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Screening Almond Rootstocks for Sources of Resistance to Armillaria Root Disease

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Screening Almond Rootstocks for Sources of Resistance to Armillaria Root Disease HORTSCIENCE 53(1):4–8. 2018. https://doi.org/10.21273/HORTSCI12038-17 Unlike most root pathogens, which produce spores that lie dormant in the soil (e.g., Phytophthora), Armillaria persists in its veg- Screening Almond Rootstocks for etative stage—mycelium—within woody roots left in the soil after clearing infected Sources of Resistance to Armillaria trees (residual roots). We have recovered viable A. mellea mycelium from residual Root Disease roots, in orchard and vineyard soil, originat- ing from tree crops and forest trees 10+ years Kendra Baumgartner1 and Phillip Fujiyoshi after they were cleared (Baumgartner and U.S. Department of Agriculture, Agricultural Research Service, Crops Rizzo, 2002; Rizzo et al., 1998). Fumigants Pathology and Genetics Research Unit, Davis, CA 95616 such as methyl bromide are only effective to the extent that they reach and penetrate Craig Ledbetter residual roots (Bliss, 1951; Munnecke et al., U.S. Department of Agriculture, Agricultural Research Service, Crop 1981). Injection of the systemic fungicide propiconazole to living trees reduced symp- Diseases, Pests and Genetics Research Unit, Parlier, CA 93648 tom severity in almond on Lovell rootstock Roger Duncan (Adaskaveg et al., 1999), but this approach can be cumbersome for large disease centers. University of California Cooperative Extension, Stanislaus County, Modesto, Another approach to postinfection manage- CA 94358 ment is root collar excavation, although this approach must be carried out in advance of Daniel A. Kluepfel severe symptom development (Baumgartner, U.S. Department of Agriculture, Agricultural Research Service, Crops 2004; Schnabel et al., 2012). Pathology and Genetics Research Unit, Davis, CA 95616 A 10-year field trial in France of ungrafted Prunus rootstocks (Guillaumin et al., 1989) Additional index words. Armillaria mellea, Armillaria tabescens, clonal rootstocks, Prunus identified A. mellea-resistant and susceptible dulcis, Prunus persica Prunus species. Peach (GF305) and peach · Abstract. Prunus dulcis (almond) is one of the most susceptible horticultural crops to hybrids [S3400 (Prunus besseyi peach) and · Armillaria root disease. Resistance to Armillaria mellea and Armillaria tabescens, the ‘Paramount’, aka GF677 (almond peach)] geographically isolated causal fungi that attack almond and closely related Prunus were most susceptible. By contrast, root- persica (peach), has been evaluated in studies of almond, peach, and other Prunus stocks with plum backgrounds [‘Myrabi’ rootstocks, but not in one comprehensive study. We evaluated the relative resistance to A. (Myrobalan), GF43 (Prunus domestica), · mellea and A. tabescens of six clonally propagated almond and peach rootstocks (Bright’s 5, Marianna GF8-1 (Myrobalan Prunus mun- Empyrean 1, Hansen 536, Krymsk 1, Krymsk 86, and Lovell) in comparison with that of soniana)] were the most resistant. Similar clonally propagated Marianna 2624 rootstock (resistant control) and clonally propa- results of lower mortality among rootstocks gated Nemaguard rootstock (susceptible control). Replicate clones used in the growth with plum parentage including Myrobalan chamber assay were micropropagated and rooted in vitro before inoculating the culture (P. cerasifera), P. munsoniana, or both were medium with Armillaria spp. At 2 months, the most resistant and susceptible rootstocks found in a field trial in California (Thomas were Krymsk 86 and Hansen 536, respectively, with 27% vs. 89% mortality. This finding et al., 1948). Interestingly, in the French was consistent among two isolates of A. mellea and one isolate of A. tabescens in three study, hybrids of peach and Myrobalan · · replicate experiments. Our finding of low mortality among Krymsk 86, Krymsk 1, and {‘Ishtara’ [Myrobalan (Myrobalan peach)], · Marianna 2624, which all share Prunus cerasifera (Myrobalan plum) parentage, is ‘Myran’ (Myrobalan peach)} were resis- consistent with past reports of resistance in the field to A. mellea, but conflicts with tant, suggesting that resistance in Myrobalan reports of susceptibility to A. tabescens. Resistance to A. tabescens of genotypes with is a dominant trait. Mortality was related to Myrobalan plum parentage in our assay may reflect the simplified rooting environment the degree of infection, such that rootstocks of tissue culture medium, which does not perfectly mimic a field trial, in which biotic and with low mortality also had very few infected abiotic factors may affect host resistance. Nonetheless, our growth chamber assay may roots, and a low proportion of plants were provide a more rapid alternative to identify sources of resistance for breeding and to girdled by root