Gypsum soil amendments significantly reduced root rot in avocado seedlings in greenhouse experiments. The cause of this reduction is not entirely clear from these tests. Increased plant growth is eliminated as a significant factor, since gypsum amendments frequently reduced root growth in uninfested soil. Root exudation was unaffected, although levels of specific exudates were not measured. The effects on water infiltration depended on the particle size of the gypsum amendment, rather than on a reaction between gypsum and soil particles. The addition of gypsum seemed to reduce root rot regardless of whether the soil drainage was good or poor, although root rot is often reduced in well-drained soils. Finally, growing the plants in gypsum-amended soil did not increase root resistance to P. cinnamomi infection. The effect of gypsum on growth and reproduction of the pathogen itself may shed more light on the mechanisms of gypsum-induced disease reduction in avocado.  

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