AVOCADO - Tropical Fruits - page 168

clonal rootstocks developed in South Africa. 'Dusa' has already been released
to nurseries since it is root rot and salinity tolerant. Several dozen selfi ng or
outcrossing seedlings showing exceptional resistance to Phytophthora root rot
are under trials.
South Africa has serious root rot problems, and they followed a path
similar to California in the early years of last century and used Mexican
seedling rootstocks. In the 1950s, because most of their Mexican mother
plants were contaminated with sun blotch, they started using the Guatemalan
'Edranol', which had no resistance to root ríot, and later 'Duke', and then
after many trials 'Duke 7', which is now the standard rootstock, providing
reasonably healthy, productive and uniform trees (Kremer-Köhne and Köhne,
2007). Recent rootstock selections from South Africa, 'Latas' (Merensky
I), 'Dusa' (Merensky II) and 'Evstro' (Merensky III) have great production
potential with 'Hass' and tolerance to Phytophthora root rot and salinity. 'Dusa'
is more root rot tolerant and productive than 'Duke 7' and it has been made
available to growers in several countries. Australia uses plants grafted on
seedling rootstocks that are very diverse; this complicates management of the
orchard. They have started a programme to identify superior rootstocks and
are looking for an ei cient clonal propagation system. They will start selecting
material from superior trees in the fi eld and will evaluate imported rootstocks.
Chilean breeders used plants grafted mainly on 'Mexicola', and other Mexican-
race seedlings are going in the same direction. While in Peru 'Topa Topa'
seedlings are generally used, and West Indian types imported from Israel are
used in areas with salinity.
A rootstock with resistance to salinity is a focus of research in Israel,
California, Chile, Australia and the north and central coasts of Peru. The
three races dif er in their ability to resist the uptake of salts (Table 7.1), in
their ability to retain the toxic salts in the root system without translocating
them to the scion and in their tolerance within their tissues. The West
Indian rootstocks have the highest level of tolerance to salinity, followed by
Guatemalan and Mexican races (Ben-Ya'acov and Michelson, 1995). Israel
has taken two approaches: (i) selection of types that produced a resistant
seedling population; and (ii) selection of individual resistant plants for
vegetative propagation. Unfortunately, the true West Indian and high-salinity-
resistant types are dii cult to propagate by cuttings. The cold-tolerant Mexican
'Toro Canyon' and 'Duke 7' have the best salinity rating; 'Thomas' and 'Martín
Grande' had intermediate ratings; and 'Barr Duke' had the lowest rating. The
recent rootstock selections from South Africa, 'Latas', 'Dusa' and 'Evstro',
show some salinity resistance.
The main problem with clonal rootstocks is that the plants lack a tap
root, and therefore special care has to be taken that water supply is timely
and adequate, especially in the most superfi cial portion of the soil, during
the fi rst year or so, otherwise the plants will suf er and many of them will not
recover properly.