Video on producing "CLONAL ROOT STOCK TREES"
Characteristics of Hass Avocado on Commercial Clonal and Seedling Rootstocks IN
Larry S. Rose
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.
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
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
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).
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.
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.
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
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 Canyon’s 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.
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).
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.
selections of the G755 debuted in the 1980’s 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 G755’s best contribution to the California avocado industry was a
stark reminder that root rot resistance is only part of the path to greater
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. Grower’s 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
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
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-Ya’acov’s rootstock work are being screened for
root rot resistance, salt tolerance and productivity in California.
Hopes are that some clonals may adapt to California’s worsening water
quality, but at this point West Indian clonal rootstocks are not utilized in
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
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