Agave

The main non-pest killer of agave: soft rot of the crown and roots in cold-plus-wet winters or from summer overwatering. Give it gritty fast-draining soil, raised berms or slopes, full sun and no winter water, and grow tender types in pots of cactus mix. If the crown is lost, start over from clean offsets.

Wilting in the middle of the day on a plant whose soil is damp - the classic tell, and the reason people water a rotting plant and finish it off. Leaves yellowing from the bottom up, dull off-color foliage, and growth that stalls for no visible reason. Whole shoots, or one side of the plant, collapsing while the rest still looks fine. Soft, dark, or mushy tissue at the base of the stem or at the crown, sometimes with a sour smell. Lift a failing plant and look at the roots: healthy ones are firm and pale, rotted ones are brown or black and slimy, and the outer layer slips off between your fingers leaving a thread of core behind. Seedlings that topple over at soil level are the same problem, called damping-off.

There is no cure for tissue that has already rotted, so act on the conditions rather than looking for a spray. Stop watering at once and fix the drainage - open up compacted soil, or move the plant to a raised, sharply drained position or a container. Lift a badly affected plant, cut away every soft dark root and rotted section with a clean blade back to firm pale tissue, and replant only the healthy parts into fresh free-draining soil or mix. Often the better move is to give up on the plant and save its offspring instead: cuttings, offsets, or divisions taken from firm healthy growth root readily and give you a clean start. Bin the rotted material rather than composting it, and correct the drainage before you replant, because the rot comes straight back in the same wet spot.

Drainage is the whole defense. Match the plant to the site first - something that wants sharp drainage will not survive heavy clay or a low, water-collecting spot however well you tend it. Where the ground is heavy, plant on a raised bed, berm, or slope, or grow in a container. Set plants at the depth they were already growing, since burying the crown or the root flare is a common cause of rot, and keep mulch pulled back a few inches from the stem base instead of piled against it. Water the soil rather than the calendar: check that the top inch or two is dry before watering again, cut back hard in cool weather and winter dormancy, and never leave a pot standing in a full saucer. Do not plant or dig in soggy ground, and go easy on rich amendments for plants that want it lean. Rotate annual crops, and start with clean healthy plants and fresh mix.

Root rot is not one organism but a group of soil fungi and water molds - Pythium, Phytophthora, Fusarium, Rhizoctonia and others - that sit harmlessly in most soils and only turn destructive when the ground stays saturated. Waterlogged soil drives the air out, the roots suffocate, and the weakened tissue is invaded and goes soft, brown, and stringy. Because the damage is underground you usually see it first as something that looks like drought: wilting, yellowing, stunting, a plant that does not respond to being watered. Overwatering, heavy compacted ground, a low spot that collects runoff, a pot with no drainage or a saucer left standing full, and cold wet soil in winter are the usual causes. Once roots have actually rotted there is no cure - this is a problem you prevent rather than treat.

A large agave suddenly wilting, leaves loosening and folding down, the base soft and foul-smelling, the whole rosette collapsing or pulling loose from a rotted core; adult weevils or C-shaped grubs at the base.

Little can save a heavily infested plant - remove and destroy it (and the grubs), and salvage any clean offsets. Protect nearby valued agaves preventively; the weevils move to new hosts as plants die.

Choose more resistant species (many stiff-leaved and smaller agaves are less susceptible), avoid stressing plants, remove and destroy infested plants promptly to stop the weevils spreading, and in high-pressure areas apply a soil systemic insecticide to valued specimens in spring.

The most serious agave pest, especially of Agave americana and other large soft-leaved species: a black weevil that lays eggs at the base of a mature rosette (often one nearing bloom), and whose grubs tunnel into the core, introducing rot that causes the whole plant to soften and collapse - frequently fatal and often noticed only when the plant is already falling apart.