaves commonly develop viral disease-like symptoms, but often for abiotic reasons. One common virus symptom is mosaic, which I generally think of as a camouflage-like appearance of leaves, where coloration shifts from yellow to green and back again. Another symptom of virus infection in tomato (and other plants) is malformation of young leaves, with veins spreading atypically through the leaf itself. However, after negative results from virus testing of the ‘Celebrity’ tomato sample shown in Figure 2, it was deduced that the symptoms are more likely due to a nutritional deficiency, although the exact nutrients involved are not obvious from the symptoms.
Tissue and soil tests would allow a more definitive diagnosis and would inform more effective fertility recommendations for the current crop. The asymmetric leaf vein development may have a genetic basis, similar to chimera.
Trellis systems like the Carolina or Florida weaves are necessary for effectively cropping tomatoes intensively in high tunnel systems. However, even carefully training tomato plants may sometimes result in breakage of tomato leaves and petioles. The petiole shown in Figure 3 actually IS infected with Botrytis gray mold but it is likely that the Botrytis fungus infected the tomato tissue after the trellis injury.
Botrytis is an extremely common fungal pathogen of tomato, but the gray mold disease has a much greater impact in high tunnels where there is excessive plant foliage, poor airflow, and a great deal of injured plant tissue. Care should be taken when training plants in the trellis system in order to prevent trellis injury and subsequent infection with the Botrytis gray mold pathogen.
This is just a sampling of tomato problems that are disease imposters, with more to come in the following weeks. While chimeras occur fairly rarely, nutrient deficiency and trellis injury are quite common.
For additional information on IPM scouting for tomato problems, please refer to IPM Scouting Guide for Common Pests of Solanaceous Crops in Kentucky (ID-172).
By Emily Pfeufer, Extension Plant Pathologist
Acknowledgement: The author thanks Julie Beale for several thoughtful comments and suggestions.