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S.M. Southwick, M.E. Rupert, J.T. Yeager, K.G. Weis, B.C. Kirkpatrick, E.L. Little and B.B. Westerdahl

Bacterial canker (BC), caused by Pseudomonas syringae pv. syringae van Hall, is a serious disease of stone fruits that occurs most commonly in young orchards. Many factors can predispose or increase the risk that trees develop BC such as sandy or compacted soils, low soil pH, inadequate tree nutrition, frost or cold injury, genetic susceptibility, and presence of ring nematode, Criconemella spp. However, questions still remain about how these factors influence disease incidence in `French' prune, Prunus domestica L. In 1991, we established a 3.64-ha plot in Winters, Calif., to determine the effects of nitrogen (N) fertigation on growth responses and yield of young prune trees. N was applied through a surface drip system at 0, 0.11, 0.23, and 0.45 kg actual N/tree per year as UN32 urea (Unocal, Sacramento Calif.) with 1/10th of the total amount delivered per application every other week from May through September starting in 1992. Two other treatments were also included: 0.064 kg N/tree per year through surface drip if % leaf N dropped below 2.3%, and 0.23 kg N/tree/year delivered in small amounts every irrigation via an automated buried drip system. Symptoms of BC began appearing primarily in the 0- and 0.064-N treatments in 1993. During 1995 and 1996, we demonstrated highly significant relationships between low N status measured in leaves and increased incidence of BC. Furthermore, we determined levels of N application via drip irrigation, which resulted in good yields, vigorous growth, and lack of BC in our test plots, but also minimized N use and potential for nitrate leaching into groundwater. These and additional results will be presented.

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G.J. Puterka, R. Scorza and M.W. Brown

Damage by lesser Peachtree borer (LPB) (Synanthedon pictipes Grote & Robinson) and Leucostoma canker that had accumulated during 6 (Orchard A) and 8 (Orchard B) years were compared in peach (Prunus persica L.) and peach-almond [P. amygdalus (Mill.) D. A. Webb] hybrids. Afterward, the main trunk and scaffold limbs of the trees received 10 wounds 26 mm in diameter and a subset of these trees in Orchard A had wounds inoculated with Leucostoma persoonii Hohn. Before wounding, Leucostoma canker infection and LPB infestations that had accumulated for 6 to 8 years on peach-almond hybrids was ≈60% and 98% less than on peach in Orchard A and B, respectively. One month after wounding the trees, no significant differences in Leucostoma canker infection and LPB infestations were found among the peach-almond hybrids, treated or not treated with L. persoonii, or untreated peach. Yet, Leucostoma- treated and untreated peach-almond hybrids had 33% and 25% less Leucostoma canker and LPB, respectively, when compared with Leucostoma- treated peach. Ten months after wounding, peach-almond hybrids treated with L. persoonii still had significantly less Leucostoma canker (60%) and LPB (25%) when compared with Leucostoma- treated peach. Wound gumming and wound closure rates seemed to influence the degree of LPB infestation and Leucostoma canker. Based on these data, peach-almond hybrids could be valuable sources of resistance to LPB and Leucostoma canker.

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Thomas R. Gordon, Dorothy Okamoto, Andrew J. Storer and David L. Wood

Pitch canker, caused by Fusarium subglutinans f. sp. pini, causes branch dieback and stem cankers in many species of pine. Monterey pine (Pinus radiata D. Don), one of the most widely planted pines in the world, is extremely susceptible to pitch canker. Four other pine species, which might serve as alternatives to Monterey pine in landscape settings, were found to be relatively resistant, based on the size of lesions resulting from branch inoculations under greenhouse conditions. Of these species, Japanese black pine (P. thunbergiana Franco) was the most resistant, followed by Canary Island pine (P. canariensis Sweet ex K. Spreng), Italian stone pine (P. pinea L.), and Aleppo pine (P. halepensis Mill.). Consistent with these findings, a field survey conducted in Alameda County, Calif., revealed Monterey pine to have the highest incidence of infection, with significantly lower levels in Aleppo, Canary Island, and Italian stone pines. Japanese black pine was not observed in the survey area.

