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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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T.G. Beckman and P.L. Pusey

Armillaria root rot is the second leading cause of peach tree mortality (after peach tree short life) in the southeastern United States. Currently, there are no commercially available rootstocks for peach with proven resistance to this pathogen in the United States. Since 1983, we have been screening rootstock candidates for resistance to Armillaria utilizing naturally infected field sites. Inoculation of peach [Prunus persica (L.) Batsch], plum (P. cerasifera J.F. Ehrh., P. munsoniana F.W. Wight & Hedr., P. salicina Lindl. or P. angustifolia Marsh.) × peach and plum × plum hybrid rootstocks with infected plant tissue (such as acorns, Quercus sp.) prior to planting has provided a significantly increased infection and mortality of candidate rootstock lines in comparison with sole reliance on natural inoculum on an infested site.

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Desmond R. Layne, Guido Schnabel, Kerik Cox, Ralph Scorza, and Karen Bussey

Armillaria root rot (ARR) of peach caused by the soil-borne basidiomycete fungus Armillaria tabescens is causing premature decline and mortality of peach trees on most southeastern U.S. peach farms. Soil inoculum may be present both in former peach orchard sites and on sites that were once in hardwood forest. The fungus is protected under the bark of dead root pieces and may survive up to 100 years at various depths in the soil profile. No commercially available rootstocks are resistant to ARR. Since 2002, we have embarked on a multipronged strategy to develop control options to combat ARR. First, we have two replicated trials on commercial grower replant sites with a history of ARR. Trial 1 compares four preplant fumigation treatments (none, Telone II, methyl bromide, and Enzone), three rootstocks (Lovell, Halford, and Guardian) and preplant root dips with endomycorrhizal fungi. Trial 2 compares the use of raised beds, root collar excavation and preplant root dips. Both trials examine long-term productivity and tree survival. Second, we are examining the use of systemic fungicide injection into infected trees to protect trees around infection foci. Third, we are trying to develop a genetically modified ARR-resistant rootstock. We have inserted the gene encoding the gastrodia antifungal protein (GAFP—a low molecular weight lectin that binds mannose and chitin) from a Chinese orchid into tobacco (model herbaceous system) and plum (model Prunus system). GAFP has antifungal activity against several basidiomycete root rot pathogens. Pathogenicity tests with transformed tobacco plants show enhanced tolerance to several root rot pathogens when compared to nontransformed plants. Transformed plums are being multiplied for pathogenicity tests.

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Francesco Loreto, Harold H. Burdsall Jr., and Alfio Tirro'

The effect of inoculating seedlings of Mediterranean cultivated trees grown under greenhouse conditions with North American isolates of Armillaria mellea (Vahl: Fr) Kumm. and A. ostoyae (Romagn.) Herink on net photosynthesis (A), stomatal conductance (gs), and water potential was examined. The effect of water stress was determined also on the same plant species independently and in combination with Armillaria infection. Red oak (Quercus rubra L.) was used as a control to indicate Armillaria virulence on North American trees. Carob (Ceratonia siliqua L.) was resistant to infection. Infection was successful in sour orange (Citrus aurantium L.), but A, gs, and water potential were unchanged over the 60-day experiment. In olive (Olea europea L.) and oak, A and gs were reduced following inoculation with A. mellea. A and gs of all species but carob were reduced under water stress. Olive and oak responses to water stress and Armillaria infection were quantitatively similar; however, the two stresses combined did not reduce A and gs further. Red oak was strongly susceptible to A. ostoyae infection, but Mediterranean trees were not infected by the same Armillaria isolate. Our results show that Armillaria infection may reduce A and gs in susceptible species.

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T.G. Beckman, W.R. Okie, A.P. Nyczepir, P.L. Pusey, and C.C. Reilly

Nearly 5000 seedling trees representing more than 100 peach [Prunus persica (L.) Batsch.] and plum (Prunus spp.) lines were planted at a 4 × 0.6-m spacing in Jan. 1983, on a site with a known history of peach tree short life (PTSL) and Armillaria root rot (ARR). Trees were arranged in a randomized complete-block with eight replicates of six trees each. Beginning in Spring 1984 and each year thereafter the cause of tree death was determined. At the end of 9 years, 50% of the trees had succumbed to PTSL and 35% had been killed by ARR apparently caused by Armillaria tabescens. Analysis of the data for trees killed by ARR showed a wide range in mortality, some peach lines appeared significantly more tolerant to ARR than others. Plum lines derived from native North American species also appeared to be a potential source of improved tolerance. We did not establish whether ARR tolerance is affected by PTSL.

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Thomas G. Beckman, Jose X. Chaparro, and Wayne B. Sherman

( Prunus persica L. Batsch) varieties on Armillaria root rot (ARR) [ Armillaria tabescens (Scop.:Fr.) Dennis et al.] infested sites where peach seedling type rootstocks often fail to provide satisfactory tree longevity. Peach varieties budded onto ‘MP-29

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Thomas G. Beckman, Jose X. Chaparro, and Wayne B. Sherman

( Prunus persica L. Batsch) varieties on Armillaria root rot (ARR) [ Armillaria tabescens (Scop.:Fr.) Dennis et al.] infested sites where peach seedling-type rootstocks often fail to provide satisfactory tree longevity. Peach varieties budded onto ‘Sharpe

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Desmond R. Layne and Guido Schnabel

In 2003, a replicated long-term research trial was established on a commercial peach replant site with a history of Armillaria root rot and other soilborne diseases. The objectives of the trial were to determine the short- and long-term effects of preplant fumigation, rootstock, and preplant root dipping with mycorrhizal fungi and beneficial bacteria on tree growth, productivity, and survival. Preplant fumigants included none (control), methyl bromide, Telone II, or Enzone. Rootstocks tested included Guardian, Lovell, and Halford. Root dipping (or not) was with MycorTree. The scion cultivar was Big Red. There were a total of 24 experimental treatment combinations and the trial site comprised more than 1500 trees on 11.5 acres. By 2 years after planting, fumigation with Enzone was disadvantageous when compared with no treatment at all. Enzone-treated blocks had higher tree mortality or were significantly reduced in growth compared to other treatments. Preplant fumigation with Telone II or methyl bromide, however, resulted in reduced tree stunting and phytotoxicity and increased tree growth when compared to the untreated control. After 2 years, 10% of the total trees planted were dead. Half of these were from the Enzone treatment. Enzone does not appear to be a viable preplant fumigation product for South Carolina peach growers, based on this preliminary data. Both Guardian and Halford rootstocks had performance superior to Lovell during the first 2 years. Although Guardian trees were smaller than Halford at the time of planting, by the end of the second growing season, their TCA was not significantly different. There was no benefit to preplant root dipping with MycorTree. Experimental results were not influenced by the location of trees on the site.

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Thomas G. Beckman, Philip A. Rollins, James Pitts, Dario J. Chavez, and Jose X. Chaparro

California where Armillaria mellea appears to be the predominant species encountered ( Adaskaveg et al., 2008 ), in the southeastern United States, the principal species encountered appears to be A. tabescens and only occasionally A. mellea (T