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Proceedings - Florida Tomato Institute 1999
History and Symptoms The problem first presented itself in the fall of 1986 when several growers in the Quincy area of North Florida encountered plants in their fields with unusual growth characteristics. Early symptoms consisted of interveinal chlorosis in the young leaves with veins remaining dark green. Subsequent top growth became severely distorted with leaflets along the midrib failing to expand properly, resulting in a "little-leaf' appearance. Additional symptoms included cessation of terminal growth, leaflets with twisted and brittle midribs and axillary buds with very little and distorted growth. These later symptoms can be confused with cucumber mosaic virus. Fruit that set when plants are mildly affected are distorted with fruit being flattened and at times radial cracks extending from the calyx to the blossom scar. There is almost no seed set in affected fruit and all fruit are unmarketable. In the more severely affected plants, blooms are distorted and fail to set fruit. Several growers commented that the same set of symptoms had been observed in the past, but only on a very few plants. Following 1986, the set of symptoms was referred to as "tomato little leaf".
Since 1986 the problem has been observed sporadically in the fall crops in the Quincy area. Occasionally the problem has affected large areas and caused significant yield reduction (as much as 80%). It has only presented a problem in one spring crop (1992) when little leaf was observed in the north Florida/south Georgia production areas. It also occurred in the East Coast tomato production area in the winter of 1998 were yield loses from 20 to 50% occurred. I have also received reports of little leaf occurrence from Ohio, Texas, Maryland, Poland, Italy and possibly Indonesia. Attempts to Determine the Etiology To try and determine a cause, numerous samples were sent to the Extension Plant Diagnostic Clinics in both Gainesville and Quincy for virus detection and all samples were negative. Some growers believed the problem to be nutritional, since from a distance early symptoms emulated iron deficiency. Many nutritional sprays including Fe, Ca, Zn and B were made but to no avail. Extensive tissue sampling and analysis were performed from affected and non-affected plants. Nutrient levels were determined for N, P, K, Ca, Mg, Fe, Zn, Mn and B. The results failed to reveal an abnormal nutritional situation. Extensive testing of soil and plant tissue for herbicide residues were also negative. One common observation was that affected plants showed up in the wetter areas of the fields. Finally, cuttings made from distorted tops developed normal growth habits after rooting proving that a virus was not responsible. Possible Explanation In tobacco there exists a nonparasitic disease called frenching. It is considered to be a nonparasitic disease because the organism or organisms that cause the disease do not live within the tissues of the plant. Early symptoms of frenching consist of chlorosis along the margins of young leaves. The chlorosis gradually spreads toward the midrib until all interveinal regions are involved, however the veins remain dark green. As the leaf continues to develop, only the midrib elongates producing a long, ribbon-like leaf. Terminal growth is greatly retarded and apical dominance is lost, resulting in a stunted plant with an increased number of small brittle leaves. The foliar symptoms on tobacco are nearly identical to the symptoms observed in tomato little leaf. Tomatoes were mentioned to be sensitive to the problem in an experiment station bulletin over 60 years ago but symptoms were never described. Other plants that have been reported to be susceptible include petunia, eggplant, ragweed, sorrel (Oxalis stricta) and squash. Chrysanthemums are also susceptible and the disorder is referred to as yellow strapleaf and was a problem of serious economic consequence to chrysanthemum growers in Florida between 1956 and 1965. The etiology of the conditions described above are not completely understood. The current hypothesis is that one or more amino acid analogs are synthesized by certain soil microorganisms and released into the rhizoshpere. These compounds, which are structurally similar to the amino acid leucine, are taken up by the plant causing morphological changes and stunting in susceptible plants at very low concentrations. It is believed that these compounds act as an antimetabolite of the amino acid leucine. Currently, three soil microorganisms have been implicated as the causal agent. The first organism implicated was the bacterium Bacillus cereus. In controlled experiments, symptoms of frenching were obtained from diffusion of a