Author: Larry S. Rose
Brokaw
Nursery, Inc. 1419 Lirio Avenue,
Saticoy, California 93007, USA
E-mail: larry@brokawnursery.com
SUMMARY
Long-term
commercial use of clonal rootstocks has confirmed many benefits but also
exposed shortcomings of various cultivars. Clonal rootstock selection has expanded to include analysis
of multiple traits of soil stress factors and productivity. The quality of new introductions is
potentially greatly improved.
Acceptance of new commercial clonal rootstocks now requires higher
production and well rounded adaptability.
INTRODUCTION
Clonal
avocado rootstocks have been commercially planted in California since
1977. By 1981 clonal rootstocks
predominated commercial plantings.
Through the 1990s, 95% of trees sold by Brokaw Nursery were on clonal
rootstocks. In spite of a 50%
price premium of clonal trees versus trees on Mexican seedling rootstocks,
avocado growers recognized the potential of various clonal rootstocks for
greater productivity, uniformity and benefits of adaptability to soil stress
factors. During this current boom
of avocado planting in California, nurseries are supplying trees for at least
3,500 new acres per year, with clonal trees representing approximately 75% of
that market.
Growing avocados in arid
Southern California is never as simple as dealing with a single stress
factor. Typically Phytophthora cinnamomi
infested soils can lack important aeration due to finely textured soils, poor
soil structure or impervious layers in the soil profile. Heavy winter rains on cold soils can
compound the problems of soil saturation and root asphyxiation. Faced with these factors rootstock
selection has generally been limited to Mexican race cultivars due to their
adaptability to poor aeration and cold winter temperatures. However, Mexican rootstocks are
generally highly sensitive to chloride and sodium toxicity thereby complicating
planting choices where poor drainage precludes leaching of salts. Although
initial clonal rootstocks were selected for tolerance to Phytophthora cinnamomi, they also demonstrated
consistent growth and canopy uniformity.
In addition to tolerance of P. cinnamomi, some cultivars also
possess resistance to other fungal diseases, reduced absorption of salts and
resistance to lime-induced chlorosis.
Most importantly, select clonal rootstocks have greater fruit productivity
when grafted to the Hass fruiting cultivar.
Over the years, the formal research and screening of potential commercial rootstocks in California has been broadened from its focus on P. cinnamomi to include tolerance to other diseases, salinity, tree size, productivity and a tendency to begin bearing early in the life of the tree. However, field experience with trees on clonal rootstocks ultimately defines the range of tolerance to soil stress factors inherent in various cultivars, their benefits and limitations. No commercial clonal rootstock today is perfect for all situations but since clonal rootstocks have come to dominate orchard plantings in California, growers and nurseries alike have come to a better understanding of custom-selection of the best rootstock cultivar for each situation.
UNDERSTANDING CLONAL ROOTSTOCK WEAKNESSES
Duke
7, the first commercially available clonal rootstock in California is a Mexican
selection made by Zentmyer over 50 years ago. The Duke 7 has had a checkered history. Assumptions made about the performance
of clonal rootstocks in general were inaccurately attributed to this premier
cultivar when planted in severe locations and given normal care. Many failures of Hass on Duke 7 were
attributed to the rootstock cultivar itself while specific sensitivities of all
clonal rootstocks were generally unknown (Coffey 1987).
Clonally
rooted cuttings have no central tap root and grow from a crown of roots
originating from a relatively short stem close to the surface of the soil. Trees grown by this method are very
sensitive to abusive irrigation and are quick to dry out especially during the
establishment phase of the tree often leading to the loss of the tree. Stressed plants have less resilience
and disease resistance. Once
clonal plants are weakened and are unable to refoliate, roots die quickly and
tree death is predictable. A
poorly established clonal rootstock of any cultivar has great difficulty
wintering over the first year. When
stressed by cold winds and exposed to heavy rains, loss of roots causes a rapid
defoliation when mature leaves are shed in the early spring. One-year-old defoliated clonally rooted
trees rarely recover, however, once past the establishment phase, clonal trees
are more resilient to environmental stress. Seedling rooted trees, though variable by nature, are not as
prone to death following defoliation, but seedling trees do not possess
consistent, positive traits replicated by clonal rooting.
Early
commercial use of clonal root rot resistant trees were often planted in low,
wet swales or in spots of poor soil quality where original trees had
failed. When normal irrigations
were applied to these areas along with existing, older trees, the less
resilient clonals often died.
Other factors such as constant influx of P. cinnamomi contributed
to poor performance, but even without Phytophthora pressure
clonal trees needed meticulous care to become established in compromised
conditions. As years passed and
new cultivars were tried, growers were further along the learning curve and
applied new techniques with better success. Their success was often attributed to the newer cultivar and
less so to better farming practices.
