AVOCADO - Tropical Fruits - page 167

Breeding for cold-hardiness is a major objective in the subtropics.
'Brooksville', a seedling of the Mexican race, has shown outstanding cold-
hardiness, withstanding artifi cial chilling to − 8.5°C. 'Gainesville', another
Mexican type, withstands fi eld temperature as low as − 9.4°C. The cold-
hardiness of these cultivars has to be combined with genes for dependable
productivity and fruit market acceptance (Knight, 1976).
Avocado rootstock breeding is an important aspect of the total breeding
programme. The avocado root rot, caused by Phytophthora cinnamomi , has
led to testing thousands of seeds and bud-wood from avocado trees that have
resisted the disease over many years and other Persea species collected in their
native habitats. Small-fruited species, such as Persea caerulea (Ruiz y Pavón)
Mez., Persea donnell-smithii Mez., Persea pachypoda Nan and Mart. and Persea
cinerascens Blake, though having strong resistance, are graft-incompatible
with the avocado. Hybridization between the small-fruited Persea species and
avocado has not been successful. California and Florida have been the most
active places doing hybridization, introducing species and varieties from their
native sites and trying existing local or foreign materials in search of better
rootstocks; Israel, South Africa, Australia and lately Chile have also been
working on this important matter.
Rootstocks
In California, several rootstocks have been tried and used, especially for root
rot resistance, with salinity and alkalinity resistance being less important.
After many years of using Mexican seedlings like 'Topa Topa' and 'Lula', with
the increase of the root rot problem in the 1970s the industry adopted the
rootstock 'Duke 7', a Mexican seedling of 'Duke' with moderate resistance
to root rot, which became the fi rst commercially successful rootstock that
was propagated vegetatively. Another resistant rootstock, 'Martín Grande'
(G-775C), a hybrid from P. americana and P. schiedeana , did not perform well
and was discarded. 'Thomas', 'D9' and 'Barr Duke' (all Mexican) have greater
resistance than 'Duke 7' and are available (Ben-Ya'acov and Michelson,
1995). The latter three were all found in infected fi elds. In the absence of root
rot, the rootstock can signifi cantly infl uence canopy growth and yield. 'Duke
7' and 'Thomas' produce signifi cantly more fruit per cubic metre of canopy
than other rootstocks, with 'D9' and 'Martín Grande' being the poorest
performers (Arpaia et al. , 1992). During the last few years 'Toro Canyon', of
the Mexican race, has become important; it is more resistant to root rot than
'Duke 7' and more tolerant to salts than the Mexican seedling rootstocks; it
also has resistance to Phytophthora citricola . Two releases from the University
of California-Riverside are 'Zentmyer' and 'Uz' of the Mexican race, which
look very promising. Additionally, California has introduced, and will probably
be using commercially, the 'Dusa' and 'Latas' (Mexican × Guatemalan)