Effects of Flooding and Phytophthora Root Rot on Net Gas Exchange and Growth of Avocado

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Abstract
Greenhouse studies were conducted to determine the effects of Phytophthora root rot (caused by Phytophthora cinnamoni) and flooding on avocado (Persea americana). In addition to standard disease assessments (root necrosis, root colonization, wilt, and defoliation), dry weight accumulations and gas exchange characteristics were monitored as indicators of host distress. In a peat-perlite potting medium with a high water-holding capacity, net CO2 assimilation, transpiration, stomatal conductance for CO2, and root and shoot dry weights were reduced by root rot (P < 0.05). In this medium, flooding alone generally did not reduce these parameters after 5 days,. In a calcerous soil used for avocado production in south Florida (with a lower water-holding capacity than the potting medium), root rot reduced assimilation, transpiration, and conductance in a series of three experiments, although not consistently. In this soil, flooding alone reduced these parameters as well. After 4 wk of flooding, assimilation, transpiration, and conductance declined to nondetectable levels. However, when plants with root rot were flooded, these physiological parameters were reduced as soon as 3 days after flooding began, and they declined to nondetectable levels within 1 wk. These plants also had reduced root, shoot, and total plant dry weight accumulations and increased defoliation when compared with nonflooded plants without root rot. Although similar reductions occurred for nonflooded plants with root rot and flooded plants without root rot, these reductions were not as great or consistent as those detected for the combined root rot and flooding treatment. In combination, Phytophthora root rot and flooding dramatically impaired photosynthesis and normal stomatal function and reduced the root and shoot biomass in avocado.