Agave with Black Spots on Leaves

Q. I cut out the leaves with the large black spots and lesions on my agave.  There are a few leaves with fairly small black dots speckling all over the underside (too many to cut out), should I cut off the entire leaf? Additionally, there is another photo showing the edge of the leaf that's kind of perforated - is this age or some kind of bug/mite/disease?

Agave with black spots

A. I think this is a fungal disease that favors higher humidity than we normally have in the desert. The disease was caused and spread by the rain and wind that we had earlier. You don't want that fungal disease spreading to the mainstem. To prevent that, you can apply a common landscape fungicide to the cactus if you think there is going to be a repeat of the rain and wind. Watch your weather app. If your weather app says the rain is returning, apply the landscape fungicide. In the meantime prune out any black spots that you see in the stems. 

I would sanitize between any cuts to eliminate those black spots that you make. Use 70% ethyl alcohol to sanitize a knife or pruning shears. Both the fungicide during wet weather and cutting out the black spots will help eliminate problems.

 Octopus agave originates in the Sonoran Desert of Mexico and Arizona. Make sure you're not watering too often. Get it off of the irrigation controller and hand water with a hose. If you using a hose, water these plants five times a year; three or four times during the summer. Water with a hose three or four times each time you water these plants to get the water 12 to 18 inches deep as it gets bigger. Otherwise the water will only get three or 4 inches deep if you water them only once. Once you establish how often to water, don't deviate from that except for the seasons. The number of minutes stays the same. How many times you water each month changes...that's all.

The leaves of agave won’t totally die back to the stem when part of their leaf is cut. It will just sit there, usually edged with brown where it has been cut. If it doesn’t look “funnier” and out of balance, I would suggest removing the leaves entirely. Your call.

It is up to you if cutting the leaves of agave is acceptable or not. Some people don't mind. Others do.

Remember that fungicides prevent fungal diseases from getting worse. Right now, the humidity is above 15% so I don’t expect diseases to get any worse unless the plant is in poor health. I would recommend “dusting” the leaves with a fungicide dust to prevent the spread of those black spots that you are talking about.

https://edis.ifas.ufl.edu/publication/PP154

Most fungicides are liquid now and not a dust. Dusts were more popular 50 years ago. If the fungicide exists as a spray, it is a different formulation from a dust. Read the directions. An alternative to dusts is to spray over the black spots as “insurance” to keep the disease from spreading if the weather or plant health changes. If it rains again, reapply the fungicide within 24 hours after it stops raining.