collar infection. These find- screen progeny of such crosses. ings suggest that the Armillaria resistance in Myrobalan, and possibly other plums, is true resistance, rather than tolerance. Resistant Prunus species are among the most sus- (Schnabel et al., 2005) and it limits the rootstocks became infected, but infection ceptible tree crops to Armillaria root disease productive lifespan of peach and necessitates was restricted to a low level. If Myrobalan in the northern hemisphere (Baumgartner premature removal and replanting of entire were ‘‘tolerant,’’ we would expect low mor- et al., 2011). In the southeastern United orchards (Schnabel et al., 2012). Outside the tality, in spite of high levels of root infection. States, the causal species is A. tabescens United States, there are similar reports of Field trials examining rootstock resis- stonefruit susceptibility from Mexico (Elias- tance to soil-borne pathogens are informa- Roman et al., 2013), Europe (Guillaumin tive, but they are lengthy and cannot be et al., 1989), the United Kingdom (Kable, modified after planting to accommodate Received for publication 18 Apr. 2017. Accepted 1974), Japan (Sasaki and Jinno, 1975), and new rootstocks. Unfortunately, inconsistent for publication 13 July 2017. China (Qin et al., 2007). The causal fungi infection in the greenhouse has been a bottle- Funding provided by the Almond Board of Cal- across these regions, A. mellea or A. tabes- neck to Armillaria research. Repeated at- ifornia. Tissue culture stocks of rootstocks were cens, colonize and kill woody roots and then tempts by researchers to modify each step provided by Javier Castillon (Duarte Nursery/Dry decompose the root wood as substrate. Such of the greenhouse infection assay have been Creek Laboratories, Hughson, CA). destruction to the root system over time unsuccessful at counteracting the high pro- Review of this manuscript and isolates of Armil- laria tabescens from Guido Schnabel (Plant & reduces yield and diminishes growth capacity portion of plants that escape infection and the Environmental Sciences Department, Clemson (Baumgartner, 2004), eventually killing in- slow rate of infection (Guillaumin et al., University) are gratefully acknowledged. fected trees. 1989; Mansilla et al., 2001; Singh, 1980). 1Corresponding author. E-mail: kendra.baumgartner@ Management of Armillaria root disease Rather than trying to recreate a field infection ars.usda.gov. focuses mainly on preplant soil fumigation. of plants grown in pots in a soil-based medium, 4 HORTSCIENCE VOL. 53(1) JANUARY 2018 | BREEDING,CULTIVARS,ROOTSTOCKS, AND GERMPLASM RESOURCES we grew plants in a tissue culture medium, could easily be selected for Armillaria in- percent mortality. ANOVA was performed which supports both plants and pathogen oculation at a later date. using the MIXED procedure in SAS, with (Baumgartner et al., 2010; Baumgartner Three Armillaria isolates (two A. mellea Kenward–Roger as the denominator df et al., 2013). With this infection assay, we and one A. tabescens) were inoculated sepa- method (Littell et al., 1996). All main and have overcome the major barriers of the rately to eight plant genotypes, in each of three interaction effects were considered fixed. F greenhouse approach, namely, eliminating experiments. The two isolates of A. mellea, values with P < 0.05 were considered signif- ‘‘escapes,’’ and bringing about consistent Sac304 and Sol310, were recovered from icant. Before ANOVA, homogeneity of var- and repeatable levels of mortality. We can symptomatic cherry and French prune, re- iance across treatments was confirmed (Box replicate experiments within a 1-year period spectively (Sacramento County, CA, and et al., 1978). After ANOVA, for significant to identify promising rootstock genotypes for Solano County, CA, respectively). Of our effects (P < 0.05), differences among means later, more lengthy evaluation in the field. collection of 16 A. mellea isolates from were assessed based on the presence/absence Our objective was to use this infection Prunus, these two isolates exhibited similar of overlap of their 95% confidence intervals, assay to identify Armillaria-resistant root- growth in culture (data not shown). Also, and means without overlapping intervals stocks for almond, by screening a set of eight exhibiting similar growth in culture was one were considered significantly different clonally propagated, commercially available isolate of A. tabescens, SC4 (aka SC.LD- (Westfall et al., 1999). rootstocks, which are graft-compatible with 1.02), which was recovered from symptomatic almond. Plant genotypes were selected based peach in Greenville County, SC (Schnabel Results on input from University of California Co- et al., 2005). Inoculum was prepared by operative Extension crop advisors and al- homogenizing for 30 s a 7-d culture grown The ranking of relative resistance of mond researchers,
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