Browsing by Author "Merwin, Ian A."

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    Malus germplasm varies in resistance or tolerance to apple replant pathogens in a mixture of New York orchard soils.
    (American Society for Horticultural Science, 2000-04) Isutsa, Dorcas K. 1; Merwin, Ian A.; ; ;
    We tested 40 seedling lots and 17 clonal accessions - representing 941 genotypes and 19 species or interspecific hybrids of Malus - for their resistance or tolerance to apple replant disease (ARD) in a mixture of five New York soils with known replant problems. Total plant biomass, root necrosis, root-infesting fungi, and root-lesion nematode (RLN; Pratylenchus penetrans Cobb) or dagger nematode (DN; Xiphinema americanum Cobb) populations were evaluated in apple seedlings and clones grown for ≃60 days in the composite soil. In addition to phytophagous nematodes, various Pythium, Cylindrocarpon, Fusarium, Rhizoctonia and Phytophthora species were isolated from roots grown in the test soil. Plant growth response was categorized by a relative biomass index (RBI), calculated as total plant dry weight in the pasteurized field soil (PS) minus that in an unpasteurized field soil (FS), divided by PS. Nematode reproduction on each genotype was defined by a relative reproduction index (RRI), calculated as final nematode populations in roots and soil (Pf) minus initial soil populations (Pi), divided by Pi. The RBI, RRI, and other responses of accessions to ARD soil were used to rate their resistance, tolerance, or susceptibility to apple replant disease. None of the accessions was completely resistant to ARD pathogens in our test soil. Seedling accessions of M. sieversii Roem. and M. kirghisorum Ponom. appeared to have some tolerance to ARD, based upon their low RRIs and RBIs. Three clonal rootstock accessions (G.65, CG.6210, and G.30), and four other clones (M. baccata Borkh. - 1883.h,M. xanthocarpa Langenf. - Xan, M. spectabilis Borkh. - PI589404, and M. mandshurica Schneid. - 364.s) were categorized as tolerant to ARD. The disease response of other accessions was rated as susceptible or too variable to classify. We concluded that sources of genetic tolerance to ARD exist in Malus germplasm collections and could be used in breeding and selecting clonal rootstocks for improved control of orchard replant pathogens.
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    Nematodes and fungi associated with apple replant disorder in sampled New York State orchards.
    (2014) Isutsa, Dorcas Khasungu; Merwin, Ian A.; ; ;
    The factors associated with unthrifty growth of apple are not fully determined, resulting i n inadequate diagnosis of apple replant disorder (ARD). The present study isolated and identified nematodes and fungi from ARD -infested soils to determine factors that interacted to cause ARD. Nematodes were extracted and identified in several orchard soil subsamples. The remaining soils were mixed to give composite soil. Transformed and non-transformed apple host seedlings were planted in the composite soil in a greenhouse, harvested after 10 weeks, and processed for isolation and identification of fungi. Pratylenchus and Xiphinema nematodes were present in the soils. Root-lesion nematode (RLN) frequency was always 100%, and dagger nematode (DN) frequency ranged from 40-70%. The RLN and DN ranged from 56-350 and 5-58 per 100 cm 3 of soil, respectively. Isolation frequency of Pythium species was highest (29%), followed by 10% for Pythium cryptogea, 4% for Pythium cambivoraand Pythium catorum, 3% for Pythium megasperma , and 2% for Phytophthora species. Both transformed and non-transformed apple hosts were infested by at least several fungi. Of the six apple hosts tested, Pythium sp. and F. oxysporum colonized six each, Pythium cryptogea, C. lucidum and C. destructans colonized three each, Pythium cactorum, Pythium cambivora and Pythium megasperma two each, and Phytophthora sp., F. solani, F. equiseti, F. acuminatum and Rhizoctonia sp. one each. Consequently, the ARD symptoms observed on diverse apple hosts were associated with combined effects of RLN, DN and various species of Pythium, Phytophthora, Cylindrocarp on, Fusarium and Rhizoctonia. Whenever unthrifty growth is observed in soil especially that under perennial fruits and continuously cultivated annual crops, similar diagnostic isolation of multiple pathogens should be performed. In developing management st rategies, germplasm for the crop in question should be evaluated in multiple sites or in composite soil in order to identify germplasm with broadspectrum resistance or tolerance to the pathogens.