Hello,

This could be a fungal issue if the agave leaves are getting wet. Plant bug feeding damage looks like this, too.

Poorly draining pots and soil can cause dieback of leaves. If the plant has been outside the spots may be from plant bug damage. Leaves that are indoor grown can be damaged by getting too much direct sun.  If the plant has no water for long periods, the leaves will begin to shrivel and dieback. Agave needs water about once per month, sometimes more depending on temperature and amount of sun. Always let soil dry before watering. 

Prune back damaged leaves, clip off brown ends.  Sanitize the pruner by dipping in a solution of rubbing alcohol, or 10:1 water to bleach solution, for each cut. If the plant hasn’t been repotted, repot in fresh media and a slightly larger pot. 

Your client probably knows how to care for agave, but here is an article on Agave care  from Extension:

Use cacti or succulent planting media, or mix a little soil with pumice or gravel to pot agave. Water should run out the bottom of the pot and any standing water in a catch pot or saucer should be removed.

Agave plants should never be watered overhead, a slow trickle of water right at the root zone can prevent the splashing and spreading of fungal spores, as well as prevent the crown rot that can begin if water pools in the crown of agave plants. Pumice, crushed stone or coarse sand can be added to the soil when planting agave to provide more drainage. Container grown agave will do best in a cacti/succulent soil mixture. Crown rot of agave may present itself as gray or mottled lesions or, in extreme cases, the plant’s leaves may entirely turn gray or black and shrivel where they grow out from the crown. Red/orange fungal spores may also be obvious near the plant crown.

Regularly inspect your agave plant for signs of insect chewing and rot. Fungal and bacterial rots can be controlled with selective pruning and treatment with fungicides such as thiophanate methyl or neem oil. **Reminder, always follow label directions with any fungicides, or chemical treatments. 

Leaves with chew marks or lesions should be cut off at the crown and disposed of immediately. When pruning away diseased plant tissues, dip pruners in a mixture of bleach and water( or rubbing alcohol)  between each cut. In extreme cases of rot, it may be necessary to repot the whole plant- remove all soil from the roots, prune off all crown and root rot that is present and treat with fungicide; replant it in a sanitized pot with fresh planting media.

To get a specific diagnosis of the fungus or possible insect damage, send pictures of the whole plant and leaf samples to MSU Plant Diagnostic lab for diagnosis. See https://pestid.msu.edu for instructions and the fee schedule.

Here are some references-

https://www.ndsu.edu/pubweb/chiwonlee/plsc211/student%20papers/articles11/MarieHagemeister/

https://edis.ifas.ufl.edu/pdffiles/EP/EP41900.pdf

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