MELeaf: A Newsletter from the Horticulture Program, April 28, 2020

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MELeaf: A Newsletter From the Horticulture Program, April 28, 2020

In this issue:

Ralstonia Solanacearum race 3 biovar 2: The Scourge of Geraniums and Potatoes

Wilting geranium infected with RsR3B2

The Situation

Last week, USDA confirmed Ralstonia solanacearum race 3 biovar 2 (RsR3B2) in a ‘Fantasia Pink Flare’ geranium from Guatemala. Although other races of Ralstonia solanaceraum are found in the US, RsR3B2 is not known to occur here and poses a great enough threat to solanaceous crops such as potato and tomato, that it is listed as a select agent on the bioterrorism list. If RsR3B2 were to become established in Maine it could mean the end of the potato industry and have a significant impact on other farmers as well as backyard and ornamental growers. When RsR3B2 is found in the US, USDA requires that strict eradication and sanitation protocols are followed. These eradication protocols include destruction (by incineration or deep burial, with oversight by USDA and State officials) of all affected host plants, any comingled host plants and comingled non-host plants, potentially leading to significant crop and income losses to the grower. 

In this latest find, RsR3B2 was identified in a single variety of geranium, Fantasia Pink Flare, in a Michigan greenhouse. Working with the importer, USDA has determined that other potentially infected plants were shipped to 288 greenhouses in 39 different states. So far, this has resulted in 58,000 plants on hold and awaiting destruction. This number is likely to grow as USDA and state officials follow-up with businesses that received potentially infected plants. While, to the best of our current knowledge, none of these geraniums ended up in Maine, from what we know about past incidents with this disease and the complexity of the geranium supply chain, it would be prudent to know and look for symptoms of this disease on your plants.


RsR3B2 causes wilting and leads to plant death. This is a heat-loving disease; plants may be infected, but not begin to show symptoms until warmer weather conditions occur. Symptoms on geranium to look for include:

  • upwardly curling leaves starting on the bottom of the plant and progressing up
  • limp, wilted or yellowed leaves
  • wedge-shaped yellowing in leaves that expands towards the leaf margin
  • vascular discoloration on the lower stem
  • browning of roots may occur in more advanced stages of the disease

These symptoms are similar to bacterial blight in geranium caused by Xanthomonas campestris pv pelargonii. While there may be some subtle differences that distinguish these two diseases, if you suspect either disease it is best to send a plant sample out for testing.

Prevention is Key

There are no effective chemical treatments for RsR3B2 and infection is a death sentence for the plant, even without required eradication procedures. Prevention and sanitation are the key to minimizing plant losses. Plant management decisions that you should consider in your geranium production include:

  • Keep geraniums from different suppliers separated on different benches or in different greenhouses.
  • RsR3B2 is easily spread with water and soil. Avoid using sub-irrigation (such as ebb and flow or flood benches) and recycled water systems in your geranium production.
  • Contaminated hands and tools can spread disease. When cleaning or working in the geranium crop wear gloves and clean/disinfect hands and tools frequently.
  • Purchase geranium cuttings from reliable and responsible suppliers that observe strict sanitation protocols in their geranium production.
  • Clean and sanitize benches and greenhouses between crops.

Keep greenhouses and areas surrounding the greenhouse weed free. Solanaceous weeds such as nightshade can be hosts for RsR3B2 and could lead to the disease persisting in the environment or may assist in the spread from a geranium in the greenhouse to a potato or tomato crop in the field.

More Information

How to Get a Pest Identified

The age of Covid-19 has provided an opportunity to use our technology more than ever and getting plant pests identified is no exception. Not only is emailing photos of insects and diseases easy, quick, and inexpensive, it has also become the preferred or only method of many diagnostic centers during the coronavirus outbreak. Although, there are some exceptions where actual specimens are required for accurate identification, many plant pest problems can be identified with emailed photos. Of course, the better the photo, the better the diagnosis. Here are some must-do photo tips for accurate plant pest identifications:

