Description of flat-top decay syndrome

Description of flat-top decay syndrome

Giant cardon (Pachycereus pringlei) populate deserts and low hills and mountains of all sizes throughout the Baja California Peninsula in Mexico, and travelers along the peninsula are familiar with this cactus. However, in some "infected areas" the cardon show various degrees of decay. By closely studying over 1000 infected young and mature cardon, it was possible to reconstruct a pattern of decay.

This pattern inspired a name for the phenomenon:
Flat-top decay syndrome of cardon

because this term describes the most common visible symptom. We add the word syndrome because it is not yet known whether the different decaying phenomena detected in this study are caused by a single or by several agents. Heavy infection with flat-top decay could create a plant that looks as if all its branches were chopped off.

Gray decay of cardon

Although flat-top decay is the more common phenomenon we detected, another form of degeneration occurs near the Pacific coast of the peninsula, about 5 km south of the fishing camp of El Conejo (Location on map). Here, the entire mature population (which is generally green elsewhere) turns gray or even white, and the epidermis cracks. Many of the plants in this area are dying from this ailment. This pattern is restricted to a 3-km2 area.

Decaying sites

Field surveys located four large, highly infected areas and three smaller areas. The four large areas are: the island Espiritu Santo-Partida (off the north coast of the city of La Paz), Mesa Prieta, El Conejo area, and south of Bah?a Concepc?on. The smaller decay areas are: near Laguna Balandra, surrounding the Tres Virgenes volcanic area, and the eastern slopes of the Sierra San Francisco (for location and points of entry, see map).

Level of decay

Not all cardon are evenly decayed. It seems that the affliction is localized in small areas (Laguna Balandra, Tres Virgenes, and the eastern slopes of Sierra San Francisco) or much larger zones (probably more than 100 km2 around El Conejo and Mesa Prieta, for example), while completely absent from other zones. Affected and unaffected zones can be close.

The density of decaying cardon varied from site to site, highest at El Conejo and on the islands of Espiritu Santo and Partida, and lowest in an unnamed arroyo north of Loreto and in the southern part of the peninsula in the Sierra de la Laguna. In the latter two sites, no plants were affected (Map).

Why decay of the population happened?

Degeneration, decay, or destruction of mature plant populations, especially ones that reproduce very slowly and can live hundred of years, is a clear indication of some major change that the old plants could no longer tolerate. This is the case for cardon, the largest plant on the Baja California Peninsula and one of the most massive of all cacti.

Only mature plants (a few meters tall and approx. 50–100 years old) flower and produce seeds. Establishment of seedlings is poor. For the cardon, a falling branch cannot re-root as is common for some other cacti. This further emphasizes the extreme importance of old-growth cardon in sustaining its population.

The reason(s) why the giant cacti are dying is unknown.

Wind and pathogenic agents

Of the two large islands in the Gulf of California that were surveyed, one (Isla Cerralvo) is virtually unaffected. The other island (Isla Espiritu Santo-Partida), which lies just 30 km to the north, but only 5 km from the site at Laguna Balandra, is heavily degenerated with dead and decaying plants. The location of the two islands and the prevailing wind patterns point to a pathogenic organism(s). However, reports of cacti pests and pathogens in nature are rare. Therefore, one has to assume that, in their natural habitat, cacti have high tolerance to pathogenic agents.

However, since the cardon may live hundreds of healthy years without any infection, this might be a new pathogen or a modified opportunistic microorganism. The appearance of the affection in limited areas resembles the spread of a pathogenic agent from a small center. This type of phenomenon is well documented for many bacterial and fungal pathogens.

Wind as the agent of transmission cannot explain the two locations of the most severe infections so far found in the wilderness, between La Paz and Cuidad Constitucion in the southern part of the peninsula: Mesa Prieta and El Conejo (map), where most of the cardon were infected, although the number of dead cardon is small. Possible explanations for the “non-lethal” nature of the phenomenon there are:

What happened to a similar columnar cactus in the Sonora Desert?

Degeneration of the cardon cactus is not an isolated case among columnar cacti. Giant saguaro (Carnegiea gigantea) of the northern Sonoran Desert of Arizona (the very same desert where cardons are growing in its southern part) have almost disappeared in the last decades. Most plants that currently grow in the Saguaro National Monument in Arizona are relatively young.

The explanations for this die-off phenomenon have varied between a depletion of the ozone layer that subjects the plants to excessive exposure to ultraviolet irradiation, copper-smelting pollution, car pollution, freezing temperatures, bacterial necrosis, and long-term precipitation patterns. There is insufficient scientific evidence for any of these hypotheses.

It seems that none of the above explanations apply to decay of cardon. Infected populations were found in extremely remote, barely accessible areas, far from human activity. In these habitats, there is no industry or intensive modern agriculture for hundreds of kilometers around, ruling out man-made pollution.

In conclusion, We discovered that this decaying phenomena of cardon is common and widespread.

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