Phytophthora ramorum in Maryland
Current Situation
Updated March 2017
An important disease called ‘Sudden Oak Death’ is caused by an exotic pathogen,
Phytophthora ramorum, which can cause rapid plant death. This pathogen is important because of its ability to survive prolonged heat in the form of tough spores, or mycelium in infected tissue, or in soil, and the ability to infect a wide host range of plant species (oaks, and other landscape and forest shrubs and trees).
The Maryland Department of Agriculture (MDA) has been continuously following up all trace forward lists provided by USDA, APHIS for exotic diseases, including P. ramorum.
In addition, MDA has been conducting monitoring surveys for P. ramorum for several years, including recent surveys from 2014-17. Until recently, P. ramorum had been detected in some of the samples collected from a list of Maryland buyers (nursery and home owners) provided by USDA, APHIS. These buyers obtained the plant materials from a nursery which was found positive in an annual compliance survey. In the past, P ramorum had been detected in trace-forwarded samples only (one each year) in Baltimore, Prince George’s, and Montgomery Counties in 2004, 2009 and 2012, respectively.
Recently, the Maryland Department of Agriculture has been monitoring exotic pathogens such as P. ramorum in Azalea,
Kalmia, Pieris, Rhododendron and Viburnum in nurseries and grape phytoplasma in vineyards. For P. ramorum, surveys were conducted in 2014, 2015 and 2016 at 30 nurseries, landscaping companies, and big box stores inspecting 115,327, 34,206, and 15,032 plants, respectively. Of those plants, 578 in 2014, 266 in 2015, and 868 in 2016 exhibiting symptoms similar to those produced by P. ramorum were collected and tested. Five percent of the 2014, 12% of 2015, and 23% of 2016 tested positive for Phytophthora spp by ELISA test. In 2016, P. ramorum was detected only in one of the nurseries surveyed, located in Howard County, MD. At this nursery, a total of 404 plants were inspected and 33 of which found symptomatic were collected, and tested, and 33% samples were found positive for Phytophthora spp. P. ramorum was discovered in one Rhododendron ‘Cunningham White’ (Fig 1) and in two mountain laurels -- Kalmia ‘Olympic Wedding’ (Fig 2 and 3). This was the first time P. ramorum was detected in a nursery in Maryland.
Fig 1. Leaf spot and blight in
Rhododendron ‘Cunningham White’, Fig 2. Leaf spot leading to blight in Kalmia ‘Olympic Wedding’, Fig 3. Leaf blight leading to dieback in Kalmia ‘Olympic Wedding’.
The Maryland Department of Agriculture, in an attempt to track the source and potential spread of the disease, found that this nursery obtained plants from two different sources. Kalmia’ Olympic Wedding’ plants were obtained from another nursery in Maryland, and
Rhododendron ‘Cunningham White’ plants came from a Oregon nursery. MDA also monitored this supplier nursery in Maryland and contacted Oregon to find a possible source of the pathogen, but no identifiable source for the disease was found. Twenty-three homeowners in Maryland purchased the same plant cultivars from this infected nursery and of those, one Rhododendron ‘Cunningham White’ tested positive for P. ramorum in one of the homeowners’ sites.
This occurrence shows the need for nurseries, landscapers, and homeowners growing
P. ramorum susceptible plants to be alert for possible symptoms as shown in the pictures.
Maryland Department of Agriculture, in collaboration with USDA, PPQ, and the involved nursery destroyed all infected plants and cleaned the affected area of the nursery and homeowner’s sites, following USDA’s protocol. MDA continues to participate in national surveys of nurseries and the environment to gather information on the potential entry and spread of
P. ramorum in Maryland.
Please help us keep our forests and nursery industry healthy and strong by excluding this devastating disease from our state. All materials received from out-of-state sources by Maryland nurseries should be separated from other stock, especially suspicious plant materials until proven negative. This summer, 2017, MDA will continue monitoring this exotic disease at both the 2016 positive
nursery and homeowner sites for
P. ramorum. In addition, five nurseries in Maryland will be selected, samples of the five major host species will be inspected and all suspects will be tested. If nurseries or plant dealers are concerned and would like to have plants sampled, please contact MDA at 410-841-5920. Our nursery inspectors will visit your nursery as quickly as possible. All Maryland homeowners, if you have suspicious plants, contact Maryland Cooperative Extension’s Home and Garden Information Center, http://www.hgic.umd.edu/ for more information. The results of these efforts will be posted on this site as they become available.
Nurseries, plant dealers, and landscapers that are not contacted or visited by MDA are not restricted from selling host plants from the regulated areas. Please be aware that additional establishments may be contacted or visited as more information becomes available.
To contain
P. ramorum on the West Coast, a USDA, APHIS Emergency Federal Order is in effect, restricting the movement of nursery stock from California, Oregon, and Washington nurseries. However, we strongly recommend that Maryland nurseries, landscapers, and plant dealers isolate all known host plant materials originating in these states separate from other shipments, in order to reduce the risk of spread to other plant material and to expedite any regulatory action that may be necessary if the source nursery is later determined to be infected with P. ramorum.
For any questions or further information, contact ramesh.pokharel@maryland.gov or the phone number provided above.
