Effects of gypsum soil amendments on avocado growth, soil drainage, and resistance to Phytophthora cinnamomi

Infection of avocado seedlings by Phytophthora cinnamomi in infested soil w as decreased by 71% by the addition of gypsum soil amendments in replicated greenhouse experiments. Root weights and total seedling weights were not s ignificantly increased by gypsum amendments compared with unamended soil; h owever, the significant reduction in total seedling weight and root weight caused by P. cinnamomi was largely eliminated by the addition of gypsum. Fr esh organic matter amendments alone did not significantly affect the total fresh weight or root weight of avocado seedlings. Root fresh weight was dec reased in uninfested soil amended with 5% gypsum and organic matter, but in infested soils, the same treatment increased root fresh weight. Root fresh weight of mature avocado trees in an uninfested grove was not significantl y affected by gypsum soil amendments. Avocado seedlings grown in gypsum-ame nded soil and the roots inoculated with suspensions of Phytophthora cinnamo mi zoospores were no more resistant than seedlings grown in unamended soil. Permeability of avocado root membranes, as determined by the amount of Rb- 86 exuded from root segments over time, was unaffected by growth in gypsum- amended soil. Infiltration of water into soil amended with fine-grade gypsu m was impeded initially; whereas soil amended with coarse drywall gypsum dr ained faster than unamended soil. Drainage was not correlated with root inf ection of avocados grown in soil infested with P. cinnamomi and amended wit h gypsum. It appears that large reductions in infection of avocado seedling s by P. cinnamomi in gypsum-amended soil are nor caused by an avocado growt h response, increased root resistance, or reduced root membrane permeabilit y. Infection is not markedly affected by poor drainage when the soil is ame nded with high levels of gypsum.