SLATER, Iowa, March 18 /PRNewswire/ -- Garst Seed Company of Slater, Iowa,
is adopting a standard that will easily inform farmers of the resistance
threshold of corn hybrids to the disease Gray Leaf Spot (GLS).  In doing so,
Garst expects other seed companies will move to a more customer-friendly, yet
scientifically responsible system of classification for resistance levels to
the fungal disease.
    "We want farmers to have a clear understanding of how we define resistance
levels in GLS hybrids," says Virgil Sparks, operations director for Garst.
"Right now each seed company has its own interpretation of how well or how
poorly a hybrid reacts to GLS."
    Seed companies use different techniques to communicate resistance levels
-- sometimes rating hybrids on a scale of one to nine with one being low
resistance and nine being high resistance to the disease.
    "It can become confusing to a farmer when he is told that the resistance
score for a hybrid is a six, when in fact a score of six may not mean
resistance at all.  Using the word resistance to cover a wide range of hybrid
response to the disease is misleading to the farmer," says Sparks.
    Because several genes are involved in conveying resistance to Gray Leaf
Spot, it is to be expected that hybrids will resist the disease to various
degrees.  Garst wants to simplify the terminology and the criteria for
categorizing hybrids according to a hybrid's response to GLS.
    Beginning this summer and communicated in various company literature,
Garst will adopt five classifications for GLS resistant hybrids:  highly
resistant, resistant, moderately resistant, moderately susceptible and
susceptible.
    This classification is determined during the screening of hybrids under
worst-case scenarios for disease exposure in "GLS hot zones" around the
Midwest.  The screening typically takes place in late August when the disease
is at its high point.
    The classification procedure is based on physical deterioration caused by
GLS of leaf tissue above the ear leaf of the hybrid.  GLS, with its
distinguished gray and tan rectangular lesions, can be visually tracked on the
corn leaf.  Garst uses a rating method of assessing these dark lesions on the
leaf area above the ear leaf.
    Garst has determined the classification of hybrids based on the following
criteria:  a hybrid with less than 21 percent of leaf area affected (LAA)
above the ear leaf is classified as highly resistant.  Other ratings:  LAA of
21 to 40 percent is classified as resistant; 41 to 60 percent is moderately
resistant; 61 to 80 percent is moderately susceptible and 81 to 100 percent is
susceptible.
    "This is a simple approach that allows Garst to easily track the
progression of the disease and determine the resistance levels of hybrids.  It
gives a measurable definition to the terminology we use when communicating the
resistance levels to farmers," adds Sparks.
    Garst also will have a performance rating for the hybrids that will rate
overall yield performance of the GLS resistant hybrids.  Garst's goal is to
deliver hybrids with the ability to resist infection to a level where economic
damage is acceptable and to deliver yield potential with or without GLS being
present.
    "At end of day, if it is a GLS-resistant product, we expect our hybrids to
perform when under the pressure of GLS, as well as in areas where the disease
is nonexistent," says Sparks.
    The classical definition of plant resistance, according to the National
Academy of Sciences, is "the inherent capacity of a plant to prevent or
restrict the entry or subsequent activities of a pathogenic agent when the
plant is exposed to inoculum under environmental conditions suitable for
infection."
    GLS is most likely to occur when infected residue overwinter in fields
where reduced tillage methods are used and where susceptible hybrids have been
planted in the past year.
    If infected residue is present in fields during the right environmental
conditions -- usually prolonged periods of humidity from June to September --
and the farmer intends to use susceptible hybrids, it could set the stage for
severe outbreaks of GLS to occur.  Garst has pinpointed 16.2 million corn
acres in the Midwest that are considered "hot zones" for GLS.
    "In these hot zones, Garst recommends the use of moderately resistant to
resistant hybrids to help stabilize performance and reduce inoculum, while
moderately susceptible hybrids may be used in areas of light exposure,"
advises Sparks.  "Plant resistance is considered the most economic and
effective means of disease control available today."
    For the 1998 planting season, Garst will introduce G-STAC(TM)
(gene-stacked) hybrids with GLS resistance, as well as herbicide resistance.
Garst has been an industry leader in developing GLS resistant hybrids since
collaborating with Virginia Polytechnic Institute in the late 1980s, which
lead to the issuance of a patent in 1996 that introduced a method of
transferring resistant genes into desirable germplasm.

    G-STAC is a trademark of Garst Seed Company.

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