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Zeinab A. El-Hamalawi and John A. Menge

The sugary exudate appearing on bark lesions of Persea americana Miller and Persea indica plants after infection with Phytophthora citricola contained viable oospores and hyphal fragments in the field and in the greenhouse. This sugary exudate was a source of inoculum and dispersal of the pathogen within and between avocado plants. Spraying water onto lesions moved inoculum from the sugary exudate to wounds below. Water from sprinkler irrigation washed propagules into the soil around the plants. Viable propagules of Phytophthora citricola were identified in the feces of snails (Helix aspersa) that had fed on infected bark tissues. When these snails were moved to healthy plants, they made wounds on succulent tissue, and the infectious feces induced cankers. Ants (Iridomyrmex humilis) were attracted to the sugary exudate and also transmitted infectious propagules to wounds on avocado stems and to the soil. Control strategy for the avocado stem canker disease should consider control of vectors.

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R. Danti, A. Panconesi, V. Di Lonardo, G. Della Rocca and P. Raddi

Common cypress (Cupressus sempervirens L.) is an integral feature of the landscape of many Mediterranean regions (particularly in Tuscany). Furthermore, common cypress has multiple uses as ornament, timber, windbreaks, recovery of deteriorated soils, protection of slopes and the production of essential oils. During the last three decades, Seiridum cardinale canker of cypress has caused severe losses in woods, windbreaks and ornamental plantings in the Mediterranean area, significantly limiting the use and cultivation of this tree.

In this paper we describe `Italico' and `Mediterraneo', two C. sempervirens varieties which were patented in 2004 for their resistance to cypress canker. Both varieties are particularly suitable for ornamental purposes because of their fastigiated columnar habit and the low production of male flowers and cones. `Italico' and `Mediterraneo' represent the recent result of the breeding programme on cypress, in progress since the 1970s. This program aims at: 1) obtaining a high number of canker-resistant cypress clones and multi-clonal varieties to be used for ornamental plantings and windbreaks and 2) finding mother trees for the production of improved seed for reforestation purposes.

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David R. Sandrock, Jean Williams-Woodward and Michael A. Dirr

Five taxa of Atlantic white cedar [Chamaecyparis thyoides (L.) B.S.P.], `Blue Sport', `Okefenokee', Raulston Form, `Emily', and `Rachel', and one cultivar of Leyland cypress [×Cupressocyparis leylandii (Jacks. and Dallim.) Dallim.], `Haggerston Grey', were screened for resistance to Botryosphaeria and Seiridium cankers. Treatments consisted of Seiridium unicorne (Cke. And Ell.) Sutton, Botryodiplodia Sacc. sp., Fusicoccum Corda. sp. and the non-inoculated control. After 8 weeks, plants were measured for change in caliper at the wound site, change in plant height, and length and width of surface and interior cankers. Seiridium and Botryosphaeria canker development on Atlantic white cedar taxa was not significantly different than that on Leyland cypress. Seiridium unicorne was more pathogenic than Botryodiplodia sp. and Fusicoccum sp. on Atlantic white cedar and Leyland cypress with infection percentages of 100%, 84%, and 80%, respectively. Well-defined, sunken, resinous cankers developed on Leyland cypress plants infected with Seiridium unicorne, whereas Atlantic white cedar showed no visible surface canker.

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A.R. Biggs

Whole-shoot water potential, osmotic potential of the xylem fluid, and bark water potential were examined from late winter through early spring for six peach [Prunus persica (L.) Batsch.] cultivars varying in relative susceptibility to Leucostoma canker. There were significant differences among cultivars for whole-shoot water potential on all 11 dates tested in 1986, but not in 1985. The date effect was not consistent among cultivars, although when averaged across dates, the whole-shoot water potential of `Loring' was significantly more negative than that of `Candor' or `Garnet Beauty'. There were significant differences among cultivars for xylem fluid osmotic potential on one of five dates tested in 1985 and three of 11 dates tested in 1986, although cultivar differences were not consistent between years. Cultivars exhibited differences in bark water potential on three of five dates tested in 1985, with `Loring' exhibiting the least negative values when averaged across dates. There were only occasional significant correlations of the water status characteristics with relative susceptibility to Leucostoma canker or suberin accumulation. Measurements of plant water status among cultivars or genotypes in peach do not appear to be reliable indicators of susceptibility to Leucostoma spp. or wound response.

