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‘MP-29’ rootstock has been jointly released for grower trial by the U.S. Department of Agriculture, Agricultural Research Service (Byron, GA) and the Florida Agricultural Experiment Station. ‘MP-29’ is suggested for trial as a rootstock for peach (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’ rootstock displayed significantly better resistance to ARR than did trees on ‘Guardian’ or ‘Sharpe’ rootstocks. Trees on ‘MP-29’ displayed resistance to peach tree short life (PTSL) comparable to that of trees budded onto ‘Guardian’ peach seedling rootstock. ‘MP29’ also provides a marked reduction in tree vigor compared with peach seedling type rootstocks and may prove useful as a semidwarf rootstock where a less vigorous tree is preferred.

Sharpe is suggested for trial as a rootstock for peach (Prunus persica) varieties on Armillaria root rot (Armillaria tabescens) infested sites where peach seedling-type rootstocks often fail to provide satisfactory tree longevity. Peach cultivars budded onto Sharpe rootstock display resistance to peach tree short life comparable to that of trees budded onto Guardian peach seedling rootstock. It is resistant to Meloidogyne incognita and Meloidogyne floridensis. Sharpe has been tested for compatibility with a range of peach and nectarine scions, including Delta, Fireprince, Flordaguard, Gulfcrest, Harvester, Majestic, Redhaven, Rubyprince and Summergrand, all of which displayed satisfactory compatibility (observation periods range from 3 to 10 years and are still in progress). It also displayed satisfactory compatibility with Ozark Premier and Stanley plum scions in short-term trials.

Twelve peach [Prunus persica (L.) Batsch] seedling rootstocks [Lovell, Nemaguard, Flordaguard, 14DR51, five Guardian™ (BY520-9) selections, and three BY520-8 selections] budded with `Cresthaven' were planted in 1994 and evaluated through 2000 to determine performance under commercial management practices. Mesocriconema xenoplax population densities were above the South Carolina nematicide treatment threshold of 50 nematodes/100 cm3 of soil after 1996. However, symptoms of peach tree short life (PTSL) were not observed. Tree mortality was less than 14% through 1999, with most of the dead trees exhibiting symptoms consistent with Armillaria root rot. About 13% of the surviving trees in 1999 were removed in 2000 due to symptoms of phony peach. There were no differences in tree mortality among rootstocks. Tree growth, photosynthesis, and suckering varied among rootstocks, but leaf conductance, internal CO2, and leaf transpiration did not. Foliar calcium, magnesium, iron, and phosphorus varied among rootstocks, but all were within the range considered sufficient for peach trees. Fruit yield varied among rootstocks, but yield efficiency did not, indicating that higher yield corresponded with larger trees. Bloom date did not vary among rootstocks, but harvest date was advanced as much as 2 days for some rootstocks, compared to Lovell. Fruit weight varied among rootstocks but skin color, flesh firmness, and soluble solids content were similar. All rootstocks performed satisfactorily for commercial peach production.

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.

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 commer- cially 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.) x peach and plum x 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. The principal causes of premature tree mortality in the peach growing areas of the southeastern United States are peach tree short life (PTSL), responsible for ≈50% of tree deaths annually (Miller, 1994), and Armillaria root rot (ARR). The latter disease is estimated to be responsible for 35% of annual tree losses with an estimated lifetime production value (value of lost crops through an expected 15 year lifespan) of $5-6 million (Miller, 1994). The peach trees in these areas are propa- gated almost entirely on peach seedling rootstocks, typically 'Lovell', 'Nemaguard', and, more recently, 'Guardian' ® the U.S. Dept. of Agriculture (USDA) and Clemson Univ. in 1993 (Okie et al., 1994a), is rapidly displacing both 'Lovell' and 'Nemaguard'. 'Guardian' ® is clearly more resistant to PTSL than is 'Lovell'. In addi- tion, 'Guardian' ® , unlike 'Lovell', appears to have root-knot nematode resistance compa- rable to that of 'Nemaguard'. Like most peach seedling rootstocks, 'Lovell', 'Nemaguard', and 'Guardian'

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.

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.