SCIENCE TO THE GROWER: No matter how you spell edema, it’s an excrescent intumescence on the plant leaf
by Richard Evans
If you see abnormal, translucent, warty growths bulging out of the leaves of one of your crop plants and you ask your local farm advisor to identify the problem, you may be told that your plant has edema. Or your farm advisor may identify it as oedema. Or an intumescence. Or an oedemata, neoplasm, enation, excrescence, leaf lesion, genetic tumor, or gall. This profusion of names illustrates the unclear status of a problem that has been attributed to physical injury, chemical injury, insect injury, fungal infection, plant nutrition, air quality, light quality or quantity, soil temperature, air temperature, humidity, excess soil water, plant growth substances, plant genetics and — for those who can imagine the existence of other causes but can’t imagine what they are — “unspecified factors.” Something about this disorder’s warty appearance and muddled status reminds the UCNFA News editors of me, and they have asked me (as the God of Agriculture) to explain what we know, and don’t know, about these curious lesions.
Intumescence May Be the Best Name
First, let’s settle on a name for them. The first published scientific description of the disorder was made in 1886 by Paul Sorauer, a German plant pathologist, who observed it on the Tasmanian snow gum (Eucalyptus coccifera). Sorauer called it an intumescence (enlarged or swollen plant part). In 1893, George Atkinson, assistant professor of cryptogamic botany at Cornell University, used the word oedema to describe a leaf disorder on tomato plants. He observed that the “veinlets as well as the midrib, petioles and the surface of the stem presented numerous elevated areas of a frosty aspect.” Then, in a scientific paper published in 1900, a Cambridge student, Elizabeth Dale, called the outgrowths on rose mallow (Hibiscus vitifolius) intumescences and noted that Americans favor the term oedemata. The other names listed above didn’t appear until twenty or more years later. Since intumescence came first, and since edema (or its older spelling, oedema) is defined by the Oxford English Dictionary as “a fluid-filled tumour or swelling,” which doesn’t always occur as a symptom of this disorder, I rule in favor of calling it an intumescence.
Cause of Intumescence Development Remains Unclear
Naming the disorder isn’t the only problem. Carl La Rue, one of the scientists who has studied intumescences, said, “There seems to be no law as to the development of intumescences within groups of plants.” Some scientists have reported that intumescences occur when the outer layer of cells in leaves (the epidermis) is ruptured because of swelling of underlying water-soaked cells (the palisade parenchyma). Other scientists have attributed intumescences to abnormal cell division, and still others have said the disorder results from a combination of abnormal cell enlargement and cell division.
As noted above, scientists have proposed over a dozen different biotic and abiotic causes of intumescences. These affect well over two dozen plant genera, including horticulturally valuable genera such as Aphelandra, Begonia, Brassica, Cleome, Cuphea, Dianthus, Eucalyptus, Ficus, Hibiscus, Impatiens, Ipomoea, Lycopersicon, Pelargonium, Peperomia, Philodendron, Populus, Ruellia, Solanum and Tibouchina (fig. 1A,B,C).
Fig. 1A, B, C. Tibouchina urvelleana (princess flower) grown in greenhouse (Feb 2015) show symptoms of what has conventionally been called edema. These symptoms are also seen under some magnification showing the warty appearance of the affected areas. These affected areas began to become necrotic a few weeks after they first appeared. Photos: Steve Tjosvold
In nearly all cases, intumescences develop on plants that are growing in controlled environments, such as greenhouses. When intumescences do develop on plants growing outdoors, such as on Eucalyptus (fig.2), they seem to require abundant moisture. In poplar, intumescences have been found that have been rolled up or bound together by insects, and it has been suggested that high humidity in the leaf “chambers” predisposes the leaves to intumescence formation. These observations have contributed to one popular explanation for intumescence development, which ascribes them to the swelling and rupture of leaf cells when water uptake occurs at a faster rate than water loss.
Fig. 2. Intumescences (edema) on eucalyptus. Photo: J.K. Clark.
Another hypothesis is that there may be a transmissible chemical stimulus that induces intumescence formation. A few researchers have reported that intumescences form in healthy leaves that have been injected with a sterilized sap extract taken from intumescent leaves. Application of plant growth substances, in particular auxin and ethylene, also induce intumescence formation in some cases.
It is difficult to determine the exact cause of intumescences because the research results are inconsistent. For example, one hypothesis is that intumescences develop in plants that have received environmental cues similar to those caused by flooding, and that the plant reaction is aimed at improving air circulation in the leaves. However, studies in which researchers manipulated air humidity, soil moisture, or both have yielded inconsistent results, and some researchers suggest that the affected cells don’t actually rupture due to swelling. Instead, their dilapidated state may be caused by drying out. Inconsistencies have also shown up in intumescence research where light has been the focus of researchers’ attention. Although studies have linked intumescences to absence of ultraviolet light (filtered out in most greenhouses) or insufficient far-red light, there is no solid, consistent evidence supporting any single cause of intumescence formation.
Damage Symptoms and Treatment
One thing people seem to agree on is that intumescences detract from the esthetic and monetary value of plants. For example, intumescences on princess flower (Tibouchina urvelleana, fig. 1A,B,C) soon turn necrotic; the intumescences that occur on the lower side of ivy-leaf geranium (Pelargonium peltatum) leaves turn brown and corky, and in severe cases cause the leaves to become distorted or drop from the plant. Intumescences can affect a grower’s pocketbook indirectly, too. Pervasive intumescences on leaves are likely to reduce photosynthesis and decrease plant growth rate.
So what can be done? Unfortunately, not much. The presence of an array of potential causes has led to a mixed bag of recommended practices, but none has been shown to work consistently. Perhaps continuing study of intumescence development will lead to a better understanding of this scourge, as well as some solutions for growers.
Richard Evans is UC Cooperative Extension Environmental Horticulturist, Department of Plant Sciences, UC Davis.
Atkinson GF. 1893. Oedema of the tomato. Cornell University Agricultural Experiment Station Bulletin 53.
Craver JK, Miller CT, Williams KA, Boyle DL. 2014. Characterization and comparison of lesions on ornamental sweetpotato 'Blackie', tomato 'Maxifort', interspecific geranium 'Caliente Coral' and bat-faced cuphea 'Tiny Mice.' Journal of the American Society for Horticultural Science 139: 603-615.
Dale E. 1900. On certain outgrowths (intumescences) on the green parts of Hibiscus vitifolius Linn. Proceedings of the Cambridge Philosophical Society 10: 192-209.
Lang SP, Struckmeyer BE, Tibbitts TW. 1983. Morphology and anatomy of intumescence development on tomato plants. Journal of the American Society for Horticultural Science108: 266-271.
La Rue CD. 1933. Intumescences on poplar leaves. I. Structure and development. American Journal of Botany 20: 1-17.
Morrow RC, Tibbitts TW. 1988. Evidence for involvement of phytochrome in tumor development on plants. Plant Physiology 88: 1110-1114.
Pinkard E, Gill W, Mohammed C. 2006. Physiology and anatomy of lenticel-like structures on leaves of Eucalyptus nitens and Eucalyptus globulus seedlings. Tree Physiology 26: 989-999.
Rangarajan A, Tibbitts TW. 1994. Exposure with far-red radiation for control of oedema injury on 'Yale' ivy geranium. HortScience 29: 38-40.
Sorauer P. 1886. Handbuch der Pflanzenkrankheiten, 2nd edition. Parey, Berlin.