Soil amendments and suppression of Phytophthora root rot in avocado (Persea americana)

The ability of organic, microbial or mineral-based soil additives to suppress root rot caused by Phytophthora cinnamomi was compared with disease reduction resulting from the use of the fungicides phosphite or metalaxyl. The effect of glyphosate (commonly used for weed control) on plant health was also examined. Avocado plants were grown in a glasshouse in pots with soils collected under mature commercial avocado trees. To simulate ‘orchard soil’ conditions, chicken manure, wood mulch, and mulch from beneath 20-year-old trees in an avocado orchard were added to the pots. The effect of P. cinnamomi on plant growth and visible root damage was assessed using plants grown under these ‘orchard’ soil conditions, and treatments with further additives (two microbial soil conditioners, one organic and two mineral-based mulches). In two of three experiments, infestation of soil with P. cinnamomi resulted in no significant reduction on fine root dry weight for plants sprayed with phosphite, or treated with a silicate-based mulch. However, when a combination of these two treatments gave no additive effect. In one experiment, a microbial-based conditioner was also beneficial. Phosphite was preferable to metalaxyl as a chemical treatment, as the latter reduced shoot dry weight by 25% and fine root dry weight by 30% of that in non-inoculated plants. Glyphosate treatment of wheat seedlings growing in the pots with the avocados also reduced shoot dry weight (20%) and fine root dry weight (20%) of non-inoculated avocados. These observations need to be confirmed under field conditions.