- Root rot of sweet corn in western Oregon and Washington is a significant disease that
can reduce yield of intolerant cultivars of processed sweet corn by fifty percent. Root rot
is caused by a complex of soilborne organisms, including Drechslera sp., Phoma
terrestris, and Pythium arrhenomanes. Processors have adopted tolerant cultivars but
farmers continue to seek cultural management strategies that reduce inoculum potential.
High rate manure and compost amendments (16.8 - 56.0 Mg ha⁻¹) suppress root rot of corn
through general suppression but this practice is not agronomically viable. General
suppression is typically associated with high rates of organic amendment and high
microbial (FDA) activity. Processed vegetable farmers currently grow winter cover crops
to improve soil and water quality and are interested in identifying cover crops that
suppress root rot of corn and increase yield. High biomass cover crops can yield up to 12
Mg ha⁻¹ dry matter; this rate of organic amendment may or may not be sufficient to
generate general suppression. However, specific cover crops, such as species and
cultivars of crucifers and oats, have been shown to more suppressive than other cover crop
species and cultivars against specific soilborne diseases. Oat is grown as a winter cover
crop in the Willamette Valley and contains avenecin, a chemical that has been shown to
have activity against pathogen propagules. In addition, in previous work in containers oat
cover crops suppressed root rot of sweet corn. However, there is a concern that oat cover
crops immobilize N and reduce corn yield.
The objectives of this research were to 1) identify high biomass cover crops with
agronomic potential for western Oregon processed vegetable cropping systems, 2)
evaluate the impact of high biomass cover crops on root rot severity and yield of sweet
corn, 3) determine whether there is a correlation between dry matter, soil microbial
activity and root rot severity and 4) determine whether cover crops immobilize nitrogen
and reduce corn yield.
Research station field trials were conducted in 2003-04, 2004-05 and 2005-06 at
the Oregon State University vegetable research farm in Corvallis, Oregon and an on-farm
experiment was conducted in 2004-05 at Kenagy Family Farm in Albany, Oregon.
Oat 'Saia' winter-killed in 2005-06 and mustard mix 'Caliente' winter-killed
every winter except 2004-05, when winter temperatures never dropped below -7 °C. Rape
'Dwarf Essex', mustard 'Braco', and arugula are reliably winter-hardy. All mustard cover
crop species are susceptible to white mold caused by Sclerotinia sclerotiorum, causal
agent of white mold of snap bean. Oat (Avena sativa) is susceptible to barley yellow
dwarf virus (BYDV), an important pathogen of grass seed crops. Mustard cover crops
could contaminate cruciferous seed crops.
All of the cover crop species evaluated demonstrated some potential to suppress
root rot of corn. Oat 'Saia' was the most consistently suppressive; it suppressed root rot in
4 of 6 experiments. Sudangrass was suppressive in the only year it was evaluated as well
as in container experiments in previous work. In general, cover crops increased or had no
impact on shoot and root dry matter in greenhouse bioassays. There was only one
significant cover crop treatment effect on yield; in 2006, the oat treatment increased yield
by 11.6% compared to the fallow.
Overall, cover crop aboveground dry matter (DM) ranged from 4.2 Mg ha⁻¹ (summer R 2003) to 12.2 Mg ha⁻¹ (winter O 2004). Overall, there was a significant
relationship between cover crop DM and radicle rot severity in greenhouse bioassay but
not in field experiment. Cover crop treatments consistently increased soil microbial
activity. Overall, there was a significant negative correlation between microbial activity
and root rot severity in greenhouse bioassays early after cover crop incorporation, but the
correlation weakened over time and ultimately was lost by about 80 days after
The C:N of oat and rape residues was 51 and 21, respectively. Soil nitrogen was
immobilized by both the oat and rape cover crops, but oat immobilized more N than rape.
Corn grown in the oat treatment soils had lower SPAD values, but it is not clear whether
foliar N was sufficiently low to reduce yield potential. There was no consistent trend in
above- or below-ground corn dry matter after oat incorporation over the three years. In
2006, the oat treatment had no significant effect on corn DM but increased yield by
11.1%. More work is required to better understand the impact of oat cover crop N
immobilization on corn N status and yield.
Oat 'Saia' has the potential to suppress root rot of sweet corn and maintain or
increase corn productivity. However, this oat cultivar is not reliably winter-hardy and is
susceptible to BYDV. Future research should screen Avena species and/or cultivars for
improved winter hardiness, BYDV resistance, and root rot suppressive potential.