Further reading for Avocado Root Health
Download the AvoResearch articles on root rot at this URL:
http://avocadosource.com/Journals/AvoResearch/avoresearch_02_01_2002.pdf
California Avocado Society Yearbooks.
Zentmeyer, G.A., Klure, L.J., Guillemet, F.B., O'Neal, E., Robeiro, O.K., Campbell, S.D. and King, G. 1977.
Studies of Phytophthora Cinnamomi. CAS Yearbook 61: 87-89.
Zentmeyer, G.A., Klure, L.J., Pond, E.C. and Guillement, F.B. 1980. Pathogenicity of different California
isolates of Phytophthora Cinnamomi to avocado. CAS Yearbook 64:131-138.
Darvas, J.M., Torien, J.C and Milne, D.L. 1983. Injection of established avocado trees for the effective
control of Phytophthora root rot. CAS Yearbook 67:141-146.
Kotze, J.M. and Darvas, J.M. 1983. Integrated control of avocado root rot. CAS Yearbook 67:83-86.
Coffey, M. and Cohen, Y. 1984. Crown and collar rot of avocado: A need for more research. CAS
Yearbook 68:69-74.
Coffey, M. 1984. An integrated approach to the control of avocado root rot. CAS Yearbook 68:61-68.
Zentmeyer, G.A. 1985. Origin and distribution of Phytophthora cinnamomi. CAS Yearbook 69:89-96.
Salazar-Garcia, S. and Cortes-Flores, J.I. 1986. Root distribution of mature avocado trees growing in soils
of different texture. CAS Yearbook 70:165-174.
Fenn, M. and Coffey, M.D. 1987. Phosphonate fungicides for control of diseases caused by
Phytophthora. CAS Yearbook 71:241-249.
Goddall, G.E., Ohr, H.D. and Zentmeyer, G.A. 1987. Mounds aid root rot replants. CAS Yearbook 71:147151.
Oudemans, P. and Coffey, M.D. 1987. Phytophthora citricola - Advances in our understanding of the
disease. CAS Yearbook 71:139-145.
Coffey, M.D. 1987. Phytophthora root rot of avocado - an integrated approach to control in California.
CAS Yearbook 71:121-137.
Ouimette, D. and Coffey, M. 1988. Fungicide levels in avocado roots and soil following treatment with
phosphonate fungicides. CAS Yearbook 72:255-264.
Coffey, M., Oudemans, P. and Ouimette, D. 1988. Phytophthora citricola: another cause of avocado
decline. CAS Yearbook 72:127-131.
Ohr, H.D. 1990. Chemical control of Phytophthora citricola. CAS Yearbook 74:99-100.
El-Hamalawi, Z.A., Pond E.C. and Menge, J.A. 1994. Effect of leaf removal and plant pruning on the
development of stem canker disease caused by Phytophthora citricola on Persea americana and Persea
indica. CAS Yearbook 78:131-142.
El-Hamalawi, Z.A. and Menge, J.A. 1994. Effect of root infection by Phytophthora citricola on avocado
root rot caused by Phytophthora cinnamomi. CAS Yearbook 78:111-120.
Messenger, B.J., Menge, J.A., Amrhein , C. and Faber, B. 1997. The effects of calcium on avocado growth
and root health. CAS Yearbook 81:69-78.
Downer, A.J., Menge, J.A., Ohr, H.D., Faber, B.S., McKee, E.G., Pond, M.G., Crowley, M.G, and Campbell,
S.D. 1999. The effect of yeard trimmings as a mulch on growth of avocado and avocado root rot caused
by Phytophthora cinnamomi. CAS Yearbook 83:87-104.
Toerien, J. 2007. The Phytophthora challenge. CAS Yearbook 90:89-101.
Avocado Root Rot
A fungus Phytophthora cinnamomi is the cause of this disease for avocado feeder roots. The main effect
of the fungus is on the feeder root system, which through its rapid destruction, destroys the ability of
the avocado tree to take up water and nutrients. Canopy symptoms include die back of the trees'
branches, wilting and yellowing of the leaves and complete defoliation. It is not realized the trees have
root rot until the canopy shows symptoms by this time much of the feeder root system is already
destroyed.
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The fungus consists of zoospores which are small mobile spores with two flagella
The zoospores have limited mobility of less than a centimeter (roughly 3/8").
