Proceedings of The World
Avocado Congress III, 1995 pp. 396 -399
EVALUATION OF WEST INDIAN
AVOCADO SEEDLINGS'S TOLERANCE - RESISTANCE TO PHYTOPHTH0RA CINNAMOMI RANDS
COMPARED WITH CLONAL RESISTANT ROOTSTOCKS DUKE 7, THOMAS, AND TORO CANYON,
UNDER FIELD CONDITIONS.
L. Gallo-Llobet and F. Siverio
Departamento de Protección Vegetal
Instituto Canario de Investigaciones Agrarias
Apartado 60 - 38200 La Laguna
Tenerife (Canary Is.), Spain
The aim of this research was
to find West Indian avocado (Persea americana Mill.) germplasm
resistant to Phytophthora cinnamomi Rands, as the West Indian race (WI)
is well adapted to the Canary Islands conditions. The trial field selected was
an old-established avocado plantation with a high degree of P. cinnamomi infection.
After planting, symptom expressions of the clonal resistant rootstocks Thomas
(TH), Duke 7 (D7), and Toro Canyon (TCY) were compared with those of WI
seedlings and seedlings of Persea indica (L.) K. Spreng wild material
(PI), the latter generally used as a susceptible tester for P. cinnamomi. A
randomized block design was used for all plantings and the trial was carried
out during three years. The recorded data include: height, trunk diameter,
disease severity, and surviving plants. The results indicate that D7 and TH
are, in our conditions, quite resistant to P. cinnamomi but TCY is not.
Most of the PI seedlings died, but the surviving plants presented a high degree
of resistance. Unselected WI seedlings seem to have a considerable natural
resistance to P. cinnamomi when compared with the clonal rootstocks.
This could be of great interest for commercial avocado plantations in countries
where the WI race is the preferred rootstock.
1. Introduction
Spain currently produces
around 52,000 t/yr of avocados (Persea americana Mill.) (FAO, 1995) on
10, 100 ha, 9, 000 ha on its southern Mediterranean coast and 1, 100 ha
distributed throughout the Canary Islands. Root rot, caused by Phytophthora
cinnamomi Rands, is the crop's most serious problem. As the best long-term
control solution is the use of Phytophthora- tolerant rootstocks
(Zentmyer, 1980), our aim was to screen for new germplasin with potential
resistance among the West Indian race avocado, which has proved itself to be
well-adapted to the subtropical conditions of the Canary Islands (Alvarez de la
Pefia, 1979). In the present study the performance of unselected West Indian
seedlings is evaluated and compared with several clonal rootstocks which have
proved resistant in California over the last 20-30 years.
2. Materials and Methods
The trial was begun in August
1992 in established avocado groves with a clay-loam soil infected with P.
cinnamomi for more than twenty years (the isolate of P. cinnamomi from
these groves was typified as A2 mating type and incorporated into the University of California
(Riverside) Collection as culture Pc296 in 1975). One-hundred-and-seventy-eight
ungrafted clonal rootstocks of Thomas, Duke 7, and Toro Canyon, 216 unselected
West Indian seedlings, and 86 viñátigo (Persea indica (L.) K. Spreng)
seedlings were planted (Table 1). Viñátigo was included as a Phytophthora-susceptible
control. A randomized block design was used with a 1 x I m planting
distance to uniformly distribute the trees in the plot. The trial was conducted
during three years, with a yearly global evaluation of the percentage of
surviving plants; plant height, trunk diameter at 10 cm height and Disease Severity
Index (DSI), using a visual scale of 0 to 5 (where 0 = healthy and 5 = dead)
(Gabor et al., 1990), were recorded.
3.
Results
The DSI totals after three
years are shown in Figure 1. Forty-one percent of the West Indian seedlings
survived, although most showed some degree of disease incidence. The seedlings viñátigo were severely affected,
but it is remarkable that the few surviving viñátigos were very healthy plants.
Of the clonal rootstocks, Duke 7 had the highest survival rate but Thomas had a
higher percentage of symptomless plants; Toro Canyon plants had the lowest
survival percentage and surviving plants were severely affected.
Height and trunk
diameter means, grouped according to DSI ratings, are given in Figures 2a and
2b. Toro Canyon development was clearly inferior compared with the other
rootstocks. At disease rates of DSI 2, 3, and 4, few differences were
appreciated between West Indian and the clonal rootstocks Duke 7 and Thomas, in
trunk diameter and plant height means. In healthy and slightly affected plants
(DSI = 0 and 1) differences were larger, in favour of the West Indian
rootstock.
Maximum and
minimum monthly average temperatures ranging between 16.6-24.3ºC and
14.4-19.4ºC, respectively, were registered throughout the trial.
4. Discussion
Based on a visual scale,
Gabor et al. (1990) found that clonal rootstocks of Thomas, Duke 7 and
Toro Canyon (among others) were generally healthier than the other rootstocks
tested by them, with Thomas being the most resistant to P. cinnamomi and
Duke 7 and Toro Canyon having an intermediate degree of resistance. Menge et
al. (1992) also found Thomas to be the best under field trial conditions,
while Duke 7 displayed a moderate tolerance to P. cinnamomi. Menge et
al. (1992) conclude that Duke 7 could be considered as a standard reference
for comparing tolerance-resistance with other rootstocks.
In the edaphic and climatic
conditions of the present study, with a high degree of infection in the trial
plots, Thomas and Duke 7 seem to be quite resistant to P. cinnamomi. Nevertheless,
rootstocks have
developed poorly during their 3 years of growth. Toro Canyon did not show an
acceptable degree of resistance, given the low percentages of surviving plants
and the advanced disease states these presented, despite the fact that fewer
plants were evaluated.
The differences
observed in plant development between the West Indian seedlings and the Mexican
rootstocks Duke 7 and Thomas were particularly patent for symptomless and
slightly- affected plants. In these stages, West Indian seedlings had a clearly
better adaptation to our environmental conditions. The survival percentages of
West Indian seedlings are higher in the present study than in previous trials
(Gallo-Llobet, 1992; 1993).
The viñátigo seedlings were in their
majority very susceptible to P. cinnamomi although there were a few
individuals which showed a high degree of resistance. This is consistent with
the results of previous trials (Gallo-Llobet, 1990; 1993).
The West Indian seedlings
which gave the best results in this study are currently being propagated to
further evaluate their behaviour once grafted with commercial avocado
cultivars.
Acknowledgements
The authors thank Rob Brokaw for
supplying the clonal rootstocks, and the University of California (Riverside)
for test agreement for Thomas rootstocks. To our ICIA colleague Maria del
Carmen Cid Ballarin for all her aid throughout the trials and to T.P. Redard
for her help in preparing the manuscript. This work has been funded by the
Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria.
References
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