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Kendra Baumgartner, Phillip Fujiyoshi, Greg T. Browne, Chuck Leslie, and Daniel A. Kluepfel

Armillaria root disease affects orchards in all Juglans regia (Persian walnut)-growing regions of California ( Gardner and Raabe, 1963 ). The causal agent is Armillaria mellea (Basidiomycota, Physalacriaceae), which attacks walnut and other

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Kendra Baumgartner, Phillip Fujiyoshi, Craig Ledbetter, Roger Duncan, and Daniel A. Kluepfel

Prunus species are among the most susceptible tree crops to Armillaria root disease in the northern hemisphere ( Baumgartner et al., 2011 ). In the southeastern United States, the causal species is A. tabescens ( Schnabel et al., 2005 ) and it

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Renata Goossen and Kimberly A. Williams

under replicated or controlled conditions. H 2 O 2 is a well-known oxidizing agent often used as a remedy by consumers to treat algae and root decay from presumed root disease on interior plants, as well as to encourage root growth and health

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R. Kasten Dumroese, Robert L. James, and David L. Wenny

Inoculum of Douglas fir root diseases caused by the fungi Fusarium and Cylindrocarpon is carried from crop to crop in reused containers. Soaking containers for 90 seconds in 80 °C water removed ≈99% of Fusarium and 100% of Cylindrocarpon inoculum between growing cycles. Overall seedling growth was also improved: seedlings grown in containers soaked between growing cycles were 10% taller and had 20% more biomass than seedlings grown in nonsoaked containers. We obtained a 13% increase in the number of deliverable seedlings from containers soaked in hot water between crops, from the use of copper coated containers, or from both practices combined.

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B.H. Ownley and D.M. Benson

Abbreviations: PB, pine bark. 1 Former Graduate Research Assistant. Current address: USDA-ARS, Root Disease and Biological Control Research Unit, 367 Johnson Hall, Washington State Univ., Pullman, WA 99164-6430. 2 Professor. This research was

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L.E. Datnoff and R.T. Nagata

The effects of corky root (CR) disease, caused by Rhizomonas suberifaciens(van Bruggen, Jochimsen, and Brown) on fresh and marketable weights of lettuce (Lactuca sativa L.) were assessed during the 1988 and 1989 cropping seasons in several commercial lettuce fields. The resistant crisphead cultivars Raleigh and South Bay and the susceptible cultivars Ithaca and Shawnee produced similar yields in fields either planted in lettuce for the first time or in continuous lettuce production for three cropping cycles. Average yields of the resistant cultivars, from two fields cropped for six cycles naturally infested with CR, ranged from 875 to 1062 g/head fresh weight and 674 to 907 g/head marketable weight. The average yields of the susceptible cultivars in these infested fields ranged from 419.8 to 668.7 g/head fresh weight (37% to 52% yield loss) and 317.5 to 488.2 g/head marketable weight (46% to 53% yield loss). CR severity ratings were highly negatively correlated with root dry matter accumulation and whole and marketable head weights of-lettuce.

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Benjamin K. Hoover and R.M. Bates

’s Studentized range test ( P ≤ 0.05); 1 g = 0.0353 oz. Fraser fir inoculated with Phytophthora had severely diseased roots at the conclusion of the 2009 experiment. Canaan fir inoculated with Phytophthora had little root disease in non-flooded treatments

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B.K. Duffy and G. Défago

Host nutritional variables were evaluated for their effects on the severity of crown and root rot of tomato caused by Fusarium oxysporum f.sp. radicis-lycopersici. Tomato (Lycopersicon esculentum Mill.) seedlings (cv. Bonnie Best) were grown in a pathogen-infested, soilless rockwool system in the greenhouse and were fertilized with a nutrient solution that was amended with macro- and microelements at various rates. Disease was evaluated after 2 weeks using an index of 0 to 4, and plant fresh weight was measured. Regression analysis indicated that disease severity was significantly increased by ammonium-nitrogen [NH4Cl, (NH4)6Mo7O24, and (NH4)2SO4], NaH2PO4·H2O, Fe-EDDHA, MnSO4, MoO3, and ZnSO4·7H2O. Disease severity was reduced by nitrate-nitrogen [Ca(NO3)2·4H2O] and CuSO4·H2O. Low rates of NH4NO3 (39 to 79 mg·L-1 N) reduced disease, but rates above 100 mg·L-1 N increased it. Disease was not affected by MgSO4·7H2O. In all cases, plant growth was inversely related to disease severity. Mineral fertilizers had no effect on nutrient solution pH. This information sheds new light on environmental factors that influence plant-pathogen interactions, and may be applied to develop a management strategy for Fusarium crown and root rot based on host nutrition.

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Ariena H.C. van Bruggen, Philip R. Brown, and Art Greathead

In growth chamber experiments with five concentrations of NH4NO3 and inoculation of lettuce (Lactuca sativa L.) cv. Salinas with Rhizomonas suberifaciens, the causal agent of corky root (CR), symptoms of noninfectious corky root induced by high rates of N were distinct from those of infectious corky root (ICR). Nitrogen toxicity was observed at 350 kg·ha-1 and above, and was not affected by inoculation with R. suberifaciens. There was a curvilinear relationship between concentration of NH4NO3 applied and ICR severity with a maximum at 525 kg·ha-1. In a similar growth chamber experiment with NH4NO3 plus urea, ICR severity decreased and N toxicity increased at increasing N levels (N at 160 to 650 kg·ha-1). In microplots at Davis, Calif., sidedressing with NH4NO3 (N at 170 kg·ha-1) increased ICR severity on `Salinas' lettuce over the nonfertilized control. There was a significant interaction between N fertilization and soil-infestation with R. suberifaciens with respect to head fresh weight: sidedressing with NH4NO3 increased head weight in noninfested plots, but decreased head weight in infested plots. In four field experiments at Salinas, Calif., sidedressing with N at 78 to 213 kg·ha-1, with N as (NH4)1SO4, NH4NO3, urea, or Ca(NO3)2, increased ICR over the control, but there were no significant differences between the forms of N. Head fresh and dry weights were either increased or unaffected by sidedressing with N fertilizers, depending on the residual concentrations of N in the soil. The increase in ICR was likely related to concentrations of soil NO3 rather than NH4.

Open access

Natalia Salinas, Zhen Fan, Natalia Peres, Seonghee Lee, and Vance M. Whitaker

various levels of anthracnose disease in the same plant ( Forcelini et al., 2016a ), and FaRCa1 effects on ARN have not been previously documented. ARN is particularly important because once root disease is found in newly planted fields, control methods