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From: Spring Branch, TX
Region: Southwest
Topic: Cacti and Succulents
Title: Lesions on Agave Leaves in Spring Branch, Texas
Answered by: Mike Tomme
Boy, am I glad you only asked what the problem is rather than what to do about it, because that would have been a really hard question.
Agave americana (American century plant) is normally resistant to most diseases, but this previous Mr. Smarty Plants answer gives some information on the anthracnose fungus which seems to be the most likely culprit in your situation. This fungus thrives in wet conditions which, I suspect, has been the situation where you live over the last year. Shade can also contribute to the problem.
Ok, what to do about it (hard questions don't scare Mr. Smarty Plants). Here is an article from the University of Arizona that (briefly) discusses the issue on page 7. This article, which was also cited in the previous Mr. Smarty Plants answer mentioned above, recommends removal of the affected foliage and also mentions a fungicide that can be used if that is your preference.
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