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Sandy Feathers: Too much fertilizer, fungal disease could be killing vinca plants

Saturday, January 03, 2004

Q. For many years, I planted annual vinca (Catharanthus roseus) and had great success. I especially enjoyed it because of its bright colors, continuous flowers and heat tolerance because my yard is mostly full sun. Lately, I get great-looking plants but they don't survive. Out of a flat of vinca, I have only five or six plants that survived. I have planted them in pots by themselves, in beds over daffodil bulbs, and around the base of a tree. I use a long-lasting granular fertilizer in the soil when I plant them. Do you have any idea what is happening?

A. There are a number of possible causes for your recent problem with annual vinca, but two stand out. Too much fertilizer can burn the roots, killing them and eventually killing the entire plant. It is always wise to read and follow label directions when applying fertilizers (and pesticides). Using more than the recommended amount of fertilizer, especially in a pot, is never a good idea.

However, there is a soil-borne disease that can cause similar symptoms. Unless you used far too much fertilizer, I would be more likely to believe that your annual vinca is succumbing to verticillium wilt. This fungal disease has a wide host range that includes many species of flowers, as well as trees and shrubs. It persists in the soil for many years and will infect any susceptible plant installed where it has become established.

The fact that you have grown annual vinca in your yard for so many years makes me lean toward this as the cause. Soil-borne diseases are one of the primary reasons that crop rotation -- even in a small yard or garden -- is so important. Crop rotation is an important tool to minimize soil-borne disease problems, as well as the buildup of insect pests that can occur when you grow the same crop in the same place year after year.

Beyond fumigating the soil, there is no chemical control for verticillium wilt. The pesticides used to fumigate soil are not registered for use in residential landscapes. They can only be purchased and used by licensed pesticide applicators. They are extremely toxic, and home gardeners should not even think about trying to obtain or use them in their gardens. You can try soil sterilization using clear plastic (see my Oct. 11 column) but you cannot use that technique around the base of your tree or over your daffodil bulbs.

Verticillium wilt may have come into your garden from plants that were infected when you purchased them, even though they looked healthy at first. Such plants often do not exhibit symptoms until subjected to stress from drought, high temperatures and other factors.

As much as you enjoy annual vinca, you should choose species of flowers that are resistant to verticillium from now on. The list below includes flowers that tolerant of full sun. A short bibliography follows so that you can learn more about plants you have not grown before.

Ageratum (Ageratum houstonianum) -- annual Hollyhock (Alcea rosea) -- biennial/perennial Anemone (Anemone spp.) -- perennial Pot marigold (Calendula officinalis) -- annual Carnation/Pink (Dianthus spp.) -- annual and perennial species Blanket flower (Gaillardia x grandiflora) -- perennial Baby's breath (Gypsophila spp.) -- annual and perennial species Flowering bulbs -- tender and hardy perennial species Sunrose (Helianthemum nummularium) -- perennial Candytuft (Iberis sempervirens) -- perennial Iris (Iris spp.) -- perennial Lantana (Lantana camara) -- annual Sweet alyssum (Lobularia maritima) -- annual Nemesia (Nemesia spp.) -- annual Baby-blue-eyes (Nemophila menziesii) -- annual Ornamental grasses -- annual and perennial species Penstemon (Penstemon spp.) -- annual and perennial species

Balloon flower (Platycodon grandiflorus) -- perennial Moss rose (Portulacca grandiflora) -- annual Pincushion flower (Scabiosa spp.) -- annual and perennial species Nasturtium (Tropaeolum majus) -- annual Pansy (Viola x wittrockiana) -- annual Verbena (Verbena spp.) -- annual and perennial species Johnny jump-up (Viola tricolor) -- annual/short-lived perennial Violet (Viola spp.) -- perennial Zinnia (Zinnia spp.) -- annual

Bibliography

"Amazing Annuals" by Marjorie Mason Hogue, Firefly Books.

"Annuals for Connoisseurs" by Wayne Winterrowd, Prentice Hall.

"Amitage's Garden Perennials" by Allan M. Armitage, Timber Press.

"Armitage's Manual of Annuals, Biennials and Half-Hardy Perennials" by Allan M. Armitage, Timber Press.

"Herbaceous Perennial Plants" by Allan M. Armitage, Varsity Press.

"Manual of Herbaceous Ornamental Plants" by Steven M. Still, Stipes Publishing Co.

"Perennials for American Gardens" by Ruth Clausen and Nicolas Ekstrom, Random House.

"Perennials: How to Select, Grow and Enjoy" by Pamela Harper and Frederick McGourty, HP Books.

"Perennial Garden Plants" by Graham Stuart, Thomas J. M. Dent & Sons.

"The Encyclopedia of Ornamental Grasses" by John Greenlee, Rodale Press.

"The Plantfinder's Guide to Tender Perennials" by Ian Cooke, Timber Press.


Send questions to Sandy Feather by e-mail at slf9@psu.edu or by regular mail c/o Penn State Cooperative Extension, 400 N. Lexington St., Pittsburgh 15208.

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