ABSTRACT
During the life of a citrus planting, the population of Phytophthora pathogens can build to significant levels in orchard soil. A study was initiated to examine the impact of some nonchemical cultural practices on survival of P. nicotianae, the most prevalent Phytophthora sp. in Arizona citrus groves, in soil formerly planted to citrus. In three trials over a 3-year period, P. nicotianae could not be detected at a depth of 10 cm after soil naturally infested with the pathogen was subjected to a dry summer fallow period of at least 31 days in the desert southwest region of Arizona. The mean temperature of soil at this depth during these trials ranged from 37 to 39°C. Furthermore, in two of these trials, after summer dry fallow periods of 38 and 45 days, the pathogen could not be detected at a depth of 15 to 20 cm and was detected in only one of 19 soil samples at a depth of 25 to 30 cm. In comparison, the pathogen was recovered from a high proportion of soil samples subjected to a dry winter fallow period or maintained in the greenhouse and planted with a seedling of citrus, alfalfa, or irrigated without the presence of any plant, where mean temperature of soil ranged from 15 to 30°C. In regions with a hot and dry summer climate, a dry summer fallow treatment of soil after removal of an existing citrus planting and before establishment of a new grove could provide a rapid and relatively inexpensive means of lowering the population of P. nicotianae to virtually nondetectable levels to at least a depth of 30 cm.