  1. Get close! Get as close to the specimen as possible while still retaining focus. You may have the urge to zoom in but it’s much better to physically get closer. Most smartphones will focus on subjects within 4 inches. For very small specimens use the macro setting (tulip icon) if that’s an option, which often allows you to get within an inch or two of the subject. Consider purchasing an inexpensive clip-on macro lens made for most smartphones.
  2. Focus! No matter how close you are able to get, make sure your subject is in focus. If the background is busy, the lens may not know where to focus. Put your hand or a piece of paper behind the subject. On smartphones, touch the screen where your subject is. This will lock the focus and the exposure of the subject.
  3. Light! If there is not enough light on the subject, the camera lens will not be able to focus well. If possible, move your specimen to a well-lit area. Natural light is always best but be aware of harsh shadows and backlighting (when there is more light behind the subject than in front of it). If shadows or backlighting cannot be avoided, use your flash.
  4. Scale! It may seem obvious how big something is in real life, but in photographs it is not always easy to tell. When you are photographing the subject in question, put in another object (like a penny) or a ruler. If that is not possible, indicate the size in the written description.
  5. Take many photos. For insects, get some shots of its top (dorsal side), its bottom (ventral side), and its profile. Photographing the insect on the plant will also help with identification. For non-insect plant problems, take photos of the whole plant (roots included), and closeup photos of the individual symptoms on the leaf or stem, etc.
  6. Emailing. Many email inboxes still have a size limit for attachments. Choose four to six good photos that have a combined size of less than 9 MB. Include in your email:
    1. name, contact information, town, county
    2. host plant, problem
    3. date when symptoms were observed

Email your photos to 


Connecting With Other Growers Through Industry Organizations

There is no doubt, the 2020 spring sales season for greenhouse growers and retailers will look very different than usual. While businesses selling plants have been deemed essential in Maine and are allowed to be open to the public during the stay at home order, CDC guidelines and social distancing rules will change the way they operate. This is new territory for everyone and will take creative planning unique to each business to safely sell plants to customers. But no one is alone in this challenge, many in the trade have similar questions and are curious how others will be handling the situation. One way to connect with others for ideas, information and collaboration is to become involved with industry organization such as the following here in Maine. 

Maine Landscape and Nursery Association (MELNA)The mission of this non-profit professional trade association for all members of the green industry, is to promote the use of ornamental plants, products and services. MELNA provides a unified voice, communicating to the public the vital role of the green industry in improving quality of life. Members exchange information, keep current through educational sessions, receive recognition through certification programs and advocate for the industry through the legislative and regulatory process. MELNA is the exclusive owner of the Maine Flower Show and launched the marketing campaign Plant Something! Plant Maine! They also hold an annual meeting and trade show with educational opportunities, called the Grow Maine Green Expo, have twilight meetings, charity golf tournaments and work projects.

During the COVID-19 pandemic, MELNA has been vital for distributing important industry information to members. This includes interim guidance for essential businesses in the horticulture industry, CDC guidelines for social distancing, what to do if an employee contracts COVID-19, and notifications for online industry discussion sessions. MELNA also put together letters for members to distribute to local and state policymakers stressing the importance for both garden retailers and landscapers to remain essential businesses.

Maine State Florist and Growers Association (MSFGA)MSFGA strives to build a stronger industry by bringing education, knowledge and positive change to florists and growers statewide. They offer the Professional Certified Florist and Maine Master Floral Designer programs as well as host a full day meeting with presentations and hands on demos at the Agricultural Trades Show for both florists and growers.

Mid-Maine Greenhouse Growers Association (MMGGA)MMGGA is the only active trade association exclusively for greenhouse growers and their suppliers in the state. They are dedicated to furthering the interests of Maine's greenhouse industry. Members take advantage of group purchasing power, educational meetings at member greenhouses, end of season meetings to view UMCE trial gardens, and friendly get-togethers. They share ideas about plants, methods, cultural practices, merchandising, and the whole spectrum of greenhouse systems management. MMGGA also sponsors Maine Greenhouse and Nursery Day, a well-publicized statewide celebration of spring to kick off the gardening season and raise the visibility of members.

Upcoming Webinars, Calls and Events for the Horticulture Industry

From Around the Web: Useful Websites and Other Resources