Article
Soil Solutions That Slow Phytophthora
by Dr Ash Martin PhD BSc(For)Hons
Phytophthora root rot is the biggest threat to avocado production worldwide. It is also a severe disease of many other horticultural crops. This review looked at how different soil amendments—organic mulches, calcium, silicon, and beneficial microbes—help manage the disease and how these treatments reshape the soil microbiome.
What they did: The authors reviewed global research on soil amendments used to manage Phytophthora cinnamomi in avocado orchards. They examined how organic inputs, mineral additives, biological control organisms, and common pesticides influence disease severity and how each treatment alters the soil and root microbiome. They also assessed why many promising biocontrol organisms succeed in the lab but fail in field conditions.
What they found:
- Phosphite remains the main control tool, but P. cinnamomi is developing resistance in multiple countries.
- Organic mulches (manures, composts, wood mulch, biochar) improve soil structure, boost microbial activity, and can reduce disease pressure.
- Biochar increases aeration, microbial biomass, and beneficial bacteria, making soils less favourable for Phytophthora.
- Calcium amendments (gypsum, limestone, calcium chelates) reduce zoospore activity and improve root health, sometimes performing as well as phosphite.
- Silicon (mulches, drenches, fertilisers) strengthens plant defences, improves root growth, and shifts the microbiome toward beneficial groups such as Actinobacteria.
- Many microbes suppress P. cinnamomi in vitro—especially Trichoderma, Bacillus, Paenibacillus, and Aspergillus—but commercial products are not always successful in the field.
- Microbial consortia (multiple species with complementary modes of action) perform better than single‑strain inoculants.
- Soil fumigation or Brassica residues can reduce pathogen loads but may also suppress beneficial microbes.
- Herbicides and fungicides alter the soil microbiome: phosphite can increase some beneficial bacteria, while metalaxyl often reduces microbial diversity.
What this means for you:
- Build organic matter. Compost, manures, and biochar improve soil structure, aeration, and microbial diversity—conditions that naturally suppress Phytophthora.
- Use calcium and silicon strategically. Both reduce disease severity and support stronger, more resilient root systems.VAM can make intercropping a more reliable, sustainable option for boosting yields.
- Think in terms of microbial communities, not single inoculants. Mixed microbial products or consortia are more likely to persist and function in orchard soils.
- Reduce disturbance. Practices that protect soil structure and biology—mulching, careful irrigation, minimal compaction—support natural suppression.
- Be cautious with chemicals. Overuse of phosphite or metalaxyl can shift the microbiome in ways that reduce long‑term resilience.
- Expect variability. What works in pots may not work in orchards; field testing and monitoring are essential.
Managing Phytophthora root rot isn’t about finding a single silver bullet—it’s about building a soil environment where the pathogen struggles and the tree thrives. Organic matter, calcium, silicon, and diverse microbial communities all contribute to healthier, more resilient avocado soils. As research advances, especially into microbial consortia and endophytes, growers will have more tools to reduce reliance on phosphite and strengthen long‑term orchard health.
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Read the full article:
Farooq et al. (2024). Soil amendments for management of Phytophthora root rot in avocado and their impact on the soil microbiome. Journal of Plant Pathology, 106:439–455.
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