Plant of the Week
Geranium Wilt
Latin: Pelargonium x hortorum
Communicable diseases and the threat they represent to the economy and human
health have become hot topics of conversation. Natural outbreaks of SARS, avian
flu and mad cow disease and terrorists threats from anthrax and smallpox have
the paranoid amongst us going to bed with knots in the stomach.
Contagious plant diseases don’t get much press play, but they also happen.
The greenhouse industry is currently in the midst of a disease outbreak that is
illustrative of how globalization is changing this segment of agriculture. The
disease, Ralstonia solanacearum race 3 biovar 2, is a bacterial wilt
disease that affects geraniums.
While they have been losing ground in recent years, geraniums (Pelargonium
x hortorum) are still the most important springtime pot crops for the
greenhouse industry. Greenhouse growers may choose to produce geraniums from
seed, but most prefer cutting-grown plants because they have larger, more
spectacular flowers and consumers prefer them.
Before WWII, greenhouses were pretty much self-contained businesses when it
came to growing vegetatively propagated crops. Every grower would save a few
stock plants over winter and propagate new starts from these plants for spring
sales. With the end of the war and the return of a fleet of DC-3's to haul
cargo, air freighting geranium cuttings from southern California to growers
throughout the nation became economically feasible.
Geranium cuttings were ideal for this type of operation because they are
compact, tolerate wilting without a fuss thus making them amenable to long
distance shipping, and are a high value product. In the early 1950s, this new
system of production lead to the outbreak and nationwide distribution of a
different bacterial disease that spawned the sale of seed-grown geraniums.
From this early experience with shipping geranium cuttings and the
possibility of inadvertent disease spread, a system called "culture indexing"
was developed to screen plants for the presence of pathogens. Like any kind of
ongoing screening program, things occasionally slip through. For the screening
program to work correctly, redundant testing is required because it is estimated
that four positive plants will slip through undetected for every 1,000 plants
tested.
The geranium cutting production industry left southern California after the
disease outbreak of the 50s and relocated to the Canary Islands, a group of
Atlantic islands 800 miles off of the southwest coast of Spain. Disease
outbreaks eventually caused problems there so production shifted to Mexico in
the 1980s.
Animal and Plant Health Inspection Service - a branch of the USDA -
regulations permit the importation of unrooted cuttings or rooted plants without
soil attached, provided they are inspected upon entry. The choice of production
sites by the three or four multinational corporations that provide almost all of
the geranium cuttings for the world market require cheap labor rates and a
subtropical, dry climate to minimize disease spread.
The current disease problem involved cuttings from one firm, Goldsmith Seeds
of California. The problem first became apparent in February 2003, and the
company and APHIS acted quickly to destroy all suspect plants. The cuttings were
produced by the Goldsmith facility in Kenya, so during the summer the company
shifted production to Guatemala.
Last month two of the Guatemala grown cultivars were found to be infected
after they were distributed to over 400 growers in 41 states. These plants were
ordered destroyed and all importation from Guatemala was discontinued. The plant
quarantine system worked as designed and there is no indication that infected
plants made it to final sales. The company involved has cooperated fully to try
to stay ahead of the problem and keep diseased plants out of the supply chain.
Though this disease could be devastating to greenhouse growers, the real
threat comes from the fact that the bacterium can infect potatoes. This race of
the disease is established in parts of Asia, Africa and South America and
seriously limits potato production in some areas.
By: Gerald Klingaman, retired
Extension Horticulturist
- Ornamentals
Extension News -
February 27, 2004
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