By: Darcy Larum, Landscape Designer
While usually an easy plant to grow in rock gardens and hot, dry areas, agave can be susceptible to bacterial and fungal rots if exposed to too much moisture and humidity. Cool, wet spring weather that rapidly changes to hot, humid summer can cause a surge in fungal growth and pest populations. Mid to late summer crown rot of agave plants can be common in cooler climates and potted plants. Read on to learn what you can do for agave plants with crown rot.
Agave, or century plant, is native to the deserts of Mexico and hardy in zones 8-10. In landscaping, they can be a stunning addition to rock gardens and other xeriscaping projects. The best way to prevent root and crown rot of agave plants is to situate them in a location with excellent drainage, infrequent irrigation, and full sun.
Agave plants should also 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 happen if water pools up in the crown of agave plants. Pumice, crushed stone, or sand can be added to the soil when planting an agave to provide more drainage. Container grown agave will do best in a cacti or 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 right where they grow out from the crown. Red/orange fungal spores may also be obvious near the plant crown.
Crown and root rots in agave can also be caused by an insect called the agave snout weevil, which injects bacteria into the plant as it chews on its leaves. The bacteria causes soft, squishy lesions in the plant where the pest then lays its eggs. Once hatched, the weevil larvae tunnel their way to the roots and soil, spreading rot as they work their way throughout the plant.
It is important to regularly inspect your agave plant for signs of insect chewing and rot, especially if it not growing in the optimal conditions. If caught early enough, fungal and bacterial rots can be controlled with selective pruning and treatment of fungicides such as thiophanate methyl or neem oil.
Leaves with chew marks or lesions should be cut off at the crown and disposed of immediately. When pruning away diseased plant tissues, it’s recommended that you dip pruners in a mixture of bleach and water between each cut.
In extreme cases of rot, it may be necessary to dig up the whole plant, remove all soil from the roots, prune off all crown and root rot that is present and, if there is any plant left, treat it with fungicide and replant it in a new location. Or it may be best to dig up the plant and replace it with a disease resistant variety.
Before planting anything in the area that an infected plant was growing in, you should sterilize the soil, which could still contain pests and disease after the infected plant has been removed.
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Agave plants (Agave spp.) generally are succulents with large leaves that end in spiny tips. There's a lot of variety in the agave genus. There are the large, stiff specimens that can grow to 10 feet or more in height and width. And there are the small dish-sized agaves, as well as a few agave species with soft leaves and no spines. Agave foliage tends toward a blue-green in hardier varieties and a gray-green in warm-climate varieties. There are also some that are variegated with gold or white markings.
It's typically best to plant this slow-growing succulent in the spring or early fall. When agave matures after several years or even several decades, a tall flower stalk often grows out of the plant’s center. The flowers are bell-shaped and long-lasting in shades of white, yellow, and green. For most agave species, once the flowers produce berry seed pods, the plant dies.
|Common Name||Agave, century plant|
|Plant Type||Perennial succulent|
|Mature Size||Different varieties average 1 to 20 feet tall and 1 to 10 feet wide.|
|Sun Exposure||Full sun|
|Soil Type||Sandy or rocky, dry, well-draining|
|Soil pH||6.6 to 6.8|
|Bloom Time||Most plants only bloom once in their lifetime.|
|Flower Color||Green, white, yellow|
|Hardiness Zones||5 to 11|
|Native Area||Hot, arid regions of the Americas also some tropical areas|
Agave may stand up to the sun and dry periods well but are quite susceptible to plant-eating insects such as the agave snout weevil, soft scale and the cactus longhorn beetle. Signs of infestation include withering or drying leaves and brown spots. Spray infested plants with a broad spectrum insecticide and monitor condition until the health of the plant returns to normal.
Several infections can also plague agave plants with symptoms including lesions, black and brown spots and rot. Agave infections include Anthracnose, root and crown rot and Phyllosticta pad spot. These infections are caused by fungus spores that find a home on the agave plants. Use an anti-fungal agent occasionally to prevent infection. Destroy already infected plants to prevent spread.
Blotchy patches appear on leaves and the top layer sloughs away. Unlike sunburn, the damaged area is lower than the surrounding leaf surface. Unlike frost, which typically "burns" tips, edema shows on the wider part of the leaves.
Edema can happen when agaves that are used to a cool, mild climate like the Bay Area's go through an intense heat spell.
Inconsistent watering also can cause edema. Blistering on one of my agaves happened after a pipe broke beneath it. If there's a way to anticipate or prevent the damage, I'm unaware of it.
Agave means noble in Greek, and one look at the roster of the some 200 species in the Agave genus gives you a notion of why the name was selected. Some of these rosette-forming plants are large and bold, with succulent leaves that emerge spirally from the plant center. Although agaves are listed as perennials, the classification does not fit perfectly since many species bloom only once in their lifetimes. Some agaves require tropical heat, but many thrive in U.S. Department of Agriculture plant hardiness zones 8 through 11. If your agave fails to thrive, your first step is to check its hardiness zone.
Ask your agave supplier or local garden store for precise hardiness zone information for your plant. Many agaves thrive in temperate climates, like the squid agave (Agave bracteosa) with its curving succulent leaves that prefers USDA zones 8 though 11, or the century plant (Agave americana) that thrives in USDA zones 8 though 10. Others, like Agave "Sharkskin," only grow in USDA zone 11.
Water the agave until the soil is saturated and watch how quickly it drains. Agaves are native desert plants and require the best possible drainage. Transplant the agave if it is growing in soil with poor or average drainage. Plant in a mix of one part gritty soil sold specifically for cactus plants or granulated pumice to three parts garden soil. Alternatively, plant the agave in a raised bed or a container where drainage can be assured.
Mark the days you water the agave on your calendar to see if you are over-watering. Generally, irrigate no more than every 10 days and always allow the soil to dry thoroughly before the next watering. Dig down about 3 inches into the soil with a kitchen spoon before you water if it is even a little damp, wait a few more days before watering. This type of limited irrigation meets the plant's needs, prevents drought stress and reduces the problem of soil-borne pathogens.
Look for a reddish spore mass on the agave foliage that indicates it is suffering from anthracnose. Disinfect a sharp knife by rubbing the blade with a rag soaked in denatured alcohol and slice off infected leaves. Dispose of the leaves. Avoid all overhead watering. Proper irrigation and eliminating over-irrigation helps prevent this infection.
Inspect the plant for wounds on its foliage made by the agave snout weevil (Scyphophorus acupunctatus). Weevil attack is particularly likely in large agaves. The mature female weevil bores holes in the plant base to lay her eggs. Decay microbes enter through the holes and rot the agave tissue, and the larvae hatch in the dying plant. Apply a broad-spectrum insecticide in early spring if your agave is infected.
Look for scale insects attached to agave leaves. You will see flattened covers around 1/8 inch long that shield the scale's body from view. Coccid and soft scale attack stressed agaves and suck out the plant juices. Provide plants with optimal irrigation and well-draining soil to avoid this problem. If your agave is already under siege, spray it with a systemic insecticide such as imidacloprid or acephate according to label directions.