The spores are attracted to the actively growing feeder roots
They become attached to the root surface, become rounded by forming a wall and penetrate
into the root tissue
Within about 6 hours the feeder root tissue is dead and the fungus grows within the dead tissue
for several days
When the food reserves of the root tissue are used up, the Phytophthora fungus forms two
further types of spores: sporangia and chlamydospores.
Sporangia can survive for days or even weeks under moist soil conditions.
Under wet conditions the mobile zoospores can be moved passively in run-off and irrigation water. As
these spores can remain mobile for over 24 hours movement in soil water can result in significant
spread of the disease. Soil already contaminated with Phytophthora cinnamomi contains rotting roots
that hold mycelium and chlamydospores that are highly infective. Phytophthora cinnamomi infects over
1000 different host plants and can survive without a host for a long time. Once in the soil or infecting a
plant Phytophthora cinnamomi does not go away.
Symptoms are more severe and appear earlier when more propagules of the fungus are present,
approximately 50% of the roots were rotted at 5 propagules per gram of soil. In California avocado areas
the fungus can produce oospores year round in the well insulated soil environment (below the top few
centimeters). Below 15cm little annual fluctuation in soil temperature occurs in California avocado
growing areas. The root systems of mature trees can extend below one meter deep. The soil type also
affects the size of the avocado root system where the roots in clay soils are less developed and are near
the soil surface while in sandy soils roots are found deeper in the soil and can be four times the amount
in clay soils. Compacted soils also have fewer avocado roots than more loose and open soils. When
there are low air spaces in the soil avocado root numbers are also reduced.
Root rot develops very rapidly in a susceptible variety such as Topa Topa.
The fungus grows best in mild temperate or sub-tropical regions. It does not grow or survive at low soil
temperatures below 43F or at high soil temperatures above 93F. Optimum temperatures are between
70F and 81F.
Root rot control has been achieved on fully grown avocado by injecting fosetyl-Al twice per season at
0.4g a.i. per m2 canopy area. The treatment showed little effect in the first year of its use, but it
dramatically improved the condition of the trees in the second year. The treatment has been tested on
Fuerte, Hass, Edranol and Ryan cultivars with good results. There is some residual overlap with the next
year's treatment; and it may be possible that after the second or third year, when the trees have
regained full vigor and are healthy, fosetyl-Al injected once a year will be sufficient to maintain their
condition.
The greatest concentration of phosphonate fungicides following trunk injection were in feeder roots 4
weeks after injection after which the time the concentration declined.
Diseased trees recover remarkably well when injected with a 8 to 10% solution of fosetyl-Al of 0.4g a.i. is
applied per m2 canopy. Two injections per season were sufficient to effect almost complete recovery.
It was found that two isolates had high survivability in soil over a period of a month. With root rot
caused by Phytophthora cinnamomi the earlier a chemical treatment is applied, the more dramatic the
results. In the laboratory with Aliette or more correctly its breakdown product in plants which is
phosphorous acid, 5-10ppm caused 50% inhibition of growth of both P. citricola and P. cinnamomi. This
level of inhibition by a low concentration of Aliette or phosphorous acid also occurred when plants were
inoculated with P. citricola. Cannot demonstrate any good efficacy against crown rot under field
conditions.
The successful management of avocado root rot depends on:
1. Disease-free nursery plants from nurseries that practice complete elimination of Phytophthora
cinnamomi
2. Maintaining clean orchard soils by minimizing the effects of excessive watering
o Use well drained deep soil
o Add in adequate organic matter (%???)
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On heavier soils use mounds or ridges 2-3 feet high and 304 feet across
Irrigate to avoid severe water stress or excessive water-logging
Add straw mulch
Use drainage channels and barriers to reduce the possibility of flooding introducing
Phytophthora cinnamomi
3. Resistant rootstocks
4. Chemical control
o the main breakdown product of fosetyl-Al is phosphonate
o phosphonate is thought to stimulate natural disease resistance
o phosphonate is active at a range of concentrations
Avocado Trunk Canker
Trunk cankers near the soil are often caused by the fungus Phytophthora mengei (formerly citricola). hey
appear as blackened areas on the bark at or near the soil line. Under the bark the canker is a deep
seated lesion characterized by the darkened appearance of the wood. The initial infections are invisible
since they occur below the soil surface. The most severe disease situations are seen groves upwards of
10 years old.