Better Rootstocks for Sturdier Citrus
Adult citrus root weevil,
Since the 1890s, ARS
has been developing rootstocksthe bottom portions of grafted trees
for the U.S. citrus industry. In recent years the industry has become
concerned with several growing threats to citrus rootstocks. Among these
are citrus tristeza virus (CTV), citrus root weevil (Diaprepes abbreviatus),
foot rot (Phytophthora nicotianae), root rot (P. palmivora),
and citrus blight as well as declines related to poor soil adaptation
The two Phytophthora specieswhich have a
very wide host rangeand Diaprepes form a lethal combination
that is spreading throughout Florida with devastating effects on the
state's $8 billion annual citrus industry.
Kim D. Bowman, a geneticist at ARS's Horticultural Research Laboratory in Fort Pierce, Florida, leads a research program on citrus rootstock breeding and genetics. He and a variety of collaborators are working to solve these underground problems.
Grapefruit trees on US-802
(left) and US-897 (right)
rootstocks growing in a
grove severely infested with
Diaprepes weevil and the
nicotianae and P. palmivora.
The two trees with tolerant
rootstocks appear to be
healthy, while neighboring
trees on common commercial
rootstocks are declining.
Robert Adair, Jr., director of
the Kerr Center for
Sustainable Agriculture, a
cooperator in the ARS-led
citrus rootstock study,
stands between the trees to
give a sense of scale.
Three new rootstocksUS-897, US-942, and US-802have
emerged from Fort Pierce as very strong candidates for eventual commercial
use. They have performed well in initial tests for combating a combination
of CTV, P. nicotianae, P. palmivora, and the citrus root
weevil in heavy (damp) coastal soil. That's a first. They've also demonstrated
adaptability to a wide range of soils, good resistance to some other
diseases, and favorable effects on fruit productivity and quality.
"These three represent the most advanced of thousands
of different rootstock selections and hundreds of rootstocks already
in field trials," says Bowman. But all three of them are still
about 3 to 4 years away from being commercialized.
A quality rootstock can defend itself against destructive
pests while producing a high yield of quality fruit for sometimes up
to 50 years. Disease-resistant, healthy rootstocks can be spliced to
mature tops (or "scions") that produce the highest quality
fruit. These grafted trees live longer and produce citrus fruits sooner
than seedling trees.
A major difference among the three candidate rootstocks
is the effect they have on tree size. Sweet orange trees 12 to 15 years
old on US-897 rootstock remain small, perhaps only 10 feet tall; trees
on US-942 rootstock will be medium-sized; and trees on US-802 rootstock
can exceed 20 feet.
Rootstock influence on tree size is a key factor because
shorter, more compact trees are easier to manage and harvest. Taller
trees can be very productive, but they cost more to maintain.
In 2001, Bowman released the US-812 rootstock, which took
longer than 30 years to develop. More than 10,000 trees were propagated
on this new rootstock in the first year after release. Current demand
for trees on this rootstock far exceeds the supply, Bowman says. It
has proven to be outstanding for well-drained soils in central Florida.
Recognizing the need for better rootstocks, the Florida
Citrus Production Research Advisory Council has provided additional
funds to expand Bowman's research over the past 5 years. Industry collaborators
provide space for widespread field-testing, while scientist collaborators
help test plants for resistance to diseases and pests.
Plant breeders, horticulturists, and plant pathologists
from the universities of Florida, California-Riverside, Arizona-Yuma,
and Puerto Rico are working with Bowman, as are geneticists Raymond
J. Schnell, in the ARS Subtropical Horticulture Research Unit in Miami,
Florida, and Robert A. Owens, of the ARS Molecular Plant Pathology Laboratory
in Beltsville, Maryland. By Alfredo
Flores, Agricultural Research Service Information Staff.
This research is part of Plant, Microbial, and Insect
Genetic Resistance, Genomics, and Genetic Improvement, an ARS National
Program (#301) described on the World Wide Web at www.nps.ars.usda.gov.
"Better Rootstocks for Sturdier Citrus" was published in the December 2003 issue of Agricultural Research magazine.