Diseases caused by species of Phytophthora are responsible for significant economic losses on a wide range of host plants, including pear (Erwin and Ribeiro, 1996; Harris, 1979; Wormald, 1919). Disease caused by Phytophthora syringae occurs during the winter in nursery stock in the Pacific northwest region of the United States (PNW), especially on trees that are harvested and stored in coolers or in outdoor sawdust beds (Erwin and Ribeiro, 1996; Pscheidt and Ocamb, 2002; Tidball and Linderman, 1990). The fungus spreads via splash dispersal of inoculum from soil to leaves, stems, and fruit (Ristaino and Gumpertz, 2000; Upstone, 1978), and infects trees through wounds on stems caused by handling or pruning or through leaves and leaf scars (Bostock and Doster, 1985; De Bruyn, 1924). In nursery stock, dark, sunken cankers occur on stems, and in severe cases, the stem is girdled (Erwin and Ribeiro, 1996; Young and Milbrath, 1959).
Most pear varieties in the PNW are grafted on selected rootstock clonally propagated in nurseries and defoliated before harvesting and cold storage, similar to production practices used for many other deciduous nursery trees (Frecon, 1982). Methods used for defoliation include manual or mechanical removal of leaves and the use of chemical sprays [e.g., chelated copper ethylenediaminetetraacetic acid (CuEDTA)] that result in early abscission of leaves, usually while the leaves are still green. Chemical-induced defoliation of deciduous trees decreases the amount of N mobilized from leaves to stems and roots in the autumn; thus, it can reduce N reserves required for growth the following spring (Bi et al., 2003; Guak et al., 2001). The combination of foliar sprays with urea and CuEDTA can be used to obtain efficient early defoliation and promote N storage without reducing plant growth performance the following year (Bi et al., 2005; Guak et al., 2001). Early defoliation before terminal buds set can also inhibit the development of dormancy and cold hardiness (Fuchigami, 1970). To survive in cold storage and grow well during the following growing season, trees must have enough reserve nutrients and develop dormancy.
Surface wounds inflicted during harvest and leaf scars caused by artificial or natural defoliation and subsequent handling can serve as infection openings for P. syringae, but the pathogen is unsuccessful in causing infection of uninjured bark (Bostock and Doster, 1985; De Bruyn, 1924; Linderman, 1986; Pscheidt and Ocamb, 2002). Recently, growers of pear nursery trees in the PNW have reported increased incidence and severity of damage to trees by P. syringae when ‘Old Home × Farmington 97’ pear (‘OHF 97’) rootstock is sprayed with urea and chemical defoliants. It was hypothesized that the combination of urea and defoliant treatment predisposes the trees to P. syringae infection by inflicting injury to the stem tissue or by indirectly making the trees more susceptible to infection by increasing tree N content. With other plant pathogens, abundant N can make trees more susceptible to pathogens and increase the length of time trees are more susceptible (Frecon, 1982; Simon et al., 2003).
Infection of bare-rooted nursery trees by P. syringae during cold storage could result from subjecting trees to environmental conditions that favor the growth of the pathogen and/or are unfavorable to maintaining optimal physiological condition of the plant (Pscheidt and Ocamb, 2002). The combination of host predisposition and cold, wet conditions in cold storage or outdoors favorable for pathogen activity increases the potential for disease. In addition, P. syringae infection might be favored by the physiological or physical condition of the plant during harvesting and subsequent handling and storage. It is possible that trees harvested before they are dormant may be more susceptible to P. syringae and that surface wounds or leaf scars resulting from urea and defoliant sprays and harvesting and handling procedures could also serve as infection openings for P. syringae.
Increased damage by P. syringae has not been reported for the majority of plant species that are sprayed with urea and defoliants during production. Some nurserymen report that early defoliation of trees treated with the combination of urea and defoliant may actually reduce the incidence or severity of diseases (L. Lyon, personnel communication). Reduced disease incidence and severity in response to early defoliation may be a result of more complete healing of leaf scars before the cold, wet conditions in the autumn when the potential for P. syringae infection is high. Additionally, early defoliation enables trees to be harvested and stored before the onset of environmental conditions that promote the activity and subsequent infection by P. syringae (Bostock and Doster, 1985). The effect of urea and defoliants on disease incidence and severity caused by P. syringae appears to be species specific and may be related to the time autumn sprays are used during production.
Common control measures for P. syringae in nursery production include a combination of good nursery sanitation practices and chemical application. Phosphonate-containing fungicides, including fosetyl-aluminum (fosetyl-Al) and its breakdown product, phosphorous acid (also referred to as phosphonate) are commonly used to control Phytophthora species (Doster and Bostock, 1988; Erwin and Ribeiro, 1996). It is applied to trees as sprays, drenches, dips, or by injection (Erwin and Ribeiro, 1996; Pegg et al., 1987; Quimete and Coffey, 1989; Tidball and Linderman, 1990). Fosetyl-Al translocates upward and downward (in the transpiration stream and in the phloem sap) and may have direct fungal toxicity activity against Phytophthora species, or induces host plant resistance (Erwin and Ribeiro, 1996). Although chemical control measures exist for this pathogen, the influence of these chemicals on controlling the disease may be affected by the use of urea and defoliant sprays during nursery production.
Our specific objectives were to determine whether susceptibility of field-grown pear trees to P. syringae is related to N concentration in stems and is altered by spraying trees with urea or the defoliant CuEDTA at different times in the autumn before and after inoculation with the pathogen. Additionally, we also evaluated whether the effects of fungicides containing fosetyl-Al and phosphorous acid on P. syringae are altered by spraying trees with urea or CuEDTA.
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