James
Downer Howard
Ohr
U.C.
Cooperative Extension Dept. Plant
Pathology
669
County Sq. Dr.
University of Calif.
Ventura,
CA 93003 Riverside,
CA 92521
U.S.A.
U.S.A.
Additional index words: Phytophthora cinnamomi, rootstock,
yardwaste
Abstract
Hass avocado on three types
of rootstock (Duke 7, Thomas, and Toro Canyon) and Zutano seedlings were
planted in root rot infested soil and treated (or untreated) with mulch,
Aliette or gypsum, as well as all possible combinations. Toro Canyon rootstocks
produced the largest trees, Duke 7 the smallest. Mulch and gypsum did not
affect tree size, however Aliette stimulated tree growth. Mulched trees had
increased diseased symptoms while Aliette-treated trees were less symptomatic.
Gypsum had no effect on disease ratings.
1. Introduction
Phytophthora root rot is the
most limiting disease for avocado production in California, where it has spread
to 65-75% of groves. Planting of root rot tolerant rootstocks has increased,
but they are not fully resistant to the disease. It is important to create the
environment for optimum growth in root rot infested soils to augment planting
of tolerant rootstocks.
Soil organic matter added to
soils can enhance suppression of root diseases and increase growth and yield of
avocado (Broadbent et al., 1974; Peg et al, 1987). Although the exact mechanism
of control is not known, absence of the fungus in tropical forests suggests
that the fungus does not occur in high organic matter soils (Shea et al.,
1975). Biocontrol may be caused by a specific suppressive organism enhanced by
the organic matter, such as Trichoderma spp. (Papavizas, 1983); a
general increase in competitiveness for resource resulting from increased
number and diversity of microflora and fauna (Cook et al.,1983); or it may be the
general effect on soil physical and chemical conditions that organic matter has
in conjunction with earthworm activity (Stephensen et al., 1944).
Although avocado trees
produce their own mulch as they mature, this has little bearing on replanting
infested lands with young trees. Growers do not mulch young plantings, yet
these trees are at greatest risk to Phytophthora attack. Application of thick
mulch layers
(>5 000 m3ha-1) has potential to create beneficial
conditions for avocado growth while retarding Phytophthora cinnamomi.
Newly planted orchards can
receive mulch within rows because there are no interfering branches. Such
orchards are potentially afforded other benefits, such as weed control, water conservation,
nutrients and soil temperature modulation.
The purpose of this project is to evaluate the suitability of mulches derived from eucalyptus tree trimmings to enhance the survivability of avocado trees planted to root rot infested soils. Eucalyptus was used because it is abundant in the Southern California landscape and is relatively inexpensive. Mulching is being tested along with fungicide mulch gypsum interactions.
2. Materials and methods
The trial was initiated March, 1994, in Ventura, CA,
an avocado growing region north of Los Angeles. The site is a 2.83 ha block (9%
slope), planted to 700 trees in a root rot infested Rincon silty clay loam
(fine, montmorillonitic, thermic Mollic Haploxeralfs).
Three resistant rootstocks with Hass scion and a
seedling Zutano were planted on alternate rows. Treatments were hand-applied as
15 cm of chipped eucalyptus mulch around the base (2 m2) of the
tree, 10 kg of gypsum and or foliar application of an Aliette solution of 3.5 g
l-1. Untreated controls were used in each treatment factor. All
trees are irrigated on the same system with 45 lhr-1 emitters. The
factorial experiment is a four factor completely randomized complete block
design with 16 replications. All treatments were reapplied in the summer of
1995.
Tree growth and mortality are monitored by trunk and
canopy volume measurements and visually. Disease severity is measured by a visual index (1=healthy,
5=dead). Soil fungal populations were assessed by dilution plating on modified
Rose Bengal media. Plant water status was measured by poromitry (LI- 1600;
Li-Cor, Lincoln, NE). Soil moisture tension (15 cm depth) was monitored under
Thomas rootstocks by pressure transducer (Soil Measurement Systems, Tucson,
AZ).
Results were analyzed with
SAS and MSTAT-C. Because of significant row and tree position effects, a
covariate was used in the factorial ANOVA. Disease Severity observations were
log transformed.
3. Results
3. 1. Trunk caliper growth
Trunk caliper growth of the
three clonal rootstocks is greatest in Toro Canyon, with Thomas being greater than
Duke 7 (table 1). Aliette increased trunk caliper, while gypsum has no direct
effect and the mulch-has a negative effect (table 1). Interactions were not
significant, but trends show an increase in caliper with applied gypsum on
Thomas.
3.2. Disease severity
The most pronounced effect
on disease severity (table 1) is with the application of Aliette. Disease
severity across all rootstocks is significantly reduced. The use of gypsum has
no significant effect, although there is a tendency for reduced disease with
its use. Mulch on Zutano, Duke 7 and Thomas increased severity of disease, but
had no pronounced effect on Toro Canyon. In spite of the higher levels of
disease associated with the mulch, the number of soil fungal genera associated with the
mulch is higher (data not shown). Most interactions tested are not significant
(Table 2), however mulch X Aliette and mulch X rootstock interactions did
affect disease severity.
3.3. Soil and plant moisture
effects of mulch
Daily soil moisture tensions
(not shown) are higher for the unmulched plots, yet poromitry indicates that
plants have similar rates of stomatal conductance.
4. Discussion
In a trial where one
treatment significantly alters soil moisture, yet all treatments receive the
same amount of water, results can be confounding. The use of mulch promotes
both increased biological activity and physical properties. During an
irrigation, the water runs off the unmulched plots, whereas it is retained in
the mulched plots. This is because surface crusting occurs in the unmulched
plots, and with the slope of the land, runoff is exacerbated. In the mulched
plots, the soil surface has a granular structure which is promoted by organic
matter incorporation by fungal breakdown, insect and earthworm activity.
Increased disease activity under mulches plots may
be due to the higher soil moisture content. Equivalent stomatal conductivity in
the mulched and unmulched plots indicates that mulch acts primarily to reduce evaporative
loss. Increased soil aeration associated with mulch and subsequent disease
reductions may only be starting to occur one year post-mulching. Such effects
may be overwhelmed by the other mechanisms of moisture retention conferred by
mulching.
References
Broadbent, K and K. F.
Baker. 1974. Behavior of Phytophthora cinnanwmi in soil suppressive and
conducive to root rot. Aust. J. Agric. Res.. 25:121-137.
Cook, RJ and K.F. Baker.
1983. The Nature and Practice of Biological Control of Plant Pathogens. Am.
Phytopath. Soc., St. Paul, MN. 539 pp.
Papavizas, G.C. 1983. Trichoderma
and Gliocladium: Biology, ecology and potential for biocontrol. Ann. Re.
Phytopathol. 23:23-54.
Pegg, K.G. and A.W. Whiley.
1987. Phytophthora control in Australia. Yrbk. S. Afr. Avoc. Growers’ Assoc.
10:94-96 .
Shea, S.R. 1975.
Environmental factors of the northern jarrah forest in relation to
pathogenicity and survival of Phytophthora cinnamomi. For. Dep. Bull. 85,
Perth,W. Aust. 83pp.
Stephensen, R.E. and C.E.
Schuster. 1944. Effect of mulches on soil properties. Soil Sci. 59:219-230.