[From the April-May 2016 issue of the UC IPM Green Bulletin]
A downy mildew caused by the fungus Peronospora mesembryanthemi has recently been confirmed by the USDA-APHIS from a red apple ice plant (Aptenia cordifolia) sample collected in San Diego. Since this first finding in San Diego County in summer 2015, 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 and later in the United Kingdom and New Zealand, but has never before been reported in North America. The host range of P. mesembryanthemi is thought to be limited to ice plants, and in California it is currently found infecting the red apple ice plant, trailing ice plant or pink carpet (Delosperma [= Mesembryanthemum] cooperi), and Lampranthus spp. These ice plant species are all native to southern Africa.
Because of their environmental hardiness, ease of growing, and bright, colorful flowers, ice plants are grown as ornamental plants or as ground covers. However, the red apple ice plant, which is sometimes considered a weed, is also listed as an invasive plant by the California Exotic Pest Plant Council.
Downy mildew appears as a mat of grey, blue, or brown fungal growth on the undersides or sometimes on both sides of leaves and other infected plant parts. Fungal growth consists of the asexual fruiting structures known as sporangia that are produced on sporangiophores, which are specialized hyphae.
Downy mildew fungi cause systemic infection and grow internally in all plant parts. Infection rapidly spreads under cool and wet conditions. Windblown rain or sprinkler splashing help disperse sporangia and aid in the disease spread. Downy mildew fungi are obligate parasites of plants and most of them have a narrow host range of one or just a few hosts.
The rapid spread of P. mesembryanthemi through southern California could be from accidental movement of infected nursery stock. Severe damage with heavy or total loss of plantings is becoming common in southern California counties where the disease is currently found.
There are relatively few fungicides effective against oomycetous fungi, and control can be very difficult. Since most of the fungicides available inhibit rather than kill the fungal growth, treatments against P. mesembryanthemi only suppress the fungus; systemic infections cannot be cured.
Downy mildew fungi evolve very quickly to form new races and can rapidly develop fungicide resistance. Good cultural practices and sanitation can prevent or minimize downy mildew of ice plants.
Some management options include:
Prune plants regularly and remove weeds to improve air circulation and reduce fungal growth.
- Avoid overhead irrigation during cool weather.
- Consider watering in the morning hours so that plants dry during the day.
- Do not over- or under-fertilize as it may increase the chances of infection.
- Monitor highly susceptible species like the red apple ice plant and remove and destroy plants with symptoms of infection.
- Consider other non-host plants as ground covers when replacing diseased ice plants.
—Heather Scheck, Santa Barbara Agriculture
Commissioner's Office, firstname.lastname@example.org
—John Chitambar, California Department of Food
and Agriculture, email@example.com
—Surendra K. Dara, UCCE Santa Barbara, San Luis
Obispo, and Ventura counties, firstname.lastname@example.org
This article was modified from the original at ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=20558