ABSTRACT
Infection of avocado seedlings by Phytophthora cinnamomi in infested soil was decreased by 71% by the addition of gypsum soil amendments in replicated greenhouse experiments. Root weights and total seedling weights were not significantly increased by gypsum amendments compared with unamended soil; however, the significant reduction in total seedling weight and root weight caused by P. cinnamomi was largely eliminated by the addition of gypsum. Fresh organic matter amendments alone did not significantly affect the total fresh weight or root weight of avocado seedlings. Root fresh weight was decreased 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 significantly affected by gypsum soil amendments. Avocado seedlings grown in gypsum-amended soil and the roots inoculated with suspensions of Phytophthora cinnamomi zoospores were no more resistant than seedlings grown in unamended soil. Permeability of avocado root membranes, as determined by the amount of 86Rb exuded from root segments over time, was unaffected by growth in gypsum-amended soil. Infiltration of water into soil amended with fine-grade gypsum was impeded initially; whereas soil amended with coarse drywall gypsum drained faster than unamended soil. Drainage was not correlated with root infection of avocados grown in soil infested with P. cinnamomi and amended with gypsum. It appears that large reductions in infection of avocado seedlings by P. cinnamomi in gypsum-amended soil are not caused by an avocado growth response, increased root resistance, or reduced root membrane permeability. Infection is not markedly affected by poor drainage when the soil is amended with high levels of gypsum.
