Phytophthora Diseases
An Introduction to Avocado Root Rot and Research on Integrated Approach to its Control
By Michael Coffey, Ph.D.
Phytophthora cinnamomi is the cause of this extremely destructive disease of the feeder roots of avocado. The infective spore of P. cinnamomi is the zoospore, small motile spores which can be moved passively in run-off and irrigation water and remain motile up to 24 hours. Within about six hours after contact with avocado feeder roots, zoospores have infected, penetrated and killed feeder roots. After root food reserves are used up, sporangia can be formed which can proliferate under warm, moist conditions producing many more zoospores. In addition, soil already contaminated with P. cinnamomi contains decaying roots which hold mycelium and chlamydospores (thicker walled, survival spores) and these structures are highly infective giving rise to sporangia and zoospores. Physical movement of wet soil is a common method of spread of the disease, both within a grove and from property to property.
P. cinnamomi is known to be parasitic on over 1000 different host plants, including ornamentals and many fruit trees. Since P. cinnamomi is widespread in most countries where avocados are grown, and as it can survive in soil apart from a host, there is no ornamental planting which does not pose a threat to avocado production.
An outline for Integrated Disease Control
There are five principal factors to consider:
1. Nursery Practice. In California, the avocado certification program outlines conditions to minimize the possibility of P. cinnamomi infection of nursery trees but does not however, cover P. citricola or other Phytophthora species.
Nursery practice should include:
a. The use of steamed or fumigated mixes.
b. The propagation of all trees on benches
c. The provision of adequate drainage within the nursery to minimize the risk of spread of P. cinnamomi.
d. The periodic sampling of tree roots for P. cinnamomi during the 2-year production period.
e. The avoidance of the use of fungicides such as RidomilR and AlietteR to prevent suppression of P. cinnamomi thereby hampering early detection.
2. Cultural Practice. Since P. cinnamomi is favored by wet conditions, irrigation and cultural practices should aim to minimize the effects of excessive watering including:
a. Provision of a well drained soil, especially in heavier soil by planting on mounds or ridges.
b. Provision of adequate irrigation and correct use of drip irrigation facilities.
3. Clonal Rootstocks. Development of clonal rootstocks Duke 7, G6 and G755 have revolutionized concepts about avocado planting. These rootstocks have moderated field tolerance to P. cinnamomi and therefore require special care in the provision of proper planting conditions including well drained soild and adequate drainage conditions (ie. planting on mounds).
4. Biological and Cultural Control. Some scientists conclude that the main effect of biological control is primarily cultural, by providing a good physical and chemical environment for root growth, and a generally suppressive biological soil environment for P. cinnamomi. An exciting possibility for the near future may be the incorporation of specific antagonistic microbes with clonal rootstocks to reduce root rot in the establishment years, possibly in conjuntion with a fungicide such as RidomilR or AlietteR.
5. Chemical control. There is no good evidence with avocados that fumigation (methyl-bromide, VapamR , or MyloneR) is necessary. With the development of chemicals such as RidomilR and AlietteR which are highly active specifically against P. cinnamomi, new possilbilities of disease control have recently emerged.
Chemical control research continues with:
a. Preventative nursery treatment of clonal rootstocks at planting time.
b. Post-plant alternation of AlietteR and RidomilR for stable disease control.
c. New methods of post-plant treatment including tree injections.
Summary of disease control. The critical factors for adequate control of avocado root rot are the planting of a good clonal rootstock, careful cultural practice and intelligent use of fungicides. This is an integrated control approach and does not rely excessively on any single method for reducing the impact of root rot.
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