Mysterious Ailment Afflicting Saguaro Cactus Baffles Experts : Ecology: Pollution, ozone depletion and loss of sheltering ‘nurse plants’ are among possible causes of the slow-growing desert giants’ decline.

The saguaro cactus, the pitchfork-shaped emblem of the Southwest, is suffering from an ailment that turns its green skin brown and black and slimy.

Some scientists say the stately cacti are in poor health because of factors ranging from copper-smelting pollution to depletion of the ozone layer.

Others are less alarmed by the brown decay known as bacterial necrosis and believe that the decline of the saguaro has simpler explanations, such as freezing temperatures or the cutting of trees that shelter young saguaros.

The tug-of-war between researchers has left more questions than answers about man’s effect on the cactus. It also has deepened the mystery surrounding the sentinel of the desert.

“We don’t understand much about the saguaro,” said Meg Weesner, research chief at this 87,000-acre national park outside Tucson. “People are just now looking into its root structure and how its habitat affects its growth rate.”

The saguaro (pronounced sa-WAH-ro) grows only in the Sonoran Desert in Arizona, northern Mexico and a sliver of southeastern California near the Colorado River, at elevations ranging from 2,000 to 4,000 feet.

The cactus has a normal life span of 175 years; it can grow to a height of 50 feet and a weight of five tons.

Fears of the disease are heightened by the plight of one stand of cacti in the park. A 1942 photo in the park’s handbook shows a landscape studded with thriving saguaros; a 1984 photo from the same vantage point shows a forest of dead cacti.

Last October came the worst blow. The monument’s prize cactus, a 300-year-old, 52-foot-tall saguaro known as “Old Granddaddy,” was diagnosed with bacterial necrosis.

“No one knows what it is,” said Kate Lajtha, a Boston University ecologist. “The cuticle just turns brown. Its outside waxy layer just depletes.”

“It’s this oozing cactus, this brown, black, slimy fluid,” said Joe McAuliffe, director of research with the Desert Botanical Garden in Phoenix. “It’s a not a pretty sight when you see a saguaro go.”

Tom Danton, chief of interpretation for the monument, said the disease weakens the cacti but doesn’t necessarily kill them.

For Arizonans, the saguaro is an emblem as potent as the American eagle or the Statue of Liberty. Its white blossom is the state flower and a resident, the cactus wren, is the state bird.

The Tohono O’odham Indians still harvest the cacti’s red fruit. They knock the fruit from the tips of the arms with long staves fashioned from the ribs of dead saguaros; then they ferment the juice into a ceremonial wine.

The cactus is often portrayed as invincible. In 1983, a prankster who peppered a 24-foot saguaro with shotgun blasts was crushed to death when the desert giant toppled on him.

Despite its awesome size, the plant is more fragile than its folklore.

A 2-year-old plant may be only one-quarter of an inch in diameter, and a 9-year-old plant on average is about six inches tall. It takes 75 years to grow its first arm.

The saguaro as a youngster needs a sheltering “nurse plant” to grow under. As an adult, it’s still susceptible to freezes and overheating.

Man also threatens the cacti. People dig them up and sell them for thousands of dollars on the black market. Builders remove them for residential and commercial developments.

Scientists have been worrying about the saguaros’ health since shortly after the monument was established in 1933. But their concern accelerated in the late 1980s when researchers studying the effects of pollution in national parks arrived in the monument.

Some suggested that pollution, either from cars or copper smelting that began in the early days of white settlement, contributed to the browning of the cacti.

Lance Evans, a professor at Manhattan College in New York City, suggested in 1991 that the depletion of the ozone layer and a corresponding increase in levels of harmful ultraviolet rays were killing the saguaro.

McAuliffe, who was hired by the National Park Service to analyze research on epidermal browning, said man has affected the saguaro; however, he said, that influence dates to a century ago.

The absence of large saguaros in the monument today, he suggests, is a result of heavy cutting of trees for firewood. Without palo verde and mesquite trees to shelter them, young saguaros are extremely vulnerable.

He cites studies showing that young saguaros are rebounding as the palo verde and mesquite return under park protection.

McAuliffe calls the browning of the saguaro’s skin the natural effect of freezing or overheating--something akin to a sunburn.

“I remember when we had that record 122-degree day in Phoenix,” McAuliffe said. “Later, I went to an area north of Phoenix and those saguaros were baked.”