September 2004
Sweet corn growers who plant rust-resistant hybrids to control common
rust should be prepared to scout fields and apply fungicides in 2000
because a new race of the rust fungus (Puccinia sorghi) probably will be
widespread in North America next season.
Sweet corn hybrids with
Rp-resistance will be infected wherever this race occurs.
New race of
rust
The new race of P. sorghi is a strain of the fungus that infects corn
with the Rp1D gene for resistance. For the past 20 years, Rp-resistance
has controlled common rust on sweet corn in North America. Nearly 150
commercial sweet corn hybrids have Rp genes (i.e., Rp = resistance to
Puccinia). Rp-resistance prevents P. sorghi from producing spores. Over
25 different Rp genes occur in corn. Most Rp-resistant sweet corn hybrids
have the gene Rp1D. This gene was preferred over other Rp genes because it
was widely effective against all races of rust in North America. How did this new race of rust get to America? Although the new race may have arisen from a natural mutation, it is
more likely that this race was introduced to Mexico or Central America from
South America, Hawaii, or southern Africa where races of P. sorghi that
infect corn with the Rp1D gene have occurred for several years. The
pattern of Rp genes that are effective and ineffective against the new
North American race is very similar to the pattern observed for Hawaiian
and South American races of rust.
If the new race did not come from Central America or Mexico, it may have
been introduced from Hawaii or South America. Spores of other rust fungi
have been collected in jet streams of the upper atmosphere. The coffee
rust fungus probably spread from Africa to South America in jet streams
associated with unusual weather patterns.
Will it make
landfall?
It is impossible to know for certain if the new race will be widespread
in North America in 2000, but it is very likely. Once introduced, the new
race should survive in Central America or Mexico even if it is not detected
there. The new race may only be an extremely small percentage of the
population of rust urediniospores that are disseminated to the US; however,
when the new race infects a field of Rp1D-resistant corn, it will be the
only strain capable of reproducing on those plants. Because each rust
pustule produces about 5,000 urediniospores, the new race will quickly
become the predominant race in that field and it will spread to other
fields. Thus, by mid- to late August, the new race is likely to spread
throughout North America if it survives in Central America or Mexico. If
the new race is not established in Mexico or Central America, Rp-resistant
hybrids should not be infected in 2000.
Susceptibility of
Rp-resistant hybrids
The reaction of Rp-resistant hybrids to the new race will depend on the
background of the hybrid. Hybrids should be moderately resistant if their
inbred lines were moderately resistant prior to being converted to Rp-resistance. Hybrids should be
highly susceptible if their inbred lines were highly susceptible prior to being converted to Rp-resistance. Rust
reactions can not be determined for hybrids developed from inbred lines for
which the background reactions are unknown. The reactions of a few of these
hybrids are being evaluated this winter in South Africa, Argentina, and
Hawaii where Rp1D-resistance is not effective. Fungicides currently on the market are
effective against this new race of rust.
Fungicides likely
Whether or not fungicides are necessary to control the new race of rust
on Rp-resistant hybrids depends on the rust reaction of the hybrid being
grown, the growth stage at which plants are initially infected, weather
conditions, and economics.
Fungicides are applied to prevent substantial yield reduction that
occurs when rust infection is severe. Under ideal weather conditions, rust
severity can reach 60 to 80% on susceptible hybrids, 40 to 60% on moderate
hybrids, and even as high as 20 to 40% on moderately resistant hybrids.
Yields typically are reduced about 5% for each 10% leaf area infected by
rust.
Fungicide applications are more essential when plants are young and
weather is wet and mild. Sweet corn plants are more susceptible to rust as
2- to 7-leaved seedlings than after tassels emerge. Also, rust is more
likely to become severe when younger plants are infected because secondary
infection is more prevalent. Rust develops fastest under cool, wet conditions. Infection occurs whenever moisture is present on leaves or in
whorls for at least 2 hours. Infection is more frequent and severe when
temperatures range from 65 to 85 F, but infection can occur at cooler and
warmer temperatures if moisture is present.
The goal of fungicide applications is to prevent severe infection.
Fungicides are NOT curative and have little benefit after infection is
severe. When conditions are highly favorable for rust, fungicide applications usually are economical. When highly susceptible hybrids are
grown, fungicide applications should begin when symptoms (pustules) are
first observed on plants that have 8 or fewer leaves if wet weather is
expected.
Fungicides and common rust
Fungicides will be more essential until hybrids with different rust
resistance are available. Several Rp-genes are effective against the new
race, including: Rp1E, Rp1G, Rp1I, Rp1K, and possibly, Rp1C, Rp1L, and Rp1N
. Also, several "compound genes" which combine two or more Rp-genes are
effective against the new race, including: Rp1GI, Rp1GDJ, Rp1JFC, Rp1G5JC,
and Rp1G5JD. Hybrids with effective individual or compound Rp-genes should
be available in a few years because several advanced breeding lines carry
these sources of resistance. Also, high levels of partial or general
resistance should be effective against the new race. A few hybrids are
available with adequate levels of partial resistance to prevent severe rust
infection. Others may be developed in response to the new race although
breeding for partial resistance is usually much more difficult, expensive,
and slower than converting inbreds to Rp-resistance.
More mutants coming?
New races of pathogens develop in response to selection pressures placed
on pathogen populations by host resistance. In other words, plants and
their pathogens are constantly co-evolving. When breeders and pathologists
incorporate new resistance in adapted hybrids and growers plant these
hybrids over large acreage to control diseases, pathogens adapt to survive.
In the case of the new race of rust in North America, the pathogen probably
adapted very little. Instead, a strain of the fungus that had adapted
previously to Rp1D resistance probably was introduced to an area where it
had not occurred before. If other Rp-genes are used to control rust in
sweet corn, this probably will occur again, but hopefully, other sources of
Rp-resistance will be effective in North America for as long as Rp1D was. Rp1D-resistance was first incorporated into processing sweet corn
hybrids in the late-1970s. Within 10 years, most late season processing
sweet corn grown in the Midwest was Rp-resistant. In the past 10 years,
the proportion of sweet corn hybrids that were Rp-resistant in the University of Illinois disease nursery increased from about 8% to nearly
40%. Thus, some sweet corn producers have benefited from Rp1D-resistance
for nearly a decade and others have used this resistance effectively for
nearly 20 years.
General resistance (i.e., high levels of partial resistance) should
remain effective much longer than Rp-resistance because it is more difficult for P.sorghi to adapt to selection pressures from this form of
resistance. However, partial resistance will not be available as soon as
or in as many hybrids as Rp-resistance.
Growers should be prepared
Stay informed. Know what type of Rp-resistance is in the hybrids you
grow. Know the background reactions of Rp-resistant hybrids if that
information is available. Know which fungicides are available to control
rust on sweet corn, and read their label for proper use. During the
growing season, try to learn if the new race of rust has occurred elsewhere
in North America, particularly in areas near you. If it has, scout fields
for rust on seedlings of Rp-resistant hybrids that have moderate to susceptible backgrounds. Be prepared to decide whether or not to apply
fungicides if seedlings are infected and wet weather is expected. |
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