After a series of winter storms dropped 11 inches of rain on the Santa Barbara area in January 2005 and this was followed by a sudden warmup, we noted damage that we associate with edema (also known as oedema) on our crops of Agave 'Joe Hoak' There was a very good explanation of this phenomenon on the University of Florida Cooperative Extension Service web site (link unfortunately no longer active) as part of their Plant Protection Pointers series that noted:
"Periodically in Florida's environment, stormy weather fronts will settle over large parts of the state bringing cloudy weather. These situations foster the development of a physiological condition called edema in landscape plants. Under overcast weather, photosynthesis is reduced which reduces stomatal openings in the leaves. Air circulation is often reduced under cloudy weather which reduces evaporation of moisture from soil. A normally vigorous plant in the above situation is absorbing soil moisture through the root system but is transpiring a reduced amount from the leaves due to the closed stomata. Internal water pressure buildup results in the rupturing of water from the lower leaf surface creating small water-soaked blisters overnight. These minute wounds quickly callus into scab-like spots. The upper leaf surface may exhibit small chlorotic spots directly above the lower leaf edema. This disorder happens literally overnight and ends abruptly with the advent of higher light intensity and air circulation that re-establishes normal transpiration through the stomata."
In this case we feel we likely compounded the situation by covering our crops of Agave 'Joe Hoak' with frost protection cloth (Agryl fabric) the nights after the rain because of frosts warning associated with the tail end of the storm. This would have trapped moisture around the plants and also limited transpiration the following day when the plants were left covered. Large unprotected plants of Agave Joe Hoak, both in the ground and in containers were unaffected.
Edema/Oedema damage is also widely reported on plants being transported during warmer months. This can happen with nursery shipments within trucks, inside mail order cardboard boxes shipped by carriers and sometimes even occurs only after short trips across a town as newly purchased plants being brought home from a nursery or when a precious pristine plant is transported for display at local cactus and succulent show. Many in the succulent plant community are aware of this issue and some have been working to resolve it. While no absolute solutions have been determined, its seems that plants transported with good air circulation fare much better than those in closed stagnant areas. It also appears to be a problem primarily with plants that utilize the Crassulacean Acid Metabolism (CAM) carbon fixation pathway which has stomata closed during the heat of the day that open for gas exchange in the color evening hours..
In an effort to determine how best to prevent edema on plants we ship we have spoken with others who have nursereies that ship succulents. One nursery shipper added vents to their truck boxes to increase air flow for this reason alone, while another noted that they no longer ships agave in the heat of summer because of this issue. One company notes that pre-chilling the plants prior to loading makes edema less likely to occur, but this technique is difficult for most to arrange for.
This type of damage also occurs occasionally to Agave attenuata and other plants in landscapes around Santa Barbara and is usually associated with cool overcast moist conditions that we have in early summer months. On October 28, 2015 we experienced odd early morning (pre-dawn) temperature flucuations brought on by a tropical depression off Mexico associated with Hurricane Patricia. Within two days we and many others in the Santa Barbara area were reported very bad edema damage to many agave, aloe plants. To us this event was furhter proof that this edemea phenomenan
In an article written by UC Cooperative Extension Horticulturist Richard Evans for the University of California Nursery and Floricultural Alliance titled SCIENCE TO THE GROWER: No matter how you spell edema, itís an excrescent intumescence on the plant leaf this phenomenon is described and the suggestion made that the name "intumescence' might be best to describe it, but no preventative solutions are offered.
Whether one calls this problem edema, oedema or intumescence, it is a major issue for those growing and shipping succulents and therefore we are still searching for solutions to this very serious problem.