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Zeinah A. El-Hamalawi and John A. Menge

At monthly intervals, plants and stem cuttings of avocado (Persea americana Miller) `Hass' grafted on `Barr Duke' rootstock and `Topa Topa' growing in a lathhouse were wounded and inoculated with the stem canker pathogen, Phytophthora citricola Sawada. The seasonal changes (measured monthly) in the extent of colonization of the avocado plants by P. citricola followed a periodic pattern, with two peaks of colonization during an annual growth cycle. Concentration of free amino acids and total soluble carbohydrates in the plant tissues followed a periodic pattern with two peaks similar to that of canker growth. Months were significantly different for canker size, free amino acids, and total soluble carbohydrates of the bark tissues. The extent of colonization was highest during May-June, after the first vegetative flush, and during November-December, after the second vegetative flush. Total free amino acids of the hark tissue was highly correlated with canker size (r = 0.89). Although the total soluble carbohydrate of the bark tissue was also elevated during the periods of canker development, it showed lower positive correlation (r = 0.45) with canker size. Plants were relatively resistant to colonization through March-April, during the first vegetative flush, and through August-September, during the second vegetative flush. Cankers formed on stem cuttings were generally larger than those of intact plants.

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Thomas R. Gordon, Dorothy Okamoto, Andrew J. Storer and David L. Wood

Pitch canker, caused by Fusarium subglutinans f. sp. pini, causes branch die-back and stem cankers in many species of pine. Monterey pine (Pinus radiata D. Don), one of the most widely planted pines in the world, is extremely susceptible to pitch canker. Four other pine species, which might serve as alternatives to Monterey pine in landscape settings, were found to be relatively resistant, based on the size of lesions resulting from branch inoculations under greenhouse conditions. Of these species, Japanese black pine (P. thunbergiana Franco) was the most resistant, followed by Canary Island pine (P. canariensis Sweet ex K. Spreng), Italian stone pine (P. pinea L.), and Aleppo pine (P. halepensis Mill.). Consistent with these findings, a field survey conducted in Alameda County, Calif., revealed Monterey pine to have the highest incidence of infection, with significantly lower levels in Aleppo, Canary Island, and Italian stone pines. Japanese black pine was not observed in the survey area.

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John M. DeLong and Robert M Crassweller

Fruit Yield and quality measurements were taken from 10 year-old `Newhaven' peach trees pruned at prebloom (preb), full-bloom (fb), and 2, 4, 6, and 8 weeks following fb. Trees were hand-pruned with dormant type thinning and heading cuts. Fruit weight and circumference tended to decrease from fb onwards, while fruit were firmer at fb+4 and fb+6 than at preb. Firmer peaches were also measured at fb+6 than at fb. Fruit color was measured on a tristimulus color scale. Value (L) indicated that lighter fruit occurred with pruning at preb, fb, and fb+4 weeks, while fruit were darker at fb+2, fb+6 and fb+8 weeks. Hue (a) indicated that trees being pruned at preb had redder fruit than those pruned at fb+2 or fb+8 weeks. Redder fruit were also measured at fb+6 than at fb+8 weeks. Chroma (b) indicated that peaches from trees pruned at fb+4 had the highest degree of yellow, while fruit from tree pruning at fb were more yellow than at fb+2, fb+6 and fb+8 weeks. Also being investigated are the pruning timing effects on cold hardiness and carbohydrate status of one year-old stems, and on Cytospora canker incidence at the pruning cuts.