compound produce by B. cereus into small tobacco plants. B. cereus is a ubiquitous soil inhabitant and has been observed in large numbers in the root zone of tobacco plants with frenching symptoms. Another organism that has been implicated is the fungus Aspergillus wentii. This organism has been shown to produce a compound (ANCPA) which is a potent antagonist of leucine. In the lab it has been shown that ANCPA in minute quantities can produce symptoms on tobacco similar to frenching and can similarly affect the growth of other crops such as bean, tomato, sunflower and chrysanthemum. Inoculations of tobacco root systems with the soilborne fungus Macrophomina phaseolina has also resulted in the appearance of frenching symptoms. It should be noted however, that often times symptoms fail to develop after inoculation with these organisms. Role of Environmental Conditions in Symptom Development The problem has been shown to be more severe with warm soil temperatures. In tobacco symptoms can appear in 8 to 15 days with soil temperatures of 95°F, but rarely appear in soil temperatures of 70°F or less. Symptoms in tobacco are more severe on neutral or alkaline soils and are rarely observed with a soil pH of 6.3 or less. Frenching of tobacco and yellow strapleaf of chrysanthemum are more prevalent on wet, poorly-aerated soils. It is apparent the for the problem to occur a fairly unique combination of the above soil conditions must occur in combination with an appropriate soil microorganism. Possible Control Methods In tobacco control measures can include, reduction of soil pH to about 6.3 or less or use of ammonium sulfate as the nitrogen source to lower the pH around the root system. In chrysanthemum production, to reduce the potential of the problem, it is advised that soil moisture be maintained so as not to cause water-logged conditions. In both tobacco and chrysanthemum, pulling the affected plants up and replanting has resulted in the plants growing out of the disease. This remission of symptoms is apparently the result of improved aeration and perhaps by root pruning which resulted in decreased uptake of toxins by the root system. In recent years in north Florida the occurrence of little leaf has decreased because growers have become aware of the need to monitor soil moisture very carefully and make sure water-logged conditions do not occur. Lacking definitive information, we cannot suggest any radical changes in tomato culture at this time for avoidance of the disorder other than monitoring soil moisture levels to avoid waterlogged conditions. Higher soil pH has been implicated in tobacco but not chrysanthemum. Changes in soil pH for tomato should be approached carefully to avoid problems that might accompany reduced lime utilization. The problem does not seem to carry over from one season to the next, since at this time we have not seen the problem in the next year after a field was affected. If it caught early, drying a field out will usually result in subsequence growth to be normal and plants will set normal fruit. Conclusions A set of symptoms consisting of interveinal chlorosis of young leaves, distortion and failure of leaves along the midrib to expand properly, fruit distortion consisting of radial cracks extending from the calyx to the blossom scar and failure of blooms to set have been observed for numerous years in the north Florida and other production areas. These symptoms are collectively referred to as "little leaf syndrome" and have resulted in significant yield loss for numerous growers. Repeated attempts to identify a plant virus as the causal agent have failed. Nutrient analysis and examination for pesticide residue in affected plants have not provided any explanation for the cause. At this time it is felt that the cause of the problem may be similar to that of frenching of tobacco, where a nonparasitic microorganism living in the soil produces a toxin under specific soil conditions that disturbs the protein metabolism of the plant, causing the symptoms to occur Lacking definitive information, we cannot suggest any radical changes in tomato culture at this time for avoidance of the disease. Three conditions appear to predispose to this class of disease/disorder, namely 1) high soil temperatures, 2) high soil moisture, and 3) the presence of a predisposing population of bacterial/fungal soil microorganisms. Higher pH has been implicated in tobacco but not chrysanthemum. Changes in soil pH for tomato should be approached carefully to avoid problem that might accompany reduced lime utilization. The single most important factor to monitor to reduce the possibility of little leaf occurring is to monitor soil moisture and prevent water logging conditions. |
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