Yuccas, various species within the genus of the same name, are prized as perennials, shrubs or trees for their ability to tolerate drought and heat. Several pests, diseases and abiotic disorders potentially plague yuccas, but only a few cause the development of brown leaf tips. The most likely cause of brown or dead tips and margins on yuccas is fluoride or salt toxicity.
Identifying a Fluoride Toxicity
Fluoride toxicity most commonly appears on yuccas first as tiny dead spots on leaf margins slightly back from the leaf tip, which then enlarge to form semi-circular dead areas before the whole tip dies. Oldest leaves may suffer the most, though all leaves are potentially affected. Fluoride has little affect on overall plant health and growth. It is mainly an aesthetic problem.
Treating and Preventing Fluoride Toxicity
Fluoride toxicity is generally not problematic if the yucca is grown in regular soil, as soil minerals quickly make introduced fluoride ions unavailable to plants. However, if the yucca is planted in soil-less media such as a combination of peat moss and perlite, high levels of fluoride could present a threat. Avoid fluoride toxicity by leaching fluoride out of perlite and other media with two or three rinses of water, keep the pH above 6.5, use nitrate rather than ammonium as a nitrogen fertilizer and soluble phosphates as opposed to superphosphate.
Identifying Salt Toxicity
Salt toxicity and related conditions such as problems with excessive sodium and chloride may appear on yucca as stunted growth, leaf tip and marginal burn, or necrosis or other leaf discoloration. In severe cases, yucca can suffer seriously or even die. In sodic, saline-sodic soils or soil in container specimens, an indicative white or black crust could form on the soil surface. Salt-related injury can occur on yuccas grown near swimming pools or surfaces treated with deicing salts or yuccas watered with softened water.
Addressing High Salt Levels
The best way to determine if soil salt is definitely the cause of the tip browning is to have a soil test conducted. Soil kept moist sees reduced salt toxicity, but constantly moist soil is not ideal for yucca cultivation. Infrequent, deep watering can leach out of the root zone if the soil is present because of heavy or improper fertilizer application or high-salinity soil amendments. Sprinkler applications of water with high salt also cause salt deposits to appear on yucca leaves, so avoid this type of irrigation, if possible. Soil with high salinity is treated with gypsum, which produces calcium that displaces sodium and allows it to leach out of the root zone.
Water Stress and Other Possible Causes
In addition to fluoride and salt toxicities, a high level of boron can cause leaf tip yellowing and eventual necrosis along margins and between veins. Water with a high boron content and borate-containing herbicides are usually responsible for high boron. Treat a boron toxicity by leaching it out of the soil. Water stress can also cause yucca leaf tips to turn brown. Although yuccas generally withstand drought well, young or recently transplanted yuccas do require regular, deep watering for establishment while older yuccas benefit from occasional, deep waterings during dry weather.
- Abiotic Disorders of Landscape Plants: A Diagnostic Guide; Laurence R. Costello
- Growing Media for Ornamental Plants and Turf; Kevin A. Handreck et al
- University of Florida, IFAS Central Florida Research and Education Center: Yucca Production Guide
- UC IPM Online: Agave—Agave spp.
- UC IPM Online: Nutrient and Mineral Excesses
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