- Author: Steven A. Tjosvold
Now through the spring is the time to take a close look at crops that are susceptible to downy mildew so that the disease can be detected early. This post follows the previous one that describes the general identification and biology of downy mildew diseases. Here are some more images illustrating the symptoms on various ornamental crops.
Downy mildew on statice ( species) caused by Peronospora statices. (BELOW) Bluish gray sporulation occurs on undersides of leaves. On upper surface of leaves, infections appear as light green areas that turn yellow, then coalesce, and eventually the leaf dies.
Downy mildew on hebe (Hebe species) caused by Peronospora grisea. (BELOW). The disease is usually noticed first by a yellowish discoloration often with purple halos on the lower leaves of the plants. Leaves become curled and distorted. A grayish- brown sporulation may develop on the underside of the leaves. Eventually the affected parts of leaves turn brown and shrivel. Plants can be entirely defoliated and killed.
Downy mildew on snapdragon (Antirrhinum majus) caused by Peronospora antirrhini. (BELOW) Young tip leaves are dull green, severely stunted, and roll downward. Gray‑purple fungus grows on undersides of leaves. Disease is common on seedling phase; large plants are less frequently attacked. Infected plants fail to produce flowers.
- Author: Steven A. Tjosvold
Downy mildew diseases have become increasingly important and common on ornamental plants over the last decade in California. The wet and cool weather we recently have had should promote the development of this disease. Now through the spring is the time to take a close look at susceptible crops. One of the newest introductions of downy mildew in California is on iceplants, and this disease provides an excellent example to talk about downy mildew diseases in general.
In general, downy mildew diseases can be severe in cool or warm (but not hot) temperatures, high humidity, and when liquid water is present on plant tissue for extended periods. Link to information on leaf wetness importance. Downy mildew pathogens are part of a larger taxonomic group of fungi-like micro-organisms that contains Pythium and Phytophthora species. They primarily cause foliar blights and rapidly spread in young green leaf, twig, and fruit tissues. They produce spores on the ends of stalks and the spores can be carried by wind and rain to new infection sites of the same or different plant. The pathogen can be present in soil associated with host and non-host plants and therefore can spread by any means that aids in the movement of soil and/or water from infected plants to non-infected ones. The pathogen can spread through contaminated plant cuttings and transplants. Some are found in seed and can be spread through seed propagation.
Learn which crops that you grow are susceptible to downy mildew diseases, and monitor them closely now through the spring.
- Author: Heather Scheck, Santa Barbara Ag Commissioner's Office
- Author: John Chitambar, California Department of Food and Agriculture
- Author: Surendra K. Dara
A downy mildew caused by the oomycetous fungus, Peronospora mesembryanthemi Verwoerd has recently been confirmed by the USDA -APHIS from a red apple ice plant sample collected in San Diego. Since its first finding in San Diego County last summer, the disease has spread to Orange, San Bernardino, and Ventura counties and is now found in the Goleta area in Santa Barbara County. Peronospora mesembryanthemi was first reported from South Africa (Verwoerd, 1924) and later in the United Kingdom (Francis and Waterhouse, 1988) and New Zealand (McKenzie and Dingley, 1996), but has never been reported in North America. The host range of P. mesembryanthemi is thought to be limited to ice plants and it is currently found infecting the red apple ice plant, Aptenia cordifolia and the trailing ice plant or the pink carpet,Delosperma (syn. Mesembryanthemum) cooperi in California. Both these ice plant species are native to southern Africa. Because of their environmental hardiness, ease of growing, and bright, colorful flowers, ice plants are grown as ornamental plants or found as groundcovers. However, the red apple ice plant is listed as an invasive plant by the California Exotic Pest Plant Council. It can sometimes grow like a weed.
See the full article with photos here.
- Author: Deborah Mathews, Department of Plant Pathology, UC Riverside
Impatiens are dying from a relatively new plant disease called impatiens downy mildew, caused by the fungus-like, oomycete pathogen Plasmopara obducens. The pathogen primarily affects varieties of Impatiens walleriana, or hybrids with an I. walleriana parent and wild impatiens (I. balsamina). Note that this pathogen does not affect New Guinea impatiens (Impatiens hawkeri) or other bedding plant genera. This disease develops rapidly, with a few leaves on apparently healthy impatiens beginning to show slight yellowing and stunting followed by development of white, powdery spores on the undersides of leaves, and later, by leaf and flower drop. Plants are likely to become completely defoliated within several weeks. The pathogen produces airborne spores, which can travel for many miles, as well as swimming zoospores and oospores, which can survive within soil and plant debris for long periods and infect healthy plants when replanted in the same area.
Early detection is especially critical for this disease since chemical control has been shown to be ineffective once sporulation begins. Scout routinely to identify and remove diseased plants before epidemics can result. Removing infected plants may limit spread to other areas of the landscape.
Consider growing alternative bedding plants that will grow well in shady areas of the landscape but that will not be affected by the disease. Some examples include Bergenia hybrids, Caladiums, Coral bells, Lobelia, New Guinea impatiens, Sweet alyssum, and wax begonias.
This article was originally published in the December 2013 issue of the UC IPM Green Bulletin. Read the entire article at http://www.ipm.ucdavis.edu/greenbulletin/
- Author: Deborah M Mathews
One of California's most adored flowering plants, impatiens, is being threatened by a serious pest. You may have noticed the common garden impatiens missing from nurseries, retail store shelves, and landscapes, parks, and gardens this year.
Impatiens are dying from a relatively new plant disease called impatiens downy mildew, caused by the fungus-like, oomycete pathogen Plasmopara obducens. The pathogen primarily affects varieties of Impatiens walleriana, or hybrids with an I. walleriana parent and wild impatiens (I. balsamina). Note that this pathogen does not affect New Guinea impatiens (Impatiens hawkeri) or other bedding plant genera. This disease develops rapidly, with a few leaves on apparently healthy impatiens beginning to show slight yellowing and stunting followed by development of white, powdery spores on the undersides of leaves, and later, by leaf and flower drop. Plants are likely to become completely defoliated within several weeks. The pathogen produces airborne spores, which can travel for many miles, as well as swimming zoospores and oospores, which can survive within soil and plant debris for long periods and infect healthy plants when replanted in the same area.
Early detection is especially critical for this disease since chemical control has been shown to be ineffective once sporulation begins. Scout routinely to identify and remove diseased plants before epidemics can result. Removing infected plants may limit spread to other areas of the landscape.
Consider growing alternative bedding plants that will grow well in shady areas of the landscape but that will not be affected by the disease. Some examples include Bergenia hybrids, Caladiums, Coral bells, Lobelia, New Guinea impatiens, Sweet alyssum, and wax begonias.
This article was originally published in the December 2013 issue of the UC IPM Green Bulletin.
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