While growing mustard as a cover crop prior
to carrots may boost yields, there are risks associated that may more
than offset the yield advantage.
University of California Cooperative Extension studies show that carrots
grown in fields following a mustard crop have consistently higher
levels of cavity spot.
Cavity spot is caused by several species of fungal-like organisms
belonging to the genus Pythium. Disease characteristics include root
blemishes, frequently manifest as vertical cracks, making the carrots
unacceptable for the fresh market.
Grant Poole, Joe Nunez, R. Michael Davis and others from the University
of California conducted field studies in 2002 and 2003 and again in
2004 and 2005 comparing performance and cavity spot occurrence following
a mustard cover crop.
Mustards have become popular cover crops in vegetable production areas
throughout Washington and California, the researchers note. Some research
suggests that the incorporation of mustard residue can ‘biofumigate’
or suppress certain soil-borne diseases, such as cavity spot, as well
as weeds and nematodes. Mustard as a cover crop certainly does have
a fit in some crop rotations.
Previous research suggests that incorporated mustard residues can
suppress certain Pythium species, but Poole and colleagues failed
see that in the field. Although Joe Nunez did show that cavity spot
levels were lower than the fallow control checks of carrots harvested
in 2003, results of mustard biofumigation studies throughout California
to date have show erratic pest suppression results (nematodes, disease,
and weeds).
Mustard tissue contains chemical compounds know as glucosinolates
(GSL), the researchers explain. Once residue is chopped and incorporated
into the soil, the GSL converts to isothiocyanates (ITC) which is
similar to m-ITC, the active ingredient in Vapam. The amount of ITC
released from incorporated mustard biomass is largely dependent on
mustard dry matter yield and variety.
Growing mustard as a bio-fumigant cover crop requires intensive management
to achieve biomass and receive any possible bio-fumigation effect,
Poole warns. Research at Washington State University has revealed
that nitrogen needs are approximately 120-150 lbs per acre. Also sulfur
is connected to the concentration of glucosinolates in the plant tissue
and should be adequately supplied at a N:S ratio of 5:1 (i.e. 25-30
lbs S). For adequate mustard growth, half of these nutrients should
be supplied to the plant within the first two weeks of active growth.
The applied nutrients should be available to the vegetable crop within
the growing season as the mustard residue decomposes.
“Mustard is quick to germinate, grow and develop, so proper
nutrient and water management during early growth stages is important,”
the researcher stresses. “Growing mustard as a cover crop requires
intensive management compared to growing other types of cover crops.
The UCCE team’s 2002 and 2003 mustard evaluations took place
at a field owned by Bolthouse Farms Inc., near Lancaster, Calif. There
were both fall and spring mustard plantings. The fall planting took
place took place on Sept. 12, 2002, and the spring planting on Dec.
16 of that same year. Fall planting incorporation occurred on Jan.
6, 2003, and the spring planting incorporation on April 7 (2003).
The carrots were planted on June 13, 2003, and harvested on Oct. 1.
Field trials in Lancaster showed mixed results, the team report. Carrot
yield was 0.8 to 4 tons higher in the mustard treatments when compared
to the fallow control treatment for the fall and spring mustard plantings.
Pythium sp. (disease) levels of the soil samples were lower in mustard
treatments 10 days after mustard incorporation. However, cavity spot
occurrence was higher in carrots from the mustard treatments at carrot
harvest.
Carrot cavity spot level could have been higher in mustard treatments
because there were six months between mustard incorporation and carrot
planting, Poole points out. Soil respiration and water infiltrations
rates were higher in the mustard treatments compared to the fallow
control treatment.
Mustard cover crops have been shown to improve soil quality and suppress
dust, Poole and colleagues acknowledge. Although total Pythium sp.
Levels in the 2002 and 2003 study were lower in the mustard treatments
10 days after mustard incorporation when compared to the fallow control,
there was a higher cavity spot occurrence in carrots following a mustard
cover crop.
“Initially it was thought that cavity spot levels in carrots
following mustard could have occurred due to the six-month period
of time between mustard incorporation and carrot planting,”
Poole explains. “During this period of time, Pythium sp. probably
re-colonized in the mustard residue resulting in greater occurrence
of cavity spot in carrots following the mustard treatments. However
when carrots were planted in 2005 (30 days after mustard incorporation),
preliminary results have indicated that cavity spot levels were again
higher in the harvested carrots.”
Field evaluations continue to show higher cavity spot levels in the
mustard treatment versus the fallow control check, Poole notes. This
year’s data collected were inconclusive because of problems
in the field, but overall yields on three of the replications were
higher in the mustard treatment than the fallow control check –
although not as high as previous trials had shown.
“Based on the results of two separate studies, we’ve found
that mustard grown as a cover crop prior to carrot cultivation has
the potential to increase carrot yield but does pose a risk,”
Poole repeats. “Preliminary data show that following a mustard
cover crop, carrots consistently had higher levels of cavity spot.”
That said, the researcher reminds growers that planting a mustard
cover crop prior to carrots provides other benefits that must be weighed.
Among them are increased water infiltration, increased water holding
capacity and dust suppression. Dust suppression, in particular, is
important in parts of California where carrots are grown.
