July 2022 - 6

GRAFTING
CONTINUED FROM PAGE 1
been looking at increasing production
efficiency, " he said. " The challenge in
particular is about increasing crop yield
and less about cutting costs, but certainly
expenses are part of the profit equation.
Grafting is one means of trying to push
that envelope into achieving higher yields,
and I think it's worthwhile looking at. "
A fresh perspective on grafted
plants
Most grafted tomato plants currently are
used for greenhouse production, where
indeterminate fresh-market varieties
are harvested over several months. In
California, on the other hand, growers
favor determinate varieties outdoors.
The fresh-market trial involved seven
scion varieties, two rootstocks and a nongrafted
check from 2016 to 2018 at three
To graft a tomato plant, scions and rootstocks are cut at about 45-degree
angles. A silicon clip is placed on the rootstock stump, with the scion placed
in the clip and aligned above the rootstock. It works best if both scion and
rootstock are about the same diameter. Photo: Brenna Aegerter, UCCE
fields in San Joaquin County and three in
Merced County.
In addition to Aegerter, researchers
included Michael Grieneisen and Minghua
Zhang in the UC-Davis Department of
Land, Air and Water Resources, and UC
Extension farm advisor Scott Stoddard in
California processing tomatoes are harvested
mechanically in one pass over the field.
Photo: Vicky Boyd
Merced County. It also marked the first
public research into grafted fresh-market
tomatoes using California production
practices. It was funded with a research
grant from the California Department of
Food and Agriculture and the Department
of Pesticide Regulation.
Across the six trials, marketable yield
increased an average of 12% when scion
was grafted onto Maxifort or DRO138TX
rootstocks, although the results were better
in some individual trials.
At the San Joaquin County sites, for
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example, yields of non-grafted vines were
similar to the statewide average yield and
grafting increased yield significantly - 25%
to 40% - depending on the year. At the
Merced sites, yields of non-grafted vines
were well above average and grafting
produced much less of an increase.
Based on an economic analysis, Aegerter
said an average yield increase of 20%
would only be economically viable when
the crop price exceeded about $9 per
25-pound carton and grafted plants cost
no more than 40 cents each.
But she said budgeting for that $9 target
could be difficult because of wild market
swings during the season, ranging from
$6 to $12 per carton for California freshmarket
tomatoes.
Aegerter said grafted fresh-market
tomatoes may be suited for suboptimal
growing conditions. Fields with heavy
soilborne disease pressure, such as from
resistance-breaking root-knot nematode or
Southern blight, or abiotic stresses may be
the best candidates to see improvements.
Processing tomatoes
Beginning in 2016, Miyao led a
three-year trial with Aegerter and UC
Extension farm advisor Zheng Wang of
Stanislaus County that examined using
grafted processing tomato plants. It was
partially funded by a U.S. Department
of Agriculture Specialty Crop Research
Initiative grant as a national team effort
under the leadership of Frank Louws of
North Carolina State University.
The rootstocks were Maxifort, Multifort,
DRO138TX and a few others. Scions
included H 8504, DRI 319, N 6428
and HM 3887. The grafted plants were
compared to non-grafted scions.
From 2016 to 2020, the grafted
processing tomato plots yielded an average
of 17% - or about 10 tons per acre - more
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6 | VegetableGrowersNews.com
than the non-grafted ones, Aegerter said.
" A 17% yield increase is pretty good -
who wouldn't want that? " she said. " But the
cost to get to that is so high. "
To try to address that, Aegerter and
Miyao in 2019 and 2020 looked at
increasing the space between plants. This
was based partly on literature reports
that grafted plants were more vigorous
than non-grafted counterparts and could
possibly compensate for fewer plants.
Although actual plant populations
vary among growers, she said a typical
processing tomato field has about 7,500
plants per acre.
Crews harvest a fresh-market tomato trial
that examines yields and sizes of fruit from
grafted plants compared to non-grafted
ones. Photo: Brenna Aegerter, UCCE
Their three trials compared 12- to
13-inch regular plant spacings to 24- to
26-inch-wide spacings on standard
60-inch beds. Many of the grafted plant
combinations on both the conventional
and wide spacings produced significantly
higher yields than the non-grafted
control on both spacings. And the
N 6428 scion grafted onto FusaPro
rootstock had statistically similar yields,
regardless of plant spacing.
Based on those results, and similar
findings from other countries, Aegerter
said reducing populations to 4,500
grafted plants per acre could potentially
work. But the wider spacings have a few
downsides. For example, not every plant
will survive, creating larger gaps for
which the plants may not compensate.
In California's Central Valley, beet
leafhoppers that transmit beet curly top
virus can be a challenge.
" They're attracted to more sparse plant
stands, " she said. " It was really clear last
year we had some curly top in our trials,
and they were attracted to our widerspaced
plots. "
Although the tomato grafting studies
have ended, Aegerter said she continues to
insert grafted plants into other trials that
look at a new soilborne tomato pathogen,
Fusarium falciforme, on an observational
basis. VGN
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July 2022

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