It is now the norm to utilize integrated management practices (Coffey
1988) of remedial site preparation, cultivar selection, generous mulching,
gypsum topdressing, fungicides and irrigation monitoring to assure replant
success (Menge, 1991). Success
rate of clonals planted in non-infested soils exceed that of trees on seedling
rootstocks.
Duke 7
Duke
7 has a tendency to remain greener in cool winter soils when chlorosis is at
its worst and begins to grow early in the spring before trees on other
rootstocks. Hass on Duke 7 bears
early in its life, bears large fruit and lots of it (Arpaia et al 1993). Duke 7 has only moderate root rot tolerance
but its horticultural adaptation is broad and its salt tolerance is strong for
a Mexican rootstock (Oster et al 1992).
It still remains a preferred variety where P. cinnamomi is not an
imminent threat.
Toro Canyon
Although Toro Canyon is a comparable producer to Topa Topa seedling rootstocks in soils not infested with P. cinnamomi, its moderate size provides for very efficient production for its canopy volume. Because it is significantly more resistant to root rot than Duke 7 and more tolerant to salts than Mexican seedlings, it has become the major commercial clonal selection. It demonstrates very good sodium exclusion and chloride tolerance for a Mexican rootstock (Mickelbart et al 2002). Toro CanyonÕs timing on the scene coincided with the introduction of integrated control of P. cinnamomi and overall better understanding of how to grow clonal rootstocks in difficult situations. Toro Canyon has significant resistance to P. citricola infection and canker.
Borchard
Borchard
rootstock, a Mexican selection from southern California, is resistant to
lime-induced iron chlorosis, a tendency in calcareous soils prevalent in
coastal groves. Were it not for
the lack of root rot resistance Borchard would be a major commercial variety
having production 25% higher than Topa Topa (Arapia et al 1993). As it is, it remains a significant
problem-solving rootstock in moderate to highly calcareous soils. Borchard is resistant to P.
citrocola canker (El-Hamalawai et al 1991).
Thomas
Thomas
was initially very impressive, being one of the best rootstocks for root rot
tolerance. Its popularity and use
has waned as its shortcomings became more apparent. Thomas is very susceptible to both P. citricola and Dothiorella
gregaria canker, a problem with cultivars of Guatemalan race. This vigorous rootstock is highly
sensitive to salinity and is a poor producer in many locations. It remains useful in severe root rot
areas where salinity is not an issue.
G755
Three
selections of the G755 debuted in the 1980Õs as the ultimate rootstocks and
nearly swept aside predecessor rootstocks. These Persea schideiana x P. americana hybrids
passed preliminary root rot screening in the field but later showed poor
adaptability especially in calcareous soils. Fruit production trials ultimately ended their use when they
showed poorly (Arpaia et al 1993). Some later limited reports of adequate production
were too late to revive interest.
The G755Õs best contribution to the California avocado industry was a
stark reminder that root rot resistance is only part of the path to greater
productivity.
WHAT LIES AHEAD
The bar has been raised and the standard for commercial release is higher than in the past. Duke 6, G6, Barr Duke, D9 and others are worthwhile root rot tolerant rootstocks but do not possess the collection of positive traits that can attract interest to become commercially important. GrowerÕs general satisfaction with current rootstocks preclude introduction of mediocre candidates. Rootstocks now in trial from promising outcrosses and isolated survivor trees must have superior production and traits of tolerance to many soil stress factors to be contenders for commercial use.
West Indian Race
West
Indian rootstocks have, in the past, not performed satisfactorily due to poor
root growth, late leaf development in the spring, generally pale color and
lower production than trees on Mexican race rootstocks. Seedlings of Lula, a West Indian
hybrid, have been used by nurserymen for their vigor and ease of production but
have no other outstanding characteristics. Significant decline occurred from trees planted on West
Indian seedling rootstocks in the 1970s (Ellstrand 1992). Now, a resurgence of research is
currently being done with this salt tolerant group. Selections from Ben-YaÕacovÕs rootstock work are being
screened for root rot resistance, salt tolerance and productivity in
California. Hopes are that some
clonals may adapt to CaliforniaÕs worsening water quality, but at this point
West Indian clonal rootstocks are not utilized in California.
Merensky
2 (Dusa)
With
only 5 years of trials in California the Dusa¨ or Merensky 2 rootstock, as it
was called, has drawn a lot of attention. Not only has it fared well in
widespread replant field trials throughout California, preliminary results with
salinity screening show it may be a well rounded rootstock. What is driving anticipation are
reports from South Africa that Hass is up to 30% more productive on Merensky 2
than on Duke 7 (Roe et al 1999).
From commercial introduction in 2003 through 2005 more than 130,000
trees were planted in California and demand is strong even in the face of
limited experience under California conditions. Time will tell.
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