Answer: It's difficult to determine your problem without more information. Cotton root rot (Phymatotrichum omnivorum) is found in low desert soils, but desert plants and cacti are usually tolerant of this fungus. Infected plants suddenly wilt and die in the heat of the summer and the dead leaves remain attached to the plant. Infected roots have a distinctive fungal growth pattern which can be seen under a microscope as a cruciform (like a crucifix) pattern. Another symptom is pancake batter-like spore mats (often confused with other harmless fungi) appearing on the soil surface of infested areas, but do not spread the fungus.
Agaves sometimes collapse into a putrid, rotting mess during late summer as a result of bacterial rot and internal infestation of Agave Weevil larvae. Adults are dusty black weevils about one inch long, with a long snout. They chew into the leaves of agave and introduce a bacterial rot that appears to be necessary for larval development. They lay eggs into the holes and the larvae burrow into the plant. But that doesn't sound like your description and it wouldn't affect the saguaro.
We often see desert plants being "overwatered" from outside sources. Is it possible that your agave and cactus are receiving water from the neighbor's lawn? In combination with heavy rains we had lately, our clay soils can remain soaked below the surface even if the ground doesn't look wet, literally drowning the roots from lack of oxygen.
I'm going to refer you to Maricopa County Cooperative Extension, 4341 East Broadway Road, Phoenix, 602-470-8086, ext. 301. I suggest you bring in a sample of the rotting roots and some of the soil. Also, have more information about the condition of the plants, how they're cared for, when you first noticed the problem, and so on. They can help you determine exactly what is happening.
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