Risk will always be a part
of fruit and vegetable production, just as it will always be a part
of life. However, there’s a big difference between investing
in blue-chip stocks and taking a flyer on a dot-com startup. Likewise,
growers can dramatically reduce the risk of seedborne diseases by
purchasing professionally tested seed from a reputable supplier instead
of taking chances with inferior seed.
“Black rot and watermelon blotch can be prevented in nearly
all situations by planting disease-free seed,” says Dr. Thomas
Kucharek, professor and extension plant pathologist at the University
of Florida.
Although seedborne diseases are one of the most predictable threats
that growers face, it also is one of the most manageable.
“Growers are aware of what is happening and which crops are
more prone to problems,” says Steve Koike, University of California
Cooperative Extension farm advisor in Monterey County. “The
seed industry has done a good job of keeping seed clean. The situation
is more or less in hand.”
Growers who have experienced problems with watermelon fruit blotch
or black rot in crucifers know all too well what can happen if the
situation doe get out of hand.
The watermelon fruit blotch bacterium can be introduced into fields
by infected seed or infected transplants. Volunteer melons or other
host crops perpetuate the bacterium within a field. The bacterium
can infect seedlings, foliage and fruit.
Black rot bacterium overwinter in and on seed or in infected debris
left in fields. Infection can spread quickly under high humidity and
crowded conditions.
Unfortunately, the problems don’t stop with these two diseases.
“Although black rot and fruit blotch are by far the major ones,
we normally test for 15 or 16 seedborne diseases,” says Darrell
Maddox, president of STA Laboratories in Longmont, Colo., which provides
independent testing for the seed industry. Other seedborne diseases
that can affect fruit and vegetable crops include:
Snap beans – common blight and
halo blight.
Tomatoes – bacterial canker and
bacterial speck.
Peppers – bacterial spot.
Triple Play
Plant pathologists speak in terms of preventing seedborne diseases,
not controlling them. This requires a team effort among suppliers,
transplanters and growers, from seed production through planting.
The first – and perhaps most important – link is the seed
supplier.
“Seed companies need to produce their seed in disease-free areas
and avoid contamination after it is harvested,” Kucharek says.
Many procedures are simply a matter of common sense, says Randall
Johnson, plant pathologist with Sakata Seeds in Lehigh, Fla.
“A lot of the recommendations are no-brainers, such as doing
good general sanitation and regular inspection,” he says. “Our
first line of defense is on-site inspection by the Florida Department
of Agriculture’s Division of Plant Industry. Our site here is
subtropical, so if disease is there, we will see it. We then produce
our seed in arid climates where there is not a lot of disease pressure.
We make sure to regularly inspect our production sites in China, Chile
and elsewhere.”
Sakata sends about 10 percent of each seed lot to an independent lab
such as STA for testing.
“It’s better to have a third party look at our seed instead
of doing it in-house,” Johnson says.
STA uses a variety of testing methods, depending upon the crop, the
specific disease pathogen and availability technology.
“With fruit blotch, the most acceptable technique is a grow-out,
where the client company sends us 30,000 to 50,000 seeds,” Maddox
says. “We grow them in greenhouses under conditions that are
highly favorable to disease. Trained scientists look for symptoms
of damage, then send plants with disease symptoms to the lab for further
testing.”
Black rot, however, can be detected without growing out seeds.
“We extract the bacteria through a seed wash, place it on a
selected medium and later evaluate for the presence of disease,”
he says.
The large quantity of seed tested ensures a representative sample,
Maddox Says.
“We look for a tolerance of less than one seed in 10,000”
he says. “Testing 30,000 to 50,000 seeds gives us a very high
confidence that the seed is clean.”
Following Through
Producing disease-free seed is no guarantee of disease-free production.
The transplant process is the next critical area.
“The greenhouse is a very important aspect of disease prevention,”
says Koike, who’s also a plant pathologist. “Fortunately,
our transplant growers have high standards for quality.”
Kucharek is concerned that some large transplanters may become lax
as their business grows.
“Many transplant production operations
have gotten so big that their quality control has suffered,”
he says. “If a transplant producer is sloppy and has inoculum
in the plant production area, the seed producer may be blamed instead
of the transplanter. Good sanitation programs around transplant operations
are critical.”
The sanitation program should include rouging out volunteer plants
that can harbor disease, steam-cleaning transplant trays after use
and switching from foam to plastic trays, which are easier to clean.
Finally growers must be careful no to expose seed to disease pathogens
carried over from previous crops.
“You can have problems with black rot inoculum carrying over
in production fields,” Kucharek says. “Old crop debris
can harbor bacteria, so you need to bottom-plow the field. With disking,
you get les breakdown of debris.”
